2014 Vol. 32, No. 3
Display Method:
2014, 32(3): 399-409.
Abstract:
Taking sedimentary filling of the Neoproterozoic rift basin in central Yunnan province as a research object, sedimentology and basin analysis researches are carried out in this paper. The results show that there are four filling sequences in the Neoproterozoic rift basin in central Yunnan province, representing four basin evolution stages respectively. The Liubatang Formation and lower Luliang Formation, which were deposited between 820 Ma and 800 Ma regarded as low density turbidity current and deep water starved deposits, belonging to the first sequence that represents the rapid subsidence stage of the rift basin; The Chengjiang Formation, upper Luliang Formation and Niutoushan Formation, which were deposited between 800 Ma and 725 Ma and regarded as fan dalta and lacustrine deposits, belonging to the second sequence that represents the mature development stage of the rift basin; The Nantuo Formation that might be deposited between 725 Ma and 635 Ma and are interpreted as continental glacial deposits, belonging to the third stage that represents the transition stage from the rift basin to the passive margin basin; The Doushantuo Formation, which were deposited between 635 Ma and 551 Ma and interpreted as tidal flat deposits, belonging to the rift cover. Further study reveals that the Neoproterozoic rift basin in central Yunnan province composed of mini unidirectional half-graben basins during juvenile stage and evolved into a large united half-graben basin during mature stage, which should be attributable to a supradetachment basin. Furthermore, it is demonstrated that the Neoproterozoic rift basin in central Yunnan province should be intracontinental rift basin, which may be related to the breakup of Rodinia Supercontinent.
Taking sedimentary filling of the Neoproterozoic rift basin in central Yunnan province as a research object, sedimentology and basin analysis researches are carried out in this paper. The results show that there are four filling sequences in the Neoproterozoic rift basin in central Yunnan province, representing four basin evolution stages respectively. The Liubatang Formation and lower Luliang Formation, which were deposited between 820 Ma and 800 Ma regarded as low density turbidity current and deep water starved deposits, belonging to the first sequence that represents the rapid subsidence stage of the rift basin; The Chengjiang Formation, upper Luliang Formation and Niutoushan Formation, which were deposited between 800 Ma and 725 Ma and regarded as fan dalta and lacustrine deposits, belonging to the second sequence that represents the mature development stage of the rift basin; The Nantuo Formation that might be deposited between 725 Ma and 635 Ma and are interpreted as continental glacial deposits, belonging to the third stage that represents the transition stage from the rift basin to the passive margin basin; The Doushantuo Formation, which were deposited between 635 Ma and 551 Ma and interpreted as tidal flat deposits, belonging to the rift cover. Further study reveals that the Neoproterozoic rift basin in central Yunnan province composed of mini unidirectional half-graben basins during juvenile stage and evolved into a large united half-graben basin during mature stage, which should be attributable to a supradetachment basin. Furthermore, it is demonstrated that the Neoproterozoic rift basin in central Yunnan province should be intracontinental rift basin, which may be related to the breakup of Rodinia Supercontinent.
2014, 32(3): 418-428.
Abstract:
Based on outcrops, drilling and logging well data, seismic data and thin sections, carbonate shoals depositional characteristics have been studied and the results have been summarized as follows: the classification criteria of the carbonate shoals have been established, and four types of carbonate shoals (intraclastic shoal, oolitic shoal, bioclastic shoal and algae shoal) have developed in Ordovician; the sedimentary characteristics of the micro and macro Ordovician carbonate shoals have been integrated: the algal peloids, pellets and the algae clumps are the main components of algae shoals, and algae shoals are interbedded with algae crystalline dolomite; the content and shape of intraclastics shows the different parts of intraclastics shoals and their depositional environment energy, and intraclastics shoals have begun to develope in large-scale in the Yingshan Formation; oolitics shoals are mainly distributed in the platform marginal area of the Tazhong and Yubei area with a few other grains developed in the main part of oolitics shoals; bioclastics shoals are composed of the reef fragments and bioclastics, and are developed with the reefs complex, and mainly distributed in the platform marginal area and the margin area of inside adjacent platform. The bioclastic shoals have developed in the Yijianfang Formation. The GR value are low corresponding to the shoals formation, and the GR mainly shaped like box, funnel and bell in the shoals, while the finger and peak shapes are of the side part of the shoals. Furthermore, the shoals shows retrogradational migration from the marginal to the platform inside. According to the sedimentary characteristics, four carbonate stacking patterns have been established, including: algae-reef-shoal interbed, thick-layer cake aggradation shoal, and thin-layer cake retrogradation shoal, reef-shoal complex. Especially, algae-reef-shoal interbed and reef-shoal complex two stacking patterns distributions have been limited by the facies, but easily to form effective reservoirs, which have been proved in Ordovician of Tazhong area. The thick-layer cake aggradation shoals and thin-layer cake retrogradation shoals widely distributed in the Yingshan Formation. Therefore, combining the distribution characteristics and reservoir forming mechanism. It is conduded that the carbonate shoal reservoirs of the Yingshan Formation have distributed with regional advantages.
Based on outcrops, drilling and logging well data, seismic data and thin sections, carbonate shoals depositional characteristics have been studied and the results have been summarized as follows: the classification criteria of the carbonate shoals have been established, and four types of carbonate shoals (intraclastic shoal, oolitic shoal, bioclastic shoal and algae shoal) have developed in Ordovician; the sedimentary characteristics of the micro and macro Ordovician carbonate shoals have been integrated: the algal peloids, pellets and the algae clumps are the main components of algae shoals, and algae shoals are interbedded with algae crystalline dolomite; the content and shape of intraclastics shows the different parts of intraclastics shoals and their depositional environment energy, and intraclastics shoals have begun to develope in large-scale in the Yingshan Formation; oolitics shoals are mainly distributed in the platform marginal area of the Tazhong and Yubei area with a few other grains developed in the main part of oolitics shoals; bioclastics shoals are composed of the reef fragments and bioclastics, and are developed with the reefs complex, and mainly distributed in the platform marginal area and the margin area of inside adjacent platform. The bioclastic shoals have developed in the Yijianfang Formation. The GR value are low corresponding to the shoals formation, and the GR mainly shaped like box, funnel and bell in the shoals, while the finger and peak shapes are of the side part of the shoals. Furthermore, the shoals shows retrogradational migration from the marginal to the platform inside. According to the sedimentary characteristics, four carbonate stacking patterns have been established, including: algae-reef-shoal interbed, thick-layer cake aggradation shoal, and thin-layer cake retrogradation shoal, reef-shoal complex. Especially, algae-reef-shoal interbed and reef-shoal complex two stacking patterns distributions have been limited by the facies, but easily to form effective reservoirs, which have been proved in Ordovician of Tazhong area. The thick-layer cake aggradation shoals and thin-layer cake retrogradation shoals widely distributed in the Yingshan Formation. Therefore, combining the distribution characteristics and reservoir forming mechanism. It is conduded that the carbonate shoal reservoirs of the Yingshan Formation have distributed with regional advantages.
2014, 32(3): 442-449.
Abstract:
During recent years, the study of deep-water depositional systems has intensively attracted worldwide interests. Deep-water sediments can record plentiful paleoceanographic information, e.g., changes of climate and/or ocean circulation conditions, which is of crucial importance for paleoceanographic reconstructions. However, in and abroad studies involving deep-water sedimentary systems of the Northwest Sub-Basin in the South China Sea is rare. In this study, we analyzed high resolution 2-D seismic data that cover an area of >4 200 km2, with the total length of >1 500 m. It reveals that there are mainly three deep-water depositional systems developed on the northwestern margin (water depth > 1 000 m) of the Northwest Sub-Basin, South China Sea. (1) There is a seamount named South Shenhu Seamount standing on the gentle slopes of the Pearl River Mouth Basin Southern Uplift Zone, where the water depths range from 1 000 m to 1 500 m and the slope is gentler than 1.2°. A “seamount related contourite depositional system” that consists of a moat (the depression close to an obstacle, produced by separate and faster bottom current cores and in genetic relation with mounded and elongated separated drifts), a mounded-elongated drift (typical type of contourites, which are mounded and elongated in shape along the slope, associating with moats), a plastered drift (produced by bottom currents with weakened dynamics due to topographic changes), contourite channels (alongslope aliened erosive features that are produced by bottom currents) and contourite furrows (alongslope aliened erosive features generate by bottom currents, with incisions of < 10 m) has been discovered developing in the vicinity of the South Shenhu Seamount. The moat runs along the northern foot of the seamount, with a mounded-elongated drift developed to its north side and a plastered drift to the south (plastered onto the northern flank of the South Shenhu Seamount). Alongslope aliened contourite furrows and channels developed on the mounded-elongated drift deposits. (2) Landslidings dominate the steep slopes (>2°) between 1 500 m and 2 500 m in the south of the South Shenhu Seamount, where gravity flow slump deposits and submarine canyons compose the “gravity flow slump system” and the “canyon system”, respectively. An NNW-SSE oriented canyon (C.1) shows an asymmetric V-shaped morphology with ~140 m of incision and ~6.5 km in width on the mid-upper slope (~1 350 m in water depth and ~1°slope). The successive erosion bases within this canyon present an obvious ENE migration. Contourite deposits, however, are rare in this region. (3) On the northwestern lower slopes of the Northwest Sub-Basin (water depth >2 500 m and slope < 2°, canyons are still common. On slopes of ~1 790 m in depth and ~2° of slope, a canyon (C.2) shows a flat-bottomed and U-shaped morphology (~135 m of incision and ~4 km in width), with a slightly mounded aggradational levee-system developed on both flanks of the canyon. The levee sediments on the WSW side of the canyon show continuous wave-shaped reflectors but flat and parallel/sub-parallel reflectors on the ENE side. The development of gravity flow slump deposits are significantly reduced in this region, instead, contourite sheeted drifts (contourites deposited across continental margins where a gentle gradient and smooth topography favor a wide non-focused bottom current) compose the sheeted contourite depositional system. The “seamount related contourite depositional system” is suggested to be generated by the anticyclone intermediate water circulation of the South China Sea. To the north side of the South Shenhu Seamount, bottom currents from west to east would be intensified after being deflected by an obstacle (the presence of the seamount). Thus the bottom current intensity is high and can erode the slope to form a moat. Due to Coriolis deflection, a mounded drift developed to the north of the moat and a small plastered drift to the south (plastered onto the northern flank of the South Shenhu Seamount). Currents away from the seamount are less/non-intensified, generating normal contourite channels and furrows on the drifted deposits north of the moat. Downslope gravity flow activities dominate steeper slopes, where the depositional environment is unsteady thus alongslope contourite deposits are missing. However, the eastward migrating downslope canyon of C.1 may indicate consistent bottom currents flowing from west to east, with intense dynamics. These high velocity currents might also belong to the intermediate water circulation. On gentle and flat slopes, e.g., the bathy plain, sheeted contourite deposits are commonly developed under the control of unfocused and low velocity bottom current in deep layer. The “sheeted contourite depositional system” in the studied area, thus, is proposed to be generated by unfocused bottom currents within the westwards South China Sea deep water circulation. Furthermore, sediment waves and aggradational levees are taken place on flanks of the canyon at the deeper depths, e.g., C.2, which suggests a dominated mechanism for their origin to be overflows of turbidity currents. Indicated from the first channelized features of consistent contourite channels on seismic reflections, the stable development of the contourite depositional system in the study area could be traced back to ~10 Ma, when local SCS bottom currents initially formed. In agreement with the general transgressional pattern in the northern margin area of the SCS,the followed contourite depositional system developed an aggradational pattern. This study provides new data for further understanding how the environmental factors, e.g., seafloor topographies, sea level/climate changes, tectonic activities and sediment sources, influencing the evolution of the SCS deep-water depositional systems.
During recent years, the study of deep-water depositional systems has intensively attracted worldwide interests. Deep-water sediments can record plentiful paleoceanographic information, e.g., changes of climate and/or ocean circulation conditions, which is of crucial importance for paleoceanographic reconstructions. However, in and abroad studies involving deep-water sedimentary systems of the Northwest Sub-Basin in the South China Sea is rare. In this study, we analyzed high resolution 2-D seismic data that cover an area of >4 200 km2, with the total length of >1 500 m. It reveals that there are mainly three deep-water depositional systems developed on the northwestern margin (water depth > 1 000 m) of the Northwest Sub-Basin, South China Sea. (1) There is a seamount named South Shenhu Seamount standing on the gentle slopes of the Pearl River Mouth Basin Southern Uplift Zone, where the water depths range from 1 000 m to 1 500 m and the slope is gentler than 1.2°. A “seamount related contourite depositional system” that consists of a moat (the depression close to an obstacle, produced by separate and faster bottom current cores and in genetic relation with mounded and elongated separated drifts), a mounded-elongated drift (typical type of contourites, which are mounded and elongated in shape along the slope, associating with moats), a plastered drift (produced by bottom currents with weakened dynamics due to topographic changes), contourite channels (alongslope aliened erosive features that are produced by bottom currents) and contourite furrows (alongslope aliened erosive features generate by bottom currents, with incisions of < 10 m) has been discovered developing in the vicinity of the South Shenhu Seamount. The moat runs along the northern foot of the seamount, with a mounded-elongated drift developed to its north side and a plastered drift to the south (plastered onto the northern flank of the South Shenhu Seamount). Alongslope aliened contourite furrows and channels developed on the mounded-elongated drift deposits. (2) Landslidings dominate the steep slopes (>2°) between 1 500 m and 2 500 m in the south of the South Shenhu Seamount, where gravity flow slump deposits and submarine canyons compose the “gravity flow slump system” and the “canyon system”, respectively. An NNW-SSE oriented canyon (C.1) shows an asymmetric V-shaped morphology with ~140 m of incision and ~6.5 km in width on the mid-upper slope (~1 350 m in water depth and ~1°slope). The successive erosion bases within this canyon present an obvious ENE migration. Contourite deposits, however, are rare in this region. (3) On the northwestern lower slopes of the Northwest Sub-Basin (water depth >2 500 m and slope < 2°, canyons are still common. On slopes of ~1 790 m in depth and ~2° of slope, a canyon (C.2) shows a flat-bottomed and U-shaped morphology (~135 m of incision and ~4 km in width), with a slightly mounded aggradational levee-system developed on both flanks of the canyon. The levee sediments on the WSW side of the canyon show continuous wave-shaped reflectors but flat and parallel/sub-parallel reflectors on the ENE side. The development of gravity flow slump deposits are significantly reduced in this region, instead, contourite sheeted drifts (contourites deposited across continental margins where a gentle gradient and smooth topography favor a wide non-focused bottom current) compose the sheeted contourite depositional system. The “seamount related contourite depositional system” is suggested to be generated by the anticyclone intermediate water circulation of the South China Sea. To the north side of the South Shenhu Seamount, bottom currents from west to east would be intensified after being deflected by an obstacle (the presence of the seamount). Thus the bottom current intensity is high and can erode the slope to form a moat. Due to Coriolis deflection, a mounded drift developed to the north of the moat and a small plastered drift to the south (plastered onto the northern flank of the South Shenhu Seamount). Currents away from the seamount are less/non-intensified, generating normal contourite channels and furrows on the drifted deposits north of the moat. Downslope gravity flow activities dominate steeper slopes, where the depositional environment is unsteady thus alongslope contourite deposits are missing. However, the eastward migrating downslope canyon of C.1 may indicate consistent bottom currents flowing from west to east, with intense dynamics. These high velocity currents might also belong to the intermediate water circulation. On gentle and flat slopes, e.g., the bathy plain, sheeted contourite deposits are commonly developed under the control of unfocused and low velocity bottom current in deep layer. The “sheeted contourite depositional system” in the studied area, thus, is proposed to be generated by unfocused bottom currents within the westwards South China Sea deep water circulation. Furthermore, sediment waves and aggradational levees are taken place on flanks of the canyon at the deeper depths, e.g., C.2, which suggests a dominated mechanism for their origin to be overflows of turbidity currents. Indicated from the first channelized features of consistent contourite channels on seismic reflections, the stable development of the contourite depositional system in the study area could be traced back to ~10 Ma, when local SCS bottom currents initially formed. In agreement with the general transgressional pattern in the northern margin area of the SCS,the followed contourite depositional system developed an aggradational pattern. This study provides new data for further understanding how the environmental factors, e.g., seafloor topographies, sea level/climate changes, tectonic activities and sediment sources, influencing the evolution of the SCS deep-water depositional systems.
2014, 32(3): 459-467.
Abstract:
There have been two views of deep-water and shallow-water facies with regard to the Dachenling Formation of Lower Cambrian in Western Zhejiang. Fossils are very rare in the Dachenling Formation. There are only a few fossils including floating and nektonic Arthicocephalus, Arthricocephalites and Changaspis in the middle to upper part of the Dachenling Formation, but without benthic fossils. Carbon and oxygen isotope analysis were conducted for the research. The result shows that there is a carbon isotope positive excursion for the interval including deep-water fossils, which is not consistent with bird’s eye structure, a sedimentary feature of shallow water carbonate platform facies. Our field investigation shows that there is a suite of seismic-tsunami deposits within the Dachenling Formation of Lower Cambrian in Kaihua, Changshan and Jiangshan of Zhejiang. In addition, the interval including tsunami deposits corresponds to the deep-water fossils and carbon isotope positive excursion. Deposits from various environments can be mixed together by tsunami events, which would result in abnormal sedimentary rocks. Thus, abnormal sediments within the Dachenling Formation of Lower Cambrian in western Zhejiang could be interpreted as a signature of tsunami events. The fossils and sediments of the deep-water zone were brought into the shallow-water zone and then deposited by the tsunami events, which also resulted in a brief, sharp positive excursion of carbon and oxygen isotope curves of the coeval sediments.
There have been two views of deep-water and shallow-water facies with regard to the Dachenling Formation of Lower Cambrian in Western Zhejiang. Fossils are very rare in the Dachenling Formation. There are only a few fossils including floating and nektonic Arthicocephalus, Arthricocephalites and Changaspis in the middle to upper part of the Dachenling Formation, but without benthic fossils. Carbon and oxygen isotope analysis were conducted for the research. The result shows that there is a carbon isotope positive excursion for the interval including deep-water fossils, which is not consistent with bird’s eye structure, a sedimentary feature of shallow water carbonate platform facies. Our field investigation shows that there is a suite of seismic-tsunami deposits within the Dachenling Formation of Lower Cambrian in Kaihua, Changshan and Jiangshan of Zhejiang. In addition, the interval including tsunami deposits corresponds to the deep-water fossils and carbon isotope positive excursion. Deposits from various environments can be mixed together by tsunami events, which would result in abnormal sedimentary rocks. Thus, abnormal sediments within the Dachenling Formation of Lower Cambrian in western Zhejiang could be interpreted as a signature of tsunami events. The fossils and sediments of the deep-water zone were brought into the shallow-water zone and then deposited by the tsunami events, which also resulted in a brief, sharp positive excursion of carbon and oxygen isotope curves of the coeval sediments.
2014, 32(3): 478-484.
Abstract:
Three common grain size analysis methods, including sieving, laser and image analysis, have their different applicability because of the varied measurement principles. Sieving and image analyses are accurate to measure size of sandy grains but not of muddy grains (<0.063 mm). In contrast, laser analysis can measure accurately the fine-grained size but not for coarse particles with size larger than 0.21 mm. It is therefore needed to combine using sieving and laser methods consecutively (SLMC) in order to measure poorly sorted natural sediment samples with wide size distribution. A nodal size of 2 mm is commonly selected to carry out SLMC size analysis, but the measurement precision is usually not good enough because of inaccurate probing of middle-coarse sand grains by laser method. It is recommended to choose 0.21 mm as the nodal size for SLMC size analysis, and the measurement accuracy can be effectively improved when poor probing size ranges of sieving and laser methods have been successfully avoided.
Three common grain size analysis methods, including sieving, laser and image analysis, have their different applicability because of the varied measurement principles. Sieving and image analyses are accurate to measure size of sandy grains but not of muddy grains (<0.063 mm). In contrast, laser analysis can measure accurately the fine-grained size but not for coarse particles with size larger than 0.21 mm. It is therefore needed to combine using sieving and laser methods consecutively (SLMC) in order to measure poorly sorted natural sediment samples with wide size distribution. A nodal size of 2 mm is commonly selected to carry out SLMC size analysis, but the measurement precision is usually not good enough because of inaccurate probing of middle-coarse sand grains by laser method. It is recommended to choose 0.21 mm as the nodal size for SLMC size analysis, and the measurement accuracy can be effectively improved when poor probing size ranges of sieving and laser methods have been successfully avoided.
2014, 32(3): 485-493.
Abstract:
High-resolution 3-D seismic data covering 1 400 km2 of the Rio Muni Basin enabled a detailed study of the Quaternary continental slope submarine topography and the configuration, origin and evolution of geomorphic elements. Three main conclusions about the area under investigation can be drawn in this paper. Firstly, based on the gradient, the seafloor topography shows a north-south partition. Secondly, submarine channels (both straight and sinuous types), submarine slides (both continental slope and channel wall) and seabed pockmarks (isolated pockmarks and sinuous pockmark belts) can be identified. Straight submarine channels developed mainly in the upper continental slope, and sinuous submarine channels developed in the middle and lower slopes. Sinuous submarine channels are mainly distributed in the north and south of Zone 2, and straight submarine channels are mainly in the south, middle and north of Zone 3. The sinuosity of submarine channel is related to the gradient of the seafloor and gravity flow supply. Thirdly, sinuous pockmark belts are distributed mainly in the north and south of Zone 2, and isolated pockmarks are mainly in the south, middle and north of Zone 3. The seismic profile of a single pockmark at any direction is characterized by U-type or V-type. The seismic events of a gas chimney linked to a pockmark show high-amplitude reflection and pull-down effect. The sinuous pockmark belts lie on the seabed above the palaeo-channel. The seafloor along the sinuous pockmark belts is characterized by a ridge-and-swale topography. Palaeo-channels and escape of the shallow gas are the main controlling factors of the sinuous pockmark belts.
High-resolution 3-D seismic data covering 1 400 km2 of the Rio Muni Basin enabled a detailed study of the Quaternary continental slope submarine topography and the configuration, origin and evolution of geomorphic elements. Three main conclusions about the area under investigation can be drawn in this paper. Firstly, based on the gradient, the seafloor topography shows a north-south partition. Secondly, submarine channels (both straight and sinuous types), submarine slides (both continental slope and channel wall) and seabed pockmarks (isolated pockmarks and sinuous pockmark belts) can be identified. Straight submarine channels developed mainly in the upper continental slope, and sinuous submarine channels developed in the middle and lower slopes. Sinuous submarine channels are mainly distributed in the north and south of Zone 2, and straight submarine channels are mainly in the south, middle and north of Zone 3. The sinuosity of submarine channel is related to the gradient of the seafloor and gravity flow supply. Thirdly, sinuous pockmark belts are distributed mainly in the north and south of Zone 2, and isolated pockmarks are mainly in the south, middle and north of Zone 3. The seismic profile of a single pockmark at any direction is characterized by U-type or V-type. The seismic events of a gas chimney linked to a pockmark show high-amplitude reflection and pull-down effect. The sinuous pockmark belts lie on the seabed above the palaeo-channel. The seafloor along the sinuous pockmark belts is characterized by a ridge-and-swale topography. Palaeo-channels and escape of the shallow gas are the main controlling factors of the sinuous pockmark belts.
2014, 32(3): 503-509.
Abstract:
A lot of sideritic and limouitic concretions with different shapes have been found in modern sediments of Lake Taihu. The former mainly consists of siderite, while the latter is predominantly composed of goethite and lepidocrocite. They contain terrigenous clast, clay, numerous fragments of aquatic plants, cell, spore and opal phytolith, as well as some shell clast and a lot of bacteria. Clearly, their origin is related to organism. The sideritic concretions were formed in a closed reducing environment, just beneath the lower mud layer of modern sediments of Lake Taihu. The 14C dating results of sideritic concretions with different shapes indicate different formation ages. In contrast, the limonitic concretions were formed in anoxidation environment in the upper mud layer of modern sediments, and bacteria play a key role in their aggregation. Therefore, these ferruginous concretions are products of diagenetic processes. The origin of Lake Taihu is still under debate. Meteorite impact hypothesis has received a lot of attention. Wang et al. (2009) regarded the above sideritic and limouitic concretions, even calcareous concretions in loess as impact ejectain the Taihu area, and claimed that they provide evidence of meteorite impact hypothesis. However, the meteorite impact is associated with instantaneous shock metamorphism, which will produce some characteristic minerals and microstructure including coesite, stishovite, planar deformation features as well as impact glass. So far the high-pressure polymorphs of quartz have not been found in the Taihu and adjacent area. The deformed laminar structure in quartz from sandstone is a common feature in sedimentary clasts, which cannot be considered diagnostics of shock metamorphism. The bottom of Lake Taihu is quite flat. The water depth of Lake Taihu does not exceed 3 m and the sediments are only about 2-m-thick, which do not agree with the topography of an impact crater. In the view of sedimentology, flood submerge is a more favorable hypothesis to interpret the origin of Lake Taihu, because it is consistent with characteristics and ages of sediments, as well as archaeological relics and historical records.
A lot of sideritic and limouitic concretions with different shapes have been found in modern sediments of Lake Taihu. The former mainly consists of siderite, while the latter is predominantly composed of goethite and lepidocrocite. They contain terrigenous clast, clay, numerous fragments of aquatic plants, cell, spore and opal phytolith, as well as some shell clast and a lot of bacteria. Clearly, their origin is related to organism. The sideritic concretions were formed in a closed reducing environment, just beneath the lower mud layer of modern sediments of Lake Taihu. The 14C dating results of sideritic concretions with different shapes indicate different formation ages. In contrast, the limonitic concretions were formed in anoxidation environment in the upper mud layer of modern sediments, and bacteria play a key role in their aggregation. Therefore, these ferruginous concretions are products of diagenetic processes. The origin of Lake Taihu is still under debate. Meteorite impact hypothesis has received a lot of attention. Wang et al. (2009) regarded the above sideritic and limouitic concretions, even calcareous concretions in loess as impact ejectain the Taihu area, and claimed that they provide evidence of meteorite impact hypothesis. However, the meteorite impact is associated with instantaneous shock metamorphism, which will produce some characteristic minerals and microstructure including coesite, stishovite, planar deformation features as well as impact glass. So far the high-pressure polymorphs of quartz have not been found in the Taihu and adjacent area. The deformed laminar structure in quartz from sandstone is a common feature in sedimentary clasts, which cannot be considered diagnostics of shock metamorphism. The bottom of Lake Taihu is quite flat. The water depth of Lake Taihu does not exceed 3 m and the sediments are only about 2-m-thick, which do not agree with the topography of an impact crater. In the view of sedimentology, flood submerge is a more favorable hypothesis to interpret the origin of Lake Taihu, because it is consistent with characteristics and ages of sediments, as well as archaeological relics and historical records.
2014, 32(3): 518-526.
Abstract:
The Yellow River brought a great deal of sediments to Diaokou area during 1964~1976 when the Yellow River changed its course to Diaokou River, deposited and formed the Diaokou lobe. It has experienced complete natural evolution process of modern Yellow River delta and has formed a complete set of modern Yellow River delta depositional system, and it is of great theoretical significance to recognize the evolution of the modern Yellow River Delta. Based on the characteristics of grain size, magnetism and AMS 14C of Cores ZK10-3 and ZK30 in the north of Diaokou lobe, combined with Cores ZK227, ZK1, ZK228 in the south of Diaokou lobe, we discussed the recent and Holocene stratigraphic evolution of the Diaokou lobe area and its formation process. Sedimentary stratigraphy of Diaokou lobe from the Holocene may include facies of shallow sea, river and lake, salt marsh, shallow sea and delta from bottom to top. The study area received deltaic deposit from 1855, experienced prodelta, delta front (lateral) area and delta plain. After 1964, when the Diaokou lobe began to form, it experienced four stages of flow deposition, single channel deposition, diversion deposition and abandoned erosion.
The Yellow River brought a great deal of sediments to Diaokou area during 1964~1976 when the Yellow River changed its course to Diaokou River, deposited and formed the Diaokou lobe. It has experienced complete natural evolution process of modern Yellow River delta and has formed a complete set of modern Yellow River delta depositional system, and it is of great theoretical significance to recognize the evolution of the modern Yellow River Delta. Based on the characteristics of grain size, magnetism and AMS 14C of Cores ZK10-3 and ZK30 in the north of Diaokou lobe, combined with Cores ZK227, ZK1, ZK228 in the south of Diaokou lobe, we discussed the recent and Holocene stratigraphic evolution of the Diaokou lobe area and its formation process. Sedimentary stratigraphy of Diaokou lobe from the Holocene may include facies of shallow sea, river and lake, salt marsh, shallow sea and delta from bottom to top. The study area received deltaic deposit from 1855, experienced prodelta, delta front (lateral) area and delta plain. After 1964, when the Diaokou lobe began to form, it experienced four stages of flow deposition, single channel deposition, diversion deposition and abandoned erosion.
2014, 32(3): 538-549.
Abstract:
The study of the dolomite texture, as the foundation of dolomitization analysis, not only indicates the origin of the dolomites but also controls the dolomite reservoir quality significantly. The main objectives of this paper are to investigate the dolomite texture types and origin of the Cambrian-Ordovician dolomite in the central Tarim Basin, based on the analysis of the core, thin-section, SEM, cathodoluminescence and C-O-Sr isotope, as well as the widely used terminology for dolomite texture in international academic circles. The results show that there are two types of replacement dolomite: one is the dolomite with well-preserved precursor lithologic fabric that can be subdivided to micritic dolomite and (relict) dolo-grainstone, the other type is the crystalline dolomite, including fine crystalline, planar-e(euhedral) dolomite, fine crystalline, planar-s(subhedral) dolomite and medium to coarse crystalline, nonplanar-a(anhedral) dolomite. Additionaly, two types of void-filling dolomite cement have been observed that are very fine to fine crystalline, planar-e(s) dolomite void-filling and medium to coarse crystalline, saddle dolomite void-filling, respectively. Different texture types of the dolomite indicate that the diagenetic environments and formation processes of the dolomite are various. For the study area, the dolomite with well-preserved precursor fabrics is associated with the mimetic dolomitization during the penecontemporaneous period. Abundance of high-supersaturation dolomitizing fluid is beneficial to the perservation of precursor fabrics. The planar-e and planar-s dolomite are probably formed from reflux of penesaline seawater during early diagenetic stage by the low-temperature dolomitization. Overdolomitization may play a vital role in planar-e crystals transferring to planar-s crystals with decreasing of pore space. The nonplanar-a dolomite is interpreted as the product of high-temperature/hydrothermal dolomitization and recrystallization during the medium or deep burial, the high temperature is the key to cause dolomite crystal boundary transferring to nonplanar-anhedral.
The study of the dolomite texture, as the foundation of dolomitization analysis, not only indicates the origin of the dolomites but also controls the dolomite reservoir quality significantly. The main objectives of this paper are to investigate the dolomite texture types and origin of the Cambrian-Ordovician dolomite in the central Tarim Basin, based on the analysis of the core, thin-section, SEM, cathodoluminescence and C-O-Sr isotope, as well as the widely used terminology for dolomite texture in international academic circles. The results show that there are two types of replacement dolomite: one is the dolomite with well-preserved precursor lithologic fabric that can be subdivided to micritic dolomite and (relict) dolo-grainstone, the other type is the crystalline dolomite, including fine crystalline, planar-e(euhedral) dolomite, fine crystalline, planar-s(subhedral) dolomite and medium to coarse crystalline, nonplanar-a(anhedral) dolomite. Additionaly, two types of void-filling dolomite cement have been observed that are very fine to fine crystalline, planar-e(s) dolomite void-filling and medium to coarse crystalline, saddle dolomite void-filling, respectively. Different texture types of the dolomite indicate that the diagenetic environments and formation processes of the dolomite are various. For the study area, the dolomite with well-preserved precursor fabrics is associated with the mimetic dolomitization during the penecontemporaneous period. Abundance of high-supersaturation dolomitizing fluid is beneficial to the perservation of precursor fabrics. The planar-e and planar-s dolomite are probably formed from reflux of penesaline seawater during early diagenetic stage by the low-temperature dolomitization. Overdolomitization may play a vital role in planar-e crystals transferring to planar-s crystals with decreasing of pore space. The nonplanar-a dolomite is interpreted as the product of high-temperature/hydrothermal dolomitization and recrystallization during the medium or deep burial, the high temperature is the key to cause dolomite crystal boundary transferring to nonplanar-anhedral.
2014, 32(3): 560-567.
Abstract:
The dolomite rocks of the Lower Cretaceous are formed under special geologic conditions, and are the key horizon that can be correlated with the whole basin. In recent years, abundant hydrocarbon shows have been found in the dolomitic rocks, but there is little research on the dolomitization of this deposit. The genesis of dolomite in the study area is related to hydrolysis of volcanic material. Bulk rock and microscopic petrologic features analysis indicate that the specific suite of rocks is rich in volcanic ash which contains a large amount of Ca2+ and Mg2+. Dolomites, which originated from the reaction of CO2, Ca2+ and Mg2+, replace volcanic material to fill pores and fractures. Crystal structures of dolomite observed under microscope, can be classified as porphyritic crystal, micritic crystal and fine-powder crystal. Dolomite with porphyritic crystal behaves dispersire state and shows irregular shape. Additionally, the size of porphyritic crystal is relatively large, with an average diameter of about 0.2mm. Dolomite with micritic crystal is characterized of laminated and agglomerate feature. The difference between porphyritic and micritic structure is related to the liquidity of pore fluids. Dolomites with fine-powder crystal concentrate in reservoir pores or fractures as cement. The problem about origin of the CO2 is as important as that of Mg2+ to the specific dolomitization. Analysis of carbon isotope show that CO2 used to hydrolysis is related to the methanation of organic matters. The carbons within original organic material are transformed to methane and bicarbonate by methanation in the absence of oxygen. The enrichment of 12C in methane results in the concentration of 13C in bicarbonate and a slightly positive of δ13C in dolomite.
The dolomite rocks of the Lower Cretaceous are formed under special geologic conditions, and are the key horizon that can be correlated with the whole basin. In recent years, abundant hydrocarbon shows have been found in the dolomitic rocks, but there is little research on the dolomitization of this deposit. The genesis of dolomite in the study area is related to hydrolysis of volcanic material. Bulk rock and microscopic petrologic features analysis indicate that the specific suite of rocks is rich in volcanic ash which contains a large amount of Ca2+ and Mg2+. Dolomites, which originated from the reaction of CO2, Ca2+ and Mg2+, replace volcanic material to fill pores and fractures. Crystal structures of dolomite observed under microscope, can be classified as porphyritic crystal, micritic crystal and fine-powder crystal. Dolomite with porphyritic crystal behaves dispersire state and shows irregular shape. Additionally, the size of porphyritic crystal is relatively large, with an average diameter of about 0.2mm. Dolomite with micritic crystal is characterized of laminated and agglomerate feature. The difference between porphyritic and micritic structure is related to the liquidity of pore fluids. Dolomites with fine-powder crystal concentrate in reservoir pores or fractures as cement. The problem about origin of the CO2 is as important as that of Mg2+ to the specific dolomitization. Analysis of carbon isotope show that CO2 used to hydrolysis is related to the methanation of organic matters. The carbons within original organic material are transformed to methane and bicarbonate by methanation in the absence of oxygen. The enrichment of 12C in methane results in the concentration of 13C in bicarbonate and a slightly positive of δ13C in dolomite.
2014, 32(3): 576-585.
Abstract:
Based on the sequence stratigraphy theory of Vail, sequence stratigraphy framework of Yingcheng Formation in Lishu fault depression is established through analysing seismic data, core data, logging data and other materials. Yingcheng Formation is divided into 4 3rd-order sequences from the bottom to the top, SQ1, SQ2, SQ3, SQ4, which are, respectively, equal to YC1, YC2, YC3, YC4 stages. SQ1-SQ3 at the bottom are T-R sequences, and SQ4 sequence is composed of transgressive system. According to the characteristics of sequence development of target strata in the study area, and based on the division and comparison of sequence formation unit, this paper studies on sedimentary types and its distribution law. This area mainly develops fan delta and braided river delta, which are characteristicted by mixed-source sedimentary and regional distribution. The whole area develops fan delta in SQ1; SQ2 exits fan delta in the east area and braided river delta in the north area of the depression. The sedimentary facies evolved to braided river delta within the scope of whole depression in SQ3 and SQ4 sequences.
Based on the sequence stratigraphy theory of Vail, sequence stratigraphy framework of Yingcheng Formation in Lishu fault depression is established through analysing seismic data, core data, logging data and other materials. Yingcheng Formation is divided into 4 3rd-order sequences from the bottom to the top, SQ1, SQ2, SQ3, SQ4, which are, respectively, equal to YC1, YC2, YC3, YC4 stages. SQ1-SQ3 at the bottom are T-R sequences, and SQ4 sequence is composed of transgressive system. According to the characteristics of sequence development of target strata in the study area, and based on the division and comparison of sequence formation unit, this paper studies on sedimentary types and its distribution law. This area mainly develops fan delta and braided river delta, which are characteristicted by mixed-source sedimentary and regional distribution. The whole area develops fan delta in SQ1; SQ2 exits fan delta in the east area and braided river delta in the north area of the depression. The sedimentary facies evolved to braided river delta within the scope of whole depression in SQ3 and SQ4 sequences.
2014, 32(3): 593-600.
Abstract:
Oil shale, shale oil and shale gas are the unconventional shale energy resources. They have already become domestic and foreign research hotspots, and brought about“shale revolution”. These mineral resources are closely related to organic rich shales. There is a connection and coexistent pattern in genesis among these mudstone mineral resources, but a big difference in metallogenic (accumulation) conditions. Thick dark mudstone developed in the upper Cretaceous Qingshankou Formation in the Songliao Basin. The distribution of strata is widespread with high abundance of organic matter, but the burial depth is of significant differences. Based on the previous studies, the organic matter content is very high in the Qingshankou Formation, and the kerogen type is I and II1. These geological conditions of this thick mudstone layer have the potential to accumulate the shale coexistent energy resources. In the southeast uplift of the Songliao Basin, it is initially thought that the Qingshankou Formation has a shallow depth (<1 000 m) and a low degree of organic thermal evolution (Ro: <0.7%), thus forming a better metallogenic place for oil shale. The shallow depth of organic rich shales maybe generate biogenetic shale gas due to microbial activities, and the weak compaction and abundant organic frameworks provide adequate space for shale gas reservoirs (porosity: 4%~14%) . From the eastern uplift to the central depression, the burial depth increased (>1 000 m) and the thermal evolution increased (Ro: 0.7%~1.2%). Organic matters mainly mature to over-mature and large quantity hydrocarbons were expelled, and they offered sufficient sources for shale oil and shale gas accumulation. Abnormal high pressure and hydrocarbon expulsion pore-creating to form well-developed reservoirs, emhancing the good conditions for oil shale and oil gas accumulation. Therefore, the energy of shale metallogenic(accumulation) regularity might be that, the southest uplift and some uplift areas of central transition zone are the potential accumulation belt of oil shale and biogenetic shale gas, and the large area of central transition zone is the accumulation belt of shale oil and pyrolysis shale gas, and the deepest central depression is the accumulation zone of cracking shale gas. These shale coexistent energy minerals are ringly distributed in the basin.
Oil shale, shale oil and shale gas are the unconventional shale energy resources. They have already become domestic and foreign research hotspots, and brought about“shale revolution”. These mineral resources are closely related to organic rich shales. There is a connection and coexistent pattern in genesis among these mudstone mineral resources, but a big difference in metallogenic (accumulation) conditions. Thick dark mudstone developed in the upper Cretaceous Qingshankou Formation in the Songliao Basin. The distribution of strata is widespread with high abundance of organic matter, but the burial depth is of significant differences. Based on the previous studies, the organic matter content is very high in the Qingshankou Formation, and the kerogen type is I and II1. These geological conditions of this thick mudstone layer have the potential to accumulate the shale coexistent energy resources. In the southeast uplift of the Songliao Basin, it is initially thought that the Qingshankou Formation has a shallow depth (<1 000 m) and a low degree of organic thermal evolution (Ro: <0.7%), thus forming a better metallogenic place for oil shale. The shallow depth of organic rich shales maybe generate biogenetic shale gas due to microbial activities, and the weak compaction and abundant organic frameworks provide adequate space for shale gas reservoirs (porosity: 4%~14%) . From the eastern uplift to the central depression, the burial depth increased (>1 000 m) and the thermal evolution increased (Ro: 0.7%~1.2%). Organic matters mainly mature to over-mature and large quantity hydrocarbons were expelled, and they offered sufficient sources for shale oil and shale gas accumulation. Abnormal high pressure and hydrocarbon expulsion pore-creating to form well-developed reservoirs, emhancing the good conditions for oil shale and oil gas accumulation. Therefore, the energy of shale metallogenic(accumulation) regularity might be that, the southest uplift and some uplift areas of central transition zone are the potential accumulation belt of oil shale and biogenetic shale gas, and the large area of central transition zone is the accumulation belt of shale oil and pyrolysis shale gas, and the deepest central depression is the accumulation zone of cracking shale gas. These shale coexistent energy minerals are ringly distributed in the basin.
2014, 32(3): 410-417.
Abstract:
The Second Member of Mantou Formation (Cambrian Series 3) exposed well in Dengfeng area of Henan and characterized by purple and isabelline thick-bedded mudstone interbedded with stromatolite limestones, oncloidal limestones and oolitic limestones. Abundant oncoids are well preserved in the lower part of the Member. The paper have done further researches on macroscopic morphological characters, microscopic cortex characters, microbial fossils and their calcification, and the formation and controlling factors of oncoids using the actualism of “the present is the key to the past”. The study provides evidence for the reconstruction of Cambrian palaeoenvironments of western Henan and the understanding of co-evolution between microbe and metazoan. There are three types of oncoids from the Second Member of the Mantou Formation(Cambrian Series 3), Dengfeng Area, Henan: the spherical and axiolitic oncoids are found in association with big columnar stromatolites and formed in high-energy subtidal environment; the long olivary oncoids are related to small columnar stromatolites and preserved in low- to moderate-energy intertidal environment; the irregular oncoids and their companion nearly horizontal and gently wavy-shaped stromatolites are dominant in low-energy supratidal and intertidal environment. The oncoids are often preserved in carbonates together with stromatolites in the study area. The setting containing oncoids and stromatolites is composed of three developing stages form bottom to top. The first stage is dominated by long olivary oncoids and small columnar stromatolites. The microbes wrapped up the carbonate particles to grow around the long olivary bioclastic and gravel core and formed long olivary oncoids under low to medium-energy condition or small columnar stromatolites under low-energy condition. The second stage consists of spherical and axiolitic oncoids and big columnar stromatolites. The microbes wrapped up the carbonate particles by intertwined grow of filaments and trapping and bounding of sticky EPS and formed evenly alternating of light and dark laminae under high-energy condition which lead to the formation of regular spherical and axiolitic oncoids and big columnar stromatolites. The third stage is characterized by stoamtolite-like oncoids and wavy stromatolites. The spherical and axiolitic oncoids in second stage stopped to agitate as the descending of sea 1evel. The laminae of oncoids began to grow upwards in place at this time. The result is that the spherical and axiolitic oncoids can gradually become to stoamtolite-like oncoids. All oncoids totally become to wavy stoamtolites and eventually disappear in carbonate sediments as the continued descending of sea level. The genetic analysis on oncoids shows that there is a coupling relationship between microbiogenic structures and metazoan disturbed structures; water energy is the determining factor for the changes of oncoid types; the influx of terrigenous clays is the most direct factor for oncoid growth and their ultimate demise.
The Second Member of Mantou Formation (Cambrian Series 3) exposed well in Dengfeng area of Henan and characterized by purple and isabelline thick-bedded mudstone interbedded with stromatolite limestones, oncloidal limestones and oolitic limestones. Abundant oncoids are well preserved in the lower part of the Member. The paper have done further researches on macroscopic morphological characters, microscopic cortex characters, microbial fossils and their calcification, and the formation and controlling factors of oncoids using the actualism of “the present is the key to the past”. The study provides evidence for the reconstruction of Cambrian palaeoenvironments of western Henan and the understanding of co-evolution between microbe and metazoan. There are three types of oncoids from the Second Member of the Mantou Formation(Cambrian Series 3), Dengfeng Area, Henan: the spherical and axiolitic oncoids are found in association with big columnar stromatolites and formed in high-energy subtidal environment; the long olivary oncoids are related to small columnar stromatolites and preserved in low- to moderate-energy intertidal environment; the irregular oncoids and their companion nearly horizontal and gently wavy-shaped stromatolites are dominant in low-energy supratidal and intertidal environment. The oncoids are often preserved in carbonates together with stromatolites in the study area. The setting containing oncoids and stromatolites is composed of three developing stages form bottom to top. The first stage is dominated by long olivary oncoids and small columnar stromatolites. The microbes wrapped up the carbonate particles to grow around the long olivary bioclastic and gravel core and formed long olivary oncoids under low to medium-energy condition or small columnar stromatolites under low-energy condition. The second stage consists of spherical and axiolitic oncoids and big columnar stromatolites. The microbes wrapped up the carbonate particles by intertwined grow of filaments and trapping and bounding of sticky EPS and formed evenly alternating of light and dark laminae under high-energy condition which lead to the formation of regular spherical and axiolitic oncoids and big columnar stromatolites. The third stage is characterized by stoamtolite-like oncoids and wavy stromatolites. The spherical and axiolitic oncoids in second stage stopped to agitate as the descending of sea 1evel. The laminae of oncoids began to grow upwards in place at this time. The result is that the spherical and axiolitic oncoids can gradually become to stoamtolite-like oncoids. All oncoids totally become to wavy stoamtolites and eventually disappear in carbonate sediments as the continued descending of sea level. The genetic analysis on oncoids shows that there is a coupling relationship between microbiogenic structures and metazoan disturbed structures; water energy is the determining factor for the changes of oncoid types; the influx of terrigenous clays is the most direct factor for oncoid growth and their ultimate demise.
2014, 32(3): 429-441.
Abstract:
Two sections, at Penglaitan and Tieqiao, where the global stratotype sections for the boundary between the Guadalupian and Lopingian are located in the Laibin area of Guangxi province, respectively, are extensively eposed along the two banks of the Hongshui River and are chosen for analyses of facies and sequence stratigraphy. At the two sections, fourteen facies are recognized in the Maokou (Guadalupian) and Heshan (Lopingian) Formations, and these facies are further grouped into four main facies associations , which are basin, lower slope, upper slope and platform margin. Based on these facies, their vertical stacking patterns and the lateral distribution of facies associations, six transgressive-regressive (TR) sequences, TR1 to TR6, are identified and well correlated by detailed conodont zones. They are the TR1 ( J. nankingensis zone), TR2 ( J. aserrata to J. shannoni zone), TR3 ( J. shannoni zone to early C. p. postbitteri zone), TR4 ( C. p. postbitteri zone to early C. transcaucasia zone), TR5 (C. transcaucasia zone to C. orientalis zone) and TR6 (C. orientalis zone to C. inflecta zone). Based on the transgressive-regressive sequences of the two sections, (high-resolution) sequence-based lithofacies and paleogeographic evolution in the study area were carried on, which provides important implications for the relationship among some events during this time, such as the global regression, the Emeishan volcanism and the end-Guadalupian mass extinction.
Two sections, at Penglaitan and Tieqiao, where the global stratotype sections for the boundary between the Guadalupian and Lopingian are located in the Laibin area of Guangxi province, respectively, are extensively eposed along the two banks of the Hongshui River and are chosen for analyses of facies and sequence stratigraphy. At the two sections, fourteen facies are recognized in the Maokou (Guadalupian) and Heshan (Lopingian) Formations, and these facies are further grouped into four main facies associations , which are basin, lower slope, upper slope and platform margin. Based on these facies, their vertical stacking patterns and the lateral distribution of facies associations, six transgressive-regressive (TR) sequences, TR1 to TR6, are identified and well correlated by detailed conodont zones. They are the TR1 ( J. nankingensis zone), TR2 ( J. aserrata to J. shannoni zone), TR3 ( J. shannoni zone to early C. p. postbitteri zone), TR4 ( C. p. postbitteri zone to early C. transcaucasia zone), TR5 (C. transcaucasia zone to C. orientalis zone) and TR6 (C. orientalis zone to C. inflecta zone). Based on the transgressive-regressive sequences of the two sections, (high-resolution) sequence-based lithofacies and paleogeographic evolution in the study area were carried on, which provides important implications for the relationship among some events during this time, such as the global regression, the Emeishan volcanism and the end-Guadalupian mass extinction.
2014, 32(3): 450-458.
Abstract:
Under the guidance of sequence stratigraphy and sedimentology theory, detailed research on Toutunhe Formation in the Toutunhe outcrop, southern margin of Junggar Basin was carried out to discuss the model of braided river transform to meandering river. On the basis of the division of sequences and the determination of river-genesis lithofacies, by the field measuring, fine dissecting and sedimentation description and observation, braided river transform to meandering river (braided to meandering river) in Toutunhe Formation was analyzed and characteristics summarized, and the control factors and depositional setting of braided to meandering river were cleared. In this study, the process of braided to meandering river was controlled by base-level cycle, sedimentary source recharge, gradient and paleoclimate. As the base level rising, sedimentary source recharge decreasing, gradient slowing down and paleoclimate changing to hot-arid gradually, the river type change from braided river to meandering river. Eventually, by the vertical sequence of Toutunhe Formation, the local and regional depositional evolution models are built up.
Under the guidance of sequence stratigraphy and sedimentology theory, detailed research on Toutunhe Formation in the Toutunhe outcrop, southern margin of Junggar Basin was carried out to discuss the model of braided river transform to meandering river. On the basis of the division of sequences and the determination of river-genesis lithofacies, by the field measuring, fine dissecting and sedimentation description and observation, braided river transform to meandering river (braided to meandering river) in Toutunhe Formation was analyzed and characteristics summarized, and the control factors and depositional setting of braided to meandering river were cleared. In this study, the process of braided to meandering river was controlled by base-level cycle, sedimentary source recharge, gradient and paleoclimate. As the base level rising, sedimentary source recharge decreasing, gradient slowing down and paleoclimate changing to hot-arid gradually, the river type change from braided river to meandering river. Eventually, by the vertical sequence of Toutunhe Formation, the local and regional depositional evolution models are built up.
2014, 32(3): 468-477.
Abstract:
The detailed sedimentological and petrological studies reveal that, for the first time, tide-dominated deltaic deposits are well-developed in Lungamachi Formation, Longshan-Yongshun regions, northwestern Hunan. Three subfacies and ten microfacies were identified by outcrops observation and petrological analysis. The prodelta subfacies consists of shelf mudstones and low density turbidite. Microfacies of the delta front include frontal sheet sand, interdistributary bay, channel mouth bar and distal sand bar. The delta plain subfacies consists of distributary channel, interdistributary bay, natural levee and marsh. The cycles of sedimentary sequences and relative sea level change are furthermore discussed. The deltaic sequence from the prodelta deposits to the delta plain deposits showed three upward-coarsening successions that, respectively, consisted of five to six sub-successions and overlain by an upward-fining succession of tidal flat. Paleocurrent data further indicate that the source of Lungamachi Formation should be located in the SE of the study area(“Xuefeng uplift”). The results show the significance of the deltaic deposits in Lungamachi Formation, which are probably initial sedimentary responses to outset of “Xuefeng uplift” during Rhuddanian-Aeronian and recordings for transition of tectonic-basin pattern on the margin of Upper-Middle Yangtze.
The detailed sedimentological and petrological studies reveal that, for the first time, tide-dominated deltaic deposits are well-developed in Lungamachi Formation, Longshan-Yongshun regions, northwestern Hunan. Three subfacies and ten microfacies were identified by outcrops observation and petrological analysis. The prodelta subfacies consists of shelf mudstones and low density turbidite. Microfacies of the delta front include frontal sheet sand, interdistributary bay, channel mouth bar and distal sand bar. The delta plain subfacies consists of distributary channel, interdistributary bay, natural levee and marsh. The cycles of sedimentary sequences and relative sea level change are furthermore discussed. The deltaic sequence from the prodelta deposits to the delta plain deposits showed three upward-coarsening successions that, respectively, consisted of five to six sub-successions and overlain by an upward-fining succession of tidal flat. Paleocurrent data further indicate that the source of Lungamachi Formation should be located in the SE of the study area(“Xuefeng uplift”). The results show the significance of the deltaic deposits in Lungamachi Formation, which are probably initial sedimentary responses to outset of “Xuefeng uplift” during Rhuddanian-Aeronian and recordings for transition of tectonic-basin pattern on the margin of Upper-Middle Yangtze.
2014, 32(3): 494-502.
Abstract:
This paper has studied in detail the characteristics of thrombolite in the Zhangxia Formation (Third Series of Cambrian), Shandong Province. Thrombolite is bioherm and biostrome. And the clot of thrombolite is clotted dendritic and meshed. The microbes formed thrombolite are Epiphyton, Girvanella and Actinophycus. Based on the analysis of sedimentary facies and paragenesis lithofacies, the author initially determines sedimentary environment of Cambrian thrombolite in the west of Shandong Province is the low-energy subtidal zone.
This paper has studied in detail the characteristics of thrombolite in the Zhangxia Formation (Third Series of Cambrian), Shandong Province. Thrombolite is bioherm and biostrome. And the clot of thrombolite is clotted dendritic and meshed. The microbes formed thrombolite are Epiphyton, Girvanella and Actinophycus. Based on the analysis of sedimentary facies and paragenesis lithofacies, the author initially determines sedimentary environment of Cambrian thrombolite in the west of Shandong Province is the low-energy subtidal zone.
2014, 32(3): 510-517.
Abstract:
The homogenization temperature of fluid inclusions reflects the temperatures of the brines from which halite crystals grew. Therefore, it is a powerful tool to reveal the paleoclimate. Northern Shaanxi Salt Basin is located in the central and eastern of Ordos Basin. There is thick-bedded salt in Ordovician Majiagou Formation. Petrographical research and the homogenization temperature measurement of the samples which come from Zhenjia-1 core have been studied in detail. The results of petrographic research show that there are three types of fluid inclusions which are square, round and irregular. They contain liquid inclusion, liquid-gas inclusion and liquid-gas -crystal inclusion. The homogenization temperatures of fluid inclusions have been measured by cooling nucleation method. It shows that the homogenization temperatures of fluid inclusions of sample zjy-1(2 738 m deep) range 18.5℃ to 27℃, sample zjy-2(2 740 m deep) range 14.9℃ to 29.9℃, sample zjy-3(2 744 m deep) range 24℃ to 39.6℃, and sample zjy-4(2 812.8 m deep) range 14.7℃ to 31℃. Comparing with previous paleotemperature, the records are basically coincident. The homogenization temperatures reflect the paleotemperature of Northern Shaanxi Salt Basin in the O2m65 stage of Ordovician period is about 27℃~36.9℃, the paleoclimate is arid and hot in low-latitude arid zone of the southern hemisphere.
The homogenization temperature of fluid inclusions reflects the temperatures of the brines from which halite crystals grew. Therefore, it is a powerful tool to reveal the paleoclimate. Northern Shaanxi Salt Basin is located in the central and eastern of Ordos Basin. There is thick-bedded salt in Ordovician Majiagou Formation. Petrographical research and the homogenization temperature measurement of the samples which come from Zhenjia-1 core have been studied in detail. The results of petrographic research show that there are three types of fluid inclusions which are square, round and irregular. They contain liquid inclusion, liquid-gas inclusion and liquid-gas -crystal inclusion. The homogenization temperatures of fluid inclusions have been measured by cooling nucleation method. It shows that the homogenization temperatures of fluid inclusions of sample zjy-1(2 738 m deep) range 18.5℃ to 27℃, sample zjy-2(2 740 m deep) range 14.9℃ to 29.9℃, sample zjy-3(2 744 m deep) range 24℃ to 39.6℃, and sample zjy-4(2 812.8 m deep) range 14.7℃ to 31℃. Comparing with previous paleotemperature, the records are basically coincident. The homogenization temperatures reflect the paleotemperature of Northern Shaanxi Salt Basin in the O2m65 stage of Ordovician period is about 27℃~36.9℃, the paleoclimate is arid and hot in low-latitude arid zone of the southern hemisphere.
2014, 32(3): 527-536.
Abstract:
In order to investigate the evolution of intermediate water in Shenhu area on the northern slope of the South China Sea (SCS) from Late Miocene, benthic foraminiferal assemblage composition and stable oxygen and carbon isotope values from core SH7B were studied. Factor and cluster statistical analysis were used to perform based on the quantitative census data of benthic foraminiferal fauna with 35 dominance genus or species in 128 samples. Four assemblages were recognized including assemblage of Globocassidulina subglobosa-Stilostomella spp.(Gs-St), Chilostomella mediterranensis-Globobulimina spp.(Cm-Gl), Hoglundina elegans-Pyrgo spp. (He-Py) and Melonis affinis-Pyrgo spp. (Ma-Py). During the warm period and higher eustatic sea level stage from Late Miocene to early Pliocene (5.8~2.8 Ma), benthic foraminiferal assemblage (Gs-St) is stable and relative medium-low nutrient environment and low oxygenated situation in this region. Caused by the current status of global ocean, the relative negative excursion of δ13C values and marked dissolution of carbonate in Core SH7B were also recognized in other marine regions from 5.8~3.79 Ma, which indicate that the low oxygenated situation of intermediate water in the northern slope of the SCS may be controlled by the presence of oxygen-depleted and corrosive pacific intermediate water. From the Early Pliocene 3.79 to 2.8 Ma, the increasing of benthic foraminiferal abundance and aerobic epifaunal species suggests the intensification of the intermediate water circulation and the increasing of oxygen content. Since Late Pliocene 2.4Ma, fluctuating changes of benthic foraminiferal assemblages and stable isotope values indicate medium-high seasonality of surface water productivity and well-oxygenation to oxygen-depleted bottom-water situation. The benthic foraminiferal distribution is mainly controlled by the organic matter flux to the seafloor and the circulation of intermediate water.
In order to investigate the evolution of intermediate water in Shenhu area on the northern slope of the South China Sea (SCS) from Late Miocene, benthic foraminiferal assemblage composition and stable oxygen and carbon isotope values from core SH7B were studied. Factor and cluster statistical analysis were used to perform based on the quantitative census data of benthic foraminiferal fauna with 35 dominance genus or species in 128 samples. Four assemblages were recognized including assemblage of Globocassidulina subglobosa-Stilostomella spp.(Gs-St), Chilostomella mediterranensis-Globobulimina spp.(Cm-Gl), Hoglundina elegans-Pyrgo spp. (He-Py) and Melonis affinis-Pyrgo spp. (Ma-Py). During the warm period and higher eustatic sea level stage from Late Miocene to early Pliocene (5.8~2.8 Ma), benthic foraminiferal assemblage (Gs-St) is stable and relative medium-low nutrient environment and low oxygenated situation in this region. Caused by the current status of global ocean, the relative negative excursion of δ13C values and marked dissolution of carbonate in Core SH7B were also recognized in other marine regions from 5.8~3.79 Ma, which indicate that the low oxygenated situation of intermediate water in the northern slope of the SCS may be controlled by the presence of oxygen-depleted and corrosive pacific intermediate water. From the Early Pliocene 3.79 to 2.8 Ma, the increasing of benthic foraminiferal abundance and aerobic epifaunal species suggests the intensification of the intermediate water circulation and the increasing of oxygen content. Since Late Pliocene 2.4Ma, fluctuating changes of benthic foraminiferal assemblages and stable isotope values indicate medium-high seasonality of surface water productivity and well-oxygenation to oxygen-depleted bottom-water situation. The benthic foraminiferal distribution is mainly controlled by the organic matter flux to the seafloor and the circulation of intermediate water.
2014, 32(3): 550-559.
Abstract:
There is multiplicity during the study of dolomite origin with conventional methods. It is over simple and patternized using the existing dolomite models or establishing new models. The crystal structures of dolomite preserved the forming environments, crystallizing velocity, crystal growing and diversification characteristics of dolomite crystals and the fluid characteristics, and so on. The crystal structure analysis of dolomite is an effective method to explore the origin of dolomite, which was concerned by few researchers. Based on petrographic and geochemical studies, this paper used X-ray diffraction, TEM and EPM methods and focused on the different characteristics of ordering degrees, crystal cell parameters, crystal lattice stripes, interplanar distances, and lattice defects of 5 different kinds of dolomite crystals (saddle dolomite, fibrous dolomite, fine crystal dolograinstone, coarse crystal dolomite in vugs and micritic dolomite) from Sinian Dengying Formation-Cambrian in Sichuan Basin after micro-fabric sampling and discussed the variation of the forming environments and upgrowth features. Different dolomites with varial crystal forms, sizes, burial depths and petrographic characteristics have disparate crystal structures. Saddle dolomites from hydrotherm have low ordering degrees (0.42~0.68), higher cell parameter c value and abnormal distribution in crystal cell parameter chart, curved lattice stripes, notable branched and ribbon-like lattice defects and narrower interplanar distances. Fibrous dolomites cements from seawater have absolute order structures (ordering degree=1), ideal cell parameters, exquisite lattice stripes, few lattice defects and narrow interplanar distances. Fine crystal dolograinstones with burial replacement origin have high ordering degrees (0.96~0.97), high cell parameters c values, regular lattice stripes, few lattice defects and near-dial interplanar distances. Coarse crystal euhedral dolomites cements from pores have higher ordering degrees (0.905~0.97), various cell parameter c values because of different burial depth, straight lattice stripes, unfrequent lattice defects and near-ideal interplanar distances. Authigenic micritic dolomites from nearsurface environment has lower ordering degrees (0.64~0.832), a little lower cell parameter c values, inlaid lattice stripes, equable lattice defects, a little narrow interplanar distances. Based on 5 kinds of dolomites crystal structure assemblages, the crystal structure identification marks of different dolomites were preliminarily established.
There is multiplicity during the study of dolomite origin with conventional methods. It is over simple and patternized using the existing dolomite models or establishing new models. The crystal structures of dolomite preserved the forming environments, crystallizing velocity, crystal growing and diversification characteristics of dolomite crystals and the fluid characteristics, and so on. The crystal structure analysis of dolomite is an effective method to explore the origin of dolomite, which was concerned by few researchers. Based on petrographic and geochemical studies, this paper used X-ray diffraction, TEM and EPM methods and focused on the different characteristics of ordering degrees, crystal cell parameters, crystal lattice stripes, interplanar distances, and lattice defects of 5 different kinds of dolomite crystals (saddle dolomite, fibrous dolomite, fine crystal dolograinstone, coarse crystal dolomite in vugs and micritic dolomite) from Sinian Dengying Formation-Cambrian in Sichuan Basin after micro-fabric sampling and discussed the variation of the forming environments and upgrowth features. Different dolomites with varial crystal forms, sizes, burial depths and petrographic characteristics have disparate crystal structures. Saddle dolomites from hydrotherm have low ordering degrees (0.42~0.68), higher cell parameter c value and abnormal distribution in crystal cell parameter chart, curved lattice stripes, notable branched and ribbon-like lattice defects and narrower interplanar distances. Fibrous dolomites cements from seawater have absolute order structures (ordering degree=1), ideal cell parameters, exquisite lattice stripes, few lattice defects and narrow interplanar distances. Fine crystal dolograinstones with burial replacement origin have high ordering degrees (0.96~0.97), high cell parameters c values, regular lattice stripes, few lattice defects and near-dial interplanar distances. Coarse crystal euhedral dolomites cements from pores have higher ordering degrees (0.905~0.97), various cell parameter c values because of different burial depth, straight lattice stripes, unfrequent lattice defects and near-ideal interplanar distances. Authigenic micritic dolomites from nearsurface environment has lower ordering degrees (0.64~0.832), a little lower cell parameter c values, inlaid lattice stripes, equable lattice defects, a little narrow interplanar distances. Based on 5 kinds of dolomites crystal structure assemblages, the crystal structure identification marks of different dolomites were preliminarily established.
2014, 32(3): 568-575.
Abstract:
This paper discusses the research idea of sedimentary facies in continental thin and interbed sandbody based on seismic sedimentology method. Seismic sedimentology mainly applies high-precision 3D seismic data to study sedimentary rock and its forming process, and improve resolution processing and frequency-division interpretation help to establish high-frequency sequence stratigraphic framework, and isochronous stratigraphic sections can be applied to divide the minimum isochronous stratigraphic unit. The 90° phase shift technology make the seismic events have lithological significance, which is used for describing the form and distribution characteristics of thin sand body. Optimization reflects the properties of sand body and characteristics of the reflection waveform, and precise sedimentary facies distribution diagram is established by combining source directions and sedimentary system types. Take three-section of Weizhou Formation in Weixi’nan Depression as an example, and carry out sedimentary facies research based on seismic sedimentology method. Seven sand groups as the minimum isochronous stratigraphic unit are recognized and established in the study area. By using source direction of east and west, shape of sand body, amplitude property of 90° phase shift data, waveform characteristics to accurately lay out seven sand group sedimentary facies diagram reflecting the law of which is braided river delta from east and west sides filling the lacustrine area.
This paper discusses the research idea of sedimentary facies in continental thin and interbed sandbody based on seismic sedimentology method. Seismic sedimentology mainly applies high-precision 3D seismic data to study sedimentary rock and its forming process, and improve resolution processing and frequency-division interpretation help to establish high-frequency sequence stratigraphic framework, and isochronous stratigraphic sections can be applied to divide the minimum isochronous stratigraphic unit. The 90° phase shift technology make the seismic events have lithological significance, which is used for describing the form and distribution characteristics of thin sand body. Optimization reflects the properties of sand body and characteristics of the reflection waveform, and precise sedimentary facies distribution diagram is established by combining source directions and sedimentary system types. Take three-section of Weizhou Formation in Weixi’nan Depression as an example, and carry out sedimentary facies research based on seismic sedimentology method. Seven sand groups as the minimum isochronous stratigraphic unit are recognized and established in the study area. By using source direction of east and west, shape of sand body, amplitude property of 90° phase shift data, waveform characteristics to accurately lay out seven sand group sedimentary facies diagram reflecting the law of which is braided river delta from east and west sides filling the lacustrine area.
2014, 32(3): 586-592.
Abstract:
Fluvial sandbody is one kind of the most important hydrocarbon reservoir in China. With the exploration of fluvial facies getting a major breakthrough at shallow squenence in Bohai bay in 1990s, fluvial facies reservoir research becomes the current hot spot. According to the analysis for outcrops, modern sedimentation, cores and well-logging, guided by the theory and technology of high-resolution sequence stratigraphy, sedimentology, reservoir geology and seismic sedimentology, stacking pattern of sandybody of the Minghuazhen Formation of S oilfield had been developed. Also, the distribution characteristics of sandbody in sequence stratigraphic framework has been illustrated, and fluvial facies sedimentary models has been set up. It is considered that there are 4 3rd and 13 4th and 38 5th sequences and the sequence stratigraphy is built up. It is identified 3 classes and 7 types of Minghuazhen Formation in Bohai Bay Basin South-middle part of fluvial facies sandstone, in which 3 classes comprise amalgamated channel complex(ACM), stack channel(SC) and isolated sandbody(IS), and 7 types of stack channel(SC) is further divided into intimated contact(IC) and evacuated contact(EC) and discreted contact(DC), and isolated sandbody(IS) includes incised channel(IIC), crevasse-splay(CS) and isolated channel(ICH), and amalgamated channel complex(ACM). It makes a strong basis, which is fluvial sand architecture to the guidance of fluvial facies high resolution sequence subdivision and correlation.
Fluvial sandbody is one kind of the most important hydrocarbon reservoir in China. With the exploration of fluvial facies getting a major breakthrough at shallow squenence in Bohai bay in 1990s, fluvial facies reservoir research becomes the current hot spot. According to the analysis for outcrops, modern sedimentation, cores and well-logging, guided by the theory and technology of high-resolution sequence stratigraphy, sedimentology, reservoir geology and seismic sedimentology, stacking pattern of sandybody of the Minghuazhen Formation of S oilfield had been developed. Also, the distribution characteristics of sandbody in sequence stratigraphic framework has been illustrated, and fluvial facies sedimentary models has been set up. It is considered that there are 4 3rd and 13 4th and 38 5th sequences and the sequence stratigraphy is built up. It is identified 3 classes and 7 types of Minghuazhen Formation in Bohai Bay Basin South-middle part of fluvial facies sandstone, in which 3 classes comprise amalgamated channel complex(ACM), stack channel(SC) and isolated sandbody(IS), and 7 types of stack channel(SC) is further divided into intimated contact(IC) and evacuated contact(EC) and discreted contact(DC), and isolated sandbody(IS) includes incised channel(IIC), crevasse-splay(CS) and isolated channel(ICH), and amalgamated channel complex(ACM). It makes a strong basis, which is fluvial sand architecture to the guidance of fluvial facies high resolution sequence subdivision and correlation.
2014, 32(3): 601-610.
Abstract:
The evolution of maturity of the Sinian-Lower Cambrian source rocks was reconstructed, and the starting time for oil cracking was also determined on the basis of the tectono-thermal history of Central Paleo-uplift, Sichuan Basin. The pressure evolution of some typical wells and 2D profile was simulated and the genetic mechanism of abnormal pressure in the Sinian-Low Cambrian was analyzed. The evolution of maturity indicated that the Sinian-Lower Cambrian source rocks experienced four stages, including first oil generation, second oil generation, kerogen cracking and late oil cracking. Oil cracking started at Middle-Late Triassic. The paleo-pressure evolution of the Sinian-Lower Cambrian consists of normal pressure, overpressure and pressure relief. The pressure coefficient of Cambrian maximized about 1.7 in Middle Cretaceous, and it was higher in Weiyuan-Ziyang area than Gaoshiti-Moxi structural belt. Rapid burial, hydrocarbon generation and late oil cracking are the primary factors for the overpressure. The excess pressure was relieved due to the tectonic uplift in Yanshan-Himalaya Periods, which resulted in the normal pressure in Sinian of the whole area and in Cambrian of Weiyuan-Ziyang area. However, the pressure coefficient of Cambrian maintains a high value about 1.5 in Gaoshiti-Moxi structural belt. The evolutions of temperature and pressure impacted gas generation, migration, accumulation and preservation significantly.
The evolution of maturity of the Sinian-Lower Cambrian source rocks was reconstructed, and the starting time for oil cracking was also determined on the basis of the tectono-thermal history of Central Paleo-uplift, Sichuan Basin. The pressure evolution of some typical wells and 2D profile was simulated and the genetic mechanism of abnormal pressure in the Sinian-Low Cambrian was analyzed. The evolution of maturity indicated that the Sinian-Lower Cambrian source rocks experienced four stages, including first oil generation, second oil generation, kerogen cracking and late oil cracking. Oil cracking started at Middle-Late Triassic. The paleo-pressure evolution of the Sinian-Lower Cambrian consists of normal pressure, overpressure and pressure relief. The pressure coefficient of Cambrian maximized about 1.7 in Middle Cretaceous, and it was higher in Weiyuan-Ziyang area than Gaoshiti-Moxi structural belt. Rapid burial, hydrocarbon generation and late oil cracking are the primary factors for the overpressure. The excess pressure was relieved due to the tectonic uplift in Yanshan-Himalaya Periods, which resulted in the normal pressure in Sinian of the whole area and in Cambrian of Weiyuan-Ziyang area. However, the pressure coefficient of Cambrian maintains a high value about 1.5 in Gaoshiti-Moxi structural belt. The evolutions of temperature and pressure impacted gas generation, migration, accumulation and preservation significantly.
