Geochronological Comparison on Indosinian Tectonic Events Between Qinling Orogeny and Ordos Basin
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摘要: 秦岭和鄂尔多斯盆地相互毗邻,秦岭的造山作用和鄂尔多斯盆地内陆湖盆的发展演化之间存在有机的联系,构成了山—盆耦合体系。印支构造活动具有多旋回演化的特征,它是秦岭全面造山的关键时期,可划分230~238 Ma、210~224 Ma和198~206 Ma三个重要构造演化阶段,其中中期构造活动留下的地质记录最丰富。本次研究采用LA-ICP-MS锆石原位U-Pb年龄分析技术,获得了延长组长7中上部和底部凝灰岩的年龄分别为221.8±2.0 Ma和228.2±2.0 Ma,通过对鄂尔多斯盆地三叠系延长组凝灰岩年龄的界定、分析,推算延长组顶底不整合面的年龄分别为199.6 Ma和237 Ma。长7中上部和长6厚层重力流砂体的形成时间为215~224 Ma。提出延长组底部和顶部的两个沉积间断面分别对应于秦岭造山带印支早期和晚期构造活动开始或结束阶段,并在盆地内识别出了中期构造事件,该事件造成盆地快速沉降,大面积分布的长7油层组底部凝灰岩和长7油层组中上部—长6油层组中下部的厚层深水重力流砂岩是该期构造活动的直接产物。因此印支构造活动在鄂尔多斯盆地形成了两个构造层序界面和两套沉积事件地层。此外,文章总结了中期事件在鄂尔多斯盆地的八大沉积耦合效应。Abstract: Qinling mountain and adjacent Ordos Basin constitute a mountain-basin coupling system. There is a close relation between Qinling uplifting and Ordos Basin forming. Indosinian, a key period of complicated Qingling orogeny, is characterized by the feature of polycyclic evolution. According to isotopic dating, Indosinian movement in Qingling area can be divided into three important stages, i.e., 230~238 Ma, 210~224 Ma and 198~206 Ma. Among them, the geological records in the middle stage are the most abundant. Geochronological correlation between orogenic belt and basin is difficult to be conducted, for it is difficult to study the age of sedimentary strata accurately. The deposition of tuff at the bottom of Ch7 oil bearing formation, an event layer, is characterized by wide spreading, isochronous and instantaneous. Abundant magmatic zircons are hosted in the tuff bed, and will act as a bridge on the isochronous correlation between Qingling orogenic belt and Ordos Basin. Based on U-Pb dating of zircons by the method of LA-ICP-MS collected from the tuff, the ages of tuff layers at the mid-upper and bottom of Ch7 oil bearing formation in Yanchang Formation are 221.8±2.0 Ma and 228.2±2.0 Ma, separately, respectively other depositional event ages are estimated further. It is put forward that the forming ages of uncomformities located at the bottom and top of Yanchang Formation are 237 Ma and 199.6 Ma, respectively, corresponding to the beginning or end of early and late stages of Indosinian tectonic events, and the age of the deepwater sandstone at mid-upper Ch7 and mid-lower Ch6 is formed during 215~224 Ma. Middle events, represented by tuff layer and mass gravity sand body, are identified in this study. The wide spreading tuff at the bottom of Ch7 oil-bearing formation and the thick deepwater sandstone, which is steadily distributed in depositional central of Ordos Basin, are the direct outcome of the middle event. Therefore, Indosinian movement resulted in two tectonic succession boundaries and two event deposits in Ordos Basin. In addition, eight coupling effects which were triggered by the middle event are uncovered in this paper. Some obvious transformations are revealed in sediment, petrology, geochemistry and biology. (1)The tuff layer at the bottom of Ch7 oil-bearing Formation is widely distributed in the west and southwest of Ordos Basin, covering more than 3×104 km2. (2)The landform altered from gentle slope and shallow-water to rapid and intense subsidence and wide spreading deep water. (3)Massive gravity sand body is distributed in the central of lacustrine, with some trace of tectonic event. (4)Asymmetry of basin become serious and the bottom shape is featured by western limb steep and the eastern limb gentle since Ch7 depositional period. (5)Compared with Ch10-Ch8 epoch, the basin central migrated counterclockwise since Ch7 epoch. (6)The sedimentary association developed from braided riverbraided river delta to alluvial fan-fan delta or alluvial fan-braided river-braided river-delta in the southwest and west of Ordos Basin. (7)Divided by the tuff layer at the bottom of Ch7 oil-bearing formation, there is a big gap in the content of quartz, feldspar, carbonate debris and chloritoid between Ch7-Ch1 sandstone and Ch10-Ch8 sandstone. (8)The sporopollen changed greatly. The sporopollen in Ch10-Ch8 strata is characterized by low diversity and fern spore stands dominant position. The sporopollen in Ch7-Ch1 mudstone is of high diversity and the content of gymnosperm pollen is banlance with fern spore. Therefore, comparison of Indosinian tectonic phases between Qinling orogenic zone and Ordos Basin shows a good agreement. However, process of mountain-basin coupling lasted long and is complicated, and further research is needed to be done.
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Key words:
- Indosinian movement /
- Ordos Basin /
- Qinling orogeny /
- mountain-basin coupling system /
- U-Pb dating of zircons /
- tuff /
- deep water sandstone /
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