Early Silurian Siliciclastic-Carbonate Ramp Deposits in NW Sichuan
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摘要: 研究区位于扬子地块西北缘，由一套巨厚的页岩、泥岩层夹生物礁及透镜状和不规则层状风暴生屑灰岩、瘤状灰岩组成。根据区内特征的岩石类型及其组合、分布和相应的化石生态，作者认为本区兰多维列期至早温洛克期时为一典型的陆源碎屑－－碳酸盐混积均斜缓坡，并且从浅到深划分为滨岸、浅缓坡、深缓坡及盆地等亚环境。滨岸区位于潮间带，以潮坪碳酸盐岩为代表；浅缓坡位于浪基面至风暴浪基面之间，岩性组合为Ａ、Ｂ类风暴岩，Ａ、Ｂ类瘤状灰岩、生物礁灰岩及粘结岩；深缓坡位于风暴浪基面至最大风暴浪基面之间，岩性以Ｄ、Ｅ类风暴岩、Ｃ类瘤状灰岩及泥页岩组合为特征；盆地处于最大风暴浪基面之下，为黑色泥页岩沉积。结合早志留世时全球冰川作用，文中讨论了海平面变化的原因及对环境的影响。Abstract: The Llandoverian to Wenlockian storm-influenced siliciclastic and carbonate ramp environment has been recognized from the NW Sichuan area. The Early Silurian deposited on the NW margin of the S-China paleoplate tectonically and it has very thick sequences of shale, silty shale, mud and silty sandstone with some reefs, bioherms and stratigraphically discontinuous lenticular and irregular bioclastic tempestites, and multigenic nodular limestone beds.According to the different petrographic assemblages and paleoecology analysis, the authors have divided the sedimentary environment into littoral zone, shallow ramp, deep ramp and basin. The littoral zone is located at intertidal area with tidal flat carbonate deposits; the shallow ramp which is between the wave base and the storm wave base, has type A, B tempestites, pattern A, B nodular limestone, reef limestone and bindstone deposits; the deep ramp is below the storm wave base and above the maximal storm wave base and has type D, E tempestites, pattern C nodular limestone, dark shale deposits; the characteristics; the area below the maximal storm wave base is the basin area with black shale. From Lower Llandoverian to Lower Wenlockian, the sedimentary environment changed from basin to deep ramp, shallow ramp and littoral zone, the representative strata are Lungmachi Formation, Chuijiagou Formation, Wangjiawan Formation and Ningqian Formation, respectively.The authors also discuss the sea level change and its influence in this area. The global glaciation may be the main factor for the influence of the sea level change. Under the action of Tamadjert and Cananiri glaciation, the sea level changed from high to low in this area during the Llandoverian to Wenlockian.
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Key words:
- Ramp /
- Petrographic Assemblages /
- Sea-level Change /
- NW Sichuan /
- Early Silurian
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