鄂尔多斯盆地西缘中部三叠系延长组碎屑岩沉积环境及物源示踪

Sedimentary Environment and Provenance Tracing of Clastic Rocks from the Triassic Yanchang Formation in the Western Margin of the Ordos Basin

【目的】鄂尔多斯盆地西缘中部三叠系延长组沉积环境和物源认识上还存在一些分歧和不足。【方法】通过地球化学方法对延长组不同层段的16件砂泥岩样品进行了主量、微量以及稀土元素测试分析,探讨了延长组碎屑岩的沉积环境、物源区源岩类型和构造背景。【结果】砂岩类型主要为岩屑砂岩、长石砂岩,源区经历了弱—中等的风化作用,样品受再旋回作用影响很小;源岩与上地壳物质相关;延长组整体处于氧化—弱还原的淡水沉积环境,古气候为干旱—半干旱,第三段部分处于弱氧化的微咸水环境和温湿古气候;源岩主要为长英质岩石,延长组三段物源较复杂,有部分中性岩;延长组源区总体处于活动大陆边缘构造背景,第三段源区构造背景较复杂,与大陆岛弧和活动大陆边缘构造背景均相关。【结论】元素地球化学特征示踪的物源和沉积环境特征,与三叠系延长组湖盆演化和区域构造环境具有较好的一致性。

[Objective]There are still some disagreement and deficiencies on understanding the sedimentary environment and provenance from the Triassic Yanchang Formation in the middle of the western margin of the Ordos Basin[Methods]Through geochemical methods,sixteen sand and mudstone samples from different sections of the Late Triassic Yanchang Formation were tested and analyzed,and the sedimentary environment,source rock types,and tectonic background of the Yanchang Formation clastic rocks were evaluated.[Results]Sandstone types are mainly lithic and feldspathic sandstone,the source areas experienced weak-moderate weathering,and the samples were little affected by the recycling.The source is related to the upper crust material.The Yanchang Formation is in an oxidation-weak reduction freshwater sedimentary environment,with an arid-semi-arid paleoclimate,and the Third member is in a weak oxidation brackish water environment with a warm and humid paleoclimate.The source rocks are primarily felsic,and the provenance of the Third member of the Yanchang Formation is complex,with a little intermediate rock.The source area of the Yanchang Formation has a tectonic background of an active continental margin,while the tectonic background of the Third member source area is complex and is related to both a continental island arc and an active continental margin.[Conclusions]The provenance and sedimentary environment characteristics traced by elemental geochemistry are in good agreement with the lake basin evolution and regional tectonic environment of the Triassic Yanchang Formation.