黔东北石阡志留系雷家屯组的生物-岩相分异

BIO-AND LITHOFACIES DIFFERENTIAIONS OF THE LEIJIATUN FORMATION(LATE AERONIAN,LLANDOVERY,SILURIAN)IN SHIQIAN,NE GUIZHOU,SW CHINA

黔东北石阡志留系兰多维列统埃隆阶上部的雷家屯组属灰岩和碎屑岩混合相,其分布限于滇黔桂古陆以北离岸几十千米范围内的浅海区。出露于枫香铺子沟、雷家屯、白沙龙口—筷子山、白沙均田和本庄岩门的5个剖面存在生物-沉积相的差异性。铺沟村粉砂岩—泥岩含量高于灰岩,达到4∶1,壳相化石丰度最低且无生物礁滩灰岩沉积;其它4个剖面的碎屑岩—灰岩比例约1∶1,雷家屯壳相化石最丰富,上部出现米级厚度的珊瑚-层孔虫点礁;筷子山剖面的雷家屯组下部开始出现雏形礁,之上的礁核灰岩现已被剥蚀掉,但从礁基的菲利普构造和礁翼塌积岩推测,点礁的正向隆起规模是很明显的;均田剖面仅出露雷家屯组上部约7m的地层,多为粉砂岩和生屑灰岩薄层;岩门剖面多为薄层细颗粒生屑滩相灰岩,近顶部出现钙质微生物形成的叠层石。偏西剖面的雷家屯组顶部展现侵蚀面、泥裂等现象,是桐梓上升期海底暴露的证据,唯有最靠东的铺沟村雷家屯组顶部未见暴露标志,与之上马脚冲组呈整合接触。

The late Aeronian （Llandovery, Silurian） Leijiatun Formation is a suite of mixed depositional sequences composed of limestones and terrigenous depositional units. Its spatial distribution is limited within the shallow belt of the seaward epicontinental sea north of the Dian-Qian-Gui Oldland. Five sections at Pugoucun of Fengxiang, Leijiatun, Longkou-Kuaizishan and Juntian of Baisha and Yanmen of Benzhuang are employed for analyzing their bio- and lithofacies patterns. Palaeogeographic configuration did control their diversity from place to place. The Pugoucun section is less in macrofossils and more turbidity showing terrigenous rock and limestone without reef-bank unit in the ratio of 4 ： 1. The other four sections are approximately of 1 ： 1 for terrigenous rock and limestone. The Leijiatun is the richest in shelly macrofossils with coral-stromatoporoid patch reef occurrences for the upper part of the formation. Small reefal unit from the lower part of the formation at the Longkou-Kuaizishan section is recognized. Even through the onward expansion growth of a bigger patch reef-core is eroded nowadays, such a morphological reconstruction is supported by the Phillip Structure at the reef-base and remained meters thick talus from reef-flank. Development of the reef-flank is considered of an essential indicator of positive relief of patch reef. Only the upper part of the Leijiatun Formation is outcropped at the Juntian section composed of silts with less thin bedded bioclastic limestones. Thin-bedded bank facies composed of fine bioclastics are characteristics throughout the sequences at Yanmen, Benzhuang. The stromatolitic layer close to the top of the Leijiatun Formation is of the proxy of the calcimicrobial community favorable inhabitation to the near-shoal setting. Erosion surfaces and mud cracks from the top of the Leijiatun Formation are more likely occurred from the western sections, they are indicators of the regional regression during the Tongzi Uplift. The Pugoucun section has a conformable boundary between the Leijiatun and overlying Majiaochong formations. Such a case suggests that it remained under the sea water during the Tongzi Uplift.