鄂尔多斯盆地奥陶系马家沟组马五_(5)亚段微生物岩盆地尺度分布特征及主控因素

Basin-scale distribution and dominant controlling factors of microbialites of the Ma_(5)^(5) sub-member of Ordovician Majiagou Formation in Ordos Basin

基于岩心、薄片和测录井资料分析发现,鄂尔多斯盆地奥陶系马家沟组马五_(5)亚段下部发育1套盆地尺度稳定分布的微生物碳酸盐岩组合,其特征为:(1)岩石类型主要由泥质灰(云)岩、泥晶灰(云)岩、凝块灰(云)岩、叠层灰(云)岩、粘结砂屑灰(云)岩及岩溶角砾灰(云)岩等构成,凝块石主要呈层状、瘤状和格架状,叠层石可识别出层纹状、波状和柱状3种类型;(2)垂向上可识别出4种与微生物岩密切相关的、向上变深复变浅的典型沉积序列;(3)高频旋回小层划分与对比分析发现,微生物岩在横向上稳定连片分布,具有盆地尺度的可对比性和等时性;(4)基于岩心刻度测井方法建立测井相模板,依据该模板对全盆地404口井进行逐一识别和解释,发现微生物碳酸盐岩的累计厚度一般在3~7 m之间,整体呈向东减薄的变化趋势,连续分布面积超过2.5×10^(5)km^(2),揭示了较短时间内盆地尺度微生物岩规模发育的独特沉积现象。沉积序列分析表明,马五_(5)亚段沉积初期存在单旋回缓慢变深复变浅、多旋回组合振荡式持续缓慢海侵的海平面变化趋势,且相对平静的古构造环境与早期蒸发岩填平补齐作用的耦合提供了相对平坦的古地貌条件,因此在马五_(5)亚段沉积时期总体持续缓慢海侵背景下,环境同质化的底形条件是规模性浅水微生物碳酸盐岩发育的根本原因。系统报道马五_(5)亚段盆地尺度微生物岩的发育与分布特征,既丰富了鄂尔多斯盆地奥陶系马家沟组微生物岩的研究,也有助于丰富完善对碳酸盐岩台地微生物岩规模性繁育机制的认识。

Based on the analysis of core,thin section and logging data,basin-scale microbial carbo-nate with stable distribution developed in the lower part of the Ma_(5)^(5) sub-member of the Ordovician Majiagou Formation in Ordos Basin.The rock types include argillaceous limestone,limestone,stromatolite,thrombo-lite,bound sand-clastic limestone,karst breccias.It contains various shaped stromatolite and thrombolite,such as laminar,wavy and columnar stromatolite,laminar,nodular and arborescent thrombolite.Vertical-ly,four typical sedimentary sequences closely related to microbialites are identified,indicating relative sea-level fluctuations.The division and comparative analysis of the small layers with high-frequency cycles sug-gest that the microbialites are stably distributed laterally,and have basin-scale distribution.Based on the logging template established by core-log calibration,the lithology of 404 wells in the whole basin were inter-preted.The result shows that cumulative thickness of microbialites range from 3 to 7 m,and the thickness shows a decreasing trend from west to east.Furthermore,the continuous distribution area of microbialites can exceed 2.5×10^(5)km ^(2) in area,demonstrating that the basin-scale microbialites developed in a relatively shorter time.Analysis of sedimentary sequence in the early depositional stage of Ma_(5)^(5) sub-member indicates that the relative sea level first slowly rises and then falls in a single cycle,while it shows an oscillating and slowly rising trend in a multi-cycle combination.Further research suggests that the relatively stable paleo-tectonic environment coupled with the filling of early evaporite provides relatively flat paleo-geomorphic con-ditions.Under the background of continuous slow transgression in the depositional stage of Ma_(5)^(5) sub-mem-ber,the bottom shape condition of environmental homogeneity is the fundamental reason for the development of large-scale microbial carbonate in shallow water.This paper reports the development and distribution of basin-scale microbialites in the depositional stage of Ma_(5)^(5)sub-member,which not only enriches the research of microbialites in the Majiagou Formation,but also helps improve the understanding of development mecha-nism of large-scale microbialites in carbonate platforms.