"海侵—海退"背景下微生物岩序列组合的差异响应:以鄂尔多斯盆地中东部马五6-10亚段为例OA北大核心

The structural and sequential complexity and variability of microbialites result in multiple interpretations of their depositional environments.This study focuses on the 6th to 10th sub-members of the fifth member of the Ordovician Majiagou Formation(Ma56-10 sub-member)in the central-eastern Ordos Basin as an example.Based on systematic core and thin-section analyses,this paper presents a detailed characterization of microbialite sequences under varying sea-level conditions,aiming to reduce uncertainties in sedimentary environment interpretation.The results show that:(1)Three types of microbial carbonates,including stromatolites,thrombolites and microbial-bonded granular dolomite,are developed in the study area,forming two major categories and six types of microbialite sedimentary sequences.(2)During a fourth-order sea-level fall,interactions among the concavo-convex paleogeomorphologic pattern,gradual differentiation of seawater salinity,and energy control the formation of diverse microbialite sequences.These sequences exhibit complex lithologic correlations composed of four upward-shallowing fifth-order(meter-scale)cycles:lagoon-tidal flat,lagoon-microbial mound-tidal flat,lagoon-microbial mound and lagoon-microbial mound-evaporite evaporative lagoon/tidal flat.(3)Under a fourth-order sea-level rise,microbialite sequences and lithological assemblages are relatively simple,comprising two fifth-order cycles:microbial mound and intermound sea-microbial mound.The higher accommodation space during this phase contributes to the thicker single-cycle deposition.These findings reveal the construction patterns and controlling factors of microbialite sequences under varying sea level changes,providing a foundation for the subsequent inversion of different geomorphologic units and the sedimentary environmental evolution through the analysis of microbialite sequence structures.