塔里木盆地西北缘露头区中-下奥陶统碳酸盐岩层序结构、沉积演化及海平面变化

Carbonate sequence architecture, sedimentary evolution and sea level fluctuation of the Middle and Lower Ordovician on outcrops at the northwestern margin of Tarim Basin

柯坪和巴楚地区出露了较完整的中-下奥陶统碳酸盐岩沉积地层,牙形石古生物地层对比表明柯坪水泥厂露头鹰山组与下伏蓬莱坝组呈不整合接触,缺失5.9 Ma地层,与上覆一间房组呈整合接触,两者共沉积约14.4 Ma地层。综合露头、前人地化指标及盆内塔中隆起区井震资料对比分析,将鹰山组至一间房组碳酸盐岩划分出2套以主要不整合面为边界的复合(二级)层序(CS1和CS2),内部包含5套以次一级暴露面、岩相突变面为边界的三级层序(Sq1-Sq5),可进行区域对比。通过对手标本和镜下薄片分析,归纳出11种和6种灰岩、白云岩沉积微相类型,它们以特定的相序组合成台缘礁滩复合体、潮坪内碎屑滩、台内浅滩-滩后、生屑滩、滩间海、云坪等7种微相组合,指示了研究区由CS1时期干旱-半干旱局限台地潟湖、云坪沉积向CS2时期开阔台地内碎屑滩、生屑滩和台地边缘礁滩复合体转变及中奥陶世末期台地被淹没的过程。利用Fischer图解重建了露头剖面相对海平面变化趋势,其与岩相旋回和地化指标所反应的古水深演化具有高度一致性,并与Haq海平面变化曲线整体上可进行对比。海平面变化对层序结构和沉积演化起主导作用,局部受构造和海平面变化双重因素制约,其中,四级海平面变化对相序组合具有明显的控制作用。

Carbonate deposits of the Middle and Lower Ordovician develops well on Kalpin and Bachu outcrops,north-western margin of Tarim Basin.Conodont biostratigraphy shows that the unconformable contact exists between the Ying-shan Formation and its underlying Penglaiba Formation with a 5.9 Ma hiatus,while the Yingshan Formation and its overlying Yijianfang Formation are conformable with a total of 14.4 Ma deposition.Comprehensive analysis of outcrops,existing geochemical indicators,and 3-D seismic and well logging data in Tazhong uplift,indicates that the carbonate rocks of the Yingshan and Yijianfang Formations can be divided into two composite(second-order) sequences(CS1 and CS2) along the major unconformity.From within,five third-order sequences(Sq1-Sq5) are identified along sub-level exposed unconformities or lithofacies mutation boundaries,which may be used for regional correlation.Eleven and six microfacies types are recognized for limestones and dolomites respectively through observation of hand specimens and thin section,which have in turn combined into seven micro-facies associations according to specific facies sequence,namely the reef-shoal complexes at platform margin,clastic shoals within tidal flat,intra-platform shallows and back shoals,bioclast shoals,inter-shoal sea and dolomite flats,indicating a transition from the arid-to-semi-arid restricted platform lagoon and dolomite flat sedimentation of the CS1 to the open intra-platform clastic shoal,bioclast shoal and reef-shoal complex at platform margin of the CS2,as well as the platform drowning in the late of Middle Ordovician.In addition,the fluctuation of relative sea level was reconstructed for the outcrop areas by means of Fischer plot.It’s highly consistent with the paleo-water depth evolution demonstrated by facies cycle and geochemical indicators,and can also be correlated with Haq eustatic curve.The eustatic variation dominates the evolution of sequence architectures and deposits,despite that local architectures and deposits are restricted by both eustatic variation and tectonic movement.And the fourth-order eustatic variation starkly controls the facies sequence association.