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

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 14.4Ma 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. © 2019, OIL & GAS GEOLOGY Editorial Board. All right reserved.