塔里木盆地奥陶系层序地层格架

Sequence Stratigraphy in the Ordovician in the Tarim Basin

在不同沉积相区典型露头、钻井及地震层序综合分析基础上,将塔里木盆地奥陶系海相地层划分出8个可全盆地对比的三级层序(OSQ1～OSQ8),首次建立了综合露头层序、钻井层序及地震层序划分的层序地层格架,建立了年代地层、牙形石生物地层、岩石地层与层序地层之间的相互关系(表1)。首次提出海相碳酸盐岩全岩或生物化石壳的碳同位素值可作为全球海平面变化的良好指标。在相似气候带及沉积环境具备相近的碳酸盐沉积速率的假设条件下,当沉积相分析所得到的相对海平面变化趋势与全岩碳同位素分析反映的全球海平面变化趋势总体一致时,说明碳酸盐层序的发育主要受控于全球海平面变化,反之则主要受控于区域构造沉降运动。鉴于这样的分析原理,我们认为塔里木盆地下奥陶统层序OSQ1及层序OSQ2属于主要受控于全球海平面变化的稳定加积型层序,而中、上奥陶统层序OSQ3～层序OSQ8则属于全球海平面总体上升背景下主要受区域构造运动控制形成的构造淹没型层序。

Based on synthetically analyzing different sedimentary faces cropped out typically （e. g. platform faces in Bachu and platform-intraplatform basin faces in Keping of NW Tarim, and basin faces in Que＇erqueke of NE Tarim） and from the seismic sequences of drilling, the Ordovician marine strata can be divided into eight sequences （OSQ1 ～ OSQS） comparable each other in the whole Tarim basin. It is first established the frame of sequence stratigraphy by synthesizing the outcrop sequence, drilling sequence and seismic sequence, and also the relationship between chronostratigraphy, Conodont biostratigraphy, lithostratigraphy and sequence stratigraphy （Table 1 ）. In addition, we first propose that the carbon isotope of marine carbonate rock or fossil shell can be used as an important index of eustatic sea level change. That is, the global reservoirs of long-term carbon cycle are chiefly preserved in sedimentary carbonate （as 79. 952% of global cycle carbon） and sedimentary kerogen （as 19. 994% of global cycle carbon） , whereas the dissolvable carbon in marine water plus carbon in biosphere and atmosphere has only 0.054% of global cycle carbon （Sharp, 2007）. Thus, the carbon circulation mainly conducted between sedimentary carbonate and kerogen. The conservation of kerogen is related to eustatic sea level change. The eustatie rise is in favor of preserving carbon in kerogen. When more and more carbon is preserved in kerogen, it would bring away more carbon isotope ^12C from paleo-ocean and leave more ^13C in ocean water. As a result, the carbon isotpe dissolved in paleo-ocean water and its sediments such as carbonate, would become weightier. Based on this assumption, we can further deduce that the weightier carbon isotopic compositions of carbonate or fossil shell reflect the eustatic rise, otherwise eustatic fall. Alternatively, the sedimentation ratio of carbonate is similar in the same climatic condition and sedimentary environment. The changing trend of relative sea level on the basis of sedimentary faces analysis is generally consistent with that of the eustatic sea level change deduced from carbon isotope of carbonate rock. In this respect, the development of carbonate sequence is chiefly controlled by eustatic sea level change, otherwise, by regional tectonic subsidence. According to such analytical principle, we conclude that the sequence OSQ1 and sequence OSQ2 of Lower Ordovician in the Tarim basin belonging to the stable aggradation sequences, are chiefly controlled by eustatic sea level change, while the Middle-Upper Ordovician sequence OSQ3 to sequence OSQ8 belonging to the tectonic drowning sequences are mainly controlled by regional tectonic movement because of generally eustatic sea level rise.