Observation and implication of the paleo-cave sediments in Ordovician strata of Well Lundong-1 in the Tarim Basin  

作　　者：SUN Shu ZHAO WenZhi ZHANG BaoMin LIU JingJiang ZHANG Jing SHAN XiuQin 

机构地区：[1]Institute of Geology and Geophysics, Chinese Academy of Sciences [2]PetroChina Exploration & Production Company [3]PetroChina Research Institute of Petroleum Exploration & Development

出　　处：《Science China Earth Sciences》2013年第4期618-627,共10页中国科学（地球科学英文版）

基　　金：supported by"Conditions for Giant Marine Carbonate Oil/Gas Fields to Form in Sichuan, Tarim Basins and the Neighboring Regions" a Key National Oil and Gas Program (Grant No. 2008ZX05004-002)

摘　　要：Well Lundong-1 is located in the periclinal area on the eastern flank of the Tahe-Lunnan paleo-uplift in the Tarim Basin. A 25-m-high cave fill sequence was observed in the Upper Ordovician interval of the well at 6800-6825m. A third cut of cores was obtained from the top of the cave. The following conclusions were obtained by studying the cave sediments and depositional sequence, and by undertaking paleontological and elemental geochemistry analyses. 1. The cave sediments contain abundant brachiopod, gastropod, echinoderm, ostracod, and acritach fossils, which can be classified into two groups: cave autochthonous and cave allochthonous fossils (from collapse breccia dissolution or transportation by underflow). The fossils indicate that the cave was formed before the Carboniferous and partly-filled and buried during Carboniferous resubsidence. 2. Elemental geochemistry shows that the mud that filled the cave is sourced from calcareous paleo-soil and weathered crust that came from a salty environment with poor water circulation. 3. The formation and evolution of the cave occurred in three stages. The first stage occurred after the deposition of the Late Ordovician Lianglitage Formation, the second stage took place after the deposition of the Late Ordovician Sangtamu Formation, and the last stage happened after Silurian deposition. Major dissolution occurred in the latter two stages as a result of bedding-confined deep underflow karstification. Based on the reconstruction of the cave formation history, favorable paleokarst targets can be predicted and estimated to aid paleogeography and paleokarstology studies.Well Lundong-1 is located in the periclinal area on the eastern flank of the Tahe-Lunnan paleo-uplift in the Tarim Basin. A 25-m-high cave fill sequence was observed in the Upper Ordovician interval of the well at 6800-6825 m. A third cut of cores was obtained from the top of the cave. The following conclusions were obtained by studying the cave sediments and deposi- tional sequence, and by undertaking paleontological and elemental geochemistry analyses. 1. The cave sediments contain abundant brachiopod, gastropod, echinoderm, ostracod, and acritach fossils, which can be classified into two groups： cave au- tochthonous and cave allochthonous fossils （from collapse breccia dissolution or transportation by underflow）. The fossils in- dicate that the cave was formed before the Carboniferous and partly-filled and buried during Carboniferous resubsidence. 2. Elemental geochemistry shows that the mud that filled the cave is sourced from calcareous paleo-soil and weathered crust that came from a salty environment with poor water circulation. 3. The formation and evolution of the cave occurred in three stages The first stage occurred after the deposition of the Late Ordovician Lianglitage Formation, the second stage took place after the deposition of the Late Ordovician Sangtamu Formation, and the last stage happened after Silurian deposition. Major dissolu- tion occurred in the latter two stages as a result of bedding-confined deep underflow karstification. Based on the reconstruction of the cave formation history, favorable paleokarst targets can be predicted and estimated to aid paleogeography and paleo- karstology studies.

关 键 词：cave filling and depositional sequence cave autochthonous fossils cave allochthonous fossils bedding karst Well Lundong-1 Tarim Basin 

分 类 号：P618.13[天文地球—矿床学]