Mid-cretaceous Shalc-microbialite In The Tibetan Tethys

Increasing evidence shows that episodes of mid-Cretaceous oceanic anoxicevents （OAEs） are associated with the eruption of methane hydrates. Therefore, itis important to study the sedimentary record in the extreme environment: blackshale-calcimicrobialite assemblages for re-interpretation of the causes of OAEs,the interaction mechanism and biological-environmental co-evolution. Shale-calcimicrobialite assemblages in Gamba area can be divided into three kinds ofargillaceous lithofacies and six carbonate microfacies, and then divided into ninekinds of sedimentary formation. Based on planktonic foraminifera, TheGucuocun Formation/Gamba Dongshan Formation boundary are close toAptian/Albian boundary in age, and The Chaqiela Formation’s age is limited in R.appenninica Zone, Lengqingre Formation is Cenomanian-Coniacian, and theGamba Cunkou Formation is later than Santonian. The Zongshan section andTingri region seems to be different, so there may be shuttle laterally. Accordingto the lithostratigraphic sequence cycles and sea level variations, it can bedivided into two sub-second-order sequence: K1SS1and K₂SS1, the overallperformance of the two transgression-regression cycles. K1SS1and K₂SS1,respectively, can be divided into seven third-order sequences （K1S1-K1S7） andfour third-order sequences （K₂S1-K₂S4）.Gamba overall located in the inner shelf-shelf edge slope environment, witha water depth of50to250m, habitat type of Ⅲ^Ⅳ and higher productivity inthe mid-Cretaceous. Three planktonic and benthic fauna were divided accordingto the stage of evolution and combination of characteristics. Water depth limitedspecies,（P/P+E）)%and other indicators of recovery paleao-water depth （sealevel） change show that sea-level change coincides with long period of GambaHaq et al （1987）, and short-period basic collaborative, but the study areaCenomanian of sea-level change much more frequently than the global changes, suggesting that sea-level change was mainly controlled by the structure. Theafter-OAE2sea-level change and Haq et al （1987）’s short cycle tends to becollaborative, demonstrating climate-driven characteristics. The benthicforaminiferal alternative indicator of dissolved oxygen concentration includingabundance, Shannon-Weiner diversity and benthic foraminiferal oxygen index（BFOI） identified eight different oxygen-depleted or anoxic periods, which offour were corresponding to OAE1d, MCE, OAE2and OAE3.Two sets of large-scale cold-seep carbonates were found in the upper part ofthe Gamba Dongshan Formantion and Lengqingre Formation for evidence ofmethane leakage. The spatial and temporal distribution, biological communitiesand geochemical characteristics show typical seep carbonates characteristics.Methane leakages were commonly caused by greenhouse effect, sea-levelchange, anoxic environment and regional tectonic instability. It has obviousinteraction between these environmental events, and jointly controlled successionof the anoxic environment, Cretaceous marine life and the development ofproductivity, and then formed the Biosphere-Geosphere feedback mechanisms.