The Late Eocene Chronostratigraphic Study And Paleoelevation Reconstruction In Coqen Basin, Tibet

The collision between the Indian and Eurasian plates that lead to the uplift of the Tibetan Plateau and global climate enormous change is one of the greatest geology events during the Cenozoic. The uplifting progression of the Tibetan Plateau has been an important area of research in the geological sciences, and significant research achievements have been achieved for the Tibetan Plateau uplift in recent decades. Quantitative palaeoelevation reconstruction can represent the tectonic history subtly and provide quantitative constraint for the uplift model. Xu Ren proposed the first quantitative palaeoelevation model for Tibetan Plateau by plant fossil at 1970s. After that this research didn’t made significant headway until Garzione Carmala initiate the oxygen isotope method for Tibetan Plateau palaeoelevation study. Nevertheless, the published paleoelevation dates from Tibetan Plateau are always based on single method by oxygen isotope, plant fossil, hydrogen isotope or △47 temperature-elevation model, but did not consider enough about the integral tectonic lithofacies paleogeographic background, biological assemblages and paleoecology study.This time we choose an upper Eocene section located in Coqen basin on the north of Lhasa terrane for quantitative palaeoelevation reconstruction. The Lhasa terrane located between the Bangong-Nujiang suture and the Yarlung-Zangbo suture is a key region for studying the uplift of the Tibetan Plateau. Coqen Basin is a depression basin located in the northern part of the Lhasa terrane. We observed and measured the lower part of Dingqinghu Formation in field work. Based on multiple division and correlation of lithostratigraphy, sedimentary facies, biostratigraphy, magnetostratigraphy and chronostratigraphy, we established the high precision chronostratigraphic framework. Focusing on the oxygen isotope and combining with the regional tectonic-lithofacies palaeogeography, ostracod assemblages? foraminifera and its paleoecology, we reconstructed the paleoelevation of this area during late Eocene. The result lays a foundation for remodeling plateau uplift model. Following are the main achievements of this thesis:(1) According to the analysis of sedimentary facies, we think this region was a shallow to semi-deep lake affected by occasional transgression event and deposited as depositional cycles of limestone and mudstone interbedded with subaqueous fan during late Eocene.(2) According to the microfossils analysis of 183 samples, we recognized 48 species Ostracod belonging to 20 genera. We divided these ostracod as Candona extima-Chinocythere-Limnocythere zone and Candoniella porrecta-Pinnocypris postiacuta zone which were similar to the late Eocene biostratigraphy in Lunpola basin, Qaidam basin, also the east and north China. Among these species of ostracod, previous research has shown that the ostracoda genus Austrocypris is an important index fossil that was short-lived in the late Eocene. This genus was widely observed in the late Eocene strata from east to west China. Their extinction could correspond tothe late Eocene variation and fluctuation event which happened at ca.34 Ma.(3) According to the detrital zircon U-Pb chronology analysis of three sand rock samples collected from layers 1,12, and 25 of Dingqinghu Formation, the three youngest detrital zircon grains yielded a mean age of ca.42 Ma, and the youngest zircon population provides the maximum depositional age for our section.(4) A total of 266 oriented palaeomagnetic samples were collected from lower part of Dingqinghu Formation (330m in thickness). After thermal demagnetization or alternating demagnetization we got the stable remanence and built up the magnetostratigraphy column. Combined with the biostratigraphy and detrital zircon age, the results can be correlated with the standard polarity time scale, which covers an age range of 39.4-35.7 Ma.(5) Based on the chronostratigraphic framework discussed above, we took systematic analysis of oxygen isotope and carbon isotope of carbonate samples from this section. After excluded the influence of spirite veins by using a microdrill to separate the micrite from the sparite, excluded the influence of recrystallization by analysis of diagenetic phases, excluded the influence of evaporation by variation coefficients analysis of 813C and δ18O values, we think the isotope data from our section are still meaningful for reconstructing its palaeoelevation.We calculated the δ18 O(water-SMOW) value in the palaeowater as -2.8±1.1‰ based on the average δ18Oc (VPDB) in our micrite samples (i.e.,-5.1%o).Previous research shows that the initial surface temperature was 27℃, the relative humidity was 90%, and the ocean was -1‰ more depleted in δ18O during Eocene than today. According to these messages, we built a suitable Rayleigh fractionation model for this period and calculated the result of paleoelevation as 1255(-1174/+1314)m. However, this calculate probablely still over estimates the paleoelevation as we used the ocean water as the mass sourse instead of inland water body. When compared the main value of δ18O(water-SMOW) of our samples to modern meteoric water Asia region, we found that this value is similar with the modern value obvioused in sites at elevation as low as sealevel.(6) We discovered 1 and 5 shells of Lagena laevis (Bandy) fossils respectively in layer 1 and layer 23. This specie of foraminifer usually live in coastal estuaries environment. Previous research on the Eocene tectonic lithofacies palaeogeography of Tibet shows that the Lhasa terrane was surrounded by sea water to the west side and south side. Only if this region was located at low altitude, the foraminifers live in estuaries can be transported to the Coqen basin through the fluvial channels by the downward flow water caused by storm surges. The discovery of foraminifera in transgression strata provides further evidence of low elevation conclusion reconstructed by oxygen isotope.(7) According to the carbon isotope and oxygen isotope analysis, we get the conclusion that these limestone were deposited in a warm-damp environment in an open lake. Based on the ecological analysis of ostracod group, we think it stand for a low salinity-warm sallow lake environment and the salinity had the trend to become higher in later time. This inference is consistent with the low paleoelevation reconstructed above.Above all, we reach the conclusion that the Coqen basin was still in elevation of near to sealevel during 39.4-35.7Ma. This region on Lhasa terrane didn’t uplift significantly until Eocene. Considering the palaeoelevation results of 3-4km during 26Ma from the nearby Oligocene basins at the same latitude, also the widespread unconformities and related basal-conglomerates were discovered at intervals of ca.34 Ma, we infer that intense uplift event occurred on Tibet during 36-26 Ma. This great uplift maybe one of the factors that cause the global significant cooling event at 34Ma.