Middle-late Ordovician Sea-level Changes In The Bachu Area, Tarim Basin, Xinjiang: Carbon, Oxygen And Strontium Isotope Records

The Ordovician underwent a series of manifold changes, e.g. the GreatOrdovician Biodiversification Event (GOBE) and the end-Ordovician extinction.Climatic and environmental changes are undoubtedly major drivers of these changes.Sea water is an important carrier of climate and environment; sea level, which canreflect the possible nature and range of climatic and environmental changes, exerts afirst order control on the architecture of marine sedments and is intimately related tosea-floor and substrate evolution, benthic ecology, and biodiversity in shallow-watercratonic seas. Among a wide array pf tecjmoqies developed for interpreting sea-levelchange, carbon, oxygen and strontium isotopes in marine carbonates have beenproved to be an effective tool to track Ordovician environmental change and correlatesedimentary sequences from different areas or even different continents. Tarim, as oneof the independent blocks, was situated in the middle and low latitudes in theNorthern Hemisphere. Continuous carbonate rocks, deposited in the sedimentaryenvironments including platform, margin and slope during Middle–Late Ordovician inthe Bachu area of Tarim Basin, provide material for studying the composition andevolution of carbon, oxygen and strontium isotopes. Detailed studies on stratigraphy,paleontology and lithofacies in this area provide the substantial basis for the presentresearch on the isotopic evolution and sea-level changes.Based on lithostratigraphy and conodont biostratigraphy from the Nanyigousection and the stratotype section of the Lianglitag Formation in the Bachu area,Tarim Basin, the Middle–Upper Ordovician biostratigraphically calibrated δ13C,δ18O and87Sr/86Sr curves and sea-level curves are reconstructed. The followingachievements are obtained: 1. The carbonate samples selected for carbon, oxygen and strontium isotopeanalysis show few crystal structure changes and paleobiological structure alterations.The values of δ18O are mostly between-10‰~-4‰(except a value of-11.7‰in theYingshan Formation). Meanwhile, the values of δ13C and δ18O without distinctivelinear relationship are quite discrete. All these indicate that the carbon, oxygen andstrontium isotope compositions involved in this study were less altered by thediagenesis, and can reflect the original isotope compositions of sea water.2. Four δ13C, δ18O positive cycles and four87Sr/86Sr negative cycles arerecognized for the first time in Middle–Upper Ordovician in the Bachu area, TarimBasin, and are named as IC-1, IC-2, IC-3and IC-4respectively, of which, IC-1cycleoccurred in the Yijianfang Formation, IC-2occurred in the Qiaerbake Formation andthe lowermost part of the Lianglitag Formation, IC-3and IC-4are mainly occurred inthe Lianglitag Formation.3. Based on the records of carbon isotope from Middle-Late Ordovician in theBachu area and the comparisons with the Kalpin and Tazhong areas, the carbonisotope evolution spectra in Middle-Late Ordovician is reconstructed for the wholeTarim Plate. The mid-Darriwilian δ13C excursion (MDICE) is recognized for the firsttime in the Yijianfang Formation in the Bachu area. The Guttenberg δ13C excursion(GICE) is recognized in the uppermost-Sandbian or lowermost-Katian in the Bachu,Kalpin and Tazhong areas. This indicates that the evolution of carbon isotope in Tarimshows a good consistency with the global carbon isotope evolution.4. The δ18O values in the Bachu area showed an increasing trend during theDarriwilian and Katian, a slight decrease in the early Katian, and another increase inthe Katian, which are correlated with the global oxygen isotope curves based onbrachiopods and conodonts. Although δ18O values are often diagenetically altered andtrend to smaller values, they still reflect part of the original signal of sea water, andare correlated to the global oxygen isotopes evolution.5. In the Bachu area, the strontium isotope curves show higher87Sr/86Sr values of0.7090in the Yijianfang Formation, rapidly drop in the Qiaerbake Formation, and getlower values of0.7080in the Lianglitag Formation. The evolution in the studed areais compatible with the global trends, which are characterized by a major drop in theDarriwilian–Sandibian.6. Comprehensive researches on carbon, oxygen and strontium isotopes withsequence stratigraphy show that the cycles of carbon, oxygen and strontium isotopes are correlated to the short-term cycles of sequence stratigraphy. The sea-levelcontinued the increased trend in the Yijianfang Formation, and got the maximum inthe Qiaerbake Formation. The Qiaerbake Formation is considered to be the condensedsection in maximum flooding. After a regression in the lower part of the LianglitagFormation, a transgression occurred to in the upper part of the Lianglitag Formation.The results show that the global sea-level fluctuations is an influencing factor in theBachu area, and the regional tectonic activities and deposition rates are also anotherdriver to the relative sea-level changes in the Bachu area.7. The reef-bank complexes show higher δ13C, δ18O and87Sr/86Sr values than thatof the coeval carbonate strata. The higher δ13C values are explained in terms ofincreased productivity and enhanced burial of organic matter. The higher δ18O valuesare mainly caused by the increasing salinity in sea water due to the enhancedbiological weathering when the reef-bank complexes are exposed out of the seasurface. The higher87Sr/86Sr values are mainly caused by the adsorption of theterrestrial material.8. The research on the relationship between relative sea-level changes and thegrowth of reefs in Middle–Upper Ordovician in the Bachu area, Tarim Basin, showsthat reefs are mainly developed in the sea-level rising periods, but too deep water isnot beneficial to the growth of reefs.9. The analysis on the porosity and permeability in different microfacies ofreef-bank complex in the Bachu area show that the reef bases, reef flanks and bankspossesses higher porosity and permeability than the reef cores, reef inners and reefcaps. The reef bases, reef flanks and banks are the favorable reservoirs. The values ofporosity and permeability have a positive relationship. When the porosity isincreasing linearly, the permeability increases logarithmically. This is helpful toexplore favorable reservoirs in the reef-bank complex.In sum, the above research is significant for the further study in Middle–UpperOrdovician in the Bachu area of Tarim Basin, particularly for promoting the accuracyin stratigraphic correlation internationally. Meanwhile, it would be also beneficial formaximize oil and gas potential in reef-bank complex in the Bachu area of TarimBasin.