Sedimentary Environments and Paleoclimate of the Middle-Late Jurassic in the Northeastern Sichuan Basin
[Objective]The Jurassic was a period of typical fluctuating greenhouse climate that resulted in the deposi-tion of sediments in terrestrial basins.In China,a number of large terrestrial basins were formed during the Jurassic,and the discrepancies in sedimentation records have been preserved in several basins.This study focused on the Middle-Upper Jurassic terrestrial redbeds in the northeastern Sichuan Basin to determine changes in the sedimentary environment and paleoclimate.[Methods]Based on the observation of outcrops,analyses of microscopic clastic com-position and C-O isotopes,and to estimate the atmospheric CO2 concentration in this area.[Results]Medium-to-fine clastic rocks are the predominant lithologies,and five sandstones are further classified by composition and lithologi-cal content.Three sedimentary environmental units are recognized:meandering river,lake,and paleosol,which represents a simpler system than in the western Sichuan Basin.The upwards facies sequence is pedogenic,beginning with low sand/mud ratio meandering mudrocks in the Middle Jurassic Shaximiao Formation,lacustrine mudrocks with siltstones in the lower Suining Formation of the lower Upper Jurassic,and upward-coarsening and thickening high sand/mud ratio meandering sandstones in the upper Suining and Penglaizhen Formations of the Middle-Upper Jurassic.Two and a half cycles of semi-arid and arid climate were distinguished in the Middle-Upper Jurassic sequence by F/Q and F/L indices,corresponding to the lower Shaximiao Formation,the upper Shaximiao Formation+lower Suining Formation,and the upper Suining Formation+Penglaizhen Formation,respectively.From the pCO2,it is postulated that the Middle Jurassic was relatively cool,becoming warmer in the early Late Jurassic and hot in the middle-to-late Late Jurassic,roughly consistent with the framework of global marine climates.[Conclusion]The climate was(semi-)arid overall in northeastern Sichuan,similar to that in other regions of the Sichuan Basin.It will be necessary to further verify that an intermittent semi-wet climate occurred in western Sichuan and that a hot climate prevailed in northeastern Sichuan.
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