Paleotemperature evolution and its driving mechanism during the formation of limestone-marl alternations in first member of Middle Permian Maokou Formation in Sichuan Basin

Abstract

From the Late Carboniferous to the end of Middle Permian, the most intense glacial event in history of Phanerozoic occurred and it is also the last transition period from icehouse stage to greenhouse stage in geological history. In this paper, the paleotemperature and paleoclimate in the first member of the Middle Permian Maokou Formation (Mao-1) were restored by means of thin section identification, scanning electron microscope, major elements, trace elements, carbon isotopes, oxygen isotopes and strontium isotopes, and the major climate evolution process and its driving mechanism in the Middle Permian were discussed. The results show that the biological assemblage type (mainly composed of foraminifera, brachiopods and molluscs, without reef-building organisms or calcareous green algae) and rock structure characteristics (supported by bioclastic and plaster, without oolitic or other non-skeletal particles) of the limestone-marl alternations in the Mao-1 member are similar to those of the international typical cool-water carbonate. During the limestone sedimentary period, the paleotemperature of seawater was 3.72 to 12.38 ℃ (8.15 ℃ in average, δ18O standard) or 13.79 to 14.28 ℃ (13.90 ℃ in average, ω(Mg)/ ω(Ca) standard), while during the marl sedimentary period, the paleotemperature of seawater was 7.00 to 14.24 ℃ (10.97 ℃ in average, δ18O standard) and 13.82 to 15.41 ℃ (14.27 ℃ in average, ω(Mg)/ω(Ca) standard). The paleotemperature changes in the sedimentary period of Mao-1 Member was mainly driven by the short eccentricity cycle of Milankovitch. The cyclical change of the short eccentricity was the driving mechanism of paleotemperature and paleoclimate cycle changes.

Keywords

• limestone-marl alternation
• cool-water carbonate
• paleotemperature
• first member of maokou formation
• middle permian
• sichuan basin