The Relationship Between Geochemistry Of Dolostone And Paleoclimate, Transgression

This paper analyses the order degree, stable isotope, major element, trace element andrare earth element (REE) of dolostones from the Paleogene Shahejie Formation in the Qikoudepression, eastern China, with the goal of determining the petrological features, genesis andcharacteristics of diagenetic fluid of dolostones in the region. These geochemical data,together with changes of paleoclimate and sea level worldwide, which are of significantimplication for revealing the palaeoclimatic and ancient environmental evolution that spansthe Eocene and Oligocene in the Qikou depression. Main achievements are as follows:Dolostones of Es3occur in the north of Qikou depression, dolomicrite play a dominantrole. Dolostones of Es2x develop in the north and west of Qikou depression, which are mainlycomposed of dolomicrite and crystallitic dolostone. Dolostones of Es2s occur in the north ofQikou depression, the main rock type is dolomicrite. Dolostones of Es1s and Es1x prevail inthe entire depression, which are mainly composed of dolomicrite, crystallitic dolostone,laminar algal dolostone and crystalline bioclastic dolostone.Dolomicrites of Es3and Es2in the Qikou depression are related to the burialdolomitization, and the genesises of crystallitic dolostones are related to the meteoric water.The genesises of crystallitic dolostones from Es1x are related to the methanation, both laminaralgal dolostones and crystalline bioclastic dolostone are related to the burial dolomitization, butthe later are buried superficially. The genesis of crystallitic dolostones from Es1s are related tothe methanation, some dolomicrites, laminar algal dolostones and crystalline bioclastic dolostones are related to the burial dolomitization, but most of them are influenced bymethanation.Cenozoic volcanic rocks are widespread in the south of Qikou depression, but decline inthe north, in the west are rather poorly developed. Dolostones from Es3almost not affected byvolcanic hydrotherm. Parts of dolostones from Es2x are affected by volcanic hydrotherm.Intense volcanic hydrotherm occurred in the Es2s, the dolostones show obviouscharacteristics of hydrothermal carbonate. Dolostones from Es1x and Es1s are with the littleeffect of volcanic hydrotherm. The depth of the most significant influence by volcanichydrotherm is2300m.A hot climate prevailed during the Es3, but this shifted to a cold climate in the Es1, andthere are two temperature rise events during the Es2x and Es1x, respectively. This change inclimate is concordant with the paleoclimate changes that spans the Eocene and Oligoceneworldwide. From Es3to Es1, the positive excursion in carbon isotope ratios caused by oceanicanoxic event and methanation, which is consistent with the positive excursions of carbonisotope at the at the Eocene–Oligocene boundary across the world.Two transgressions occurred during the Es3to Es1in the Qikou depression. Thesmall-scale transgression in Es2x had influenced dolostones from north and west of thisdepression. During Es1x, almost all the dolostones underwent an interaction of sea-water,even some samples show typical characteristics of marine carbonate. The influence oftransgression lasted until Es1s, both the scale and intensity are reduced.On a global scale, the sea level fell gradually from Eocene to Oligocene. However, therewere obvious sea levels rised during the late Eocene and early Oligocene, respectively.Time of the two transgressions happened in the Qikou depression are concordant with the sealevel rise events, this as a result of temperature rise events during this period.