Mineralogical Genesis Investigation Of Two Types Of Cretaceous Oceanic Iron-Containing Sediments

Cretaceous has been considered as the typical representative of extreme climate for the geological period. A series of special sediments in response to this extreme climate was developed in this period. In this thesis, the author carries out a systematically mineralogical study of Cretaceous Oceanic Red beds (CORBs) and glauconite-bearing beds, which widely appeared in Cretaceous ocean and both contain iron-bearing mineral, to achieve a better understanding of the paleoceanographic environments during their deposition.Three sections, including the Chuangde section in Tibet, the Vispi Quarry section in Italy and the ODP Hole1049C Core from the North Atlantic, are considered as the representatives of CORB in eastern Tethys, western Tethys and Atlantic Tethys, respectively. The studies such as identifying the species and coloration mechanism of pigments, the controlling factors and origin of CORB for these typical sections can provide an insight to acquire the paleoceanographic environmental information of Tethys during Cretaceous time.Based on studies of mineral compositions of Chuangde section, the Vispi Quarry section and the ODP Hole1049C, a detrital mineral and clay mineral assemblage are similar but their amount slightly varied comparing with adjacent non-redbeds of the same section. This may indicate that the formation of CORBs was not directly correlated with terrigenously detrital input. As shown by the element geochemical data, the CORBs have higher ferric iron concentration than adjacent non-redbeds, and ferric iron mainly present in the form of hematite (or goethite). We consider that the red color of CORBs was mainly caused by finely, dispersed ferric oxide, but there are some differences in coloration mechanism and concentrations of ferric oxide in these sections. In Chuangde section, hematite is the unique ferric oxide which endows the shale with red color. Further quantitative analysis reveals that the hematite concentration in shales was fairly high but varied within a range from3.81%to8.11%. Hematite can also be subdivided into two forms:detrital one or finely and poorly crystallized one. The former was directly derived from terrigenous input and preserved under oxidized conditions. The latter precipitated in pores of sediments in a highly oxygenated bottom water during syn-deposition or early diagenesis in the deep ocean basin. The latter one has a much higher coloration ability than the former one, and therefore mainly endow the red color of shales in Chuangde section.In the Vispi Quarry section, hematite present as primary pigment in red limestones. The concentration of hematite estimated about0.1%, was much lower than shales in Chuangde section. The origin of hematite are characterized by its distribution features:the irregularly distributed region was derived form terrigenous input with detrital minerals such as quartz, illite, and boehmite, and the homogenous distributed region was formed under oxidized condition during syn-deposition and early diagenesis. Both of them imparted a red color to the limestones. Mn2+-bearing calcite also endowed the limestones with a pink tinge induced by irradiation.In the marl from ODP Hole1049C in North Atlantic, hematite and goethite are the minerals responsible for the color change of sediment from white, green to red. However, they performed differently in terms of determining the color of sediment: hematite and goethite bring a red or yellow color to sediment, respectively. Relative change in amount between hematite and goethite can cause the color change of sediment from orange to brown.CORBs both in Chuangde section in Tibet and in Vispi Quarry section in Italy are belong to long period CORBs which span a few million years, may appear as possible consequence of Oceanic Anoxic Events (OAEs). High-cyclic CORB in ODP Hole1049C in North Atlantic as the representative of short period CORBs which are comparable with Milankovitch cycle periods, may be associated with the fluctuation in solar insolation arose by Milankovitch cycles. The hematite which endowed red color to sediments was deposited under oxidized condition during syn-deposition and early diagenesis. A supply of iron into a depositional basin which ultimately transforms to red pigmenting hematite, favorable redox condition and diagenesis can also largely influence the formation of hematite. No matter which factor is dominant, the formation of CORB depends on the dissolved oxygen in the bottom water. When the dissolved oxygen content is far greater than the consumption of decomposition of organic matter, the pigment hematite finally appeared or survived.Visible light diffuse reflectance spectroscopy (DRS) proved to be a rapid and precise method for identification of hematite and goethite. In this thesis, the author preformed a quantitative analysis using DRS with multiple linear regression fitting to estimate the absolute concentration of hematite and goethite in CORB from ODP Hole1049C in North Atlantic. The successful application of this method provides a new way for quantitative analysis of iron oxides in CORBs and coloration mechanism of iron oxides.Glauconite is the common mineral in Cretaceous marine strata. As an iron-containing mineral, it contains both reduced Fe2+and oxidized Fe3+. The Fe2+/Fe3+ratio in its structure reflects the redox condition which is decisive for its formation. Increasing attention has being focused on its formation mechanism since the important application of glauconite as facies mineral in sequence stratigraphy and isotopic chronology. Early Cretaceous glauconite from Xiala section in southwestern Tibet, China, has been studied by systematic mineralogical analyses. The investigations revealed that the glauconites are at different stages of glauconitization and both in glauconitic sandstone and limestone are highly evolved. The glauconites in glauconitic sandstone are autochthonous origin, but the ones in limestone may come from the underlying glauconitic sandstone. The autochthonous glauconite may be associated with the final separation of the Indian Continent from the Australian-Antarctic Continent during the Late Albian.Investigation of glauconite at different stages of glauconitisation in the underlying strata reveals that the process of glauconitisation is mainly characterized by the replacing K-rich proportion of alkali feldspar, in which the interface migration reaction controlled by the solution-precipitation-recrystallization processes was the main mineralogical reaction mechanism. The lowly evolved glauconite in quartz sandstones and lithic sandstones has the almost same compositional characteristics, namely higher K2O contents and lower TFeO contents. This implies that the process of glauconitisation in Xiala section does not support the neoformation theory and two-stage model in which the glauconite results primarily from a poorly crystallized low-K, high-Fe smectite-like layer silicate following by the progressive enrichment of K during the long evolution. The replacement of K-rich minerals is different from the replacement of K-poor minerals, as the second step, which result in an enrichment in K, is not necessary. The higher K2O contents were associated with the extensive dissolution of K-rich minerals in the marine environment, and the lower TFeo contents were the indicator of redox environment at that time. When the local micro environments become weak reduced, an overwhelming majority of Fe as Fe2+enter into the lattice structure of berthierine.