Ages, Lithofacies And Depositional Environments Of Cretaceous Oceanic Red Beds

Cretaceous Oceanic Red Bed (CORB) deposition has been one of the most attractive topics in Cretaceous research in the last several years, because it has a significant implication on paleoceanography evolution. As a globally distributed sedimentary succession, CORBs need to be correlated globally for their age ranges, lithologies and depositional environments. However, the temporal and spatial evolution of CORBs is not clearly documented except those in Tethyal realm. In this dissertation, to correlate CORBs globally, the author compiled the data of CORBs recovered from the DSDP and ODP sites and the cropped out in the Tethyan realm, Boreal realm and New Zealand. Nonetheless, the characteristics of CORBs cropped out in a typical area, Gyangze Basin were correlated in detail. The ages, lithofacies and depositional environments of CORBs in Gyangze Basin were described as well. The geochemical approaches were used to study the condition of bottom water during the CORB deposition as well as the paleoceanography evolution of eastern Tethyan Ocean.CORBs in DSDP and ODP sites mainly consist of clay(stones), oozes and chalks with a few exceptions of mudstones, limestones, shales or dolomites. Volcanogenetic debris could also be intercalated in the CORB sediments. Colors are mainly brown hues including brown, reddish brown, yellowish brown and pale brown, although the red, reddish, orange hues are found in some of the sites. In some cases, CORB color is characterized by alternation of different colors. However, CORB outcrops are generally composed of red, reddish or purple shales and limestones. CORBs were deposited at bathyal to abyssal depth, i.e. 200 m to thousands of meters, below or above CCD. The depositional environments are mainly slpoes to abyssal plains. Sedimentation rates vary from a few to 10 mm/kyr in most of sites, while the average rate can reach 30 mm/kyr with the tubidite intercalations. We suggest that the low sedimentation rate is a potential controlling factor of the CORB deposition.The common age of CORB is late Cretaceous, i.e. between Turonian and Maastrichtian. In oceanic drilling sites, CORBs are spread widest during early Campanian-Maastrichtian. In the outcrops, CORBs are mainly aged early Turonian-Santonian. The tempo evolution of CORB can be divided into 3 stages: sporadic occurrence, regional distribution, and global widespreading. The onsets of each stages are early late Aptian, early Turonian, and Campanian, which follow respectively the OAE1a, OAE2, and OAE3. It is suggested that the OAEs are the trigger of CORB deposition, because the significant increasing of oxygen content as well as the decreasing of carbon dioxic followed the burial of organism during the OAEs. Paleogeographically, in late Cretaceous South Atlantic Ocean and Indian Ocean were formed and resuled from the opening of circum-Africa Seaway, which extended from the western Tethys region through the North and South Atlantic into the juvenile Indian Ocean in early Cretaceous. This pattern increased the connection between oceans; therefore, we suggest that the formation of South Atlantic Ocean and Indian Ocean is one of the important contributing factors of CORB distribution.In Gyangze Basin, CORB successions share the general features described above, but they are differentiated from the others with the re-deposition of carbonate rocks. They are mainly Campanian in age. The occurrence of CORB in south is earlier than in the north basin. CORBs are composed of red shales interbedded with thin tubiditic marls bands, and variegated slide or slump limestones. Very few benthic foraminifera found in the limestones indicate that the paleodepth of CORB is bathyal to abyssal, i.e. below 200 m. The calcic carbonate is quite low in the red beds, which indicates that CORBs were deposited between CCD and lysocline, or immediately below the CCD. Eight microfacies types were identified in the limestones, which mainly belong to the standard microfacies 3 of Wilson indicating a basin or deeper shelf margin environment. According to the correlation of the facies between Gyangze CORB and Bahama slope, we suggest that the depositional environment is out slope-base-apron to basin.The Cretaceous marine sediments in Gyangze Basin were spectacularly expressed by black, white to red succession. The geochemistry of each unit was discussed in the dissertation to study the redox condition and paleoceanography evolution of deep eastern Tethyan Ocean. The black unit was deposited under an anoxic to dysoxic environment with a warm and humid climate. The white unit stands for the suboxic to oxic bottom water, but the pore water of sediments was dysoxic. Therefore, the Mn was enriched in the sediments immediately below the redox boundary that is close to the sediment-water interface. An increased eolation is suggested. The red unit was deposited under a more oxic condition, which is significantly differentiated from the white unit by the relative position of redox boundary. In the period of red unit deposition, the redox boundary is verge on the bottom of the soft sediments so that the Mn enriched as Mn-Ca under the boundary while a small portion of Fe is reduced to Fe(Ⅱ). More eolian sediments input to the basin. The Mg/Al and K/Al ratios indicate that the paleoclimate changed from the alternation of cold and warm of white unit into relative cold and dry in the red beds. The thesis suggests hyperoxic to newly name the bottom water condition of red beds, because it is distinctly more oxic than normal oxic state of the whit unit's.In addition, the dissertation studied the relationship of color and isotope event. The red-green chromaticness a* is controlled by the content of Fe(Ⅱ) and MnCO3 in the dark gray shales and pale brownish white limestones of studied section. The yellow-blue chromaticness b* is controlled by the content of goethite. The positive excursions ofδ13C correlate to the negative excursions of a* curve. It is interpreted that the positive excursions ofδ13C is caused by the anoxic event, that represent the reduce state of the basin. Therefore, the iron presents mainly as Fe(Ⅱ) in the sediments, which in turn leads to the decrease of a* value. Otherwise, the Fe(Ⅲ) minerals, goethite or hematite lead the increase of b* or a* value. I suggest that the curve of chromaticness can be a useful tool of stratigraphic correlation, especially for the carbonate free sediments like CORBs in southern Tibet.