Paleocene to Lower Eocene sediments of the eastern Neo-Tethyan Ocean : sedimentary and geodynamic evolution as well as biostratigraphy of the larger benthic foraminifera Lockhartia and the genesis of nodular limestones

Abstract

Larger benthic and symbiont-bearing foraminifera are known as important producers of shallow-water carbonate sediments. Their depth distributions are influenced by the type of symbiont they inhabit and controlled by parameters like temperature, hydrodynamic energy, topographic conditions or substrate type. That is why larger benthic foraminifera and their association with other organisms are used to reconstruct paleoenvironmental as well as paleobathymetrical conditions. Paleocene to Lower Eocene sediments of the eastern Neo-Tethyan Ocean are used to compare differences and similarities of the passive and active continental margin and to determine the sedimentary and geodynamic evolution of the sedimentary basin. The observation of larger benthic foraminifera and their association showed the distribution of eleven microfacies in four sections of the passive Indian (Tethys-Himalaya) and four microfacies in one section of the active Asian (Xigaze forearc basin) continental margin of the eastern Neo-Tethyan Ocean. Based on the distribution of the observed microfacies, depositional environment ranges from very shallow tidal flat and restricted lagoonal part of the inner carbonate ramp to the outer carbonate ramp, showing a two-stepped deepening event from Lower Paleocene to Lower Eocene within the entire eastern Neo-Tethyan Ocean. Besides paleoenvironmental and paleobathymetrical reconstructions, larger benthic foraminifera are an important biostratigraphic tool due to their rapid diversification and abrupt extinctions. In India the Paleocene Ranikot Formation is characterized by the high diversity of the larger benthic foraminifera Lockhartia, therefore the eastern Neo-Tethyan Ocean is also called "Ranikot Sea" or "Lockhartia Sea". Shallow-water sediments of three profiles from the Tethyan Himalaya reveal the stratigraphic ranges of seven Lockhartia species. Based on the stratigraphic distribution of these species five interval biozones (called "Lockhartia biozones") are established showing a stratigraphic conformity of sediments from the Tethyan Himalaya. Correlation of established biozones with the well-established shallow benthic zones used for biostratigraphy of shallow-water environments suggest an earlier evolution of some $ textit Lockhartia $ species in the eastern Neo-Tethys compared to the western Tethyan Ocean. Additionally, $ textit Lockhartia $ shows an increasing diversity from Middle to Late Paleocene and a reduction of diversity within the Lower Eocene of the Tethys-Himalaya. In literature changes in foraminiferal diversity and assemblage at the Paleocene-Eocene boundary are interpreted as being the result of climatic and geochemical changes called Paleocene-Eocene Thermal Maximum and carbon isotope excursion. In the south of Tibet, the sharp negative shift of the carbon isotope excursion representative for the Paleocene-Eocene boundary is located in a nodular limestone bed of the Zhepure Shan Formation in Tingri. Based on field appearances five categories of nodular limestones and some transitional members can be classified. Processes responsible for the genesis of nodular limestones in Tingri are differential diagenesis followed by chemical and mechanical compaction. Differences in clay content are assumed to be responsible for the various types of nodular limestones.