Bowen Song

The early-Eocene climate optimum (EECO) event in the Qaidam basin, northwest China: clay evidence

Abstract Clay mineralogy and its palaeoclimatic interpretation of the early-Eocene (~53.3-49.70 Ma) sediments at Lulehe, Qaidam basin, northwest China, were investigated using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The interval of ~53.3–49.70 Ma, including the early-Eocene climate optimum (EECO) with isotopic events, was the transition period of ‘‘greenhouse’’ to ‘‘icehouse’’. Climate changes during the episode were documented in the sediments and were expressed by the proportion of clay species and clay indices, as well as by the proportion of non-clay minerals, gypsum, halite and calcite. Our results suggest that a warm and humid climate prevailed over the period ~53.3–52.90 Ma, followed by a warm and seasonally dry and humid climate in the period ~52.90–51.0 Ma and a subsequently warm and humid climate in the period ~51.0–49.70 Ma. Three warmer and more humid intervals were observed at 52.7, 51.0 and 50.5 Ma based on clay indices. The climate evolution in the Qaidam Basin during the period derived from the clay mineralogical study is in good agreement with the early Eocene global climate change, and the warm and seasonally dry and humid episode in the early Eocene in Qaidam basin is a regional response to the global early-Eocene climate optimum.

Climatic and tectonic evolution in the North Qaidam since the Cenozoic: Evidence from sedimentology and mineralogy

Clay mineralogy and bulk mineral composition of Tertiary sediments in Qaidam were investigated using X-ray diffraction (XRD) and scanning electron microscopy in order to better understand regional climate change resulting from uplift of the Northeast Tibetan Plateau. Climate change in Qaidam since ∼53.5 Ma could be divided into four stages: a warm and seasonally arid climate between ∼53.5 and 40 Ma, a cold and arid climate from ∼40 to 26 Ma, a warm and humid climate between ∼26 and 13.5 Ma, and a much colder and arid climate from ∼13.5 to 2.5 Ma, respectively. The illite crystallinity and sedimentary facies suggested that uplift events took place around >52–50, ∼40-38, ∼26-15, ∼10-8, and <5 Ma in the Qaidam region, respectively. The climate in Qaidam Basin could have been controlled by global climate prior to 13.5 Ma. As the Tibetan Plateau reached a significant elevation by ∼13.5 Ma, and the climate cycles of the East Asian monsoon might add additional influence.

Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin.

The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ18O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time.

Detrital Zircon U-Pb Geochronology from Greywackes in the Niujuanzi Ophiolitic Mélange, Beishan Area, NW China: Provenance and Tectonic Implications

The Niujuanzi ophiolitic mélange represents the remnant of oceanic crust between the Dunhuang massif and Mingshui-Hanshan massif. Greywacke from different tectonic slices in the Niujuanzi ophiolitic mélange were analyzed for detrital zircon U-Pb geochronology, trace elements and whole-rock trace elements to infer their provenance and the evolution of the Niujuanzi Ocean. Sample N-76s containing Carboniferous spores has the youngest zircon age of 323 Ma, while sample N-85s without fossils has the youngest zircon age of 449 Ma. The two samples were deposited no earlier than 323 and 449 Ma, respectively. The greywackes are depleted in large ion lithophile elements, and are relatively enriched in high field strength elements. The age spectra and trace element concentrations indicate that the sediments may have been deposited near the trench. The Hf, U, and Yb contents of zircons from sample N-76s vary widely, whereas those from sample N-85s have a narrow range. Sample N-76s contains both continental and oceanic zircons, while sample N-85s contains only continental zircons. The sediments were derived from the continental arc and accretionary wedge. The Paleozoic oceanic crust zircons have ages of 430–500 Ma, indicating the timing of the expansion of the Hongliuhe-Niujuanzi-Xichangjing Ocean expansion. The oldest Paleozoic continental zircon has an age of 470 Ma, suggesting that the northward subduction of the oceanic crust may have started at that time.

Palaeogene–Neogene Stratigraphic Sequences of the Tibetan Plateau and Their Response to Plateau Uplift

A total of 98 remnant basins and 5 stratigraphic domains with 13 stratigraphic subdomains have been recognized on the Tibetan Plateau. Through investigating the types of remnant basin, the tectonic setting, the stratigraphic sequences and sedimentary characteristics, and the evolution of the sediments, we divide the uplift process and sedimentary response for the Tibetan Plateau into three stages and eight substages. These are the subduction–collision uplift stage (65–34 Ma) with three substages, the intercontinental convergence and compressive uplift stage (34–13 Ma) with three substages, and the intercontinental isostatic adjustment uplift stage (since 13 Ma) with two substages.