Sedimentology, sedimentary petrology, and paleoecology of the monsoon-driven, fluvio-lacustrine Zhada Basin, SW-Tibet

Abstract

In this paper we present sedimentologic, petrographic, and paleoecologic data from a ~ 810-m-thick section of late Neogene (ca. 9–7 to 1 Ma) fluvio-lacustrine deposits in the high-elevation Zhada Basin of SW-Tibet. The tectonic evolution of the Zhada Basin region, located between the Himalayan Mountains in the S and the Transhimalayan Mountains in the N, is poorly understood, but sediment accumulation was probably initiated in response to the middle/late Miocene uplift of the Gurla Mandhata and Leo Pargil domes in the E and W, respectively. The sedimentary record can be subdivided into five stratigraphic units with distinctly different lithofacies associations (LA); these are, from bottom to top: a bedload-dominated fluvial LA (Unit 1, 0–90 m); a nearshore lacustrine, deltaic LA (Unit 2, 90–220 m); an offshore lacustrine LA (Unit 3, 220–500 m); a nearshore lacustrine, fan-delta LA (Unit 4, 500–600 m); and a fan-delta/alluvial fan LA (Unit 5, 600–810 m). Clast and heavy mineral compositions revealed sediment supply from two different provenance areas, one from the south and one from the north/east, based on catchment geology of the Himalaya and Transhimalaya. Ostracode faunal assemblages are largely comparable to those of modern Tibetan lakes and allow the reconstruction of paleo-lake environments, including lake-level changes and salinity. The sedimentary pattern shows strong cyclicity at various scales (m, 10 s m, and >100 m). Small-scale cycles are interpreted as intrinsically-controlled depositional processes of deltas (lobe switching), whereas medium- and large-scale cycles are most likely forced by climate fluctuations. The overall evolution of the sedimentologic record includes: a strong and rapid increase in fluvial discharge in the uppermost portion of Unit 1, a transition from fluvial to lacustrine deposition during deposition of Unit 2, rhythmic variations in the style of lacustrine sedimentation during deposition of Unit 3, an increase in the supply of coarse clastics starting with deposition of Unit 4, and a transition from lacustrine to alluvial sedimentation during deposition of Unit 5. We speculate that the increase in discharge and, presumably, precipitation leading to the establishment of lacustrine conditions in the lower part of the section (Unit 2) is largely due to regional summer monsoon intensification during the late Miocene, and that the transition from lacustrine to alluvial sedimentation in the uppper part of the section reflects the onset of Northern Hemisphere glaciation and winter monsoon strengthening at ~ 2.6 Ma.