Magnetostratigraphy and sedimentologically derived geochronology of the Quaternary lacustrine deposits of a 3000 m thick sequence in the central Qaidam basin, western China

The Qaidam basin is a large basin in western China where the thickness of Quaternary deposits has a range of 2000-3000 m at its subsiding depocentre. This study investigated the chronology of the Quaternary deposits, including palaeomagnetics, biostratigraphic correlation, rates of sediment accumulation, seismic reflection surveys and electrical property correlations. The results give a full interpretation of ages for the sequence from bottom to top. The oldest sequence age was defined by the first appearance of Microlimnocythere sinensis Huang, and was placed in the Gauss Epoch, estimated at approximately 3 Ma by extrapolation of accumulation rates. The M/G boundary occurs at the base of the third fossil zone of Ostracoda (the first appearance of Qinghaicypris crassa Huang), or at the marker layer K-10 of the electrical log in the Sebei anticline area, considered here to be 2.48 Ma. The Olduvai event in the fifth fossil zone, spanning electrical layers K-4 and K-3 in the Sebei area, occurs at reflection layer T-0(-1) of seismic stratigraphy which could be stratigraphically correlated in the whole basin. The boundary of Early/Middle Pleistocene such as the Brunhes and Matsuyama (B/M) boundary, falls within the eighth fossil zone and coincides with electrical marker K-0(1). Two boundaries corresponding to Middle/Late Pleistocene and Pleistocene/Holocene occur at the bases of the 11th and 12th biozones respectively, neither of which can be correlated with seismic stratigraphy and electrical logs in the basin due to the lack of widely distributed lacustrine deposits since the Middle Pleistocene. The ages of the boundaries can be tested by deposition rates of different facies derived from the varves and synchronous deposits. As a result, geochronological sequences of subsurface stratigraphy could be established within a basin-wide area on the synchronology of seismic reflectors and correlations of electrical logs as well as biozones. (C) 1998 Elsevier Science B.V. All rights reserved.