The sequence stratigraphy of the East China Sea: Where are the incised valleys?

Jeffrey D. Warren and Louis R. Bartek, III: Department of Geological Sciences, University of North Carolina at Chapel Hill

Abstract

Four separate seismic surveys conducted during the past decade have acquired an overlapping grid of more than 13,000 km of 2D, high-resolution seismic data in a 300,000 km2 study area within the East China Sea (ECS). In addition, more than 10,500 km of higher frequency chirp sonar exist over the same grid allowing sub-meter resolution of the uppermost strata masked by the seismic bubble pulse (i.e., acoustic source signature). This extensive, nested-frequency database allows a regional, sequence-stratigraphic analysis of shallow subsurface sediments (up to 150 m) deposited during the Holocene and late Pleistocene through oxygen isotope stage 10 approximately 350 Ka before present. The shallow nature of the ECS (ave. depth = 72 m, shelf-slope break between 150 and 192 m) has allowed differing degrees of shelf exposure during stages 2 through 10. Periods of maximum exposure have occurred during stage 2, 6, and 8 lowstand systems tracts (LSTs) when sea level was approximately 120 meters below present. This subaerial exposure, combined with a near-horizontal seaward dip (0.37 m/km) and abundant fluvial input from the Yangtze and Yellow rivers (presently contributing 1.6 billion tons of sediment per year), offers a set of boundary conditions that do not produce incised valley systems. Instead, the ECS system has repetitively produced laterally extensive (>300 km) and amalgamated LST fluvial systems that are up to 60 m thick and truncated by transgressive surfaces and transgressive systems tracts (TSTs). Even though the LST bases are erosional and regionally incisive to a certain degree, the volume of sediment deposited on the shelf and not bypassed into the Okinawa Trough, in addition to the lack of focused trunk channels and tributaries, produces an unincised fluvial system. Limited examples of localized incision are present but constrained to topographic highs from paleo depocenters. This antecedent topography/bathymetry, along with subsidence driven by sediment loading and tectonics after stage 6 and tectonically uplifted strata prior to stage 6, also appears to have controlled the lateral extent of the LST fluvial systems. Interpretations of older strata (i.e., stages 10 and 12) are also possible on a limited basis due to the shallow multiple yet also appear to have the same massively amalgamated, unincised LST characteristics.