Shelf Sand Body Distribution and Transgressive Systems Tract Development on a High Energy Shelf: An Example from the East China Sea

Louis R. Bartek, Greg Condray, Thomas M. Rose

Approximately 3,428 km of Chirp-sonar data and 39 gravity core (28 coring stations occupied) were acquired during a survey of the East China Sea in September of 1993. The East China Sea continental margin is characterized high tidal currents, frequent and intense storm events, and an extremely high sediment supply relative to other margins (e.g Atlantic Coast, Gulf of Mexico). Therefore, a detailed seismic and coring survey of this area was undertaken to observe distal "end member" type continental shelf stratigraphic and sedimentologic conditions. The combination of these highly dynamic conditions and abundant supply of sediment greatly increases the probability of rapidly burying and preserving the stratigraphic record of these storm and tidal events, as well as the Qua ernary response of such a system to eustatic sea-level change.

The transgressive systems tract (TST) associated with the last glacial cycle is thin (<5 m) to absent on this shelf. Isopach mapping has revealed a series of highs. In the SE portion of the study area, Pleistocene fine-grained deposits appear to be exposed at the seafloor. In this area, strong tidal currents erode the seafloor and apparently mold it into it's present ridge-like form. The eroded sediments are transported to the north were they are deposited as tidal ridges. This type of ridge is characterized by shallowly inclined seismic reflections, that are interpreted as foresets. Tidal currents apparently continue to transport a portion of these Pleistocene sediments further to the north, where they in fill pre-existing topographic lows. These sediments are characterized by a l ck of internal reflections and a flat upper surface. The three types of late-glacial TST isopach highs documented on the ECS shelf formed under very different hydrodynamic conditions. For example, the molded, ridge-like topography in the SE portion of the shelf is a destructional feature, while the ridges in the north are active to inactive, constructional features. Without knowledge of the high-resolution seismic stratigraphy in these areas, these two radically different features could easily be grouped together.

Ongoing statistical analysis of the sedimentologic changes in cores and horizontal and vertical facies transitions and their relationship to sequence stratigraphic surfaces on sonar sections are being conducted to ascertain the minimum amount of data necessary to identify the nature and orientation of reservoir sands and seals that develop under typical high energy "end-member" type depositional conditions.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995