YING, XUDONG, and DA ZHOU, Department of Geology, Stanford University, Stanford, CA

ABSTRACT: Tectonic Control on Basin Development and Sedimentation During Syn-Rift and Post-Rift Stages In The Beibu Gulf Basin, China

The Beibu Gulf basin is the most landward of Cenozoic rifted basins on the northern continental shelf of the South China Sea. Active faulting and rifting, as a result of N-S extension, started in the latest Cretaceous to early Paleogene and ceased at late Oligocene due to formation of oceanic crust at the southern shelf

margin and a subsequent shift of extension center to the south. Reflecting this tectonic story, a thick Paleogene syn-rift stratigraphic megasequence, confined to five isolated or connected half-graben and grabens, is truncated by a regional angular unconformity (breakup unconformity of 27 +/- 2 Ma) and is overlain by a blanket-like upper Paleogene to Neogene post-rift megasequence. The syn-rift megasequence is characterized by alluvial, fluvial, and source-rock-bearing lacustrine clastic sediments, whereas the post-rift megasequence consists of paralic andshallow-marine sediments that were deposited during the stage of thermal subsidence.

A pattern of east-west trending basins and ranges set the framework for drainage systems and syn-rift basin filling patterns in the Beibu Gulf basin. The spatial association of half-grabens and related accommodation zones determined local drainage pattern, local sediment supply, and the distribution of sediments in the basins. During active extension of syn-rift stage, deep and highly asymmetric lacustrine basins filled with dark-colored mudstone and deep-water turbidite facies, occupied the rapidly subsiding downthrown blocks of the half-graben systems. Coarse-grained sediments were generally trapped in a narrrow belt along the steep, faulted side of the lakes in front of bounding ranges. As extension slowed, fluvial and shallow-lacustrine systems dominated the decreasingly asymmetri basins. During this stage of basin development, migrating meandering streams as well as progradation and lobe switching of deltas were best-developed. Marine transgression took place as the rifting terminated and the whole region underwent thermal subsidence. Several large progradational deltaic and shallow marine sequences, fed by streams of large drainage basins, were deposited on this new passive continental margin.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.