GEOLOGICAL STRUCTURE AND SEDIMENTARY SYSTEMS OF THE WAN'AN BASIN,NANSHA WATERS

The Wan'an Basin is a strike-slip and pull-apart basin formed by the tensile stress derived from the dextral strike-slip movement of the Wan'an fault on its east side. Main faults extend in NE、NEE and SN directions that formed a tectonic pattern of "three swells-four depressions" in the basin. The basin experienced a successive tectonic evolution of basement formation, initial rifting, rifting development (first phase of rifting), early postrifting(second phase of rifting)、tectonic inversion and postrifting (regional subsidence). Based on the relationship of two-way travel time vs. depth between stacking velocities from the seismic data and drilling data from wells,four megasequence boundaries were identified:MB1,the top of the acoustic Pre-Tertiary basement;MB2,the top of the Oligocene (24Ma);MB3,the Late Miocene (8Ma);MB4,the Early Pliocene (4Ma). Above each megasequence boundary(MB),there is a megasequence(MS). They are referred to as MS1 to MS4 from the oldest to youngest, corresponding to a lacustrine-deltaic depositional system of Late Eocene-Oligocene; a transitional system of non-marine-littoral-shelf in Late Miocene;a system of carbonate-shallow water shelf and deltaic system of Miocene-Early Pliocene and a shelf-slope-deep marine system of Early Pliocene-Quaternary,respectively.