Seismic and Geologic Characteristics of Drowning Events on Carbonate Platforms(1)

Abstract

Carbonate platform drowning events are recorded in Upper Jurassic-Lower Cretaceous carbonates of the Baltimore Canyon area, offshore United States East Coast, and lower Miocene carbonates of the Pearl River Mouth Basin, offshore People's Republic of China. Cursory examination of seismic data from both areas would indicate that the platforms fit the classic definition of a drowning unconformity. However, detailed lithologic and paleontologic data indicate that the depositional/erosional hiatuses vary widely in each area, from 0-25 m.y. in the Baltimore Canyon area, to a condensed section with no hiatus in the Pearl River Mouth Basin. The sedimentary sequence produced during drowning (the drowning sequence) in both areas is gradational with underlying shallow platform carbo ates and, in some places, overlying deeper marine limestones and shales. The generalized facies models proposed for drowning events suggest that they may appear as instantaneous or gradual changes in geologic and seismic data. However, high-resolution seismic data across the Baltimore Canyon and Pearl River Mouth basins show that some drowning sequences may be masked on conventionally processed seismic sections. Because of this resolution problem, a distinction should be made between seismically and geologically defined unconformity surfaces. The geologic characteristics of drowning events on carbonate platforms include a gradational lower (and sometimes upper) contact, chemical sedimentation (glauconite, phosphate), open-marine shelf sediments, and a variable hiatus at the upper boundary. Late-growth shelf margin reefs also can be a diagnostic feature of drowned platforms. Seismic characteristics include horizontal to sub-horizontal basinal marine onlap, basin-parallel reflector continuity above the carbonate sequence boundary, and downlap of later highstand units. Evidence from platforms adjacent to continental margins (Baltimore Canyon) indicates that drowning occurs in a step-wise fashion, advancing more or less symmetrically across the platform from shelf to nearshore environments. Data from isolated open-ocean atolls (Pearl River Mouth Basin) indicate that they respond differently to drowning, showing an asymmetric decrease in shallow platform environments as drowning occurs. Proper recognition by a geoscientist of the seismic and geologic characteristics of drowning events can lead to better reservoir and seal estimates, and the correct reconstruction of the depositional, tectonic, and eustatic histories of an area.