AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratigraphic Evolution and Heterogeneity of a Holocene Ooid Tidal Sand Shoal: Lily Bank, Bahamas
(1) Geology, University of Kansas, Lawrence, KS.
General factors controlling the distribution and plan-view geometry of ooid shoals are reasonably well understood, but the relation between bar form morphology (i.e., linear bars and parabolic bars) and their stratigraphic signature are less well constrained. Yet, such understanding is essential for accurate interpretation and prediction in the stratigraphic record. To explore such relations, this study integrates subbottom seismic imagery and cores from Lily Bank, a Holocene ooid tidal sand shoal on the northern edge of Little Bahama Bank.
The data reveal several distinct facies elements. The base of the succession is a continuous, high amplitude, flat, horizontal reflector at ~9 m below water level, a depth coincident with oceanward top-Pleistocene bedrock outcrops. Above this basal reflector is a package of ~ 4 m of low-amplitude to seismically transparent deposits. This interval includes predominantly bioturbated muds and muddy sands. The elevation of this reflector, ~5 m below water level, is coincident with top of the sediment column in channels between bar forms. It continues beneath the active ooid shoals, however, where it is capped by a 4 m thick sediment package of predominantly oolitic sand with well-imaged dipping foresets up to 4 m tall that downlap onto this sub-horizontal reflector. Geometries in this upper sediment package indicate the direction of bedform migration consistent with those predicted by surface form.
Collectively, the succession is interpreted to reflect the Holocene evolution of this complex. The basal flat horizontal reflector at 9 m is interpreted as top Pleistocene. Following the flooding of this surface, the active shoal initiated in a more oceanward position, flanked platformward by lower energy, muddy sediments. With continued sea level rise, the active ooid factory migrated platformward, depositing the ooid sands that now cap the succession. Within individual bars, trends in grain size, sedimentary structures, bedding thickness, cross strata orientation, and contacts illustrate systematic relationships with plan-view bedform geometry.
These results provide enhanced understanding of the nature and controls of three dimensional depositional heterogeneity, insights that could be applied to enhance prediction of shoal geometry and continuity in subsurface oolitic reservoir analogs.