Anatomy and Growth History of Holocene Ooid Shoal: ABSTRACT

Abstract

Facies anatomy of the Joulters ooid shoal is strikingly End_Page 462------------------------------ similar to many Jurassic oolite reservoirs of the Gulf Coast. Extensive coring of the shoal, lying north of Andros Island on the margin of Great Bahama Bank, has documented six subsurface facies: (1) skeletal grainstone; (2) ooid grainstone; (3) ooid packstone; (4) fine-peloid packstone; (5) pellet wackestone; and (6) lithoclast packstone. The relief of the shoal over the surrounding seafloor is the result of contributions by these different facies in differing amounts throughout the area, but in broadest terms the relief is a result of ooid sands in one facies or another. Basically, the facies anatomy consists of a fringe of ooid grainstone bordering a much wider shoal composed of two opposing sand wedges--an upper bankward-thinning wedge of ooid packstone overlying a muddier seaward thinning wedge of fine-peloid packstone. During sea-level rise in the last 5,000 years, topography of the underlying Pleistocene limestone has affected shoal growth by initially localizing ooid formation and structuring the shoal's bankward-curving trend. Growth of the shoal occurred in three stages: (1) bank flooding, from 4,000 to 5,000 years B.P., when fine-peloid and pellet muddy sands were deposited in platform interior; (2) shoal formation, from 3,000 to 4,000 years B.P., the beginning of ooid accumulation along the platform margin; and (3) shoal development, during the last 3,000 years, when growth of a marine sand belt established size and physiography of the shoal and changed platform sediments from muddy sands to ooid sands. This change was a result of increased agitation produced by a combination of topographic bu ldup and rising sea level. The anatomy and growth history of the Joulters ooid shoal suggest that present patterns in surface sediments are a product of changing subenvironments throughout the late Holocene. The development of the shoal provides one possible scenario for the evolution of a common facies package--a narrow belt of ooid grainstone bordering a much wider belt of ooid packstone that becomes increasingly muddy with depth. End_of_Article - Last_Page 463------------