2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 15
Presentation Time: 1:30 PM-5:30 PM


LEWIS, Ronald D., Department of Geology and Geography, Auburn Univ, 210 Petrie Hall, Auburn, AL 36849-5305, lewisrd@auburn.edu

Pleistocene (Sangamonian) reefs on San Salvador, Bahamas, such as the Cockburn Town fossil reef and Sue Point reef, are recognized by the abundance of in situ coral heads and coarse coral debris. The fossil reefs occur in shallowing-upward sequences, typically consisting of the following facies: lagoon, reef, shallow subtidal, storm, beach, and eolianite. Determination of reef type according to location ( e.g., fringing, patch reef, or bank barrier) has been based on dominant coral taxa, reef geometry, and stratigraphic sequence. However, coral taxa are not always diagnostic, facies geometry is often not well preserved, and stratigraphic successions are not necessarily simply progradational. Another approach is the study of reef sediment including its foraminifera; samples taken from an array of modern reefs at San Salvador provide an actualistic model against which fossil reefs can be evaluated.

Nearshore reefs (e.g., Dump Reef) are characterized by widely varying grain size distributions, a high percentage of grain aggregates, and foraminiferal assemblages dominated by the milioline Archaias angulatus and the attached rotaliine Homotrema rubrum. Mid-shelf patch reefs, located in approximately 5 meters water depth and dominated by the corals Montastrea and Diploria (Snapshot Reef and distal Monument Reef), have a high percentage (over 50%) of grain aggregates and a high, but variable, proportion of polished grains. Foraminiferal assemblages are strongly dominated by Archaias angulatus (over 50%), with Homotrema rubrum making up less than 20% of the fauna. Bank-barrier reefs, located offshore at 5 meters depth and dominated by Acropora palmata are represented in this study by Gaulins Reef. Here the sediment lacks grain aggregates and has a low-diversity foraminiferal assemblage strongly dominated by Homotrema rubrum. Relatively deep-water (15+ m) bank-edge reefs in Fernandez Bay (e.g., Vicki's Reef) have poorly sorted, finer-grained sediment. Their conspicuously diverse assemlages of well-preserved foraminifera include a relatively large proportion of free and attached textulariines. Samples from the Cockburn Town fossil reef contain a high percentage of grain aggregates (over 50%) and polished grains (20%), and are strongly dominated by Archaias angulatus. Thus the fossil reef compares most favorably with a mid-shelf patch reef.