86 D53. Sediments, Carbonates (Posters)

Sunday, 22 October 2017
9:00 AM - 5:30 PM
Halls 4EF

DEPOSITIONAL HISTORY OF EEMIAN REEF DEPOSITS (COCKBURN TOWN MEMBER, GROTTO BEACH FORMATION) ON SAN SALVADOR ISLAND, BAHAMAS

Paper number: 86-5
Program: Technical Sessions
Time: 9:00 AM – 5:30 PM

Authors

Abstract

In addition to coral geochronology and the mid-reef “Devil’s Point” erosional discontinuity, smaller-scale depositional features within the Eemian (MIS 5e) Cockburn Town Member fossil reef deposits yield important insights into paleoenvironmental changes potentially associated with short-term sea-level fluctuation during the last glacial maximum. On San Salvador Island, lower reef (Reef I) exposures in Cockburn Town quarry exhibit a facies transition from coral floatstone (with minor coral bafflestone/framestone) to coral-microbial bindstone, whereas the upper reef (Reef II) is dominated by coral-mollusc-corallinacean rudstone (with essentially no microbialite). Although subtle horizontal discontinuities are common within Reef I, isolated areas display several inclined grainstone-draped discontinuities that define decimeter-scale lateral accretion surfaces. Each lateral accretion deposit contains branching coral fragments coated by micritic stromatolites (laminated microbialite) and poorly-laminated/clotted sandy microbialites (typically 30-80 mm thick). Microbialite-coated corals are separated by discontinuous, mm- to cm-scale drapes of ooid-peloid-skeletal grainstone similar to foreshore deposits. Reef I features suggest the following history of reef development: 1) establishment of branching coral stands in a bank-barrier setting; 2) development of backreef and lagoonal subenvironments; 3) subsequent restriction of those subenvironments, likely from a minor sea-level drop or a stillstand; 4) coating of coral clasts by microbial communities thriving in the backreef; 5) repeated introduction of new coral clasts via storms from reef-front settings followed by microbial coating of the new rubble; and 6) gradual migration of foreshore sand draping the shallow backreef. Repeated episodes of Reef I microbialite development, microbialite abundance and thickness, and the truncation of Reef I by the “Devil’s Point” discontinuity, indicate a significant period of backreef restriction followed by erosional truncation (sea-level drop). In contrast, the pristine coral preservation and the internal discontinuities of Reef II rudstones suggest episodic but rapid accumulation of reef debris in an unrestricted, energetic nearshore setting.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-307524

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