Doctoral Dissertations

Date of Award

12-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Michael L. McKinney

Committee Members

Kenneth R. Walker, Thomas W. Broadhead, John Gittleman

Abstract

Shallow marine Paleogene carbonates of the Eastern Gulf Coastal Plain contain a variety of unique and largely undocumented facies. The Late Paleocene Salt Mountain Limestone of southwestern Alabama is a coral-algal-sponge buildup composed of large foram-algal packstone, algal bindstone, and sponge-coral bafflestone. Abundant coralline algae and a low diversity coral fauna are characteristic of Paleocene reefs, but sponges are rare in earliest Tertiary sediments. Their abundance in the Salt Mountain could indicate an opportunistic response of sponges as reef constructors after the extinction of rudist-coral communities in the Late Cretaceous.

The Bridgeboro Limestone, a rhodolith and coral-bearing limestone in southwestern Georgia, contains the larger foraminifera Lepidocyclina (Lepidocyclina) Mantelli (Morton), L. (Nephrolepidina) yurnagunensis Cushman, and L. (Eulepidina) undosa Cushman. an assemblage indicative of the Lepidocyclina (s.s.) subzone of the Eulepidina Zone of Early Oligocene age. These foraminifera also characterize the Duncan Church bed of the Florida panhandle, and an assemblage of isolepidine and eulepidine Lepidocyclina occurs in the Glendon Limestone of Alabama. This and other stratigraphic evidence indicates that the Bridgeboro, Duncan Church, and Glendon are biostratigraphically and lithostratigraphically correlative.

Throughout the Early Oligocene, the eastern Gulf Coast was characterized by three paleogeographic and carbonate facies provinces: (1) Shelf Province (Alabama / NW Florida), with foramol/bryomol limestones and local red algal pavements; (2) Gulf Trough/Apalachicola Embayment Province (N Florida / SW Georgia), a deep, current-swept structure with shallow, flanking coralgal reefs; and (3) Florida Platform Province (peninsular Florida / SE Georgia), a miliolid, peloidal chlorozoan limestone with local patch reefs and coral thickets.

During the mid-Oligocene highstand, the Shelf Province became a drowned ramp with a shelf margin condensed section (Glendon LS). This carbonate system was unable to keep-up with sealevel rise because of its slow-growing, temperate water (30°N paleolatitude) fauna. Around the Gulf Trough, however, coralgal reefs (Bridgeboro LS) kept pace with sealevel rise and formed a rimmed platform. Despite their comparatively high paleolatitude (29-32°N), the tropical fauna of these carbonates thrived because of the influence of warm Gulf Trough waters originating in the Caribbean. The Florida Platform also kept pace with sealevel rise, and was partially emergent. During the 30 Ma eustatic sealevel fall, coralgal reefs moved from the flanks of the Gulf Trough (Bridgeboro LS) into the Trough (Okapilco Mbr., Suwannee LS). This time-transgressive shift in reef development continued to step down the bathymertic gradient (tracking sealevel fall) until by the Late Oligocene, reefs existed along the northern Gulf Shelf margin in southern Mississippi and Alabama (Heterostegina Zone) as a rimmed shelf.

Lower Oligocene larger benthic foraminifera from the Gulf Coastal Plain provide a means of testing the morphologic and life history effects of environmental stress. Stress-selection occurs in environments where disturbance is low, but conditions are persistently suboptimal for normal growth and function. In stable (i.e., low energy) but stressful (i.e., low light for photosynthesis) conditions, the semelparous, symbiont-bearing larger foraminifera will delay reproduction, grow to larger sizes, and produce larger (or more) embryons during asexual reproduction. Test size for Nummulites panamensis, Lepidocyclina mantelli, L. yurnagunensis, and L. undosa, examined along a forereef-to-deep shelf paleoenvironmental transect, generally was found to increase in both the shallow and deeper extremes of the range of three out of four species. Embryon size show the same pattern in two species.

Under shallow, high energy conditions, large adult size (to produce larger young) and large embryon size are advantageous because small juveniles can easily be dislodged from their substrate. However, deep, low energy / low light conditions also favor large adult size and large juveniles which inherit much symbiont-rich protoplasm from the parent embryon size are predicted at the limits of a species' range, but for different reasons.

Along the shallow to deep ecogradient, differences in growth rate and reproductive schedule should occur. At the shallow end, high light intensity and water energy facilitate relatively rapid growth. In deeper waters, low light reduces the efficiency of symbiont photosynthesis and test classification. The latter populations are stress-selected, but in both cases, large size is attained by hypermorphosis.

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