HAMMES, URSULA, University of Colorado, Boulder, CO

ABSTRACT: High-frequency Cyclicity in Shallow-Water Carbonate Ramp Deposits, Oligocene, Florida, USA

High-frequency cycles in shallow-water carbonate ramp deposits of the Oligocene Suwannee Formation in southwest Florida were controlled by Milankovitch-forced sea-level oscillations. Peritidal, lagoonal, grainstone, and subtidal cycles are the basic building blocks of this carbonate ramp. Peritidal cycles are characterized by thick peritidal caps and a thin subtidal base, whereas lagoonal cycles are capped by thin peritidal deposits and a thick subtidal base. Subtidal and grainstone cycles do not contain any peritidal caps but are distinguished by thick, subtidal shallowing-upward, mud-rich, and grainstone facies, respectively. Many cycles, regardless of type, are capped by subaerial exposure surfaces. Ages derived from 87/86Sr ratios indicate that average cycle duration is between 11 ky and 600 ky. Assuming 80% of the temporal record is missing due to non-deposition or exposure, these periods reduce to 22,000 and 120,000 years, respectively.

The primary origin of the Suwannee small-scale cycles is interpreted to be high-frequency, eustatic sea-level oscillations. Spectral analyses demonstrate a three-fold signal of cyclicity across the entire ramp. The cycle ratios from spectral analyses are 20:5:1. Assuming the frequency of 1 is 20,000 years, these ratios support orbital forcing with periodicities of long eccentricity, short eccentricity, and precession cycles with approximate rhythms of 400 ky, 100-125 ky, and 20 ky, respectively. These periodicities are in good agreement with average cycle duration derived from the Sr isotopic ages. In those time frames, regional subsidence would not create enough space for the observed thickness of the cycles. Thus, autocyclic controls are considered to have been negligible.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.