Rate of Sea Level Rise as a Control on Physical versus Biological Sedimentation: Examples from Holocene of South Florida

Randall W. Parkinson

Decelerating late Holocene sea level rise over the south Florida platform has been accompanied by changes in sediment composition which reflect a transition from physically emplaced (allochthonous) sediment to biologically emplaced (autochthonous) sediment. This interpretation is based on a south Florida submergence curve which suggests that the rate of sea level rise has decreased from 26 cm/100 years, prior to 3,200 y.B.P., to 3.5 cm/100 years thereafter. These compositional changes are recognizable within both mixed siliciclastic/carbonate and pure carbonate depositional systems.

For example, mangrove islands located in the Ten Thousand Islands area of southwest Florida are overlain by a Holocene sediment sequence which consists of (in ascending order) (1) thin basal mangrove peat, (2) thin oyster zone, (3) shelly quartz packstone, 4) oyster or vermetid packstone to boundstone, and (5) mangrove peat. The lower sequence (units 1 through 3) reflects an early deepening phase which accompanied a rapid rise in sea level. Biological sediment production was not rapid enough to keep up with rising sea level and, as such, the mangrove-fringed shoreline was overstepped. The overlying subtidal shelly quartz packstones reflect local reworking of Pleistocene quartz sands and, later, the introduction of quartz sand by suspension from offshore. As sea level rise slowed, biol gical sediment production and accumulation began to exceed rates of sea level rise and subsequently deposits built up to and kept pace with rising sea level (units 4 and 5). Carbon-14 dates confirm this interpretation as the thin basal peats date at 4,000 y.B.P. (rapid rise) and oysters at the base of unit 4 date at 1,000 y.B.P. (slow rise).

Carbon-14 dates and island stratigraphy from areas within the pure carbonate realm of southeast Florida, such as Tavernier and Rodriguez Banks, support the hypothesis that sea level deceleration is recognizable within the sedimentary record as a compositional transition from physically to biologically emplaced sediment.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.