CONTROLS ON THE DEPOSITIONAL BALANCE BETWEEN CARBONATES AND SILICICLASTICS ON THE SOUTHEASTERN FLORIDA PLATFORM

Seismic-reflection data collected along the length of the Caloosahatchee River show a major Late Miocene(?)-to-Early Pliocene fluvial deltaic depositional system that prograded southward across the carbonate Florida Platform, interrupting relatively constant carbonate deposition since Early Cretaceous. Six coreholes verify a seismic-sequence stratigraphy, which is age constrained with biostratigraphy and Sr-isotopes. The sand-rich delta system is greater than 100-m thick. Its width (approximately 35 km) compares with the Holocene Balize and Plaquemine deltaic lobes of the Mississippi River.

Late Tertiary connection of the top of the Florida Platform to a continental source of siliciclastics, and significant paleotopography and accommodation combined to focus progradational accumulation of the immense supply of siliciclastics onto the southeastern part of the Florida Platform. From the Caloosahatchee area, the siliciclastics prograded about 200 km southward to the Florida Keys without significant bypass to the platform margin. Here, Late Miocene-to-Pliocene siliciclastics form the foundation of the well-known Quaternary carbonate shelf and shelf margin of the Florida Keys. The stable platform formed of these far-traveled quartz sands allowed Quaternary recovery of carbonate sedimentation on the southeastern Florida Platform.

Chronostratigraphic constraints suggest that the thick deltaic sands accumulated during overall “long-term” transgressive and highstand portions of eustatic cycles TB3.2 to TB3.5. This contrasts with predictive sequence stratigraphic models that stress siliciclastic accumulation is greatest in mixed carbonate-siliciclastic systems during sea level lowstands with carbonate sedimentation dominating during highstands. Accumulation of thick, Late Tertiary siliciclastics on top of the Florida Platform with apparently minimal transport to the platform margin also contrasts with ancient analogues, where typically thickest siliciclastic deposition on mixed carbonate-siliciclastic platforms occurs on the lower slope and basin. These exceptions to stratigraphic and ancient models emphasize the role of antecedent paleotopography and accommodation on siliciclastic accumulation during invasion of siliciclastics onto a carbonate platform.