Depositional Themes of Mixed Carbonate-siliciclasticsinthe South Florida Neogene

Abstract

A recent drilling project to evaluate the Neogene stratigraphy of south Florida has provided refined insight to the depositional controls and facies patterns of a heterogeneous, mixed carbonate-siliciclastic system. Six key themes have emerged that may have implication for reservoir development and facies architecture in similar depositional systems. These modern depositional themes are compared to some ancient mixed system examples. Although mixed systems are complex and spatially unique, similarities in the basic lithofacies deposition and their associated physical properties can aid in prediction of reservoir distribution and in refinement of geologic models in ancient mixed systems. The deposition-related themes recognized in this study of the Florida Neogene include (1) Concept of Template Control on Both Carbonate and Siliciclastic Deposition—precursor topography controls depositional geometry and location of subsequent depocenters for both carbonates and siliciclastics; (2) Distal Transport of Coarse Clastics and Influence of Currents on Grain-size Segregation—conditions can exist for the long-distance transport (fluvial?) of extremely coarse siliciclastics (flat-pebble quartz in this Neogene example) from the source area, and regional currents help segregate grain-size populations and partition grain types; (3) Demise of the Carbonate Platform/Ramp: Smothered by Siliciclastics?—in this Neogene example, we recognize a hiatus of several million years bounding the top of a carbonate ramp, which indicates that demise of the ramp and subsequent input of siliciclastics are temporally distinct; (4) The Mixing Transition: Abrupt Vertical and Lateral Facies Changes—the lateral transition of carbonate to siliciclastic strata highlights the potential for abrupt facies changes both laterally and vertically. Interfingered carbonates and siliciclastics may form stratigraphic traps based on lithologic differences and differential diagenesis and can result in alternating reservoir pay zones and nonreservoir intervals; (5) Cryptic Sequence Boundary in Shallow-marine Siliciclastics and Carbonates—in cases where no distinct change in lithology exists, it may be inherently difficult to recognize major disconformity based only on lithologic changes. In settings dominated by admixing, sequence-boundary confirmation may require the integration of biostratigraphic and chemostratigraphic markers with any available textural indicators; and (6) Similarity in Acoustic Properties of Laterally Equivalent Siliciclastics and Carbonates—shallow burial and early diagenesis have produced an almost identical acoustic signature for the two admixed sediment types. This acoustic similarity may make it difficult to distinguish specific lithofacies on seismic profiles and sonic logs. In ancient mixed-system deposits where only seismic data exist, problems in specific lithofacies or geometric characterization may occur.