Sedimentary Characteristics of Low-Energy

John Ragan, Richard Smosna

Abstract


The recognition of carbonate-beach deposits may be important in understanding ancient stratigraphic sequences in that they can serve as an environmental reference point for interpreting adjacent facies. However, low-energy beach deposits are generally difficult to distinguish due to (1) the low contrast in energy regime from onshore to offshore and (2) extensive bioturbation which destroys primary structures (such as inclined bedding and ripple marks). Wave-induced longshore currents along three modern Florida beaches (Lower Matecumbe, Bahia Honda, and Big Pine Keys) are weak, and only small beaches are maintained. In cross-section the sand bodies appear as lenses, no more than 33 m wide and 1.5 m thick. A sedimentological study shows that these skeletal sands are marked by four distinct, but subtle, textural and compositional properties. (1) Backshore and foreshore sands contain some mud (up to 8%), but the mud content increases significantly across the shoreface and into the offshore sands (up to 25%). (2) Only the backshore sediments are moderately well sorted; most sediment deposited below mean high tide is poorly sorted. (3) Mean grain size of foreshore sands (coarse) is greater than for sands of the other subenvironments (fine to medium). Foreshore sands are also bimodal, thus accounting for their poor sorting, and these several properties result from the population of molluscs that live and die at the strandline. Waves of this zone are simply incapable of transporting these large shells. (4) One final feature of the beach sands is their low skeletal equitability, that is, the sediments are commonly composed of just one kind of skeletal grain, predominantly eurytropic gastropods or pelecypods. Results of this study indicate that a combination of petrographic characteristics is needed, because of their subtlety, to interpret low-energy beach facies in the rock record.


Keywords


Carbonate beach sediments; constituents; Florida Keys; low-energy environment; sedimentary model; textures

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