|Peter E. Sedgwick, Department of Geology, University of South Florida, Tampa, FL
Richard A. Davis, Jr., Department of Geology, University of South Florida, Tampa, FL
Washover fans are an important aspect of the barrier island depositional system, and often comprise a significant percentage of barrier island stratigraphy on the west-central Florida coast. Despite this importance, the identification of the washover facies in the stratigraphic record is often difficult due to an incomplete knowledge of the specific textural and compositional framework of the deposits. Identification is further complicated by the high rates of bioturbation of washovers in the backbarrier intertidal and subtidal environments.
Twelve washover deposits were cored on the northern west-central barrier coast of Florida to provide data that will permit means for identifying the washover facies in the stratigraphic record. Typical modern washover stratigraphy displays landward-dipping plane beds comprised of well-sorted quartz sand with distinct laminae of shells and heavy minerals. Additional factors that permit recognition of washover deposits include low mud content in an otherwise relatively mud-rich back-barrier facies, faunal assemblages indicative of mixed shoreface and back-barrier shell species, and bioturbation typical of low-energy backbarrier systems.
Five subfacies can be delineated which show variations in composition, texture, and bioturbation throughout the washover facies (Figure 1). These variations are due to differences in flow conditions during overwash and variable degrees of reworking after deposition. The distribution and relative amounts of each subfacies varies as a function of barrier island elevation, sediment type and supply, frequency of storms and the extent of overwash into backbarrier environments.
Figure 1: Factors used to characterize each of the five washover subfacies used in this study.
In addition to the five subfacies, three shell assemblages were defined to aid in identification of washover deposits (Figure 2). Backbarrier sediments containing shell material comprised of shoreface/open water species or mixed shoreface/lagoon species may potentially be washover in origin. Sediments with purely lagoonal shell species are likely to have been deposited independent of washover activity.
Examination of an artificial time-series of differing ages of washover deposits revealed that preservation of washover stratigraphy is not exclusively a function of time. In as little time as a decade, reworking may eradicate small-scale stratification, leading to potential difficulty in delineating washover events and the facies in general. Preservation of this small-scale
Even after the destruction of small-scale stratigraphic features, washover deposits may still be identified as such due to their characteristic texture, composition, and shell assemblages. Key features in recognizing the facies after bioturbation and reworking are the presence of clean sands in otherwise muddy lagoonal sediments, the wedge-shaped geometry of the facies, and the presence of shoreface shells or mixed shoreface/lagoonal shells in the backbarrier environment. These features can be represented in a stratigraphic model for washover deposits in aggradational or retrogradational barrier systems (Figure 3). If reworking is severe and/or there is limited subsurface data, distinguishing washovers from genetically similar deposits in the stratigraphic record may be problematic. A retrogradational barrier island includes substantial washover facies with a somewhat predictable sequence of washover subfacies (Figure 3).