Transgressive Sequences

Worldwide the history of Holocene sea level change is written in the sediments left as a trace of its progress across continental shelves. The nature of these sequences varies by coastal type and locale, and the breadth of deposits and their possible interpretation would easily require a separate chapter. However, whatever the coast, transgressive sequences are generally the only way information can be obtained on sea level changes predating the last several millennia, when sea levels often stood many meters below modern limits on what today are inner continental shelves.

The transgressive stratigraphy of the barrier island coasts of the U.S. Atlantic and Gulf Coasts can serve as an example of the potentials and liabilities of these data sources. In these systems, sandy barriers—such as the long thin barriers like Atlantic City, New Jersey, and Assateague Island, Maryland—

and their associated lagoons and marshes migrated tens of kilometers across the inner continental shelf (Fig. 2.2). The lagoonal marsh deposits are the keys to deciphering these transgressive sequences, as they provide the medium for radiocarbon dating, and there has been considerable discussion concerning their preservation potential during a transgression. Depending on wave energy and the resulting depth of shoreface erosion, these materials either may be preserved intact or may suffer total removal (Kraft, 1971). If the latter occurs, the only vestiges of the transgression are nearshore marine sands, left after beveling of the shoreface by erosion.

According to the model proposed by Belknap and Kraft (1981) and Kraft and Chrzastowki (1985), the usefulness of a coastal transgressive record is linked to its preservation potential. If we assume a relatively smooth, nondis-sected preexisting topography where transgressive facies cannot hide in antecedent valleys, extensive preservation of barrier and lagoonal facies is indicative of rapid sea level rise where quick submergence isolates these materials from wave erosion. Alternatively, loss of the transgressive facies suggests belated submergence below wave base and slower rates of sea level rise. In theory, therefore, a sea level record punctuated by periods of slow and rapid rise could be gleaned from the transgressive sequence. Ideally, also, a true picture of former sea level elevation could be obtained. In the original Fischer (1961) model, landward migration of a barrier progressively buries the peats of marshes fringing the transgressing mainland shoreline of the lagoon. Dating

Outer Lagoon Inner Lagoonal

Figure 2.2 Transgressive stratigraphy of the Delaware coast, showing the present-day barrier island and seaward sand sheet overlying older lagoonal materials. Radiocarbon dates are on lagoonal materials (basal peats, shells, and wood) which can be used to estimate relative rates of landward shift of the transgressive barrier-lagoon coast as sea levels rose (from Kraft et al., 1973).

Figure 2.2 Transgressive stratigraphy of the Delaware coast, showing the present-day barrier island and seaward sand sheet overlying older lagoonal materials. Radiocarbon dates are on lagoonal materials (basal peats, shells, and wood) which can be used to estimate relative rates of landward shift of the transgressive barrier-lagoon coast as sea levels rose (from Kraft et al., 1973).

the peats at the contact of the pretransgression surface of the shore profile could furnish a dependable means of securing reliable information on sea level position, similar to the spreading of coastal marshes across upland surfaces with rising sea level described later. Nevertheless, existence of a continuous peat layer seaward of modern barriers has not been reported for many areas. More often the peats are spotty in occurrence, localized in old valley systems (Riggs etal., 1992), and most importantly, not in irrefutable contact with pretransgression surfaces. In the Virginia barrier islands, Oertel et al. (1992) found the pre-Holocene (i.e., pretransgression surface) within a few meters of the contact of the sand prism of the modern barriers, with no evident buried peats. Seaward of these islands, the shoreface appears to consist mostly of muds—no convincing transgressive stratigraphy, and little material to date.

The impact of a fall in sea level, or regression, in barrier coasts theoretically can result in progradation or seaward growth of the system, creating a series of beach ridges if sediment supply is adequate. Marshes often develop in the swales between the ridges, and by dating the basal peats in these marshes age limits for the ridges can be obtained. But interpreting these complex morpho-stratigraphic features with respect to sea level history can be daunting, as "regressive" barriers like Kiawah Island, South Carolina, or Galveston Island, Texas, have evolved during the overall transgression of the last several millennia. High sediment supply, in each case for different reasons, simply overwhelmed sea-level-driven shoreface processes that drive barrier retreat, and the barriers prograded.

The terminological morass of "regressive-transgressive" barriers— prograding barriers in the face of rising sea levels—is one that continues to confuse the unwary into interpretative cul de sacs. The problems are certainly illustrative of the pitfalls transgressive/regressive sequences pose for elucidating the sea level history of an area. Barrier transgressive sequences are demonstrably the result of prolonged sea level rise, but the precision with which the transgression that produced them can be deduced is wholly dependent on the preservation of lagoonal peats that provide dating control. This, in turn, is a function of the wave power, antecedent geology, and sediment supply, all of which may or may not have operated in ideal combination in any one area. By comparison, regressive sequences are not demonstrably the result of a prolonged regression, or fall in sea level, and in older shelf deposits, absent other supporting evidence, they can lead to erroneous conclusions regarding past regressions that never occurred.

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  • Zula Omar
    Is assateague island a transgressive or regressive island barrier?
    8 years ago

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