Holocene Gulf Levels: Recognition Issues and an Updated Sea-Level Curve

Document Type

Article

Publication Date

6-1-2004

Department

Coastal Sciences, Gulf Coast Research Laboratory

Abstract

Several new concepts have challenged the traditional view of northern Gulf coast sea-level history in recent years. Claims for higher than present mid and late Holocene sea-levels implied substantial sea-level oscillations. Such highstands, known globally in low latitudes, have never been credibly documented on the Gulf or on unglaciated Atlantic coasts. Lacking direct indications, numerous lines of indirect proof have been suggested. Proxy evidence was based on items as the reinterpreted geological record of NW Florida lakes, estuaries, and depositional conditions of Texas coast landforms. Pleistocene barrier sectors, barrier islands, relict foredune ridges, washover fans /flood-tidal deltas, and other sedimentary units and their morphology were claimed as highstand indicators. Unrealistically old or young radiocarbon and luminescence dates, the result of diagenetic changes and other contamination have resulted in questionable conclusions. A large body of sediment, stratigraphic and geomorphic data collected during two decades, including more recently obtained Holocene sea-level dates indicated the absence of mid and late Holocene marine highstand stages on the Gulf. Late Holocene variations in coastal sediment supply, on the other had, are apparent from shore progradation-recession episodes on the SW Alabama and adjacent NW Florida coast. One, recently revived school of thought proposes a step-like pattern of sea-level rise. Such prolonged stillstands that alternated with abrupt, large-amplitude sea-level rise episodes more recently were based on the genetic interpretation of broadly dated backstepping parasequences, emplaced in incised Pleistocene shelf valleys. Three loosely constrained multimillennial stillstands of 1.2-3.4 ka duration were inferred, each associated with a given Gulf paleoshoreline. These and postulated intervals of abrupt high-amplitude (1.5-6.0 m) sea-level rise conflict with the actual mid to late Holocene coastal development and sea-level history. The absence of backstepping parasequences in the incised Mobile Valley suggests that changes in sedimentation and erosion rates in such settings had greater impact on sequence development than sea-level changes had. Newly acquired information helped the formulation of a new regional sea-level curve that replaces two critical intervals in the old Nelson-Bray record. It conforms with more recently established sea-level histories in tectonically stable west Atlantic-Gulf areas, primarily Bermuda, the Bahamas, and Florida.

Publication Title

Journal of Coastal Research

Volume

20

Issue

3

First Page

680

Last Page

699

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