Assumed Holocene Highstands, Gulf of Mexico: Basic Issues of Sedimentary and Landform Criteria: Discussion

Abstract

Record high mid and late Holocene relative sea levels, significantly above the current sea level have been widely documented in certain areas of the Australo-Pacific region, Brazil (Lessa et. al. 2000), and parts of Africa and India that were not characterized by tectonic uplift and/or glacio-isostatic rebound. Such elevated sea levels are yet to be reliably identified on our Gulf and Atlantic shores. Radically departing from traditional Gulf of Mexico sea-level curves, Morton et al. (2000) now suggest recurring highstands throughout the entire second half of the Holocene Epoch. Their findings echo earlier ideas in the same vein (see Otvos 1995, 1997, 1999). However, a detailed analysis of several lines of evidence presented by Morton et al. raises serious doubts about these conclusions. Morton et al. accept the position of beach-ridge summits as indicative of higher-than-present elevations. Thus, seaward-declining beach ridge summits on Bolivar Peninsula, Galveston Island, and other coastal areas of Texas in their view reflect late Holocene sea levels that fell from their highest elevations. Although beach ridges are of wave-constructed (berm ridge), foredune, or composite origins (Otvos 2000), Morton et al. fail to distinguish adequately among these categories by sediment and landform analysis. The problem is that under favorable circumstances, wave-built berm ridges may be accurate markers of ancient sea levels or lake levels. Foredunes, on the other hand, aggrade to varying supratidal elevations, rising several meters above the associated high-tide level. For this reason, summit elevations of relict foredunes can not provide precise data on corresponding sea level (Otvos 1995). ### Foredune Ridges Foredune ridges often overlie, mask, and bury wave-built berm ridges (Beal and Shepard 1956). Because sand is usually readily available for wind transport on most Gulf shores, the upper interval in most beach ridges tends to be of eolian origin. Except when wave-built …