Responses of stable bay-margin and barrier-island systems to holocene sea-level highstands, Western Gulf of Mexico

The microtidal, wave-dominated coast of the western Gulf of Mexico displays a variety of Holocene geomorphic features indicating higher-than-present w ater levels that were previously attributed to storm processes while geoida l sea level was at its present position. Field and aerial-photograph examin ations of bay margins, barrier islands, and beach-ridge plains following ma jor hurricanes show that the elevated features are inundated periodically b y high storm surge. Despite their inundation, these highstand features are not modified by modern storm processes, Instead, storm-related erosion and deposition are always seward of and lower than the highstand features and a re always limited to the extant shorezone, where elevations typically are l ess than 1.5 m above present sea level. Bay-margin and lagoonal highstand indicators include raised marshes and sub tidal hats, wave-cut benches, abandoned wave-cut scarps with fringing marsh es and/or beach ridges, and accretionary islands and recurved spits. Other emergent marine features include abandoned compound flood-tidal delta and w ashover fan complexes attached to barrier islands and anomalously high beac h ridges within both the barrier-island complexes and beach-ridge plains. T he highest beach ridges, raised marshes and hats, and erosional scarps and benches are manifestations of one or more rising phases and highstands in s ea level, whereas the lower marshes and accretionary topography are mainly products of the falling phases and shoreface adjustment to present sea leve l. Different elevations of beach-ridge sets, discordant truncation of beach ri dges, and elevated marine- and brackish-water faunal assemblages preserved in beach ridges, raised marshes and hats, and natural levees are compelling evidence of sea-level fluctuations of +/-1 to 1.5 m from about 5500 to 120 0 cal yr BP, Independent evidence from studies of geodynamic, climatic, and glacio-eustatic processes can explain the mid-Holocene highstands and late Holocene lowering of sea level that is observed in tectonically stable coa stal regions far from former centers of glaciation.