Alternate interpretations of barrier island evolution; Apalachicola coast, Northwest Florida
Otvos, Ervin G
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The transgressive-regressive sequences of two Quarternary [Sangamon and Late Holocene] high sea level episodes were identified in numerous island, lagoon, and mainland drillholes from Apalachicola area core samples. As elsewhere on the Gulf coast, no conclusive indications of a Mid-Wisconsin [Farmdalian?] higher sea level stand had been preserved in marine units. By the use of biotope profiles, based on abundant salinity-sensitive foraminifer taxa, and strandplain configurations, it is suggesed that all four original islands [including now-relict "Little St. George" island] evolved through shoal aggradation in the Late Holocene. Seasonally alternating strong fresh and salt water influences on the microfauna account for the smaller salinity range of biotopes. In contrast with the Mississippi Sound area, very low and relatively high salinity biotopes occupy smaller areas, and intermediate salinity foraminifer biotopes dominate. Consequently, modern Apalachicola area lagoonal sediments generally show smaller contrasts in biotope salinities [with underlying untis that formed before the islands were established], than do surface units in other nearshore areas [e.g. Mississippi Sound] with lesser stream runoff. The extent of subsequent lateral or seaward progradation depended on the spatial relationship with stream and littoral drift-semdiment sources and on shelf bottom configurations. St. Vincent and "Little St. George" islands became strandplain [beach ridge plain]islands, while narrow Dog and St. George Islands acquired an essentially linear character. Alternate theories [spit segmentation and mainland beach detachment-migration] offer less convincing genetic explanations for these islands.