Barrier Island Genesis - Questions of Alternatives for the Apalachicola Coast, Northeastern Gulf of Mexico

Abstract

The transgressive-regressive sequences of two Quaternary (Sangamonian and Late Holocene) high sea level episodes were identified in numerous island, lagoon, and mainland drillholes from Apalachicola area core samples. No mid- Wisconsinan (Farmdalian) deposits, formed during a relatively higher sea level stand had been preserved in marine units. By the use of biotope profiles, based on salinity-sensitive foraminifer taxa and strandplain configurations, it is suggested 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, biotopes with extremes of very low and relatively high salinities occupy smaller areas. For this reason, intermediate salinity foraminifer biotopes dominate. Consequently, modern Apalachicola area lagoonal sediments generally also show smaller biotope-salinity contrasts with underlying units that formed before the islands were established, than do surface units of 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-sediment 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 far less convincing genetic explanations for these islands.