Ground-Penetrating Radar Study of North Padre Island: Implications for Barrier Island Internal Architecture, Model for Growth of Progradational Microtidal Barrier Islands, and Gulf of Mexico Sea-Level Cyclicity--Discussion

Abstract

Garrison et al. (2010) are the latest among several authors to claim significant Gulf level fluctuations in the last several millennia (see discussions in Otvos 2001, 2004, 2005a). The onus in their particular case rests on the rather unusual assumptions by Tanner (1992), who linked granulometric kurtosis values and his subjective age estimates of the progradation rate of a St. Vincent Island, NW Florida ridgeplain sequence to late Holocene climate and sea-level changes. The recurring major sea-level and shoreline shifts proposed by Garrison et al. can not be reconciled with the straightforward subsurface documentation presented. ### Proposed Evidence for Late Holocene Sea-Level Fluctuations Tanner (1992), on whose assumptions Garrison et al. base their sea-level chronology starts with three sweeping claims: (1) Essentially all St. Vincent strandplain ridge lithosomes were wave-built. Eolian contribution (so-called “dune decoration”) of unspecified thickness was not significant. (2) Low beach-ridge sets formed during lower sea levels; taller ones during highstands. Tanner provided no specific ridge elevation figures. Lacking corroborating dates of his own, he has arbitrarily assigned ridge-set ages by postulating a subjectively conceived rate of steady strandplain progradation. (3) Based on statistical moment-measure calculations, Tanner assumed a direct relationship between ridge sets of different average elevations, fluctuating late Holocene sea levels, and values of granulometric mean kurtosis from ridge sands. As foundation for their own contention, Garrison et al. (their Fig. 16) also repeat misstatements by Tanner (1992) that late Holocene highstands and lowstands are also reflected in Mississippi subdelta complexes. Frazier (1967), Otvos and Giardino (2004), Otvos (2005a), Tornqvist et al. (2004), Rogers et al. (2009), and others provided no stratigraphic base to support this contention. A similar assertion, equally unfounded, was applied to the SW Louisiana chenier plain as well (Tanner 1992). However, significant lateral elevation differences along these shell-dominated ridges are …