Pleistocene Calcarenite Lithosomes of Bermuda: ABSTRACT

Abstract

Bermuda's subaerially exposed limestones consist of eolianites, littoral calcarenite lithosomes, and accretionary soils deposited during Pleistocene high sea levels. The geometry and structure of these deposits were studied to evaluate sea-level fluctuations during Pleistocene high stands, and to provide criteria for recognizing the eolian-marine facies in surface and subsurface rocks of earlier age. Eolianites are dune ridges trending parallel with the present coastline and arranged in decreasing age from the center of the Bermuda islands outward. Eolianites are subdivided into 2 structural lithosomes: foreset wedge and windward-topset wedge. The foreset wedge is characterized by strata steeply inclined (35°) landward and concave and downward. Foreset curvature is used to divide the wedge into a row of adjoining lobate bodies that represent the hillocky coastal dunes from which the dune ridge was constructed. The windward-topset wedge is characterized by complexly festooned cross-stratification in the part and more regular, seaward-dipping, gently inclined (5-15°) cross-stratification near the dune crest. Littoral calcarenites that overlie or are transitional with windward eolianite strata are termed seaward shore whereas those that onlap foresets are called inland shore. Seaward-shore calcarenites are subdivided into (1) depositional coastline deposits that represent beaches fed by reef-derived detritus, and (2) erosional coastline deposits that represent pocket beaches fed by erosion of headlands. The former are wedge-shaped bodies of regular, seaward-dipping, gently inclined cross-strata and interfinger with eolianites. The latter are conglomeratic pods overlain by accretionary soils and found between eolianites. Accretionary soil is unbedded, uncemented, organic-rich calcarenite containing land snails and rhizoconcretions. The soil records invasion of vegetation and land crabs onto freshly deposited calcarenite. Environments of soil development include the supralittoral of pocket beaches, interdune swales, and the slopes of inactive, unlithified dune trains. Analysis of the carbonate eolian-marine facies in Pleistocene and older rocks can provide data necessary to interpretation on a worldwide basis of (1) sea-level End_Page 873------------------------------ fluctuations, (2) shoreline position and physiography, (3) paleowinds and paleoclimates, and (4) sediment sources. End_of_Article - Last_Page 874------------