Publication Date

2014

Document Type

Honors Thesis

Department

Geosciences

Keywords

Calcite, Calcretes, Diagenesis, Eolian processes, Sand dunes-Bahamas-San Salvador Island, Carbonate rocks-Bahamas-San Salvador Island, Caliche, Bahamas, San Salvador, Geology, Stratigraphic-Holocene, North Point, Rice Bay, Eolian, Dune, Carbonate, Grainstone

Abstract

Numerous thin caliche crusts are present in carbonate eolianites of the North Point Member of the Rice Bay Formation exposed at North Point on San Salvador Island, Bahamas. These eolianites formed during Holocene transgression about 6 to 5 ka and consist of complexly stratified deposits produced by wind-ripple migration, grainfall and sandflow processes. Caliche is a hard micritic crust formed by the dissolution of carbonate and subsequent reprecipitation of low-Mg calcite on the surface and/or at shallow depth. In the study area caliche coats most of the prominent bedding planes of eolian ooid-skeletal grainstone deposits in the upper part of the exposure including dune crests and both windward and steep (up to 34О_) leeward surfaces. In weathered exposures caliche also forms a step-like pattern of up to 12 successive crusts, 1-4 mm thick, separating eolian beds that generally thin upsection from about 32 cm to about 3 cm. Most of these crusts are parallel to wind-ripple lamination and vary in orientation from horizontal to 18О_ slope. Others encrust cross-laminated beds as well as vertical fractures forming caliche dikes. Well-preserved caliche crusts have a sharp, smooth upper surface, and are quite uniformly thick. In most cases caliche crusts are associated with dense and laterally extensive rhizoliths. Similar caliche successions were also observed in the Cockburn Town Member deposits of the Pleistocene Grotto Beach Formation found along the southern coast of San Salvador. These caliche crusts are found in the uppermost part of eolianite succession, immediately below the paleosol marking the Pleistocene/Holocene boundary. Such rare successions of multiple caliche horizons, likely formed postdepositionally by precipitation in shallow subsurface as penetrative caliche rather than as synsedimentary features on exposed dune surfaces as has been previously proposed. Plant roots penetrated through fractures in lithified eolianite and then spread laterally along bedding planes in search for water and nutrients as indicated by common rhizoliths and lateral continuity between bedding-parallel and crosscutting caliche. The presence of water and plant material facilitated formation of caliche and produced a unique stratification pattern with numerous thin crusts delineating distinct beds of mainly wind-ripple laminated strata. Well-preserved and largely undisturbed eolian lamination suggests the absence of abundant vegetation during dune deposition and the postdepositional formation of caliche with rhizoliths after lithification of eolian sediment. Distinguishing between synsedimentary surficial versus postdepositional penetrative origin for caliche has major implications for understanding and interpreting the style and nature of eolian deposition and diagenesis, and their relation to changing sea levels, sediment supply, and climate.

Language

English

Comments

v, 45 pages : illustrations (some color), maps. Honors Project, Smith College, 2014. Includes bibliographical references (pages 36-38)

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