Title

Topographic, vegetative, and climatic controls on the petrography and geochemistry of calcretes in the Bahamas and south Florida

Date of Award

1990

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Harold R. Wanless, Committee Chair

Abstract

Vegetation and topography of the limestone islands on Caicos Platform, British West Indies are decisive factors in controlling the morphologic, petrographic, and geochemical characteristics of calcretes forming on Pleistocene exposure surfaces. Valley calcretes form in low areas and develop as a result of vegetative and soil forming processes which penetrate downward and reconstitute host limestone. Valley calcretes vary in degree of maturity which is reflected petrographically and geochemically. Enrichment in Fe and Al in valley calcretes is caused by gradual concentration of aerosol-derived material in this stable topographic setting.Ridge calcretes form on ridge crests and ridge slopes and are also associated with downward penetrating roots, but they undergo a cycle of formation and destruction (ridge calcrete cycle) which prevents them from achieving the progressive maturity seen in valley calcrete profiles. Occurrence of multiple calcrete horizons previously considered individually representative of true subaerial exposure, are in fact false "penetrative calcretes" which are collectively related to only one major subaerial exposure event. Individual subaerial exposure periods can produce a true surficial calcrete, plus a series of penetrative calcretes horizons which can be mistaken for surficial features in the underlying 2-6 meters of sequence. Although penetrative calcrete horizons are not laterally traceable over long distances, they are very similar to surficial calcretes, commonly occur at lithologic boundaries (permeability anomalies), and can be easily misinterpreted as representing individual exposure periods in core borings. Differentiation of penetrative and surficial calcretes can commonly be made by lateral correlation of calcrete horizons or by lower concentrations of Al and Fe in penetrative calcretes.Climatic controls on calcrete formation throughout the Bahamiam region is expressed as variations in petrographic and geochemical aspects of calcretes. Along a climatic gradient between south Florida and the Turks and Caicos Islands, a geochemical trend is documented which manifests itself as an increase in the $\sp{18}$O/$\sp $O isotopic ratio. This is attributed mainly to higher temperatures and faster evaporation rates in the Turks and Caicos islands. Isotopic compositions of meteoric waters along the climatic gradient have not been measured directly, but are believed to play a secondary role in the observed isotopic trend in calcretes.

Keywords

Geology; Geochemistry

Link to Full Text

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