Topographic and Vegetative Controls on Calcrete Formation, Turks and Caicos Islands, British West Indies

Abstract

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 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 expressed petrographically and geochemically. Enrichment in Fe and Al in valley calcretes is caused by gradual concentration of aerosol-derived material in topographically low and therefore stable settings. Ridge caleretes form on ridge crests and slopes and are 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. The vegetative role in the calcrete cycle is twofold. First, root mats spreading out laterally within the host limestone enhance dissolution of limestone and subsequent reprecipitation of carbonate cements. These processes result in well indurated calcrete layers. Second, once formed, these indurated layers are uprooted by downward and laterally displacive rooting and subjected to physical and chemical erosion. Recognition of the two different modes of calcrete formation in the rock record can yield valuable information about paleotopography and erosional history.