Calcareous crusts on exposed Pleistocene limestones: A case study from Grand Cayman, British West Indies

Abstract

Calcareous crusts, which include laminar (up to 6cm thick) and non-laminar calcretes, have developed on ooid grainstones and skeletal wacke/pack/grain/floatstone in the Pleistocene Ironshore Formation exposed at Morgan's Harbour, Tarpon Springs, and Rogers Wreck Point on Grand Cayman, British West Indies. Non-laminar calcretes, which consist largely of micrite with variably altered skeletal fragments and lithoclasts derived from the host limestone, contain peloids in their basal parts. In contrast, the laminar calcretes are formed of porous laminae and low-porosity laminae. Calcified roots are common in the porous laminae but rare in the low-porosity laminae. The δ18O values of calcretes, from ‐5.22‰ to +0.16‰, have a positive correlation with the δ13C values that vary from ‐10.03‰ to ‐6.41‰ at Tarpon Springs, from ‐11.57‰ to ‐5.43‰ at Morgan's Harbour, and from ‐9.89‰ to ‐8.68‰ at Rogers Wreck Point.The non-laminar calcretes reflect progressive micritization of the bedrock with a basal peloidal zone that passes upward into the micrite zone. In contrast, laminar calcretes are attributed to accretionary build-up dictated by microscale processes that were controlled largely by the interplay of plant roots and their associated microorganisms. The positive correlation between the δ18O and δ13C values from the calcareous crusts indicates that evaporation influenced calcite precipitation. The δ13C values of the laminar calcretes record a change from vegetation dominated by C3 plants that grew in a cool, wet climate, to a C3/C4 vegetation that grew in a hotter, drier climate. The Cayman calcretes resemble those found on the Pleistocene Miami Limestone in Florida, and highlight the possibility that these calcareous crusts may reflect regional climates and may therefore enable regional stratigraphic correlation.