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Confocal and Transmitted Light Petrography of Cementation and Grain Types in a High-Energy Upper Shoreface to Foreshore Carbonate Strandline, Pleistocene (MIS 5e) West Caicos Island, Turks and Caicos, BWI

Abstract

Confocal petrography provides a new tool for the imaging of cements, pores, and internal structures of carbonate grains (including ooids and composite grains). A vertical section at Boat Cove, West Caicos Island contains a burrowed upper shore face to foreshore system (MIS 5e) and provides excellent diagenetic control on a single sea level cycle that has experienced constant exposure for approximately 120,000 years. This allows the effects of cementation and dissolution on a single exposure event, which can be isolated and documented without the complexity of overprinted events as seen in the ancient rock record. Blocky cements, as recognized in transmitted light (TL) images, are recognized as prismatic crystals in confocal imagery. The crystals are strongly zoned and display variable fluorescence, similar to zoning observed in cathode luminescence (CL) petrography of older carbonate cement examples (controls on the zonation are currently being investigated). Imaging of isopachous cements are also greatly improved. Micritized ooid nuclei (in TL imagery) are observed to have a complex history including previous lamellar coatings and microborings. Many ooids that appear to have single nuclei in transmitted light are observed to be composite grains when viewed with confocal microscopy. Different morphologies of microborings can be identified and at times are concentrated along single ooid lamina. Cements observed include a coarse, clear blocky cement (low Mg calcite), a finer crystalline blocky cement (low Mg calcite), an isopachous blocky cement (multi-generational, high-Mg calcite), and rare needle cements (aragonite). The clear blocky cements are observed in confocal imagery to be prismatic in shape and highly-zoned. Cements are most common in areas where ooids are being dissolved, like in burrows or in finer grained laminae (due to capillary forces). Alveolar structure is observed in foreshore sediments and meniscus fabrics are present throughout the section. The cements are interpreted to be primarily meteoric cements with minor isopachous and mold filling cements precipitated from marine fluids (probably beachrock). Moldic porosity is common and is represented by partial to total dissolution of ooid cortices.