PETROGRPAHIC ANALYSIS OF HIGHLY CEMENTED PARTIALLY DOLOMITIZED CHALKS FROM BARBADOS, WEST INDIES

Samples of chalk and dolomitized chalk of the Oceanic Group were analyzed from Cove Bay on Barbados, West Indies to determine the digenetic history. The petrographic characteristics of samples from one outcrop in Cove Bay contained partially dolomitized chalk were determined. This outcrop was a well indurated, very fine grained carbonate rock with visible conchoidal fractures. The goal of this study was to determine the cause of the highly cemented nature of this outcrop compared to poorly indurated proximal outcrops. Standard petrographic microscopy was conducted to determine characteristics of this outcrop. The dominant allochem identified was Globigeriinia, an upper slope-dwelling planktonic foraminifera. Coccoliths, Discoaster, and Astraeospongia were other microfossils identified within the sample. Other petrographic features include residues of dead oil in fractures, pyrite, and fibrous hematite. Stable isotope and XRD analyses indicate sporadic dolomitization by thermogenic methane seeps interacting with slightly modified seawater (Dolomite δ¹³C =-1.4 permil (PDB) and δ ¹⁶O=-2.9 permil; Calcite δ13C=-0.5 and δ18O=-3.2 permil (PDB)). Shortly after deposition, uplift caused fracturing in the overlying chalks. These fracture networks allowed migration of fluids including slightly modified seawater carrying thermogenic methane and hydrocarbons.