EOGENETIC KARST AQUIFER WITH FRACTURE MATRIX POROSITY IN THE MIOCENE CHALKS ON BARBADOS

Barbados provides an excellent example of early diagenetic processes within the Miocene chalks of the Oceanics Group. Caves were found in dolomitized outcrops of the chalk along fractures at Cove Bay West, and vugs were identified along fracture networks located at Cove Bay East and Gent’s Bay. Based on the morphology of the caves and vugs, they are interpreted as hypogene in origin that formed from an ascending fluid carrying dissolved methane (δ ¹³C values down to about -25 ‰ PDB). The caves were later enlarged along a horizon as a mixing zone invaded the fractures. Overprinting occurred once the caves were breached in the modern intertidal zone. These caves represent multi-phase void development in a fracture matrix porosity dominated karst aquifer.

The chalks of the Oceanics Group were deposited as a drapery over the clastics of the accretionary prism of the Scotland District. Uplift of the accretionary prism resulted in fracturing of the chalks, allowing hydrocarbon rich fluids to migrate through the chalk. Dolomitization occurred via oxidation of methane interacting with oxygen rich seawater along fracture networks. After dolomitization, the chalks were rigid enough to host extensive fracture networks. Deposition of reef limestone occurred once the island was uplifted into the photic zone. Once outcrops were subaerially exposed, a mixing zone (zone of diffusion) would have been contained almost exclusively within the fracture matrix porosity rather than in the interstitial pore spaces, considering that the chalk was and still is acting as an aquitard with very low matrix permeability. Flank margin caves developed within the fracture networks of the chalks. Ultimately, cliff retreat and modern overprinting destroys evidence of these features as seen in sea caves at Cove Bay East.

The chalks were never buried in the traditional sense, only capped by reef limestone during uplift into the photic zone. Therefore, they represent a eogenetic system with pervasive fractured matrix porosity acting like a telogenetic karst aquifer.