Modern Deepwater Mounds in the Atlantic: The Porcupine Seabight and the Straits of Florida

Miriam Andres, Gregor P. Eberli, Mark Grasmueck, and the IODP Exp. Shipboard Scientific Party
University of Miami, Miami, FL

Deep-water carbonate mounds are prominent features throughout the Paleozoic and Mesozoic with a decline in the Cenozoic. Modern imaging techniques also reveal their abundance in todays world oceans but few examples are well-documented. A much-favored hypothesis for their origin predicts that hydrocarbon seepage rising through underlying fractures initiate and fuel a microbial-dominated ecosystem covered by corals.

This hypothesis was tested during IODP Expedition 307 along the shelf edge of the Porcupine Seabight, SW Ireland where detailed sea-floor mapping and video imaging identified ~4000 mound structures. The expedition for the first time successfully drilled through one of the mounds, called Challenger Mound. The recovered cores contain an impressive range of siliciclastic and carbonate sediments of Miocene to Pleistocene age archiving the pre-, syn- and post-mound depositional systems. Challenger mound initiated on a sharp Pliocene erosional unconformity but is constructed by a thick Pleistocene deep-water coral succession.

A role for hydrocarbon fluid flow in the initial growth phase of Challenger Mound is not obvious, as preliminary analyses detected no significant quantities of gas in the mound or in the underlying sediments. Detailed geochemical analyses are underway to detect possibly remaining hydrocarbon signatures.

Deep-water mounds or lithoherms are also common in the Straits of Florida. They are comparable to their North Atlantic counterparts in size and geometry and setting. They are constructed by the same deep water coral species. Seismic data indicates that they are located preferentially in areas where the subsurface is faulted and hydrocarbons migrated upwards.