COOL WATER IN THE TROPICS? BRYOMOL CARBONATES ON THE EOCENE ATLANTIC SHELF

Carbonate sediments deposited in Eocene tropical environments usually abound in Nummulites banks, red algal accumulations and sometimes in hermatypic corals. The east coast of North America enjoyed tropical climate in the Eocene, still none of these organisms are found in the Castle Hayne Formation of the Carolinas. This peculiar rock is dominated by species-rich bryozoan fauna associated with molluscs, solitary corals, echinoids, and brachiopods (bryomol association). The less than 20 m thick succession contains repeated phosphate crusts and pebbles. The best-known bryomol limestones occur in the Eocene to Recent succession of the South Australian shelf and slope: there is well-documented evidence that these are products of cool-water currents of polar origin. We studied faunal composition, carbonate microfacies, chemical and stable isotope composition of the limestone and phosphate of the Castle Hayne Formation and found little evidence for cool-water sedimentation. How can a cool-water origin of the Castle Hayne bryomol carbonates fit their position at tropical latitudes in Eocene time? The flow of the Gulf Stream passing along the east coast of America was adjoined by the flow of the Suwannee Stream - passing from the Gulf of Mexico to the Atlantic across the neck of Florida - during Tertiary sea-level highstands. Barrages extending into the current (e.g. Cape Fear) caused significant turbulence and eddies. The subsequent upwelling yielded high organic productivity, suppressing the growth of organisms adapted to oligotrophic seas (larger foraminifers, algae, hermatypic corals). Phosphate crusts are evidence for condensed sedimentation caused by rapid currents. When sea-level fall weakened or stopped the Suwannee Stream, a rich benthic fauna was formed and preserved. A similar, turbulent current regime has been documented in detail for the Miocene phosphate-rich beds of the Atlantic shelf. A present-day analogue is below the warm Agulhas Stream offshore South Africa: it helps to develop a disturbed environment with bryomol sedimentation. We suggest that the bryomol Castle Hayne Limestone is an azonal carbonate deposit within the tropical climate zone, displaying cool temperate characters due to local oceanographic conditions.