Integrated bio/geo-chronology and paleo-environments of Paleogene short-lived carbonate shoals (SLSC) on active margins of the Caribbean Plate - relations with tectonic and volcanic processes

The CARIBCARB is designed to work on Palaeocene to Oligocene small scaled and short-lived carbonate shoals (SLCS), widespread along the active margins of the Caribbean Plate. The growth of Caribbean SLCS seems to be largely controlled by tectonic and volcanic processes, rather than eustatic sea-level changes, but they are today poorly understood and dated.

Caribbean Paleogene SLCS are characterized by a rapid appearance and disappearance of stable conditions favourable to chlorozoan carbonate production, modest size and a geologically short life span. SLCS were mainly built by rock-forming larger benthic foraminifera (LBF) and coralline red algae, while coral frameworks seem to be restricted to some Oligocene formations. SLCS are fundamentally different from long-lived, large carbonate shelves set on (often thinned) continental crust along passive margins, such as Florida, Yucatan, Nicaragua Rise etc. Hence, sedimentological models developed for the latter do not apply to SLCS. At best, they can be compared to modern guyots or atolls. Oceanic basements, such as basaltic plateaus, oceanic seamounts and proto-island arcs formed at aphotic depths and subsided thermally, unless tectonic or volcanic activity produced uplift or buoyancy to make them temporarily reach the photic zone. Once oceanic or arc rocks reached the subaerial realm – either by accretion/collision or in volcanic edifices – they were highly affected by tropical weathering, producing high discharge of clay and silt into the surrounding seas. In addition, explosive volcanic activity of island arcs produced suspended ashes in surface waters, as well as abundant dissolved nutrients, creating unfavourable conditions for the growth of SLCS. Local or regional upwelling may also have enhanced eutrophication of surface waters. Hence, the formation of SLCS could take place only under special conditions, such as rapid tectonic uplift and subsequent subsidence, the stalling of nearby volcanic activity and/or the bypassing of siliciclastic sedimentation. Biofacies and accurate dating of the rapid appearance and disappearance of chlorozoan carbonates on uplifted accretionary prisms, island arcs and intra-plate volcanoes, as well as the nature of encasing detrital sediments, will provide valuable markers for of the tectonic, geodynamic, and paleo-environmental evolution of oceanic islands and active margins of the Caribbean Plate.

Peculiar, partly endemic assemblages of LBF, a main constituent of SLCS, are difficult to date, because the proposed biozonations for LBF are based on Tethyan taxa and are not applicable to Caribbean SLCS. Their paleo-geographic position may help to refine the paleo-biogeographic known extension of particular genera and bioprovinces of LBF, and to assess the role of SLCS as "stepping stones" for the migration of benthic communities.

In addition to an improved biochronology of Caribbean larger benthic foraminifera we aim for independent chronologic calibration of SLCS and their LBF assemblages by other fossil groups (i.e. planktonic foraminifera, nannofossils), stable isotope (C, O, Sr) stratigraphy, and radiogenic isotope (Ar/Ar) dating of volcanic and intrusive rocks frequently associated with SLCS. Based on our previous reconnaissance work, we plan field campaigns in S-Central America and the Antilles, in sections where SLCS are interbedded with volcanogenic sediments, lava flows or intruded by well-crystallized sills/dykes, that will allow for Ar/Ar-dating of mineral separates