TAPHONOMIC CRITERIA FOR RECOGNIZING DOWNSLOPE TRANSPORT OF ECHINODERMS: NEOGENE OF THE ANTILLES

Deep water deposits in the Neogene of the Antilles typically include both fragmentary and, rarer, complete fossil echinoderms. Determining the likelihood of whether the origin of any given specimen is allochthonous or autochthonous may be problematic. For example, chalks of the Lower Miocene Montpelier Formation of north central Jamaica have yielded only disarticulated ossicles, whereas more complete echinoids occur in reefal slide blocks. Thus, the parautochthonous ossicles in chalks are not associated with in situ complete echinoderms; these only occur in the allochthonous slide blocks. In contrast, fragmentary echinoids in the Upper Pliocene Bowden Formation of Jamaica, a submarine mass flow deposit, nevertheless show fine preservation of microstructure. The only complete tests are either juvenile regular echinoids (1-2 mm diameter) or small, robust Clypeaster cf. carrizoensis Kew (<10 mm long). Apparently, size matters in determining preservation in this allochthonous accumulation.

The Middle Miocene Grand Bay Formation of Carriacou, the Grenadines, has a particularly diverse echinoderm fauna. This is a turbidite basin succession dominated by chemically fresh igneous grains, and with a mixture of bioclasts from terrestrial and shallower to deeper water environments. Parautochthonous crinoids are locally abundant, albeit fragmentary. Although the many hundreds of disarticulated columnals give the rock a Paleozoic-like appearance, it is important as the only crinoid-rich deposit known from the Antilles. Rare articulated asteroid and ophiuroid tests in a ‘starfish bed’ may be in situ or derived by downslope transport. Only one other occurrence of articulated ophiuroids has been reported from the region; this is the first asteroid recognized apart from marginal ossicles. The echinoids are more diverse, ranging from large, robust Clypeaster sp. to long and complete cidaroid spines. This mixed assemblage includes both allochthonous and autochthonous tests and spines, each specimen having to be interpreted on its own merits.