THE GROWTH FABRIC OF GREGARIOUS RUDIST ELEVATORS (HIPPURITIDS) IN A SANTONIAN CARBONATE PLATFORM IN THE SOUTHERN CENTRAL PYRENEES

The growth fabrics of living congregations of erect sessile epifauna r ange between two extremes. With superstratal growth, the growing skele tons project well (similar to metres) above the ambient sedimentary su rface, creating an upstanding framework, which may be secondarily fill ed by sediment, as in modern tropical coral reefs. With constratal gro wth, by contrast, the growing tips of the skeletons project little (si milar to centimetres) from the seafloor, such that the bulk of the ske letal fabric is embedded in, and supported by, the accumulating inters titial sediment, as in Pinna, and oyster beds. Here we investigate the original growth fabrics and palaeorelief of hippuritid congregations in the Santonian rudist formations around the Sant Corneli anticline, near Tremp in the southern Central Pyrenees. The study particularly co ncentrates on a hippuritid lithosome, exposed over some 0.25 km(2), on the northern flank of the anticline. Four aspects are analysed. (1), The lithosome is shown to have a tabular (biostromal) geometry, and, l ike others in the area, is covered by a laterally accreted bioclastic blanket. (2) Specimens in upright life position show that the pioneers of the paucispecific congregations grew in bouquets, but later recrui ts settled on the flanks of established, and fallen shells. Adults wer e only loosely clustered and supported by the accumulating interstitia l sediment. (3) Most shells, however, are now horizontal with respect to the bedding and densely stacked (''dense horizontal fabric'', or '' d.h.f.''). This fabric is shown to be a result of current-induced remo val of sediment and toppling of shells. (4) In contrast to the open ma rine aspect of the overlying bioclastic sediments, the fine matrix sed iment of the hippuritid lithosomes is faunally restricted, and appears largely derived from the in situ biodegradation of shells. We conclud e that the hippuritid congregations grew constratally (i.e. without an y supporting biogenic framework), and that they formed biostromal accu mulations lacking relief. So there is no justification for calling the m reefs as some authors have done. Rather, these rudists grew as grega rious sediment-dwellers in the restricted waters of the platform top, where they were sporadically disturbed by storms, and eventually blank eted by bioclastic material swept in from the platform margin.