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 range between two extremes. With superstratal growth, the growing skeletons project well (∼ metres) above the ambient sedimentary surface, creating an upstanding framework, which may be secondarily filled by sediment, as in modern tropical coral reefs. With constratal growth, by contrast, the growing tips of the skeletons project little (∼ centimetres) from the seafloor, such that the bulk of the skeletal fabric is embedded in, and supported by, the accumulating interstitial sediment, as in Pinna, and oyster beds. e 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 concentrates on a hippuritid lithosome, exposed over some 0.25 km2, on the northern flank of the anticline. Four aspects are analysed. (1), The lithosome is shown to have a tabular (biostromal) geometry, and, like 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 recruits settled on the flanks of established, and fallen shells. Adults were only loosely clustered and supported by the accumulating interstitial 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 removal of sediment and toppling of shells. (4) In contrast to the open marine aspect of the overlying bioclastic sediments, the fine matrix sediment of the hippuritid lithosomes is faunally restricted, and appears largely derived from the in situ biodegradation of shells. clude that the hippuritid congregations grew constratally (i.e. without any supporting biogenic framework), and that they formed biostromal accumulations lacking relief. So there is no justification for calling them reefs as some authors have done. Rather, these rudists grew as gregarious sediment-dwellers in the restricted waters of the platform top, where they were sporadically disturbed by storms, and eventually blanketed by bioclastic material swept in from the platform margin.