Géologie Méditerranéenne

Tome XXVm, n° 1-2, 2001, pp. 169 -171.

The geometry of rudist lithosomes

Cretaceous carbonate platforms and rudist lithosomes

Cretaceous carbonate platform deposits typically show a tabular stratal architecture, with minor sedimentary cycles predominating (Figure 1; e.g. HUNT & TUCKER, 1993; GILI et al., 1995a; RUBERTÏ, 1997; SANDERS & PONS, 1999, and many others). There is usually a lateral gradation from low energy inner platform deposits -sometimes with metre-scale peritidal cycles (e.g. MORO & JELASKA, 1994) -to thicker outer platform cycles (generally <10 m), incorporating higher energy deposits (e.g. STÔSSEL & BERNOULLI, 2000). Distally, the outer platform units may curve down and grade into low-angle slope deposits (e.g. VAN BUCHEM et al., 1996), though tectonically induced steepening sometimes produced a sharper change (e.g. MASSE & LUPERTO-SINNI, 1987).

In the outer platform, rudist and/or coral lithosomes of moderate diversity comprise tabular to lenticular biostromes intercalated between bioclastic units (e.g. GILI et al., 1995b). Each biostrome may extend laterally for hundreds of metres, even kilometres, but is rarely more than a few metres thick (e.g. SKELTON et al., 1995). Sometimes there are more localized lensoid bioherms (e.g. PERKINS, 1974 ; MASSE & PHILIP, 1981 ; SCOTT, 1990), but never a constructed ('reefal') rim -contrary to frequent misrepresentations in the literature (ROSS & SKELTON, 1993 ; GILI et

Controls on deposition

P. W. SKELTON*

al., 1995c). However, some secondary, including karstic, rims have been recorded (e.g. WINTERER et al., 1995). Thin (usually < lm) but laterally extensive rudist biostromes of the inner platform are typically pauci-to mono-specific (e.g. PLATEL, 1998).

This characteristic depositional geometry reflects two historical contingencies of the Cretaceous. First, prolonged periods of relative sea-level rise, with infrequent falls, left little scope for antecedent karst genesis, in contrast to its major role in the development of modern tropical reefs, due to Pleistocene glacioeustacy (PURDY, 1974). Secondly, the small increments of accommodation typical of Cretaceous greenhouse conditions (TUCKER, 1993), combined with high rates of carbonate production, especially by late Cretaceous rudists (STEUBER, 2000), promoted rapid shoaling, with over-production and extensive lateral redistribution of bioclastic sediment (CARANNANTE et al., 1999). Consequently, platforms usually assumed a more or less flat-topped profile (as in Figure 1), with broad outer zones across which bioclastic debris was swept both inwards across the platform top (e.g. SKELTON et al. 1995) and down flanking clinoforms (e.g. TISLJAR et al., 1998 ; LAVIANO et al., 1998). Rudists, primarily adapted as sediment-dwellers, frequently carpeted the platform top to form vast 'meadows' of clustered shells. Constratally growing elevators dominated in areas of net sediment accumulation, while recumbents were favoured in zones of sediment by-passing, frequently at the tops of shoaling cycles (GILI et al., 1995c). In life the rudists were supported by, and on death contributed significantly to, the ambient mobile sediment.