Eulàlia Gili

Rudists as gregarious sediment-dwellers, not reef-builders, on Cretaceous carbonate platforms

Abstract It has sometimes been claimed that rudist bivalves competitively displaced corals from reef frameworks during the Cretaceous. This hypothesis combines two assertions: (1) that the autecology of rudists was convergent with that of reef-building corals; and (2) that rudist formations commonly developed as reefs. We dispute both assertions, and thus reject the hypothesis of competitive displacement. We argue instead that mobile sediments, rather than frameworks, dominated the margins and tops of most of the extensive carbonate platforms of the period, and that it was on these, non-reefal, biotopes that the rudists flourished. Definitions of reefs tend to combine two major elements: (1) a robust biogenic framework (with accompanying sedimentary and diagenetic components); and (2) topographical relief. Such definitions are clearly rooted in Recent coral reefs, in which endosymbiotic zooxanthellae permit the extensive growth of colonial coral frameworks in shallow but relatively nutrient-poor waters, coralline algae and cementation may contribute significantly to the growth of rigid structures, and topography is largely the legacy of Pleistocene changes in sea-level. In rudist formations, in contrast, individual rudist congregations are volumetrically limited, relative to sediment. They are often loosely constructed, and they evidently showed little, if any, original relief. Tabular and small lenticular units predominate. These differences in structure and palaeoenvironmental situation between rudist and modern coralgal associations reflect the different autecologies of the constituent organisms. The clonal growth of corals predisposes them to the development of frameworks projecting above the sediment surface (herein termed superstratal growth). By contrast, the aclonal development of rudists was better suited to the opportunistic occupation of a variety of temporarily available substrata, by large numbers of individuals. In particular, elevator rudists (in which the entire commissure exhibited upward growth) evidently grew with their shells largely embedded in, and supported by, the ambient sediment (herein termed constratal growth). Moreover, the tolerances and growth responses of rudists to such factors as water turbidity, nutrients and current regime were quite different from those of the majority of reef-building corals. Despite repeated assertions in the literature that rudists possessed zooxanthellae, only a few species show any evidence for such a symbiosis and other evidence suggests that most lacked them. In view of these differences in their preferred biotopes, competition between rudists and corals is doubtful, even though members of both groups co-occur in many areas. The relative decline of coral frameworks in the Cretaceous was thus probably independently caused.

Sedimentary evolution of an Aptian syn-rift carbonate system (Maestrat Basin, E Spain): effects of accommodation and environmental change

We report an integrated study of an expanded and relatively complete syn-rift continental to epeiric marine succession of Aptian age, cropping out in the western Maestrat Basin (eastern Iberian Chain). Four transgressiveregressive sequences are recognized throughout this mixed carbonate-siliciclastic succession, with excellent age control provided by ammonite biostratigraphic data. The transgressive systems tracts consist mainly of alternations of marls and limestones rich in orbitolinids. The regressive systems tracts are essentially characterized by wave- and tidally influenced siliciclastic and carbonate deposits, and by the development of carbonate platforms with rudists, corals, orbitolinids and green algae. Carbon and oxygen isotope curves were established in order to identify the global d13C perturbations related to the Early Aptian Oceanic Anoxic Event (OAE1a). These perturbations commence with a horizon of coral rubble encrusted by Lithocodium aggregatum and Bacinella irregularis with widespread large-sized discoidal Palorbitolina lenticularis. Associated d18O values indicate high-frequency cooling-warming climatic cycles. The fault-controlled rapid syn-rift subsidence recorded during this stage was the most important factor in producing accommodation. However, the major transgressions, sea level falls and biotic changes recorded in the eastern Iberian Chain are in agreement with those registered in other contemporaneous basins of the Tethys. Thus, the resulting sedimentary succession faithfully reflects the major oceanographic and climatically-driven global changes that characterized this stage albeit within a context established by regional tectonics. Hence, this well-documented record of the evolution of an Aptian epicontinental sea provides a useful comparative case study for the analysis of other Aptian epeiric sedimentary successions.

The growth fabric of gregarious rudist elevators (hippuritids) in a Santonian carbonate platform in the southern Central Pyrenees

Abstract 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. 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 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. We conclude 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.

Corals to rudists an environmentally induced assemblage succession

Abstract Corals and rudist bivalves commonly coexisted on the shallow carbonate platforms of the Cretaceous Tethyan Realm. Usually, the carbonate platforms show lateral and vertical biotic zonations. In general, the vertical biotic zonation relates directly to a consistent lateral biofacies pattern: coral or coral-rudist assemblages occupy more open zones of the platform, and rudist-dominated assemblages, the more restricted zones. Several corresponding coral-to-rudist assemblage successions in carbonate platforms have previously been widely interpreted as ecological successions. We document some coral-to-rudist assemblage successions, in a well preserved Upper Cretaceous section, in the Tremp area of the South Central Pyrenean Unit, which are interpreted, by contrast, as environmentally-induced faunal replacements. A repeated assemblage succession in the eastern part of the section begins with a coral-dominated assemblage, and passes up gradually to a mixed coral and rudist assemblage that is succeeded by a paucispecific unit of hippuritid elevator congregations. The lower coral-rudist units of such successions represent relatively more open, marine conditions, and the uppermost hippuritid unit relatively more restricted marine conditions. This assemblage succession was formed in response to a change in the depositional setting caused by sediment accumulation. Two consecutive coral-dominated assemblages, overlain by mixed coral and rudist assemblages, constitute a second kind of a sedimentary coral-to-rudist assemblage succession discussed in this paper. Its regular recurrence makes up the uppermost part of the section in the East. The predominance of platy to low domal coral colonies in the lower units suggests that conditions were initially favourable for horizontal coral growth. As the habitat changed, these coral assemblages were replaced by massive-tabular to large domal coral colonies, with the large hippuritid Vaccinites, and were sometimes succeeded, in turn, by clustered hippuritid elevators. The most important factor causing the replacement was probably the increase in sediment flux, associated with turbidity. This change in the depositional environment eventually favoured the establishment of rudist elevators, well adapted to moderate and intermittent to continuous accumulation of sediment.

North African Cretaceous carbonate platform systems

REGIONAL SYNTHESES.- High resolution North African Cretaceous stratigraphy: status.- Mesozoic carbonate platforms and associated siliciclastic spreadings in Morocco.- Genesis and diagenesis of the Gattar carbonate platform, Lower Turonian, northern southern Tunisia.- The growth and migration of two Turonian rudist-bearing carbonate platforms in Central Tunisia. Eustatic and tectonic controls.- Cretaceous coral-rudist formations in Tunisia. Paleogeography and Paleoecology.- Turonian rudist-coral limestones in Jebel Bireno, Central Tunisia.- Upper Cretaceous platform-derived conglomerates in Central Tunisia: significance and genesis mode.- Palaeogeographic context of Cenomanian and Turonian carbonate platforms in the eastern Atlasic domain.- Stratigraphic and geographic distribution of rudists in Algeria: a state of the art.- Cretaceous - Paleogene sequence stratigraphy of the Levant Platform (Egypt, Sinai, Jordan).- Sedimentological and taphonomic characterization of low-energy rudist-dominated Senonian carbonate shelves (southern Apennines, Italy).- TOPICAL REVIEWS.- Integrated stratigraphy of the lower Aptian and applications to carbonate platforms: a state of the art.- Rudist evolution and extintion - a North African perspective.- Strontium isotope chemostratigraphy of rudist bivalves and Cretaceous carbonate platforms.- Use of hippuritids for interpreting carbonate platform environments.

Quantitative study of a hippuritid rudist lithosome in a Santonian carbonate platform in the southern Central Pyrenees

Abstract The Santonian carbonate platform deposits of the Sant Corneli anticline in the southern Central Pyrenees contain distinctive lenticular to tabular lithosomes formed by congregations of slender hippuritid rudists. Here we make a quantitative study of one of the lithosomes in the Sant Pere de Vilanoveta Member, in the northern side of Sant Corneli. It is exposed over some 4 km, along a WNW–ESE section, and shows a remarkably persistent thickness up to a maximum of 5.15 m, though for the most part it is less than 3 m thick. In the lithosome, hippuritid shells are mostly preserved inclined about 30°–60° from the perpendicular with respect to bedding. The majority of inclined shells lean towards the southwestern to southeastern quadrants, the inferred principal downstream direction. Hippuritids are loosely clustered and supported in a fine bioclastic matrix. Percentage cover of hippuritids ranges from 19.8% at the base to 31.9% in the middle and top of the lithosome. The preliminary data on numerical densities (number of individuals per unit area) of hippuritids show some clustering around values of about 290–750 individuals per m2. The inclined orientation of hippuritid shells seems original and the result of active growth by the animals. The high numerical density of hippuritids at the base of the lithosome may indicate that the hippuritid congregation grew rapidly. The congregation appears to have reached an optimal level of density by the middle of the lithosome and to have maintained it through time during the development of the rudist congregation. The lithosome lacks relief and has no evidence of any rigid framework.

Factors regulating the development of elevator rudist congregations

Abstract Upper Cretaceous carbonate platform deposits show a widespread development of distinctive lenticular to tabular bodies of rock (lithosomes) formed by congregations of upright tubular (‘elevator’) rudist bivalves. Here we discuss the factors that regulated the initiation, consolidation and termination of such lithosomes, based on examples in the Santonian of the southern Central Pyrenees. In the study area, rudist congregations developed between phases of sediment influx from neighbouring source areas. For the initiation of settlement, sedimentation rate evidently had to be very low or nil. The first rudist settlers both provided more hard substrates for subsequent recruitment, and fuelled the in situ formation of bioclastic sediment, leading to the embedding and consolidation of the congregation (both positive feedbacks to establishment). Thereafter, rudist density correlated with inferred sediment destabilization at the benthic boundary layer, which in turn affected rudist recruitment. Thus, successful rudist larval settlement declined with increasing numerical density of individuals in the congregation — a crucial negative feedback mechanism. The density of the rudist congregations could then have been maintained at more or less the same level through time by this stabilizing process. Finally, development of the rudist congregations ceased with progressive shallowing, usually involving reworking of their upper parts and/or burial by renewed influxes of sediment.

Calcite/aragonite ratio fluctuations in Aptian rudist bivalves: Correlation with changing temperatures

Understanding how bivalves responded to past temperature fluctuations may help us to predict specific responses of complex calcifiers to future climate change. During the late-Early Aptian, aragonite-rich rudist bivalves decreased in abundance in northern Tethyan carbonate platforms, while rudists with a thickened calcitic outer shell layer came to dominate those of Iberia. Seawater cooling and variations in calcium carbonate saturation states may have controlled this faunal turnover. However, our understanding of how rudist lineages responded to changing environmental conditions is constrained by a lack of quantitative data on the evolution of thickness, size, and mineralogy of the shell. This study is based on volumetric measurements of the shell and shows the transition in lineages of the family Polyconitidae from aragonite-rich mineralogy in the earliest Aptian, to low-Mg calcite-dominated mineralogy in the middle Aptian, returning to aragonite-dominated composition in the latest Aptian. The platform biocalcification crisis that occurred at the Early-Late Aptian boundary in the Tethys was marked by a relative increase of calcite and a decrease in skeletal thickness and commissural diameters. The highest calcite/aragonite (Cc/A) ratios in polyconitid rudists accompanied the late Aptian cold episode, and the lowest values were reached during the warmer intervals of the earliest and latest Aptian. These results imply a correlation between Cc/A ratio values and temperature and suggest that some bivalves adapted to less favorable calcification conditions by changing calcite and aragonite proportions of their bimineralic shells and decreasing skeletal thickness, thereby reducing the metabolic cost of shell growth.

Hydrodynamic behaviour of hippuritid rudist shells: Ecological consequences

Abstract Laboratory flume observations were conducted to examine the influence of the projecting shells of hippuritidrudists on water movement in the benthic boundary layer. Visualization of flow about fossil and model hippuritid shells revealed no significant differences; both showed similar, characteristic flow patterns dependent primarily on the orientation (vertical, inclined upstream, inclined downstream) of the cylinder. All orientations produced similar general flow patterns: (1) fluid near the top of the cylinder accelerated over the cylinder, separating from the cylinder at the highest edge; (2) fluid encountering the cylinder between the top and the base passed around the cylinder and a portion was entrained in the wake; (3) fluid near the substrate passed around the cylinder, was entrained in the wake, and then rose in a pair of alternating vortices to the top of the cylinder and was accelerated downstream. For both vertical and upstream-inclined cylinders, the top of the cylinder was bathed with water drawn from the mainstream flow. Flow around downstream-inclined cylinders, however, showed two distinct differences from the other orientations. First, flow separated from the top of the cylinder at the upstream edge so that the top of the cylinder was bathed with fluid drawn from the wake. Second, the vortices behind downstream-inclined cylinders were greatly amplified, even briefly lifting sediment grains at mainstream speeds in which the other two orientations showed no disturbance of the sediment behind the cylinder. The enhanced vortices strongly drew water from the wake and off the sediment surface which fed the flow within the separation bubble over the top of the cylinder; the top was thus bathed in a mixture of fluid from both the mainstream flow and the surface of the sediment. Flow from the excurrent openings of the models did not mix with fluid in the separation bubble. Because most water filtered by hippuritids entered the pores penetrating the upper valve, animals oriented vertically or inclined upstream would primarily sample water from the mainstream flow; animals inclined downstream would sample a mixture of water from the mainstream flow and water drawn from the sediment-water interface (which should be enriched in bacteria and detrital organic particles). A downstream-inclined posture would have been advantageous in nutrient poor waters because the quantity and variety of available organic particles would have been enhanced.

Revised lithostratigraphy of the Upper Cretaceous (Santonian) carbonate platform succession on the northern flank of Sant Corneli, southern Central Pyrenees

En este trabajo se presenta una revision formal de la litoestratigrafia de la Formacion Sant Corneli (Santoniense) en el flanco norte del anticlinal de Sant Corneli al ENE de Tremp, en los Pironeos centro-meridionales. Esta formacion comprende una sucesion de plataforma carbonatada que yace sobre los depositos de plataforma de las calizas de Montagut y esta cubierta por las arcillas y margas de Herba-sabina. Se han reconocido cinco miembros, tres de los cuales se han incluiido en una nueva serie tipo, que se localiza en la parte occidental del area de estudio. Esta serie, de 196 m de potencia, comprende la parte inferior y superior del Miembro Aramunt Vell, separadas por una cuna que se abre hacia el Este, y que esta constituida por dos miembros: el Miembro Sant Pere de Vilanoveta (nombre nuevo) y el Miembro l’Aubagueta (nombre nuevo), suprayacente al anterior. El Miembro Aramunt Vell comprende numerosos ciclos granocrecientes, de pocos metros dc potencia de margas arenosas y calcarenitas bioclasticas ocres, ricas en miliolidos. Las dos partes de este miembro se unen hacia el Oeste para abarcar toda la potencia de la formacion. El Miembro Sant Pere de Vilanoveta esta constituido por ciclos granocrecientes de margas ricas en corales y calizas margosas, litosomas de rudistas y grainstones y floatstones bioclasticos. El Miembro L‘Aubagueta comprende calizas margosas y arcillas arenosas. Hacia el Este, aparecen dos nuevos miembros, que representan depositos de pendiente originalmente contiguos a la plataforma. Sin embargom, en la actualidad aparecen separados de esta por una falla normal de direccion NW-SE. El Miembro Llau de Castellet (nombre nuevo) comprende margas pardas y calizas margosas lateralmente al equivalentes a la parte inferior del Miembro de Aramunt Vell. El Miembro Llau de Joncarlat (nombre nuevo), suprayacente al anterior, esta formado por margas blancas y calizas margosas, lateralmente equivalentes al resto de las unidades de la plataforma. Proponemos que las unidades ‘Formacion Carreu’ y miembros ‘el Grau’ y ‘Prats de Carreu’ sean dejadas de utilizar.