Antonio Obrador

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.

Late Miocene breccia of Menorca (Balearic Islands) a basis for the interpretation of a Neogene ramp deposit

Abstract Neogene (Tortonian) ramp facies associations crop out in the southern sector of Menorca (Balearic island). These are made mainly by sigmoidal and oblique clinoform units comprising rhodoliths, bryozoans, molluscs and foraminifera. These units are interpreted as outer-ramp deposits. A breccia deposit infilling an erosional surface is found at the top of the carbonate ramp sequence. The breccia components (mainly rhodoliths and oolite clasts) may represent erosion of the underlying ramp deposits and inner-ramp counterparts, which do not crop out on the island. The study of components in the breccia deposits confirms the indigenous character of the outer-ramp facies associations, which suggests that the ramp was steepened distally. The breccia deposits correspond laterally to a discontinuity surface locally showing karstic features. Transgressive sediments (including a phosphatic crust) are found above the discontinuity surface. All together these features, and the dolomitisation of the uppermost ramp sediments and breccia deposits, suggest that the breccia originated from erosion of the ramp after a major relative sea-level fall. The breccia and discontinuity surface separate the ramp sequence from an overlying prograding sequence. A correlation of this sequence boundary to other areas in the western Mediterranean is proposed.

No Title Provided: ABSTRACT

Miocene calcarenites of Menorca exhibit abundant terrigenous dolomite, both as medium-grained polycrystalline rock fragments and as fine-grained individual abraded crystals. Petrographically, this detritus looks both like neighboring Jurassic dolostones and like Triassic Muschelkalk dolostones of the same general area. To determine which stratigraphic unit(s) provided this terrigenous dolomite we characterized the iron contents of Jurassic and Triassic dolostones, using the electron microprobe, and then, with the probe, we matched the composition of the Miocene detritus with its source. As was expected from their present greater surface exposure, Jurassic dolostones proved to be the principal source. The trace of Triassic dolomite detritus in Miocene samples analyzed is p oportionate to the small area of Triassic exposure relative to that of the Jurassic. Dolomitization of the Miocene resulted in (1) pervasive fine to medium-crystalline dolomite and (2) overgrowths on terrigenous dolomite grains. In some samples, dolomite overgrowths occur in the absence of pervasive dolomite, indicating that seeding by terrigenous dolomite grains enhanced dolomitization. Later calcitization of Miocene dolomite favored centers of the pervasive variety of dolomite crystals and the inner margins of the dolomite overgrowths. These patterns of dedolomite indicate that the two occurrences of dolomite, the pervasive variety and the dolomite overgrowths, are behaviorly correlative, and suggest that they are temporally correlative as well. End_of_Article - Last_Page 927------------