Coral-sponge-microencruster-microbialite associations in the Upper Jurassic reef: quantitative characterization of a case study from Eastern Sardinia (Italy)

The Late Jurassic records one of the largest reefal expansions of the Phanerozoic, with major diffusion and differentiation in the Tethys realm (WOOD, 1999; KIESSLING, 2002; CECCA et al., 2005). Several depositional and compositional models about Upper Jurassic reef types (see INSALACO et al.,1997; LEINFELDER et al., 2002, 2005; RUSCIADELLI et al., 2011 for a revision) have been published but little knowledge is available about the Eastern Sardinian reefs. This study focuses on the compositional and sedimentological characterization of the Upper Tithonian reef complex presently exposed in the area of Cala Gonone (Orosei Gulf) (Fig.1). The Upper Jurassic carbonate succession of Eastern Sardinia consists of three Bathonian-Callovian to Berriasian (DIENI & MASSARI, 1985; JADOUL et al., 2010 and references therein) carbonate depositional systems developed on the southern Europe passive margin (Fig.1): 1) the first (Dorgali Fm.) is characterized by ooidal grainstone, accumulated above wave base on structural highs (Variscan basement), capped by an Upper Bathonian-Callovian condensed succession with a few Fe-phosphatic hardgrounds; 2) a low-angle Oxfordian-upper Tithonian depositional system: the shallow ramp deposition (Tului Fm.) is characterized by basal oolitic facies overlain by prograding coral-stromatoporoid reefs, interfingering with outer ramp-basinal peloidal packstone-wackestone (S’Adde and Baunei Fms.); 3) the third depositional carbonate system (Bardia Fm.) developed after an Early Tithonian regressive trend, locally marked by carbonate breccias indicative of subaerial exposure. The lower part of the Bardia Fm. (upper Tithonian) is locally characterized by gentle slopes (3-15°) with bioclastic-coral-sponge facies associations (LANFRANCHI et al., 2011). This progradational unit is followed by up to 400-500 m of back reef and inner platform shallow water carbonates.REEF COMPONENTS AND FACIES Compositional and sedimentological analysis of the Bardia reef has been carried out through the combination of “macroscopic” (outcrop-scale) and “microscopic” (microfacies-scale) observations on exceptionally exposed saw-cut quarry walls, over a surface of a few hundreds square metres in three different locations. The external surface of each macroscopically detectable component has been emphasized on the quarry walls (Fig.2). The areal distribution of each portion has been stored as vector images, defining frequency, density and area occupied by the reef components. Microfacies and paleontological analyses have been performed on 280 thin sections. Reef components were grouped into three broad categories: 1) macroscopically detectable organisms (mainly corals, sponges, bivalves, gastropods, echinoderms); 2) microscopically detectable components (microencrusters and microbialites); 3) fine- to coarse bioclastic debris and mud-supported facies. Corals show different degree of reworking, from in life-position skeletons more than 2.5 m2 in size to centimetre-sized rubble. The 49 recognized genera of corals have been classified according to external morphology and corallite type. Calcified sponges (Stromatoporoids) are a few centimetres to tens of centimetres in size, occurring as isolated specimens and in densely-packed assemblages. Siliceous sponges and spiculae are replaced respectively by precipitated automicrite and calcite spar. Microbialite and microencruster organisms form domal, columnar or irregular accretionary crusts, few millimetres to several centimetres in thickness. Frequently, crusts bind neighbouring skeletons of large biota, developing metre-scale bioconstructions. These components combine in various proportions within and among quarries, reflecting abrupt lateral and vertical changes of environmental conditions. Quarry 1 is characterized by large branched and massive coral colonies partially or totally encrusted by centimetre thick microencruster crusts and well-washed bioclastic facies, indicating sediment reworking in a high–energy environment. This facies abruptly passes laterally into a densely packed massive microsolenid coral and calcareous sponge assemblage and microbialite and microencruster (Tubiphytes. and other nubeculariids) boundstone. Microsolenid assemblages are commonly interpreted to have formed in deeper water (LATHULIÈRE & GILL, 1995; GILL et al., 2004) or alternatively related to poorly illuminated shallow-water cave environments, and adapted to low-sedimentation, low-energy and nutrient-rich conditions (INSALACO, 1996; DUPRAZ & STRASSER, 2002). Quarry 2 is characterized by progressive vertical variations from facies dominated by densely packed platy and flat coral colonies (microsolenids and others) to facies dominated by calcareous sponges and loosely packed phaceloid coral colonies. Microbialite and microencrusters (Tubiphytes and other nubeculariids) envelope and bind large biota. Platy growth forms are generally interpreted as a response to poor illumination (INSALACO, 1996). Consequently the whole association seems to be compatible with poorly illuminated water, low sedimentation rate in a low energy environment. Quarry 3 records large scale bedding (from 1 m to several metres) defined by six intervals dominated respectively by 1) bivalves; 2) dasycladacean algae; 3) reworked massive thamnasterioid coral colonies; 4) thin phaceloid corals and calcareous sponges in growth position; 5) branched ramose coral colonies and calcareous sponges; 6) reworked massive plocoid coral colonies and gastropods. Large biota within intervals 3, 4 and 5 are largely encrusted by different microencrusters such as Koskinobulina, Thaumatoporella, light-dependent Lithocodium-Bacinella, and microbial accrectionary crust. Coral assemblages and microencruster association reveal a progressive increasing of the energy regime and sediment reworking in well-lit waters (INSALCO, 1996; SCHIMD & LEINFELDER, 2002), while the presence of bivalve, algae and gastropod floatstone represents the temporary shifting to “peri-reefal” environments. Despite variability of facies associations in the three quarries, a paleoecological evolution from quarry 1 to quarry 3 emerges, reflecting the change from moderate energy environment with more protected, poorly illuminated cave environment (quarry 1), through a low energy, poorly illuminated environment characterized by low sedimentation rate (quarry 2), to a high energy, well illuminated environment, characterized by high sedimentation rate and reworking (quarry 3). The relative position of the studied quarries along an ideal depositional profile remains speculative, although a medium-scale progradational trend, from distal to proximal setting, seems to be compatible with the long-scale stratigraphic trend of the Bardia Fm. Spectacular outcrop conditions, amount of data collected, biota taxonomic classifications and the observed stratigraphic evolution provide the solid base for paleo-biogeographical comparisons with other Upper Jurassic Tethyan reef complexes. 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