Coral-sponge-microencruster-microbialite associations in the Upper Jurassic reef: quantitative characterization of a case study from Eastern Sardinia (Italy)
Abstract
Data di Pubblicazione:
2012
Citazione:
Coral-sponge-microencruster-microbialite associations in the Upper Jurassic reef: quantitative characterization of a case study from Eastern Sardinia (Italy) / C. Ricci, F. Jadoul, A. Lanfranchi, G. Rusciadelli, G. Della Porta, B. Lathuiliere, F. Berra. - In: RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA. - ISSN 2035-8008. - 21:2(2012), pp. 1014-1016. ((Intervento presentato al 86. convegno Congresso Nazionale della Società Geologica Italiana tenutosi a Arcavacata di Rende nel 2012.
Abstract:
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 co
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
Corals; Eastern Sardinia; Microbialite; Microencrusters; Sponge; Upper Jurassic reef
Elenco autori:
C. Ricci, F. Jadoul, A. Lanfranchi, G. Rusciadelli, G. Della Porta, B. Lathuiliere, F. Berra
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