Biostromal Coral Facies -A Miocene Example from the Leitha Limestone (Austria) and its Actualistic Interpretation

Coral-rich strata of the Leitha Limestone of Badenian age (Middle Miocene) at the type-locality in Grosshoeflein (Burgenland province, Austria) were re-interpreted from an actualistic viewpoint in comparison with Recent Bahamian and Arabian Gulf coral communities. The investigated outcrop (quarry "Fenk'j is interpreted as a sequence of framebuilding coral carpets and non-framebuilding biostromal coral communities, alternating with bryozoan and corallinacean calcarenites and bivalve beds. No indication for the presence of a reef was found. In total, a sequence of 1 0 coral intervals was observed, which can be grouped into 2 coral carpet types (high carpet framework, low bushy framework), four non-framework communities, and one bivalve / coral community. Within this sequence, no unequivocal indication for sea-level changes at the time of deposition was apparent. The different coral facies are interpreted as having been deposited within roughly the same water depth as Recent coral systems observed in the Bahamas (Andros, Exuma Cays) and the Arabian Gulf(Dubai, UAE). According to the model presented, the facies merely represent different lateral positions within the same spatially highly complex environment. A distribution map of benthic biota in the Arabian Gulf ( Jebel Ali, Dubai, UAE) was used to illustrate the likely lateral distribution of the Miocene facies. it is important to evaluate its coral deposits as either reef or non-reef. The investigated Leitha Limestone was deposited as a shallow subtidal ramp sloping into a basin (Piller and Vavra, 1991; Piller et al., 1996). The corals grew on a shoal or close to a string of islands, in a relatively low-energy environment. Because the coral fauna and its biogeographic affinity (Atlantic or Indo-Pacific) is poorly known, analogous coral formations in both biogeographical provinces were evaluated. The most comparable Recent environments were found in the Bahamas and the Arabian Gulf (Dubai, Abu Dhabi, UAE). The aims of the study were: (1) to establish whether the deposits are reefs or non-reefs, in particular coral carpets, and clearly describe and define the system; (2) to find Recent analogs to the coral species and framework/nonframework types observed in the Leitha Limestone; and (3) to interpret the coral-bearing strata paleoecologically and paleobiologically. The intention is to demonstrate that, despite a limited outcrop situation and taxonomic difficulties in the comparison of badly preserved Miocene corals with Recent counterparts, it is still possible to gain considerable insight into the depositional environment (whether reef or non-reef) and the paleoecology of the corals. This is done by comparison of fossil with Recent growth fabrics (Insalaco, 1998) and by analogies of fossil with Recent corals. Also provided is a fossil example for the still relatively new coral-carpet concept, which has been developed and defined only in the Recent (Riegl and Piller, 1997a, 1999). Coral carpets represent biostromes (Riegl and Piller, 1999, in press), and biostromes, in general, are important throughout the Phanerozoic (Kershaw, 1994; Kiessling et al., 1999). However, in comparison to reefs, little is known about such structures to date. Riegl and Piller (1999, in press) defined coral carpets as framework veneers following the contour of the seafloor with a primarily lateral growth component without a distinct vertical ecological zonation. The primarily vertical growth component of a typical reef, which spans several depth zones with regard to environmental variables, causes an internal ecological differentiation. Presented herein is a detailed study of the paleoecological, facies, and environmental structure of a coral biostrome. INTRODUCTION