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predavanje 9. januarja 2003 ob 17:00,
Mala dvorana ZRC SAZU, Novi trg 4, II. nadstropje

Michael W. Rasser
Institut für Geologie und Paläontologie, Universität Graz
E-mail: michael.raser@paleoweb.net
 
Upper Jurassic to Lower Cretaceous carbonate platforms of the Northern Calcareous Alps: 
structure and evolution


Upper Jurassic to Lower Cretaceous carbonate platforms of the Northern Calcareous Alps are interpreted as steeply bordered, isolated "Bahamas-type“ platforms, which are interfingered with bathypelagic basin sediments. This facies differentiation and the Upper Jurassic onset of carbonate platform development was caused by tectonic events at the Oxfordian/Kimmeridgian boundary. Gravitational sliding masses caused a structural relief within radiolarite basins that enabled the first shallow-water development in the Northern Calcareous Alps since the drowning of Upper Triassic carbonate platforms. Shallow water sedimentation takes place from the Kimmeridgian to the lower-most Cretaceous.

Basinal sediments overlying the Oxfordian radiolarites are represented by the Oberalm Formation, characterized by micritic limestones containing Coccolithophorids, Radiolaria, Spiculae, Saccocoma, and Calpionellids. Interbedded with the Oberalm Fm. are the allodapic Barmstein Limestones composed of bio- and lithoclasts derived from the platform. The Oberalm Fm. can be overlain by the "Oberalm transitional facies", interpreted as a periplatform ooze; it is partly comparable to the Barmstein Limestones but shows thicker bedding and low amounts of shallow water bioclasts in a micritic matrix. The overlying Tressenstein Fm., composed of ruditic shallow water lithoclasts, is explained as a platform slope breccia. Platform carbonates are represented by the Plassen Fm. and the Lerchkogel Limestone. Reefal structures are interpreted as isolated patch reefs, while continuous reef belts along the platform margins are unknown. Open platform environments are characterized by bioclastic and peloidal grainstones and coral-”chaetetid” patch-reefs; ooids are usually rare. Micritic sediments, partly rich in oncoids, as well as small patches and carpets of “stromatoporoids” prevail in protected lagoons. Bacinella bindstones stabilizing the reef rubble are present in all environments.

This presentation provides an overview of the Upper Jurassic to Lower Cretaceous carbonate platforms of the Northern Calcareous Alps, which are investigated in the course of a project funded by the Austrian Science Foundation (project leader: A. Fenninger, Graz). The main topics of this project, which are presented and discussed herein, are: (1) basin - platform transitions, (2) platform geometries, (3) shallow-water carbonate facies dynamics, (4) palecology of reefal structures, and (5) biostratigraphic problems including biogeography.

Two crucial problems will be discussed in detail: (1) the platform geometry and (2) the "framework problem". Our results raise doubts on the traditional interpretation of platform geometry. We suggest that Kimmeridgian carbonate platforms may have represented carbonate ramp structures without steep slopes, but not "Bahamas-type“ platforms. Only the Tithonian - Berriasian platforms showed steep slopes with a topographic relief sufficient for the production of wide-spread turbidites (Barmstein Limestone).

The "framework problem" means that the interior of most modern coral reefs represents more a pile of reef rubble than an in-situ framework. This is because reefs are periodically destroyed by heavy storms and reef recovery leads only to a thin cover of living framework. In fact, in-situ coral reef frameworks are rare among the Alpine platforms in question, but accumulations of fragmented framework-builders bound by microencrusters are abundant. Upper Jurassic - Lower Cretaceous reefs of the Northern Calcareous Alps may thus represent ancestors of modern reefs not only by having enhanced phototrophy, as formerly discussed by other authors, but also by undergoing a cycle of destruction, rubble binding, and reef regeneration, which is an essential process among present-day reefs.