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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.
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