SHELL BEDS OF DICERATID RUDISTS AHEAD OF A LAW-ENERGY GRAVELLY BEACH (TITHONIAN, NORTHERN CALCAREOUS ALPS, AUSTRIA): PALAEOECOLOGY AND TAPHONOMY.

In the Tithonian Lofer unit (Northern Calcareous Alps, Austria), Heterodiceras shell beds accumulated ahead of a low-energy gravelly beach. This is the first detailed description of diceratid shell beds from the Northern Calcareous Alps. In the Eastern Alps, during Late Jurassic tectonic deformation, parts of structurally highest preserved thrust nappes became islands. During the following latest Jurassic transgression over vegetated land, a variegated succession of shore zone to shallow neritic lithologies accumulated. Near the village Lofer, the transgressive succession records a low-energy gravelly beach ahead of a shallow subtidal, protected bay or lagoon with a bottom of organic-rich, argillaceous lime mud. The proximal area of the bay/lagoon received abundant phytoclasts and clay from land. On organic-rich lime-muddy substrata that may have supported fleshy algal meadows, the larger textulariine foraminifer Anchispirocyclina thrived. Seaward, level-bottoms dominated by the rudist Heterodiceras or by milleporidian hydrozoans were present. The milleporidian banks interfingered with storm spillover lobes shed from oolite dunes farther offshore. The Heterodiceras beds consist of floatstones to rudstones of toppled, disoriented shells in highly different taphonomic states, as a result of episodic toppling and reworking by high-energy events, burrowing, bioerosion, and in-situ shell disintegration. The matrix of lime mudstone to bioclastic wackestone is characterized by terebratulacean brachiopods and microgastropods. While the brachiopods thrived as suspension feeders attached on toppled shells of dead rudists, the microgastropods perhaps scavenged on microbial biofilms and/or on fine-grained dead particulate organic matter. A few of the diceratid shells are variably overgrown by sessile textulariine foraminifera, by microblalites, by milleporidians, and by the microproblematica Lithocodium and Bacinella. The majority of the diceratids had been largely to completely stripped of their thin calcitic outer shell layer. Spelling of the calcitic layer took place early, when the shells were exposed on the sea floor, and during shallow burial within soft sediment. Effective spalling probably was associated with decomposition of organics embedded between the aragonitic, inner shell layer and the calcitic, outer layer. [ABSTRACT FROM AUTHOR]