Carbonate deposition in mixed siliciclastic-carbonate environments on top of an orogenic wedge (Late Cretaceous, Northern Calcareous Alps, Austria)

In the Middle Turonian to Lower Campanian (Lower Gosau Subgroup; LGS) of th e Northern Calcareous Alps, Austria, in the highstand systems tract of mixe d siliciclastic-carbonate depositional sequences, small carbonate shelves l ocally developed that were mainly controlled by a differentiated morphology of the substratum and siliciclastic input onto a storm-dominated shelf. The LGS was deposited above thrust nappes of the Alpine orogen. The transgr essive systems traces of depositional sequences consist of local fan delta or fluvial deposits, overlain by a succession mainly of siliciclastics that records deepening from paralic to outer shelf environments. Locally, rudis t biostromes and coral-rudist mounds accumulated in lagoonal/bay to inner s helf environments, but were buried by paralic siliciclastics. Along coastal sectors of low siliciclastic input, because of a high-relief truncation su rface at the base of the LGS, the transgressive systems tracts consist of a n upward-deepening succession of elastic carbonates deposited from gravelly to rocky carbonate shores. The highstand systems tracts are dominated by s iliciclastics, and record shoaling to inner shelf depths and, near the basi n margins, to lagoonal to marsh environments. During highstand conditions, delta progradation combined with shoreline compartmentalization by headland s locally led to establishment of inner shelf compartments of low silicicla stic input. In these compartments, carbonate shelves developed. Along strik e, the carbonate shelves were up to more than 10 km in length (beyond limit s of larger outcrops), and about 1 km to, possibly, 10 km wide down dip. Th e carbonate shelves consisted of (a) an inner shelf belt with coral-rudist mounds, rudist biostromes and bioclastic sand bodies, (b) a dissipative sho re zone of bioclastic sand bodies, (c) open lagoons/bays with radiolitid bi ostromes, and (d) narrow, micro-tidal hats or lithoclastic/bioclastic beach es. Up-section, inner shelf to shore zone carbonate parasequence tracts con sist of a coral-rudist mound, a rudist biostrome, and of shore zone bioclas tic limestones. Parasequence tracts deposited in lagoons/bays commonly shoa led incompletely, and mainly consist of more-or-less marry limestones depos ited in shallow subtidal environments with radiolitid biostromes, substrata of bioclastic sand or lime mud, and with local mass accumulations of gastr opods or of epibenthic non-rudist bivalves. The carbonate shelf successions are up to 100 m thick and, in vertical sect ion, consist of stacked parasequence tracts that become thinner up-section and record a shearing of mean depositional water depth. Carbonate depositio n was confined to the actual inner shelf to tidal flat/beach compartment. D own dip, the coral-rudist mounds scattered along the seaward fringe of the carbonate shelves graded into small haloes composed mainly of disoriented, fragmented rudists and corals. Larger carbonate slope depositional systems were not individuated, and the carbonate shelves interfingered with and pin ched out into inner shelf siliciclastics. The development of larger carbona te slopes was prevented by the low relief of the coral-rudist mounds, combi ned with high input and effective dispersal of siliciclastics ahead. Carbon ate shelf progradation over a possible distance of up to a few kilometres w as linked with aggradation of shelf siliciclastics. (C) 2000 Elsevier Scien ce B.V. All rights reserved.