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

Abstract

In the Middle Turonian to Lower Campanian (Lower Gosau Subgroup; LGS) of the Northern Calcareous Alps, Austria, in the highstand systems tract of mixed siliciclastic–carbonate depositional sequences, small carbonate shelves locally 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 transgressive systems tracts 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, rudist biostromes and coral–rudist mounds accumulated in lagoonal/bay to inner shelf environments, but were buried by paralic siliciclastics. Along coastal sectors of low siliciclastic input, because of a high-relief truncation surface at the base of the LGS, the transgressive systems tracts consist of an upward-deepening succession of clastic carbonates deposited from gravelly to rocky carbonate shores. The highstand systems tracts are dominated by siliciclastics, and record shoaling to inner shelf depths and, near the basin margins, to lagoonal to marsh environments. During highstand conditions, delta progradation combined with shoreline compartmentalization by headlands locally led to establishment of inner shelf compartments of low siliciclastic input. In these compartments, carbonate shelves developed. Along strike, the carbonate shelves were up to more than 10 km in length (beyond limits of larger outcrops), and about 1 km to, possibly, 10 km wide down dip. The carbonate shelves consisted of (a) an inner shelf belt with coral–rudist mounds, rudist biostromes and bioclastic sand bodies, (b) a dissipative shore zone of bioclastic sand bodies, (c) open lagoons/bays with radiolitid biostromes, and (d) narrow, micro-tidal flats or lithoclastic/bioclastic beaches. Up-section, inner shelf to shore zone carbonate parasequence tracts consist of a coral–rudist mound, a rudist biostrome, and of shore zone bioclastic limestones. Parasequence tracts deposited in lagoons/bays commonly shoaled incompletely, and mainly consist of more-or-less marly limestones deposited in shallow subtidal environments with radiolitid biostromes, substrata of bioclastic sand or lime mud, and with local mass accumulations of gastropods or of epibenthic non-rudist bivalves. The carbonate shelf successions are up to 100 m thick and, in vertical section, consist of stacked parasequence tracts that become thinner up-section and record a shoaling of mean depositional water depth. Carbonate deposition was confined to the actual inner shelf to tidal flat/beach compartment. Down 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 pinched out into inner shelf siliciclastics. The development of larger carbonate slopes was prevented by the low relief of the coral–rudist mounds, combined with high input and effective dispersal of siliciclastics ahead. Carbonate shelf progradation over a possible distance of up to a few kilometres was linked with aggradation of shelf siliciclastics.