Devonian carbonates, Carnic Alps

Research project P 14131 Devonian carbonates, Carnic Alps H.P. SCHÖNLAUB 08.05.2000 Devonian carbonates of the Carnic Alps are stacked in a series of imbricated thrust slices bringing into close proximity platform-, slope-, and basin sediments of a pure carbonate system. This constellation is rarely preserved in the geological record and provides a unique opportunity to study platform evolution and factors governing sedimentation and deposition through the entire Devonian at the northern margin of Gondwana. Previous work has established the framework of geological, structural and biostratigraphic data, that is necessary for the application of modem sedimentological concepts, such as sequence stratigraphy and basin analyses. The investigation focusses on an area encompassing the Giramondo Pass to the west, Pizzo di Timau to the east, Gamspitz to the south and Gamskofel-Mooskofel Massif to the north. In this area are the various depositional environments preserved in Kellerwand Nappe (carbonate platform), Cellon Nappe (foreslope/slope) and Rauchkofel Imbricate Nappe Complex (basin). Through structural superposition of the nappes are some of the facies transitions, particularly the architecture of the shelfedge from Emsian to Eifelian time, not exposed. The composition of sediments deposited by gravity flows (mostly turbidites), which originated at the shelfedge should enable reconstruction of the "missing link" (= shelfedge) between platform and slope. This is particularly important for the understanding of slope and basin sedimentation which is to a large degree, controlled by shelfedge architecture. Furthermore, study of changes in the composition of the backreef-derived components of the turbidites should help correlation between the biostratigraphically poorly constrained platform-successions, and the well constrained basinal limestones. The strongly facies controlled tabulate corals from the region will be investigated, to determine their possible contribution to paleogeography, biostratigraphy, and paleoecology on a regional and supra-regional scale. The results of this investigation will lead to a better understanding of the evolution of the Carnic carbonate system in time and space and contribute to the comprehension of mechanisms governing sedimentation in Paleozoic carbonate systems, which are, in many respects different from Recent ones and to date, poorly understood.

The Mid Paleozoic limestones exposed in the Central Carnic Alps preserve the whole range of carbonates encountered on a shelf to basin transect, a scenario rarely encountered in the geologic record. This provided an opportunity to investigate the consequences of sea level changes, shelf sedimentation and margin architecture on a Devonian carbonate system covering a time period close to 50 million years. Devonian carbonates were investigated in an area extending from Giramondo Pass in the west to Findenigkofel in the east and from Pizzo di Timau in the south to Gamskofel-Mooskofel Massif in the north. This area encompasses the majority of well-preserved Devonian carbonates in the Carnic Alps. A NNW- SSE oriented differentiation of facies can be recognized with backreef sediments in the south, separated by reef complexes from slope (or ramp) and basin sediments in the north. Tectonic shortening brought the different facies into close proximity and the various depositional environments of the Devonian carbonates are now located in different structural units. In the Central Carnic Alps numerous sections were measured through reef- and backreef facies (Kellerwand-Hohe Warte Nappe), forereef-, ramp- and/or slope facies (Cellon Nappe) and through pelagic and hemipelagic facies with common gravity flow deposits and interbedded fine-grained siliciclastic units (Findenig Nappe). A pelagic facies with few or no gravity flow deposits occurs in the vicinity of Mount Rauchkofel and at Zollner Lake cherts and siliceous shales of deep water aspect are exposed (Rauchkofel- and Bischofalm Imbricate Nappe Complexes respectively). The successions reflect the development of a carbonate ramp which was slowly drifting into low-latitudinal warm waters to a tropical carbonate shelf platform with shelfbreak and segmented slope. Masswasting is extensive on the slope and characterizes slope sedimentation. Upper Devonian strata are characterized by overall deepening of the water and backstepping of the shelf edge assembly. The Famennian carbonates of deepwater aspect dominate in all depositional environments and platform drowning is implied.