Autochthonous facies and allochthonous debris flows compared: Early Oligocene carbonate facies patterns of the Lower Inn Valley (Tyrol, Austria)

This study presents a microfacies analysis and palaeoenvironmental interpre tations of Early Oligocene carbonates from the Lower Inn Valley Tertiary (" Unterinntal-Tertiar") of Austria. The well preserved biogenic components al low detailed investigations of component relationships and controlling ecol ogical parameters. The carbonates are dominated by coralline algae, corals, small and large benthic foraminifers, bryozoans and lithoclasts. Bivalves, gastropods, echinoderms, brachiopods and serpulids are subordinate. The li mestones are present as A) autochthonous carbonates transgressing directly above the Triassic basement and B) allochthonous debris flows within deeper -water marls. These carbonates are found within the Paisslberg Formation. T he Werlberg Member within this formation, pertains to the autochthonous car bonates and larger debris flows. Five facies types are separated following fabric analysis and statistical t reatment (correlation, cluster analysis, principal components analysis) of semi-quantitative data consisting of component frequencies of thin sections . Facies distribution patterns are principally controlled by variations in substrate characteristics, turbulence and light along a depth gradient. Rec onstruction of facies pattern distribution reveal both lateral and proximal -distal facies trends: coral-coralline algal facies, coralline algal facies as well as foraminiferal facies were situated in shallower environments, l aterally adjacent to each other. These grade distally into coralline algal- bryozoan facies, bryozoan facies and finally into mollusc rich marls. Debri s flows consisting of reworked material from all of the known carbonate fac ies were deposited within the marls. A single facies (bioclastic packstone facies) is restricted to the debris flows and possible represents transport induced differentiation of components and grain size within distal debris flows.