Hemipelagic and turbiditic mudstone facies associations in the Upper Cretaceous Gosau group of the Northern Calcareous Alps (Austria)

Turbiditic and hemipelagic mudstones from the Upper Cretaceous to Lower Paleocene Nierental Formation of the Northern Calcareous Alps (Austria) can be differentiated by employing relatively simple laboratory techniques. Hemipelagic mudstones turn out to be characterized by finer grain-sizes of the non-carbonate fraction and higher carbonate contents. Sediment colour, bioturbation, distribution of microfossils, weathering characteristics, and bed-sequence analysis were used as additional criteria to recognize the genetically different mudstone types. For acquiring the non-carbonate grain-size data the samples were leached in hydrochloric acid and the remaining siliciclastic material was used for analysis with a Sedigraph(R). The median values of the grain-size distributions of the turbidite mudstones reach up to 9.7 Phi, whereas the hemipelagic median values lie between 9.5 and 10.8 Phi. Carbonate contents of the turbidite mudstones range between 22% and 68%, those of the hemipelagic mudstones between 58% and 82%. Single-bed profiles show distinct vertical trends of both grain-size and carbonate content values for the turbidite mudstone division. No such trends exist in the hemipelagic mudstone beds. Five basic types of turbiditic and/or hemipelagic mudstones can be recognized: (1) turbidite-hemipelagite couplets; (2) pure hemipelagic mudstone sequences; (3) hemipelagic mudstone sequences with very thin turbiditic intercalations; (4) a thoroughly-mixed turbiditic/hemipelagic mudstone type, which is characterized by intense bioturbation; and (5) mud-turbidites without any hemipelagic intervals. Most of these types occur simultaneously at different localities of the Nierental Formation.