Paleobathymetry of Cretaceous Turbidite Basins of the East Alps Relative to the Calcite Compensation Level

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Abstract
The paleobathymetry of ancient turbidite basins has traditionally been dealt with in terms of paleontologic depth indicators. The calcite compensation depth now provides an additional bathymetric reference level. The concept of carbonate compensation levels (CCL) is for the 1st time being applied in detail to Cretaceous turbidite basins. Out of the 4 E Alpine basins studied 2 (i.e., Flysch Zone of the E Alps and Koessen Gosau Basin) have calcite-free hemipelagic green claystones of Campanian-Early Maestrichtian age and were below the CCL. Alternative causes such as climatic variations and terrigenous influx of carbonate-free detritus are unlikely to be major controlling factors for the absence of calcite in the hemipelagic claystones. Distinction between hemipelagic and turbiditic mudstones is facilitated by the high calcium carbonate content of the latter. Hemipelagic and pelagic layers of the same age in the other 2 basins (Reichenhall Gosau Basin and Ultrahelvetic flysch) contain calcite and were deposited above the CCL. Supportin evidence comes from microfaunal analysis. The hemipelagic layers of the Flysch Zone and Koessen Basin contain agglutinating Foraminifera, Radiolaria and some fish debris only. This fauna corresponds to Saidova''s Astrorhizidae-Ammodiscidae taxocoenosis of oceanic type from the abyssal Pacific at depths greater than 3500-4500 m. It is largely identical to Brouwer''s (1965) deep-water Rhabdammina fauna. With a few exceptions the hemipelagic layers lack calcareous planktonic and benthonic foraminifera. The turbidites, however, locally abound in them. In the Reichenhall Basin and Ultrahelvetic flysch the (hemi-) pelagic rock-types are generally richer in biogenic constituents than the turbiditic lithologies. Calcareous planktonic foraminifera prevail over benthonics. These findings support a proposed deep-sea trench environment of the Flysch Zone of the E Alps. The Ultrahelvetic Zone supposedly represents the N slope facies of the Tethyan Sea. The Gosau Basins are located on the S carbonate platform of the N Calcareous Alps which broke up into basins and highs and was tectonically displaced Northward during the Cretaceous-Early Tertiary closure of the Tethyan Sea. That 2 of these basins of the N Calcareous Alps had apparently subsided to depth below the CCL is an unexpected finding.

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