Ternate Formation, a Late Eocene Bioclastic Submarine Fan of the Lombardian Basin (Southern Alps)

Daniel Bernoulli, Rene Herb, Angela Grunig

The Ternate Formation consists of some 150 m of redeposited bioclastic limestones, channelized rubble beds, and pebbly mudstones and subordinate hemipelagic marls with a late Eocene fauna of planktonic Foraminifera (Globorotalia cerroazulensis cerroazulensis Zone). Parallel-bedded massive or graded bioclastic limestones are overlain by channelized deposits. Large channels are up to 12 m deep; some contain pebbly mudstones with large slabs and boulders of older marls and limestones in a matrix of middle Eocene marls. More typically, the channels are filled by pebbly calcirudites grading into calcarenites and laterally wedging out over very short distances. Interchannel deposits are thin-bedded, graded calcarenites and thin green marls.

The bioclastic limestones contain mainly coralline algae, larger Foraminifera (nummulitids, Discocyclina), bryozoans, echinoderms, and other shallow-water biota. All lithic fragments are of local, south-Alpine origin. By far the most abundant are spiculitic limestones and marls of Liassic age and Upper Cretaceous to middle Eocene limestones and marls.

Most of the redeposited calcareous sand and gravel was deposited in ephemeral channels which cut across each other in a very complex way and were briefly active. This depositional system matches the suprafan environment of a prograding submarine fan of low transport efficiency. The facies of the conglomerates, the local source of the lithic fragments, and comparison with other lower Tertiary bioclastic deep-water deposits suggest short transport and deposition in one (or a series) of local bioclastic submarine fans. The benthic Foraminifera associated with the hemipelagic marls indicate deposition in the upper bathyal zone (600-1,000 m).

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.