Abstract

Two large ancient submarine canyons buried below the Neogene foredeep and the Carpathian flysch belt are recognized within the ancient Tethyan margins. The features, as much as 10 km wide and 1,060 m deep, were cut into crystalline and sedimentary rocks of the Variscan platform and filled with late Eocene and early Oligocene sediments. The axes of both canyons follow the northwest-southeast structural trend of the platform. Farther downslope the two canyons probably join to form a single canyon-fan valley system.

The submarine valleys are located within a northwest-southeast-trending tectonic depression which is filled with autochthonous Tethyan strata of Mesozoic and Paleogene ages. In terms of plate tectonics, this long-lived depression on the borderland between the Alps and the Carpathians, extending from the Tethyan seaway far into the platform, has been interpreted as an aulacogen.

The sedimentary fill of the canyons is composed of silty mudstone in the upper part of the sequence, whereas sandstones and conglomerates are distributed mainly in the lower axial part of the valleys. Proximal and distal turbidites, grain-flow, and debris-flow deposits and pebbly mudstones have been recognized within the fill. The presence of pebbly mudstones, slump folds, and other mass-movement phenomena indicate relatively steep paleoslopes.

The foraminiferal thanatocoenoses comprise: (a) indigenous canyon dwellers, (b) synsedimentary Foraminifera rewashed from estuaries and open-shelf environments, and (c) faunas reworked from older sediments. On the basis of indigenous planktonics, the duration of the canyon filling is estimated at 10 to 12 m.y. The fossil record, however, represents only the final stage of the canyon development; the early erosional stages have left little evidence within the canyons.

The Tethyan canyons are considered to be submerged land valleys further developed by submarine processes. Their cutting and filling in late Eocene to early Oligocene time coincided with worldwide cooling of climate and possible eustatic sea-level changes, which also may have affected the canyon development. The Paleogene canyons also coincided with widespread accumulation of subsea-fan deposits in the Tethyan region (e.g., Annot Formation of the Alps, Krosno Formation of the Carpathians).

The presence of ancient submarine canyons within the Tethyan continental margins provided means for coarse sediment to bypass the shelf environment. This strongly supports the writer’s view that the tectonically fragmented marginal zone of the Variscan platform was much more important as a source of clastic materials than previously was believed.

The canyon fill comprises both organic-rich petroleum source rocks and sandstone traps such as the proximal turbidites and grain-flow deposits. The tectonically controlled canyons also may have served as migration paths from the mobile belt to potential reservoirs in the foreland.

Fossil submarine canyons can be identified elsewhere within the ancient continental margins; attention to the frontal zones of orogenic belts might thus prove fruitful in the search for petroleum.

First Page Preview

First page PDF preview
You do not currently have access to this article.