Many submarine canyons and channels have been described in modern seas, but little conclusive evidence about their existence has been found in ancient rock series. Like the modern submarine canyons, the ancient counterparts were situated on unstable edges of continents. During their further geologic history, they usually underwent extensive tectonic and erosional destruction or were buried below younger sediments.
The western part of the Carpathian Flysch Belt in the territory of Czechoslovakia in central Europe is one of the convenient places where the critical zone between the platform and the former geosynclinal trough can be studied. Among the most interesting contributions of this investigation is the discovery of two large buried depressions described as Nesvacilka (N) and Vranovice (V) grabens (fig. 1). Their existence was proved both by geophysical measurements and drilling operations. The depressions, traditionally regarded as tectonic structures, however, show many similarities with modern submarine canyons.
The depressions are cut in Paleozoic and Mesozoic carbonate rocks covering the crystalline complexes of the Bohemian Massif. They are filled with Eocene and Oligocene deposits and overlain by Neogene sequences of the Carpathian foredeep. These autochthonous formations deposited on marginal sectors of the platform dip below the Carpathian flysch nappes that comprise Cretaceous and Paleogene miogeosynclinal scries (fig. 1). The longitudinal axes of the depressions are oriented in a northwest-southeast direction perpendicular to the margin of the platform. The canyons probably join each other farther dovvndip to form one channel system not unlike the Scripps and La Jolla Canyons along the coast of California. At their upper ends, the depressions are surrounded by steep, high walls resembling the heads of modern canyons. The thickness of the sedimentary fill near the head of the Nesvacilka Depression exceeds one thousand meters. Even though this thickness does not wholly correspond to the original depth, it indicates the rugged relief of the structure. Downward the canyons become shallower and their walls less steep. The measured width of the depressions varies from about 2 km at their heads to as much as 7 km in their distal parts. Both structures have been followed for a distance of about 25 km, their further courses being hidden beneath 4- to 7-km-thick flvsch and molasse sequences.
The Eocene and Oligocene sediments filling the depressions are composed predominantly of dark-brown calcareous silty shales rich in organic matter. They contain abundant planktonic microfauna, proving the marine origin of these sediments. The shales are intercalated with laminae and thin beds of siltstones and fine sandstones. The lamination is a common structure present elsewhere. Scour-and-fill structures of small size found in many sandstone beds indicate the activity of strong, erosional, bottom-seeking currents sweeping through the canyons. No turbidites showing the characteristic succession of internal structures have been identified, but, because of the small number of drill cores available, their presence cannot be excluded. Thick beds of massive sandstones and boulder conglomerates were found at the bottom of the Nesvacilka Depression.
Though the oldest deposits found inside the canyons are of late Eocene age, such huge structures should have originated much earlier. Redeposition of Cretaceous and Paleocene micro fauna indicates that the canyons could have been active during the entire time of existence of the flysch miogeosyncline from Cretaceous to Oligocene. The early stage of canyon development was characterized by erosion and transportation of material but was followed during Oligocene time by sedimentation inside the canyons, which brought about the end of their development. The origin of submarine canyons has not been explained satisfactorily, though several hypotheses have been submitted. The Carpathian canyons, oriented parallel to the fault system of the platform, are believed to be of combined tectonic and erosional origin. They intersected the shelf and entered the former geosynclinal flysch trough, which, considering the morphology of the canyons, must have been at least one thousand meters deep.
There is an open question what role the canyons played in geosynclinal sedimentation. According to the commonly accepted paleogeographical concept, the flysch trough of the Western Carpathians was supplied predominantly from internal sources (cordilleras), while the platform yielded mostly only fine pelitic material. The distribution of sandy and shaly facies and the composition of clastics apparently support this hypothesis. Modern oceanographic explorations show, however, that canyon-derived sediments can be transported long- distances before finally being deposited. In the San Diego Trough, for example, the coarser sediments accumulate at the distant oceanic side of the basin far from the mouth of supplying canyons along at the coast of California (Shepard, Dill, and Rad, 1969). Also, in the Carpathian Trough the sandy facies were not necessarily related to some nearby sources such as cordilleras. The potential existence of large submarine canyons at the continental side of the Carpathian flysch trough, serving as conduits for flysch sediments, therefore must be taken into consideration in any paleogeographical reconstruction.