ABSTRACT

The Oberalm Beds, an Upper Jurassic - Lower Cretaceous (Tithonian-Berriasian) sequence of limestones in the Northern Limestone Alps of Austria and Germany, are representative of a widely distributed Tethyan lithofacies. In the Unken syncline of Austria, this formation consists largely of pelagic limestones composed of planktonic micro-fossils (Radiolaria, calpionellids), sponge spicules, and micrite; electron microscopy shows the latter to contain abundant nannoplanktonic fossils, many partly recrystallized. Most of these pelagic limestones formed through particle-by-particle deposition. They alternate regularly with thin calcareous shales in a manner suggesting cyclic sedimentation. However, in detail, the sedimentation pattern was complicated by the action of bottom currents, which resulted in some redeposition of pelagic deposits and which produced allochthonous rocks of two kinds: allodapic limestones and limestone breccias. The former are turbidites composed of pellets and shallow-water skeletal fragments; flute casts and composition indicate a northern source on a shallow shelf. Beds of limestone breccia, consisting largely of allochthonous limestone clasts, are interpreted as turbidites derived from a southern shelf. For the Tithonian part of the formation, the proportions of pelagic and allochthonous rocks are estimated as 87 and 13 per cent, respectively. Aragonitic shells of ammonites in this formation were destroyed by solution, possibly the result of great depth of water. Calcitic aptychi of ammonites, in contrast, are well preserved and common, especially along soles of redeposited beds where they were concentrated by currents.

The basin in which these sediments were deposited is interpreted as an elongate, east-west trending trough. This trough was one of a number of such linear basins which formed the Alpine geosyncline in this part of the Alps during Late Jurassic to Early Cretaceous time, and which were separated by submarine ridges. The Unken trough was bordered on both northern and southern sides by shallow-water banks and perhaps attained abyssal depths in its central part. The geometry of this basin, as inferred from orientations of flute casts and slump folds, resembled that of the present Unken syncline, which, however, represents only an erosional remnant of the trough.

Estimates of sedimentation rates for the pelagic deposits of the Oberalm Beds are comparable with, or slightly higher than, those for calcareous oozes of present deep-sea basins. This suggests that biogenic calcium carbonate was delivered to deep-sea sediments in significant amounts via the nannoplankton at least as early as Tithonian time.