Deep-Water Limestones and Radiolarites of the Alpine Jurassic
The Northern Limestone Alps contain essentially continuous sedimentary sequences extending through the Triassic and Jurassic into the Neocomian, and leading from rocks of shallow-water origin to rocks of deep-water aspect. The section here studied is that of the Unken syncline, near the western border of the Land Salzburg in north-central Austria. Here shallow-water reef complexes of Rhaetian age gave way to Early Jurassic sedimentation of red carbonates with mixed benthonic-pelagic faunas, these to Mid-Jurassic red, manganiferous coccolith-rich ooze, and these to Mid- to Late Jurassic radiolarian oozes. These were again succeeded by red coccolith-rich and locally manganiferous carbonates, and then, in Tithonian time, by drab, radiolarian-rich, coccolith and coccolith-calpionellid oozes, interspersed with turbidite calcarenites of shallow-water derivation. Slump structures occur throughout, and the constituents of interspersed breccias become more exotic upward through the section. Thickness-time relationships of these sediments, as well as mineral content and incidence of cosmic spherules, indicate very low rates of deposition.
Sedimentological and paleontological criteria indicate a progressive starvation and deepening of the basin, at least into late Middle Jurassic time. Of two quantitative models of bathyal and tectonic development, one is based on present-day “compensation depths” of aragonite and calcite in oceanic sediments, the other on a projection of subsidence rates established for the Triassic. Abyssal depths are reached in both models. While neither model is accepted at face value, the development of depths in excess of 3,000 to 4,000 meters seems very likely. From the tectonic viewpoint, this was a starved foredeep formed in early stages of orogeny, on what had previously been a continental rather than oceanic area.
Figures & Tables
Depositional Environments in Carbonate Rocks
One of the principal tasks of the geologist is to determine the depositional environments in which rocks are deposited. Although regional environmental interpretations of transgressions and regressions, movements of shoreline, and gross aspects of continental and marine sedimentation have been understood since stratigraphy became an established branch of geology, only recently has the science of sedimentology come up with criteria for environmental recognition of specific outcrops, wells, or even hand samples. This observation is especially true of carbonate rocks. The papers in this volume will provide a key to the subject of recognition of depositional environments in carbonate rocks. Based on a symposium held in Los Angeles, California, on April 1967, at the joint meeting of AAPG and SEPM.