Alpine, Mediterranean, and Central Atlantic Mesozoic Facies in Relation to the Early Evolution of the Tethys
Daniel Bernoulli, Hugh C. Jenkyns, 1974. "Alpine, Mediterranean, and Central Atlantic Mesozoic Facies in Relation to the Early Evolution of the Tethys", Modern and Ancient Geosynclinal Sedimentation, R. H. Dott, Jr., Robert H. Shaver
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The main Alpine-Mediterranean Mesozoic lithofacies, excluding flysch, are outlined and placed in their paleogeographic setting within the broader context of an evolving ocean basin. In the external (ophiolite-free) zones of the Alpine-Mediterranean orogen, Mesozoic pelagic facies almost invariably overlie kilometers-thick successions of Bahamian-type platform carbonates. Wherever the basement of these platform carbonates is exposed, it is continental, comprising low- to high-grade metamorphic rocks and granites. We suggest that the pelagic sediments of these zones were deposited on a deeply submerged continental margin of the Atlantic type. Palinspastic reconstructions of the central Alpine-Mediterranean area place the depositional setting of most of these pelagic facies on the southern continental margin of the Tethys. In this area supply of clastics and organic matter was minimal, thus encouraging pelagic conditions.
The earliest pelagic sediments of the Alpine-Mediterranean region are of Triassic age and comprise gray and red limestones or cherts that commonly are associated with volcanics. These sediments were deposited in embayments and basins between extensive carbonate platforms and reefs. During the Liassic Epoch, a phase of block faulting, probably related to rifting in the oceanic Tethys, destroyed many of these shallow-water sites, and pelagic conditions became more widespread. During the Jurassic Period a basin-swell morphology was produced by irregular subsidence of the different blocks. On submarine highs, or seamounts, the following stratigraphically condensed facies were developed: pisolitic ironstones, red biomicrites containing ferro- maganese nodules and crusts, crinoid-pelagic bivalve-gastropod-ammonite biosparites, pelagic pelmicrites and micro-oncolitic sparites, and certain red, fine-grained, nodular limestones. In the neighboring basins, more expanded successions containing slumped blocks and turbidites accumulated; the basinal facies were developed as red, more clay-rich, nodular limestones, gray limestone-marl interbeds, radiolarian cherts, and white nannofossil limestones. The Cretaceous Period saw a smoothing of submarine topography and a general deepening of the water as the continental margin continued to founder. Deposition of varicolored marls and red and white coccolith limestones was widespread.
True ocean-floor lithofacies are represented by those rocks associated with, or lying upon, ophiolites. In the central Mediterranean area they comprise ophicalcites, umbers, radiolarites, white nannofossil limestones, and black shales. Their age is Jurassic and Cretaceous.
In the western central Atlantic pelagic facies of Late Jurassic and Cretaceous ages occur. These facies resemble both the continental margin and ocean-crust lithologies of the Alpine-Mediterranean Tethys. A section through the Mesozoic portion of this undeformed continental margin and ocean-basin complex comprising the Bahamas, the inter-platform straits, and oceanic realm illustrates a paleogeographic arrangement that strongly resembles the reconstructed section for the Alpine-Mediterranean Tethys. This resemblance illustrates the parallel evolution of these two now widely separated areas, so that they both can be considered as representatives of an east-west Mesozoic seaway, or Tethyan realm, that stretched from the Caribbean to Indonesia.
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The Kay Conference was held in Madison, Wisconsin, November 1972. This symposium volume contains the texts of papers presented at Madison. It is organized in a topical manner, and in most areas of discussion, modern analogues and ancient examples together provide a comparative basis for evaluating sedimentary models for geosynclines. In the 1970s students of both modern and ancient sediments have compiled an immense body of knowledge relevant to the geosynclinals concept. Moreover, the new theory of plate tectonics has required a complete reassessment of the geosynclines as well as orogenesis. The purpose of this volume is to evaluate by comparison of modern and ancient sediments a number of depositional models applicable to the great variety of strata seen in orogenic belts also called geosynclinal.