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Global Devonian Geology
The State of the System: An Economic Survey of the Whole Devonian
The Devonian Period occupied the central slice of a 250 Ma interval during which more than a dozen narrow, intercratonic oceans were undergoing closure, while an enormous northern hemisphere ocean was being widened by marginal subduction. The period was preceded, interrupted, terminated and followed by major orogenic events of differing types and affecting different belts of rock. The Devonian was consequently a period of remarkable geological variety, with early and late Devonian geologies quite different over most areas of Devonian deposition.
The most important consequence from the viewpoint of the economic geologist was the division of Devonian rocks among three distinct tectofacies. In the Hercynian tectofacies the Devonian is essentially complete and continuous between the Silurian and the Carboniferous. In the Caledonian tectofacies some lower part of the system is missing below a sub-Devonian unconformity. In the Bretonian tectofacies the upper part of the system is missing above a post-Devonian unconformity (though the Bretonian must originally have been in one of the other tectofacies). The Hercynian tectofacies is confined to surprisingly narrow bands almost entirely restricted to the northern hemisphere. The other two tectofacies occupy huge areas within deformed belts and the whole of the Devonian coverage of the cratons.
The relations between the tectofacies and the stages of the tectonic cycle are obvious. Their effects on Devonian mineral deposits are striking. Paleozoic mineral deposits of the separation and early closure stages of the cycle are nearly all pre-Devonian. Syngenetic deposits (especially of massive sulphides) are restricted to the Hercynian tectofacies; they are much commoner in the pre-Devonian, and even somewhat commoner in the Lower Carboniferous, than they are in the Devonian. Epigenetic deposits of Devonian origin occur wherever Devonian felsic igneous rocks occur, but they are very likely to be in pre-Devonian host rocks. Deposits in Devonian host rocks are almost certain to be of post-Devonian origin; the largest are of Mesozoic or Tertiary origin. Devonian of the Bretonian tectofacies is especially unlikely to be host to large epigenetic deposits. Only a handful of important deposits, like Murdochville in Quebec, represent the combination of circumstances permitting an epigenetic metalliferous deposit of Devonian origin to be in a Devonian host rock.
Metallic deposits unrelated to the tectonic cycle, and nonmetallic and sedimentary deposits also possibly independent of it, illustrate the same apparent paradox. From Mississippi Valley-type lead-zinc to phosphorite deposits and from helium to building stone, the Devonian is endowed with less than its proper share of large deposits. Its compartmentalization by orogenies reduced it as a target for mineralization; it is the victim of its own variety.
The exceptions are potash and petroleum, both reflecting great marine transgression over previously segmented basements. The Devonian is strikingly oil prone because it is superlatively endowed with rich source sediments. Surprisingly, of all the Paleozoic producing areas of the world, only seven basins have more than 50% of their recoverable oil in Devonian reservoir rocks. Of the approximately 12 × 109 m3 of reserves in these seven basins, 75% is in the Ural-Volga and Alberta basins. Conventional natural gas is almost foreign to the Devonian, but the black shales that provide some of its oil source sediments also represent a large potential future supply of unconventional natural gas.
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