FACIES AND TIME IN DEVONIAN TROPICAL AREAS

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Abstract
Summary: Europe, North America and Australia are thought, largely on palaeo-magnetic evidence, to have been mostly within the tropics during Devonian times. The marine rocks of some of these areas are reviewed to elucidate the age, nature and extent of facies movements and transgressions of the period, especially in relation to cratonic centres. It is shown that a pattern of increasingly successful transgressions followed the late Silurian Caledonian orogeny and culminated in the transgressions of the late Devonian. Evidence for the international correlation of these movements is analyzed. Lower Devonian movements, broadly general in northern Europe, can tentatively be recognized in the Appalachian area, but evidence does not permit convincing correlation elsewhere. The early mid-Devonian transgression, which brought a substantial facies change in Western Europe, is the date given for the major transgression across the Russian platform; of about the same age are the extensive equivalents of the Onondaga Limestone in the eastern U.S.A. There is broad evidence of transgressional Middle Devonian in western U.S.A. and Canada, but it is not so precisely dated. The transgression close to the Middle–Upper Devonian boundary is the clearest dated internationally and the onset falls in the early lunulicosta Zone almost everywhere; this is the Frasnian transgression of Russia and the Taghanic onlap of the U.S.A. Local areas show slightly later, or earlier commencement. Late Frasnian and early Famennian times together covered a general period of deepening and this movement terminated reef and massive limestone deposition almost everywhere except in Australia. Certain orogenic effects (Acadian and Antler movements, etc.) change local regimes but do not mask entirely the evidence of global eustatic change. Of all the explanatory mechanisms, that of volumetric changes at spreading plate margins seems the simplest cause for large-scale, and perhaps many small-scale, facies change patterns.