The Ordovician marginal basin of Wales
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Summary The Lower Palaeozoic Welsh Basin was founded on immature continental crust. During late Precambrian-early Cambrian times, volcanism and sedimentation were influenced by NE-SW-trending faults which defined the NW and SE margins of the basin. During the Cambrian, marine sediments infilled a graben and at the end of the Tremadoc widespread tectonism was associated with an island-arc volcanic episode. In the Ordovician this subduction-related activity was succeeded by mainly tholeiitic volcanism related to back-arc extension, with the locus of arc volcanism sited further N, in the Lake District—Leinster Zone of the Caledonides. In Wales, the Ordovician volcanic activity shifted in time and space. In S Wales volcanism persisted from the middle Arenig through the Llanvirn. In N Wales the volcanism can be broadly divided into dominantly pre-Caradoc activity in southern Snowdonia and an intra-Caradoc episode in central and northern Snowdonia. In eastern Wales, including the Welsh Borderland, and in Llŷn, both episodes are represented. In all areas faults greatly influenced both volcanism and sedimentation. Intrusive activity was dominated by high-level emplacement of sills. Granite ( s.l. ) stocks are restricted to central and northern Snowdonia and Llŷn and many were coeval with extrusive volcanism. Volcanism in the basin was essentially bimodal with voluminous eruptions of tholeiitic basalts with ocean-floor affinities, and of rhyolites. Minor volumes of andesite to rhyodacite resulted from low-pressure fractional crystallization of the tholeiitic basalts. Available evidence suggests that the rhyolites resulted mainly from crustal fusion, although in some instances evolution by crystal fractionation from intermediate magma has been proposed. Calc-alkaline assemblages are petrographically distinct, of minor occurence and, contrary to previous conclusions, are relatively insignificant in the characterization of the tectonic environment of the basin. Throughout the basin, volcanism was generally succeeded by deposition of black muds and then turbidite-dominated sequences.Ordovician carbonates of the Nashville dome and Ozark dome regions have long been considered typical shelf deposits. In the subsurface to the south, in the Black Warrior basin, Mississippi Embayment, and Arkoma basin, these shelf carbonate units changed facies. The most significant change is the occurrence of a thick limestone unit characterized by a faunal and floral assemblage of Nuia, Girvanella (isolated long strands), Sphaerocodium, a delicate stacked-chambered organism (.algal), and sponge spicules and sponge mudstone clumps. In ascending order, the complete Ordovician sequence consists of: a lower dolostone, the Nuia-sponge limestone, a dolostone, and a limestone. The upper part of this four-fold sequence changes character westward into the Arkoma basin. The lower two units maintain their character for long distances along depositional strike and occur in parts of the Appalachians as far north as Newfoundland, and on the opposite side of the continent in Nevada. The Nuia-sponge assemblage is a distinctive petrographic marker and seems to be a persistent Ordovician rock and fossil assemblage of widespread occurrence. In Nevada, it occurs on the surface where it is associated with slump and slide features that suggest that it is an outer shelf or upper slope deposit. Coeval carbonates in themore » Ouachita Mountains are different and show indications of being deep water (basinal). Biostratigraphic evidence indicates that the succession in the subsurface is continuous; the regional Lower Ordovician-Middle Ordovician unconformity is absent. The Lower Ordovician-Middle Ordovician boundary falls near the top of the Nuia-sponge mudstone unit and not at the top of the underlying thick dolostone unit.« less
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