Sequence stratigraphy, tectonostratigraphy and basin analysis

摘要: Abstract The Dyfed Supergroup of SW Wales ranges from Terreneuvian to Furongian in age and represents a prolonged nearly continuous phase siliciclastic sedimentation on the southern margin Cambrian Welsh Basin. lower 564 m are included an extended Caerfai Group (six formations), embracing strata previously assigned Solva Group. Predominantly arenaceous, has important units conglomerate pebbly sandstone at its base, middle top. is overlain by 687 mainly argillaceous sedimentary deposits Porth-y-rhaw (five newly erected unit that includes ‘Menevian Group’ ‘Lingula Flags’. This two-fold division comparable with distinction laterally equivalent Harlech Grits Mawddach groups exposed around Dome North Wales. High resolution sequence-stratigraphical techniques, constrained biostratigraphical data wherever possible, applied across Britain: revised lithostratigraphy thereby integrated slightly modified version Avalonian chronostratigraphy initially developed for western successions maritime Canada. Sequences 1 2 not recognized supergroup Wales, reflecting inner platform setting area. Those sequences represented however, which was situated outer platform. relative ease Britain can be incorporated into general framework reflects shared epeirogenic history, sediment sources accumulation history microcontinent unified early times. gross lithological similarities exist between Avalonia, particular, better understood when basin placed within sequence stratigraphic defined systems tracts. particularly evident during Unnamed Series 3 (C3) times, clear links demonstrated paralic depositional environments deeper turbidites 3–7 (Terreneuvian–C3) Basin hinterland were dominated deposition epeirogenically active An extensional rift-like tectonic regime proposed, where elongate basins mudstone have time equivalents formed areas apparent uplift probably tilt-block highs. Local development regional unconformities, extent marine onlap, secular succession lithofacies controlled principally phases fault-accommodated subsidence along NE-trending lineaments bounded these their intervening horsts or platforms. In Britain, movements first Menai Straits Fault System then Borderlands System, both terrane boundaries, responsible dominantly coarse-grained arenaceous pre-Drumian Basin, this highlighted change source Monian Superterrane Wrekin Terrane. A rapid switch marginal accompanied major reorganization boundary 4. Although role variations eustatic sea level as control architecture been dismissed previously, prominent lowstands such those recorded basal Ptychagnostus gibbus Biozone, ‘ Solenopleura ’ brachymetopa Biozone (mid- Paradoxides forchhammeri Biosuperzone), upper part Biosuperzone also influenced stratigraphy. broad subdivision stratigraphy tectonically passive allows two megasequences distinguished. Megasequence (sequences 1–7) transform termination subduction following oblique convergence, accretion island arcs onto northern periphery Gondwana. Under transpressional regimes, late Precambrian arc-related inverted recycled pull-apart successor basins. Initially, margins alluvial fans fan-deltas built flood-generated dispersal processes. later stage Cadomian–Avalonian orogenesis, there supplied highlands east. Sequence 8, Pembrokeshire Aber Llong Ogof Velvet formations, lies base (late C3–early Ordovician). Excellent sections occur through formations interpretation significantly improves understanding younger Avalonia. These reflect culmination Avalonian–Cadomian orogenic cycle. Deposition occurred subaqueous delta mouth huge river system drained substantial West Gondwana, Avalonia acting sink vast quantities fine-grained sediment. Secular and/or supply, rather than tectonism, main mechanisms controlling time.