Abstract: A Welsh Meguma? Comparison of the Meguma Supergroup of Nova Scotia with the Harlech Dome succession of North Wales
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The Meguma Supergroup of southern mainland Nova Scotia is an Early Cambrian to Early Ordovician deep-water peri-Gondwanan succession. The base is not seen, but isotopic and geochronologic evidence suggest broadly ‘Avalonian’ basement. The lower part of the succession (Goldenville Group) comprises relatively quartz-rich, thickly bedded turbidites and interbedded green slates. Abundant turbidite structures indicate sedimentation continuously below wavebase, and the great thickness of the succession attests to major subsidence. The trace fossil Oldhamia indicates Early Cambrian age. At the top of the group the stratigraphy is more diverse; alternating finer and coarser units show enrichment in manganese and spectacular bioturbation. A rare trilobite-bearing horizon indicates Middle Cambrian age. A unit of manganiferous laminated slate marks the transition into the overlying Halifax Group. The overlying Cunard Formation comprises rustyweathering black pyrite-rich slate, siltstone, and sandstone with turbidite structures. Higher parts of the group possibly show a transition to shallower water conditions, with more abundant trace fossils, and rare Tremadocian graptoloids. The group is overlain with angular unconformity by the Early Silurian White Rock Formation. The succession in the Harlech Dome in North Wales is also an Early Cambrian to Early Ordovician deep-water peri-Gondwanan succession. Underlying basement is not seen but drilling has revealed ‘Avalonian’ volcanics in the subsurface. The lowest part of the Harlech Grits Group comprises Early Cambrian cross-bedded, probably deltaic sandstones which pass up into fully marine slates. The overlying Rhinog Formation consists of relatively quartzose thickly bedded turbidites and green slates. Abundant turbidite structures indicate sedimentation continuously below wavebase, suggesting significant subsidence. At the top of the group the stratigraphy is more diverse; alternating finer and coarser units show enrichment in manganese and spectacular bioturbation. Rare trilobite fragments indicate Middle Cambrian age. A unit of carbonaceous laminated slate marks the base of the overlying Mawddach Group. The overlying Maentwrog Formation comprises rusty-weathering black pyrite-rich slate, siltstone, and sandstone with turbidite structures. Trilobites indicate Late Cambrian age. Higher parts of the group show a clear transition to shallower water conditions above wavebase, with linguloid brachiopods and trilobites indicating Tremadocian age. The group is unconformably overlain by the Early Ordovician Rhobell and Aran volcanic groups. Lithological and chronological analogies between the two successions are striking. However, significant differences include: (1) the greater thickness of the Meguma succession; (2) the presence of shallow marine to non-marine strata at the base and top of the Harlech succession; (3) the possible presence of an unconformity between the two groups in Harlech; and (4) the presence of volcanics near the base and top of the Welsh succession. Nonetheless, the similarities indicate closely similar evolution for the two basins, and would be consistent with deposition in connected parts of a peri-Gondwanan rift system.Keywords:
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ABSTRACT The sedimentary succession in the Pensacola Mountains, Antarctica, which is more than 9000 m thick, ranges in age from late Precambrian to Permian. The Patuxent Formation of late Precambrian age is a thick sequence of weakly metamorphosed eugeosynclinal slates and subgraywackes. The Wiens Formation of Cambrian(?) age contains subgraywackes, which are rich in volvanic detritus; this detritus was derived from nearby Cambrian volcanic rocks. The overlying Neptune Group and Dover Sandstone of early and middle(?) Paleozoic age consist mostly of quartzite deposited on a slowly subsiding epicontinental shelf; upward decrease in amount of volcanic rock fragments indicates a shift in time from local provenance to a sediment source in the East Antartic Shield. The Gale Mudstone of Permian age i a tillite, which has the composition of a subgraywacke. The youngest rocks in the succession comprise the Pecora Formation (new name) of Permian age and consist of graywackes and carbonaceous siltstones.
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The allochthonous Cow Head Group of Cambrian-Ordovician age is a well-exposed but laterally discontinuous succession of continental-margin deposits. Spectacular megabreccias that punctuate the succession at discrete levels can now be correlated throughout the proximal Cow Head region to the more distal Humber Arm area, and tentatively into the metamorphic terrane of the Fleur-de-Lys, far to the east. The oldest breccia is of latest Early Cambrian age (Irishtown Formation). It is composed of shallow-water limestone and sandstone (Labrador Group) as well as Precambrian basement clasts in a quartz-sand matrix. Deposition coincides with progradation of supermature quartz arenites out to and over the edge of the shelf margin (Hawke Bay Formation). The main pulses of megabreccia deposition after the Early Cambrian produced carbonate-rich units. They occurred in the medial Middle Cambrian, near the transition between Cambrian and Ordovician time, and at the transition between the Early and Middle Ordovician. These breccias contain a mixture of clasts, predominantly from the shelf margin and upper part of the slope. The youngest carbonate megabreccia, lying above the Middle Ordovician Table Head Formation, is autochthonous; it records a massive rearrangement of the continental margin during ophiolite obduction. Normal platform-margin sediments in this succession, periplatform ooze (ribbon limestone), carbonate turbidites, breccia deposits (of ribbon-limestone clasts and some platform-margin clasts), and shales coincide with flooding of the platform. Megabreccias coincide with major changes in shelf sedimentation or regression, now apparent as formation boundaries. End_of_Article - Last_Page 474------------
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this resource is available as a downloadable pdf file a web map service and as an esri service the resource includes a bedrock map tectonic map cross section and a text on the geology of the bennington area vermont abstract the bed rock geology of the bennington area consists of an unknown thickness of pre cambrian gneisses and schists overlain by approximately 10 000 feet of younger sediments these range in age from lower cambrian to middle or upper ordovician the basal cambrian is composed of arkose quartzite and phyllite some of which was locally derived from the underlying gneisse overlying this basal section is a thick sequence of lower and upper cambrian dolomits and interbedded quartzites the non carbonate portion of these rocks appears to have been derived from a westerly source probably the pre cambrian craton a marked change in lithology occurs along the cambrian ordovician contact the lower ordovician sediments are dominantly calcitic limestones and marbles with interbedded calcitic dolomites overlying these limestones witha profound regional unconformity are middle and upper ordovician phyllites and shists containing lens like masses of limestone in their lower part no rock younger than the ordovician has been recognized in the area
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Abstract The classic Cambrian succession of SW Wales comprises a succession of clastic rocks some 1250 m thick that make up part of the Dyfed Supergroup. The existing lithostratigraphical framework owes much to the Nineteenth Century researchers, with relatively little detailed work having been published since. We here present a detailed, rationalized and revised lithostratigraphy for the Cambrian part of the Dyfed Supergroup of the area. Where possible and appropriate, we have retained original and well-known names for formations and other units in harmony with current usage. However, the recognition of four lithostratigraphical groups is overly complex and sometimes unworkable, and a two-fold subdivision is proposed instead. The Caerfai and much of the Lower and Middle Solva groups of earlier usage comprise conglomerates, pebbly sandstones and brightly coloured fine- to coarse-grained sandstones: all are here combined into a revised Caerfai Group. By excluding the silt-dominated Upper Solva division, the Caerfai Group is readily divided into lower and upper parts comprising conglomerates and/or pebbly sandstones, with sandstones and siltstones dominant in between. These rocks comprise the Ogof Golchfa (new), St Non's, Caerfai Bay, Caer Bwdy Bay, Newgale (new) and Porth Clais (new) formations. The following members (all new) are recognized within the Newgale Formation: the Trwyncynddeiriog, Pen-y-Cyfrwy and Cwm Mawr members. Ichnofossils from the upper part of the Ogof Golchfa Formation suggest that Cambrian deposition in the area began in Unnamed Cambrian Series 2 (C2) times. Body fossils and radiometric dating show that the Caerfai Bay to Porth Clais formations span the interval from Cambrian Series 2, Unnamed Cambrian Stage 3 (such stages are denoted by CS3, CS4 etc. below), probably to the Ptychagnostus gibbus Biozone (CS5). The former Upper Solva Group, Menevian Group and Lingula Flags comprise the newly erected Porth-y-rhaw Group, a 687 m-thick unit of fine sandstones, siltstones and mudstones, within which a further five formations are recognized. These are the Whitesands Bay, Menevia, Aber Llong (all new), Ogof Velvet (revised) and Treffgarne Bridge formations. The Porth-y-rhaw Group spans the Tomagnostus fissus Biozone (C3, Drumian) to the Olenus cataractes Biosubzone (Furongian, Paibian). The overall two-fold subdivision of the Cambrian part of the Dyfed Supergroup in the area accords broadly with the first lithostratigraphical scheme proposed for the area, and is comparable with the separation of the Harlech Grits and Mawddach groups in northern and central Wales. The oldest part of the Caerfai Group formed in a tectonically active context and records a transition from alluvial fan deposits, through braided stream environments, into transgressive nearshore marine sandstones. These sediments were derived from the west, and as well as locally-sourced material, include lithic clasts and grains derived from a presumed southwestwards extension of the Monian Composite Terrane. Above lie sedimentary deposits formed under a wide range of conditions, ranging from tidally-influenced to turbidites. Further evidence of active tectonism is found in the easterly-derived fan-delta sedimentary deposits of the higher part of the Caerfai Group and lower part of the Porth-y-rhaw Group. Deposition of the latter began with fine-grained turbidites deposited in a mid–outer shelf setting. These pass up first into hemipelagites, then into sedimentary rocks deposited on a storm-dominated shelf, and finally a very extensive shallow subaqueous delta platform formed in a passive margin setting.
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Research Article| August 01, 1982 The late Proterozoic Tindir Group, east-central Alaska: Evolution of a continental margin GRANT M. YOUNG GRANT M. YOUNG 1Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information GRANT M. YOUNG 1Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1982) 93 (8): 759–783. https://doi.org/10.1130/0016-7606(1982)93<759:TLPTGE>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation GRANT M. YOUNG; The late Proterozoic Tindir Group, east-central Alaska: Evolution of a continental margin. GSA Bulletin 1982;; 93 (8): 759–783. doi: https://doi.org/10.1130/0016-7606(1982)93<759:TLPTGE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The lower Tindir Group in east-central Alaska is composed of six formations with a cumulative thickness of as much as 2,000 m. These rocks are mainly carbonates and quartz sandstones of shallow-marine origin; a black shale unit is a deeper water deposit. They are similar to the lower part of the Mackenzie Mountains supergroup in the northern Canadian Cordillera. An unconformity between the two parts of the Tindir succession is indicated by erosion of the lower Tindir during deposition of the upper.The upper Tindir has a maximum thickness of ∼ 2,000 m; it can be subdivided into five formations which thicken to the west. Paleocurrent data suggest westerly transportation. Tholeiitic mafic pillow lavas and volcaniclastic rocks make up unit 1. Unit 2, an assemblage of purple hematitic mud-stones, includes turbidites and purple diamictite beds; the latter are coarser and thicker near the top. The stratified diamictites are interpreted as mass-flow deposits, but striations on some lonestones indicate a glacial provenance for some of the clasts. Unit 3 comprises crudely stratified purple diamictites of glaciomarine origin. This facies is only locally preserved and is considered to be allochthonous, having slid westward in response to contemporaneous fault activity. The iron-rich nature of the two glacially influenced formations, units 2 and 3, and local development of iron-formation in unit 3 are interpreted as being due to hydrothermal activity at a volcanic center somewhere to the west.Overlying units display rapid lateral facies variations, and so both local and regional correlations are tentative. In western parts of the study area, unit 4 consists mostly of shales and turbidites; to the east, it is represented by thin platformal dolo-stones. It includes some thin diamictites with granitic and other exotic clasts. Unit 5 consists mainly of basinal shales, resedimented carbonates, and siliclastic turbidites which pass rapidly eastward into rocks of shallow-water aspect. These include pebble and cobble conglomerates with carbonate, quartzite, and chert clasts probably derived from the lower Tindir, and shallow-water sandstones and shales. A break in sedimentation is proposed between unit 5 and overlying Cambrian carbonates. Rocks of the upper Tindir resemble those of the Rapitan Group and succeeding upper Proterozoic rocks of the Mackenzie Mountains area and may be the northwestern termination of a late Proterozoic-early Phanerozoic continental margin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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The uppermost Precambrian(?)–Lower Cambrian of the Avalon Zone in the northern Antigonish Highlands is composed of two dissimilar sequences in thrust contact. These include the sandstones and slates of the Doctor's Brook allochthon and the volcanoclastic-rich Malignant Cove authochthon. Lithostratigraphy of the “Black John Formation” (designation abandoned) in the Doctor's Brook allochthon is comparable to the uppermost Precambrian–Lower Cambrian in eastern Placentia Bay, southeastern Newfoundland, and Cape Breton Island. A unified stratigraphic nomenclature is appropriate in these Avalonian areas. The lower part of the “Black John” is an unconformity-bounded depositional sequence with subaerial rift facies (Rencontre Formation, 178+ m), overlying marine siliciclastic mudstones and fine sandstones (Chapel Island Formation, 59 m), and a quartzite cap (Random Formation, 2.05 m). The Chapel Island Formation has the oldest faunas from mainland Nova Scotia ( Watsonella crosbyi Zone, lower Placentian Series). A post-Random unconformity known in Newfoundland and Cape Breton Island lies at the Random-Bonavista Group (Cuslett Formation) contact in the upper “Black John Formation.” Fossiliferous Lower Cambrian limestones and siliciclastic mudstones previously reported from the Malignant Cove autochthon are actually clasts in basalt pebble-dominated slope deposits of the Arbuckle Brook Formation. These clasts were eroded from shallow-marine facies comparable to those in the Doctor's Brook allochthon during local uplift associated with Middle Cambrian(?) extension and mafic volcanism. Eight species are illustrated from the Placentian and Branchian Series. Anabaritellus Missarzhevsky, 1974, emend. (= Selindeochrea Valkov, 1982) is a Lower Cambrian calcareous tube-dwelling metazoan(?) known from tri- through multisulcate conchs that are morphologically intermediate between Anabarites and Coleoloides .
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Research Article| June 01, 2008 Stratigraphy and facies of Cambrian to Lower Ordovician strata in Saskatchewan James Dixon James Dixon Geological Survey of Canada 3303 – 33 Street NW Calgary, AB T2L 2A7 Search for other works by this author on: GSW Google Scholar Bulletin of Canadian Petroleum Geology (2008) 56 (2): 93–117. https://doi.org/10.2113/gscpgbull.56.2.93 Article history received: 17 Dec 2007 accepted: 06 May 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation James Dixon; Stratigraphy and facies of Cambrian to Lower Ordovician strata in Saskatchewan. Bulletin of Canadian Petroleum Geology 2008;; 56 (2): 93–117. doi: https://doi.org/10.2113/gscpgbull.56.2.93 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract Cambrian to Lower Ordovician strata in Saskatchewan are present only in the subsurface and are mostly siliciclastic. They are divided into three units which, in ascending order, are the informally named Basal Sandstone Unit overlain by the Earlie Formation and Deadwood Formation. Strata of the Basal Sandstone Unit and Earlie Formation are Middle Cambrian and those of the Deadwood Formation range from the Upper Cambrian to Lower Ordovician. Sedimentary facies indicate that all units were deposited in environments ranging from shoreface to shallow-water shelf.Two major transgressive-regressive sequences are identified in these strata; the older is comprised of the Basal Sandstone Unit and most of the Earlie Formation, and the younger one is mostly the Deadwood Formation. The boundary between the two sequences shows very little evidence of erosion but the younger sequence overlaps the older sequence in an easterly direction. The upper boundary of the younger sequence is a major erosional surface. Facies trends in the sequences indicate an eastern to northeastern source of the siliciclastic sediment. The Saskatchewan succession is comparable to similar age strata in north-central and western USA, and the Interior Plains of Western Canada and the Northwest Territories.Many sandstone beds in the succession are porous and would be suitable petroleum reservoirs but the source rock potential of the associated shale appears poor. However, the reported presence of Gloeocapsomorpha sp., a fossil with known hydrocarbon-producing potential, could be a positive indicator if it can be found concentrated in organic-rich laminae or thin beds. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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New mapping has resulted in a detailed and revised stratigraphic subdivision of the late Neoproterozoic (Ediacaran Period) marine sedimentary succession in the eastern Bonavista Peninsula. The oldest exposed unit, assigned to the Drook Formation of the Conception Group, is the Shepherd Point member, characterized by mainly parallel-laminated, siliceous siltstones, mudstones and fine-grained sandstones. The overlying, fossiliferous Mistaken Point Formation (upper Conception Group) is divided into a lower, siliceous and variegated tuffaceous, silt- to fine-sand-rich Goodland Point member and an upper, grey to red, argillaceous Murphy’s Cove member. The conformably overlying Trepassey Formation (lower St. John’s Group) is divided into a lower, grey-green argillaceous Catalina member and an upper, sand-rich Port Union member. The upper Trepassey Formation rocks are overlain conformably by thin- to medium-bedded, dark grey shales and disrupted or slumped units of sandstone and shales of the lower Fermeuse Formation, designated Back Cove member. Laminae and thin beds of ash tuffs are a prolific and diagnostic part of the rock record of the eastern Bonavista Peninsula. The tuffaceous volcanicity is recorded at multiple stratigraphic levels throughout the marine section; fossiliferous beds are everywhere associated with thin tuff units. The known lateral and vertical extent of Ediacaran biota in the eastern Bonavista Peninsula has been extended significantly. More than twenty separate fossil localities from multiple stratigraphic levels throughout the late Neoproterozoic marine succession have been identified. The vertical range of Charnia-related discs and associated frond-like forms, spindleshaped fossils, bush-like fossils and radial forms in the Bonavista Peninsula succession is now known to extend throughout the Mistaken Point Formation, upward through the Trepassey Formation and into the Fermeuse Formation. Morphological variants of Aspidella occur throughout the same stratigraphic range, but extend higher into the upper St. John’s Group than the other Ediacaran forms. New mapping of the upper Rocky Harbour and overlying Crown Hill formations of the Musgravetown Group in the western Bonavista Peninsula has established locally developed to regionally extensive facies subdivisions of potential member status within the thick, shallow-marine to terrestrial succession. Some units may correlate with significant parts of the Signal Hill Group, 150 km farther east, thus highlighting the large dimensions of the late Neoproterozoic basin. The basin fill is characterized by pronounced east-to-west facies variations related in part to syn-sedimentary faulting. The development of laterally consistent, reduced, sulphide-bearing (and variably cupriferous) grey beds within the thick redbed succession was an important aspect of basin evolution.
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Abstract The Talladega slate belt, the frontal crystalline thrust sheet of the Blue Ridge allochthon in Alabama, contains a Silurian(?)- Lower Devonian stratigraphic sequence not preserved elsewhere in the region that has important bearing on the tectonic evolution of the southern Appalachian orogen. A metamorphosed upper Precambrian(?) to Lower Cambrian clastic and Lower Cambrian to Lower Ordovician carbonate sequence in the lower part of the Talladega belt represents the Appalachian miogeocline (western margin of Iapetus), which progressed from a rifted basin into a stable carbonate trailing margin platform. A second major metclastic sequence of Silurian(?)-Early Devonian age (Talladega Group) developed as a successor basin unconformably atop the carbonate shelf facies following a period of uplift, tilting, erosion and subsidence of the Cambro-Ordovician carbonate platform. The unconformity at the base of the sequence bevels through the older rocks at a low angle, regionally cutting out the entire 3.5 km thick carbonate bank facies, and locally placing the Talladega Group directly above the lower clastic sequence. Significant deformation predating the unconformity is not apparent. The base of the successor basin is represented by a thick (2-3 km) submarine fan-like debris apron containing a proximal olistostromal facies of bouldery mudstones which grades distally into and is intercalated with turbidite-like sequences and various deep water sandstone facies. Clasts within the olistostromes represent the various clastic and carbonate units below the basal unconformity, as well as a suite of Precambrian basement lithologies which include granite and felsic gneiss, probably derived from the crystalline basement of the miogeocline. Overlying the fan system is a shallow marine to lagoonal sequence of sparsely fossiliferous Lower Devonian metasandstone and conglomerate, black slate, metachert and siliceous argillite. These rocks are succeeded by a thick (~2-5 km) sequence of bimodal metavolcanic rocks. The Silurian(?)-Lower Devonian successor basin and associated rocks are interpreted to have formed as a result of back- arc extension during the onset of Acadian subduction beneath the southeastern margin of Laurentia.
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