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- Knight, Ian
(författare)
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The Lower Cambrian Forteau Formation, southern Labrador and Great Northern Peninsula, western Newfoundland: Lithostratigraphy, trilobites, and depositional setting.
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The upper Lower Cambrian Forteau Formation in southern Labrador and the Great Northern Peninsula (GNP) is a successionof shale, limestone, siltstone and sandstone accommodated by rising sea levels during the early drift stages ofNewfoundland’s Laurentian passive margin. Its Dyeran trilobite fauna, characterized by Elliptocephala logani (Walcott, 1910)that ranges throughout the formation, indicates it was mostly deposited in the middle Bonnia‒Olenellus Zone. Its three lithostratigraphicdivisions, the Devils Cove member, Middle shale and Upper limestone, preserve a transgressive system tract(TST; Devils Cove and lower part of the Middle shale) and the early stages of a regressive high-stand system tract (HST; upper Middle shale and Upper limestone) that hosts a carbonate ramp shelf.A mudstone-dominated succession characterizes the TST, comprised of an inner belt of archeocyathid patch reef andcyclic, well-stratified, fine-grained mixed clastic and carbonate shelf rocks in southern Labrador. To the southeast on the GNP,the shale succession along with minor limestone and no reefs suggests a deeper water shelf basin in which shale accumulated across the GNP. Maximum flooding on the GNP is linked to dark shale midway through the basinal succession, and to athick shale bed that overlies the reefal strata in Labrador.Thin-bedded siltstone, sandstone and limestone in Labrador, and extensively bioturbated siltstone, sandstone and rare limestone on the GNP, support a shelf shallowing above storm-wave base as it prograded during the early stages of regression. Shallow-water carbonate of the Upper limestone supports a prograding shelf, at first dominated by an archeocyathid reefal tract and oolitic shoal complex. The reef tract and carbonate sand shoal complex prograded southeastward to justbeyond the northeast-trending Ten Mile Lake–Long Range fault system. Evidence of slumping in the underlying fine clastic sediment in the same area suggest that this fault zone may coincide with a hinge zone, beyond which the shelf steepened intomostly deep-water clastic sedimentation. The archeocyathid tract in southern Labrador is a broad biostromal complex confined within an erosional recess in the shelf. On the GNP, however, the tract is characterized by high-energy bioherms associated with crossbedded grainstone channels that can be traced for over 60 km along a northeast strike length. East of the reef tract‒shoal complex, the succession appears to be dominated by deeper water shelf mudrock, nodular carbonate and little evidence of shallow-water carbonate facies. The facies transition suggests the Forteau Formation insouthern Labrador and the GNP was laid down in a high-energy shallow-water, inner ramp setting that was up to 75 km wide. Above the carbonate sand shoal complex, the succession is marked by decametre-thick parasequences of intercalated carbonateand clastic intervals. The sequences support a shelf of fine grained to grainy carbonate deposited on an open shelframp overlain by intervals dominated by coarsening upward high-energy siliciclastics that suggest barrier complexes alongthe landward margin of the ramp. Thick units of crossbedded quartz arenite, in the upper half of the Upper limestone, suggest terrigenous sediments initially encroached along the inner part of the shelf, and eventually smothered the Forteau shelf leading to the low-stand deposits of the overlying Hawke Bay Formation. Trilobites recovered from this transition throughout the GNP, indicate that it occurred very late in the upper part of the Bonnia‒Olenellus Zone, likely between the Bristolia mohavensis Biozone and the top of the zone.
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| 2. |
- Skovsted, Christian, 1974-, et al.
(författare)
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Depth related brachiopod faunas from the lower Cambrian Forteau Formation of southern Labrador and western Newfoundland, Canada
- 2017
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Ingår i: Palaeontologia Electronica. - : Coquina Press. - 1935-3952 .- 1094-8074. ; 20:3, s. 1-52
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Tidskriftsartikel (refereegranskat)abstract
- A diverse fauna of organophosphatic brachiopods is described from the late early Cambrian (Series 2, Stage 3-4) Forteau Formation of southern Labrador and western Newfoundland. The total fauna includes 11 species representing a wide selection of Cambrian brachiopod groups. Three distinct assemblages are recognized: Assemblages1 and 2 are found in shallow water carbonates in association with archaeocyathans in southern Labrador and the western side of the Great Northern Peninsula of western Newfoundland. Assemblage 3 is found in a distal shelf setting of Gros Morne National Park. Assemblages 1 and 2 are found in stratigraphic continuity and definetwo brachiopod biozones, a lower Hadrotreta taconica zone and a higher Paterina zone, respectively. The presence of H. taconica in brachiopod Assemblage 3 possibly indicates time equivalence of Assemblage 1 but in a deep water setting. The identification of distinct time equivalent brachiopod Assemblages (1 and 3) in shallow and deep water environments of the Forteau Formation allow for the first time an analysis of environmental constraints determining the distribution of individual brachiopod taxa in the lower Cambrian succession of eastern Laurentia. Comparison to faunas from other areas indicates that the identified distributional patterns can be extended to other brachiopod faunas found along the Cambrian palaeocoast/margin of eastern Laurentia. The study indicates that specific brachiopod taxa can be used as indicators of palaeodepth; Botsfordia caelata in shallow environments and Eoobolus priscus and Eothele tubulus in deeper water. The following new taxa are described: Kyrshabaktella diabolan. sp., Pustulobolus triangulus n. gen et n. sp., Acrothyra bonnia n. sp.
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| 3. |
- Sperling, Erik Anders, et al.
(författare)
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On the edge of exceptional preservation: insights into the role of redox state in Burgess Shale-type taphonomic windows from the Mural Formation, Alberta, Canada
- 2018
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Ingår i: Emerging Topics In Life Sciences. - 2397-8554 .- 2397-8562. ; 2:2, s. 311-323
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Tidskriftsartikel (refereegranskat)abstract
- Animals originated in the Neoproterozoic and ‘exploded’ into the fossil record in the Cambrian. The Cambrian also represents a high point in the animal fossil record for the preservation of soft tissues that are normally degraded. Specifically, fossils from Burgess Shale-type (BST) preservational windows give paleontologists an unparalleled view into early animal evolution. Why this time interval hosts such exceptional preservation, and why this preservational window declines in the early Paleozoic, have been long-standing questions. Anoxic conditions have been hypothesized to play a role in BST preservation, but recent geochemical investigations of these deposits have reached contradictory results with respect to the redox state of overlying bottom waters. Here, we report a multi-proxy geochemical study of the Lower Cambrian Mural Formation, Alberta, Canada. At the type section, the Mural Formation preserves rare recalcitrant organic tissues in shales that were deposited near storm wave-base (a Tier III deposit; the worst level of soft-tissue preservation). The geochemical signature of this section shows little to no evidence of anoxic conditions, in contrast to published multi-proxy studies of more celebrated Tier I and II deposits. These data help confirm that ‘decay limited’ BST biotas were deposited in more oxygenated conditions, and support a role for anoxic conditions in BST preservation. Finally, we discuss the role of iron reduction in BST preservation, including the formation of iron-rich clays and inducement of sealing seafloor carbonate cements. As oceans and sediment columns became more oxygenated and more sulfidic through the early Paleozoic, these geochemical changes may have helped close the BST taphonomic window.
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