Delamerian Orogeny and potential foreland sedimentation: A review of age and stratigraphic constraints
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Abstract:
A review of available geochronology and biostratigraphy leads to the conclusion that a considerable thickness of Cambrian sedimentary rocks exposed in the Arrowie and Stansbury Basins, South Australia, was probably deposited in a foreland setting during early phases of the Delamerian Orogeny. In contrast to most previous stratigraphic correlation schemes, we consider that the pre‐tectonic Kanmantoo Group was deposited synchronously with the locally thick upper Hawker Group in essentially en echelon basins during a final phase of extensional sedimentation within the Adelaide 'Geosyncline'. The base of the locally overlying 'redbed package' (base of the Billy Creek and Minlaton Formations) is interpreted as the sedimentological signature of the onset of convergent deformation and associated uplift within the Delamerian Orogen at about 522 Ma. This early ('Kangarooian') phase of the Delamerian Orogeny is interpreted as the progressive development of a coherent sigmoidal fold‐thrust belt within the combined Fleurieu‐Nackara Arcs, with locally developed high‐temperature‐low‐pressure metamorphism and granitoid intrusions dating from about 516 Ma. The 'redbed package' is absent from the Fleurieu‐Nackara Arc region and displays isopach, palaeocurrent and facies trends consistent with derivation from this uplifted area or from the associated flexural bulge to the west. From seismic evidence we conclude that thick foreland basin deposits are present beneath Gulf St Vincent. Late phases of the Delamerian Orogeny led to local and relatively mild deformation of the early foreland deposits.Keywords:
Orogeny
Geochronology
Imbrication
Isopach map
Diachronous
Paleocurrent
Abstract Competing end‐member models for the late Paleozoic Variscan orogeny (ca. 360‐290 Ma) alternatively suggest moderate 2–3 km elevations underlain by relatively thin crust (<50 km) or a thick crust (>55 km) that supported high 4–5 km elevations. We tested these models and quantified the crustal thickness and elevation evolution of the Variscan orogeny using igneous trace element geochemical proxies and geochronologic data. These data suggest that thick crust (55–70 km) capable of supporting 3–5 km elevations developed diachronously from east to west between ca. 350 and 315 Ma. Crustal thinning occurred from ca. 315 to 290 Ma across the orogen. Crustal thickness and elevation changes at ca. 340‐325 and 315‐290 Ma correspond with increases in silicate weathering recorded by Sr and Li isotopes, consistent with models in which silicate weathering of the Variscan orogen contributed to global cooling associated with the late Paleozoic ice age.
Orogeny
Diachronous
Elevation (ballistics)
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Paleocurrent
Imbrication
Lineation
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Yilgarn Craton
Orogeny
Diachronous
Geochronology
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An investigation of sedimentary features found within the Pleistocene Older Alluvium was undertaken on an alluvial fan located just north of the City of Santa Barbara. The fan is cut by the Mission Ridge fault, and other geologists interpret folding of the Older Alluvium south of the fault. The goal of this study was to determine if any post depositional rotations of the deposits had occurred to form folded features, as postulated by other workers. The estimated trend and plunge of the long axis of 182 elongated clasts was measured and recorded, as shown by Compton (1982, p. 227, fig 12-10B), and plotted on a Schmidt Equal Area stereonet. One such channel is located 200 feet east of the intersection of Mountain Drive and Tremonto Road at GPS coordinates: 34.445N, -119.7055E. The measurement of 15 long axes trend and plunge within this channel deposit were compared, and contains a moderate to strong correlation coefficient. The correlation coefficient is 0.434, and indicates a moderate to strong correlation. The long axes moderately dip towards the north, and show a lack of rotation by structural forces such as faulting or folding. An examination the internal fabric of the older Alluvium deposits within the Rattlesnake Fan with an interest in pebble imbrication. Imbrication of elongated clasts was noted in our study, as it was also noted by the US Geological Survey et.al. (2001, p. 3). The imbricated clasts are contained within pebble and boulder conglomerate deposits. These deposits typically form paleochannels with contacts between beds forming concave shaped contact facing upward. Chaotic deposits typical of debris flows are the dominant feature in the Rattlesnake fan. Debris flows are the downslope flow of relatively coarse material: more than 50 percent of the particles are coarser than sand (Keller, 2011, p. 182). A total of five outcrops on the Rattlesnake Fan were examined for elongated clasts, and most were found to lack imbrication because of the chaotic alignment of clasts.
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Imbrication
Conglomerate
Pebble
Alluvial fan
Cobble
Basement
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ABSTRACT Detailed studies of cross-stratification, pebble imbrication, mineralogy, textures, phenoclast size, and individual mineral size all indicate a single paleocurrent direction within the Tertiary Allenby Formation. It is concluded that the paleocurrents were in a southerly direction, and that the main source of the sediments was the Osprey Lake Intrusion.
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Imbrication
Pebble
Heavy mineral
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Paleocurrent
Imbrication
Conglomerate
Paleogene
Alluvial fan
Basement
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Paleocurrent
Imbrication
Lineation
Deposition
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The paleocurrent system of the Devonian clastics of the central and northern Appalachian basin is uniformly oriented to the west judging by the orientation of sole marks on interbedded siltstones and sandstones and by the available directional data from interbedded black and gray shales. Paleocurrent indicators are at right angles to isopach of total Devonian thickness, which decreases westward from 12,000 ft. in eastern Pennsylvania to a few hundred feet in west-central Ohio. This clastic wedge is largely of Upper Devonian age and includes alluvial and delta plain environments (in the east) as well as shelf (east-central), turbidite slope, and basin plain environments (west-central and west), the latter representing most of the black shales. Lithologies within the wedge are more continuous north-south parallel to depositional strike than east-west. The gradient of carbon isotopes, which shows more marine than terrestrial carbon in the western part of the basin, closely parallels the average paleocurrent direction of the basin. The methodology of paleocurrent studies in shaly basins based on both outcrops and oriented cores, is set forth as is the relationship between paleocurrents and gas potential.
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Devonian
Isopach map
Alluvial plain
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The Central Maine Basin and Merrimack Trough are Silurian basins that formed adjacent to or were accreted to the Laurentian margin during the Acadian orogeny. The Early Devonian Carrabassett Formation of the Central Maine Basin and the Kittery Formation of the Merrimack Trough have major and trace element compositions indicative of a passive continental margin provenance, not unlike the older formations of the Central Maine Basin that are thought to have been derived from Laurentian sources. However, both the Carrabassett and Kittery formations have paleocurrent indicators of outboard sources. The Carrabassett Formation is one of the youngest formations of the Central Maine Basin and was deposited just prior to the Acadian orogeny. The Carrabassett and Kittery formations have major and trace element concentrations suggestive of passive margin turbidites derived from intermediate to felsic sources, inconsistent with a juvenile Avalonian provenance. The Carrabassett Formation contains detrital zircon grains that match the ages of peri-Gondwanan Ganderia. Unlike the dominance of positive bulk-rock εNd values that are characteristic of Avalonia, Ganderia has negative εNd values that are a better match for the negative εNd values of the Carrabassett and Kittery formations. However, Ganderia accreted to Laurentia during the Salinic orogeny, prior to the deposition of the Carrabassett Formation, and was basement to the sediments of the Central Maine Basin upon which the Carrabassett and other formations were deposited. Wedging of Ganderia by Avalonia during the initial stages of the Acadian orogeny may have uplifted Ganderia, forming highlands outboard of the Central Maine Basin that served as the source of the Carrabassett Formation sediments.
RÉSUMÉ
Le bassin central du Maine et la cuvette de Merrimack constituent des bassins siluriens s’étant formés le long de la marge laurentienne ou s’y étant accrétés au cours de l’orogenèse acadienne. La Formation du Dévonien précoce de Carrabassett, dans le bassin central du Maine, et la Formation de Kittery, de la cuvette de Merrimack, présentent des compositions en éléments majeurs et traces signalant une provenance d’une marge continentale passive, à l’instar des formations plus âgées du bassin central du Maine qu’on pense originaires de sources laurentiennes. Les formations de Carrabassett et de Kittery comportent toutefois des indicateurs de paléocourants de sources extérieures. La Formation de Carrabassett constitue l’une des formations les plus récentes du bassin central du Maine; elle s’est mise en place juste avant l’orogenèse acadienne. Les caractéristiques géochimiques et géochronologique des formations de Carrabassett et de Kittery pourraient par conséquent permettre l’identification du terrane de collision. Les formations de Carrabassett et de Kittery possèdent des concentrations d’éléments majeurs et traces évoquant les turbidites de marge passive en provenance de sources intermédiaires à felsiques, ce qui est contradictoire avec une origine avalonienne juvénile. La Formation de Carrabassett comporte des grains détritiques de zircon correspondant aux âges du Ganderia périgondwanien. Contrairement à la prédominance de concentrations εNd positives de roche en vrac caractéristiques d’Avalonia, Ganderia présentent des concentrations εNd négatives qui cadrent mieux avec les concentrations εNd négatives des formations de Carrabassett et de Kittery. Ganderia s’est toutefois accrété à Laurentia au cours de l’orogenèse salinique, avant le dépôt de la Formation de Carrabassett, et il a constitué le socle des sédiments du bassin central du Maine sur lesquels Carrabassett et d’autres formations se sont déposées. L’enfoncement d’Avalonia sous Ganderia au cours des stades initiaux de l’orogenèse acadienne pourrait avoir soulevé Ganderia, formant un massif à l’extérieur du bassin central du Maine qui a servi de source aux sédiments de la Formation de Carrabassett. [Traduit par la redaction]
Orogeny
Laurentia
Paleocurrent
Passive margin
Devonian
Trough (economics)
Felsic
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Paleocurrent
Imbrication
Lineation
Settling
Bedding
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