Initiation of Cordilleran miogeocline of western North America
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A better understanding of the events that initiated the Cordilleran miogeocline has been obtained by combining quantitative subsidence analyses, geologic field studies, and isotopic analyses of rift sequence volcanics. Tectonic subsidence of post-rift Cambrian-Ordovician strata is thermal in form and began 575 +/- 25 Ma. The initial steep slopes of the subsidence curves indicate high cooling rates, and when considered in light of finite rift models, they suggest that rifting could not have lasted more than 10-20 m.y. prior to the onset of thermal cooling. Together with a preliminary age of 762 +/- 44 Ma for volcanics near the base of the Windermere Supergroup, these data indicate that the Windermere Supergroup does not represent the rift deposits that led directly to the onset of thermal subsidence. Instead, Windermere sedimentation was initiated by an earlier rift event, probably of regional extent. This event was part of a protracted, episodic rift history that culminated with final rifting and the onset of thermal subsidence in the latest Proterozoic-earliest Cambrian. Geologic field studies in the southern Canadian Cordillera uncovered evidence for younger rifting, which agreed with the subsidence analyses. The Hamill Group unconformably overlies the Windermere Supergroup (rift onset unconformity) and contains coarse-grained, feldspathicmore » sandstones at its base that pass up-section into mature quartzarenites. Similar sedimentologic evidence within the base of the Gog Group is interpreted to represent uplift of basement sources, followed by a transition to more stable tectonic conditions associated with the initial stages of post-rift thermal subsidence. The Hamill Group also contains evidence of syndepositional tectonism that was accompanied by the extrusion of mafic volcanics. The block that rifted from North America has yet to be identified.« lessKeywords:
Thermal subsidence
Supergroup
Rodinia
Tectonic subsidence
Passive margin
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The Ross Sea contains three major depocenters, each underlain by a sediment-filled rift graben and an overlying glacial sedimentary sequence. The sedimentary sections are up to 14 km thick with up to 8 km in the rift grabens and up to 6 km in the presumed-glacial sequences. The rift grabens were downfaulted and filled probably during the late Mesozoic to early Cenozoic continental breakup of Gondwana; their early history may be analogous to coeval rift basins of southeast Australia, Tasmania, and New Zealand. The rift-graben sediments are unconformably overlain by glacial-marine sequences deposited since middle Eocene(?) to early Oligocene time. Renewed down-faulting has occurred along the west and east margins of the Ross Sea probably since Eocene time. The hydrocarbon potential of the Ross Sea is poorly known because only post-Eocene glacial rocks have been sampled offshore. The age and type of rocks filling the rift grabens, below the glacial sequence, is unknown. Source beds do not occur in the glacial sequence, but may exist in the rift grabens. Structural and stratigraphic traps are likely near basement structures and unconformities, which are common, and near large sedimentary structures found only in the Victoria Land Basin and along margins of the Eastern Basin. Reservoir rocks are unknown but sands could occur throughout the glacial and rift sequences. Lopatin-Waples models indicate that hydrocarbons could be generated presently at End_Page 47------------------------- depths of 2.5 to 4.0 km, if source beds exist. Migration is likely in dipping strata along rift-graben flanks and in late-rift fault zones of the Terror Rift. Hydrocarbon seeps and accumulations are unknown in the Ross Sea. The preglacial strata that are deeply buried within the early-rift grabens have the best hydrocarbon potential; however, a definitive assessment awaits sampling of these deep rift deposits.
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Correlation of the upper Paleozoic to Mesozoic successions of northern Alaska and the Canadian Arctic Islands has revealed close stratigraphic and tectonic links between these two petroliferous areas. Depositional and tectonic trends have been reconstructed for Arctic North America, and such interpretations can assist petroleum assessments of unexplored areas in the region. Five regional unconformities are recognized, and these allow the succession to be divided into four tectonic sequences: Carboniferous-Lower Permian, Lower Permian-lowest Cretaceous, Lower Cretaceous, and Upper Cretaceous. The first sequence, Carboniferous-Lower Permian, developed during a phase of rifting when a series of pull-apart basins formed along the eroded Ellesmerian deformation belt. Fan deltas and shelf carbonates with equivalent basinal shales and evaporites characterize this sequence. An episode of uplift and faulting terminated the first sequence. The second sequence, Lower Permian-lowest Cretaceous, developed under conditions of thermal subsidence over the rifted areas. Clastic sedimentation was dominant with alternating shelf and deltaic deposition. Significant uplift reflecting the initiation of the Amerasian basin by rifting began in earliest Cretaceous. Sequence three, Lower Cretaceous, was deposited during the rifting phase of the Amerasian basin and consists of thick, deltaic, clastic wedges derived from either the craton or the uplifted Brooks Range. Themore » onset of sea-floor spreading in the Amerasian basin in earliest Late Cretaceous resulted in widespread uplift. The fourth sequence, Upper Cretaceous, was deposited coincident with sea-floor spreading in the Amerasian basin. Initial deposits were bituminous shales which were followed by thick clastic wedges that prograded into the ocean basin. This sequence was terminated by uplift in Cretaceous-earliest Tertiary when sea-floor spreading switched to the Eurasian basin.« less
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Red beds
Pull apart basin
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Abstract The Sydney Basin, a large depocentre about 350 by 150 km in present-day area, was active from mid-Devonian to Permian times. It contains a relatively undeformed fill 4 km thick that rests upon continental crust of the Appalachian Orogen. Mid-to Upper Devonian terrigenous strata and volcanics (McAdam Lake Formation) are present locally. The Carboniferous to Permian succession is composed of two fining-upward megasequences, which are separated by a major hiatus. The lower megasequence (Horton, Windsor and Canso groups) accumulated over about 27 My, and contains fanglomerates that can be related to contemporaneous strike-slip faulting. Subsidence to below sea level allowed a prolonged marine incursion, and a subsequent lacustrine phase was followed by uplift of the basin floor, gentle folding and faulting, erosion and karst weathering. The upper megasequence (Morien Group and overlying redbeds) accumulated over about 25 My, and indicates renewed subsidence and largely inactive local faults. Alluvial strata containing coals, deposited from a long-lived drainage system, record stable tectonic conditions within the basin and the surrounding uplands. Subsidence is attributed to contemporaneous motion on distant fault systems, especially the Hollow, Long Range and Minas Geofracture systems. The basin originated as a rhomb-shaped extensional depocentre where the Minas Geofracture, a major transcurrent fault oriented parallel to the Appalachian Orogen, generated a series of divergent splay faults. The position of the basin also coincided with a major, long-lived dislocation that ran into the continental margin from offshore. The 25 to 30 My duration of the megasequences may approximate the duration of phases of activity on the major fault systems. Correlation with sequences elsewhere in Atlantic Canada suggests that the basin history represents a regional response to continent-wide tectonic events, especially the docking of the Meguma Terrane to North America along the Minas Geofracture, and the North American/Gondwana collision. The hiatus between the megasequences represents a phase of renewed tectonic activity and uplift in the Meguma Terrane, with the development of a major sediment source and subsequent alluvial sedimentation across the Atlantic region. The increase in abundance of redbeds in Stephanian to Permian times may be attributed, in part, to an increasingly continental climate following landmass suturing and the termination of accretion.
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The Larsen Basin developed in Jurassic times as a result of continental rifting during the early stages of Gondwana break‐up. Lower‐?Upper Jurassic non‐marine sedimentary and volcanic rocks constitute a syn‐rift megasequence recording initial largely amagmatic extension and subsequent widespread extension‐related silicic volcanism. A succeeding, Kimmeridgian–early Berriasian transgressive megasequence, consisting largely of anoxic‐dysoxic hemipelagic mudstones, is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak Jurassic eustatic sea levels served to maximize sediment starvation. The fragmentary record for late Berriasian–Barremian times suggests that a ?regressive megasequence may have developed in the earlier part of this period, recording increased sediment yield to the Larsen Basin from the increasingly emergent Antarctic Peninsula arc. Subsequently, strata in the southern, but not the northern, part of the basin underwent relatively intense eastward‐verging deformation, possibly during the formation of a retro‐arc fold‐thrust belt. Where exposed, the lower part of the succeeding Aptian–Eocene megasequence consists of a deep‐marine clastic wedge deposited along the fault‐bounded western basin margin during a phase of arc uplift and related differential subsidence. Following partial basin inversion in Late Cretaceous times, regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward.
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The geology of North-East Greenland (70–78°N) exposes unique evidence of the basin development between the Devonian collapse of the Caledonian Orogen and the extrusion of volcanics at the Paleocene–Eocene transition during break-up of the North-East Atlantic. Here we pay special attention to unconformities in the stratigraphic record – do they represent periods of stability and non-deposition or periods of subsidence and accumulation of rocks followed by episodes of uplift and erosion? To answer that and other questions, we used apatite fission-track analysis and vitrinite reflectance data together with stratigraphic landscape analysis and observations from the stratigraphic record to study the thermo-tectonic history of North-East Greenland. Our analysis reveals eight regional stages of post-Caledonian development: (1) Late Carboniferous uplift and erosion led to formation of a sub-Permian peneplain covered by coarse siliciclastic deposits. (2) Middle Triassic exhumation led to removal of a thick cover including a considerable thickness of upper Carboniferous – Middle Triassic rocks and produced thick siliciclastic deposits in the rift system. (3) Denudation at the transition between the Early and Middle Jurassic affected most of the study area outside the Jameson Land Basin and produced a weathered surface above which Middle–Upper Jurassic sediments accumulated. (4) Earliest Cretaceous uplift and erosion along the rifted margin and further inland accompanied the Mesozoic rift climax and produced coarse-grained sedimentary infill of the rift basins. (5) Mid-Cretaceous uplift and erosion initiated removal of Cretaceous post-rift sediments that had accumulated above the Mesozoic rifts and their hinterland, leading to cooling of Mesozoic sediments from maximum palaeotemperatures. (6) End-Eocene uplift was accompanied by faulting and intrusion of magmatic bodies and resulted in extensive mass wasting on the East Greenland shelf. This event initiated the removal of a thick post-rift succession that had accumulated after break-up and produced a peneplain near sea level, the Upper Planation Surface. (7) Late Miocene uplift and erosion, evidenced by massive progradation on the shelf, resulted in the formation of the Lower Planation Surface by incision below the uplifted Upper Planation Surface. (8) Early Pliocene uplift raised the Upper and the Lower Planation Surfaces to their present elevations of about 2 and 1 km above sea level, respectively, and initiated the formation of the present-day landscape through fluvial and glacial erosion. Additional cooling episodes of more local extent, related to igneous activity in the early Eocene and in the early Miocene, primarily affected parts of northern Jameson Land. The three earliest episodes had a profound impact beyond Greenland and accompanied the fragmentation of Pangaea. Younger episodes were controlled by plate-tectonic processes, possibly including dynamic support from the Iceland Plume. Our results emphasise that gaps in the stratigraphic record often reflect episodes of kilometre-scale vertical movements that may result from both lithospheric and sub-lithospheric processes.
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The early Mesozoic marine province of Nevada includes a shallow marine shelf terrane to the east, the Black Rock arc terrane to the west, and an intervening deep marine basinal terrane. A new integrated analysis of the Triassic record across the northern marine province indicates the following history. From the Early to late Middle Triassic, the province was affected by differential uplift and subsidence, subaerial to basinal sedimentation, and intermittent volcanism. Regional subsidence then occurred in the late Middle to early Late Triassic, leading to marine deposition on top of previously exposed areas. During this time, carbonate deposition occurred in all areas, coarse clastics from nearby basement uplifts were shed into the margins of the province, and volcanism occurred in the basinal and shelf terranes. Regional subsidence then continued in the latest Triassic, as manifested by the accumulation of very thick (up to 6 km) successions of marine strata: but in the Black Rock terrane, these strata consist of volcanogenic arc deposits; whereas, in the shelf and basinal terranes, these strata consist exclusively of continentally-derived clastics. Based on these data, the Black Rock, basinal and shelf terranes are interpreted to have evolved together in the Triassic in an extensional tectonic regime that culminated in the opening of a wide basin between the shelf and a volcanic arc. Extensional tectonism led to differential uplift and subsidence in the early history of the province, regional subsidence and accumulation of thick marine successions across the province in the late Middle to Late Triassic, and progressive isolation of the Black Rock terrane from the basinal and shelf terranes as the extensional basin widened.
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Both the eastern and western margins of North America record late Proterozoic/early Paleozoic rifting related to the development of the Iapetus and proto-Pacific oceans. Both margins were subsequently deformed when the adjacent ocean basis closed during the Paleozoic and Mesozoic eras, respectively. Detailed mapping projects in central Vermont and southeastern British Columbia provide an opportunity to compare and contrast the stratigraphic and structural evolution of the two margins, and to investigate similarities and differences in the tectonic processes. Lateral facies and thickness variations imply that the presently exposed basement acted as a crustal high'' during rifting and experienced little subsidence prior to the onset of passive margin sedimentation. The Purcell anticlinorium of southeastern British Columbia exposes a sequence of immature siliciclastic Upper Proterozoic rocks (Windermere Supergroup), up to several km thick, which unconformably overlie the Middle Proterozoic Belt Supergroup. Upper Proterozoic rocks are regionally unconformably overlain by a sequence of dominantly mature quartzites and overlying shallow water carbonate (Lower Cambrian Badshot Formation). Although this sequence is markedly similar to that observed in Vermont, there are several critical differences. Sedimentation in the adjacent basin to the west was punctuated by several episodes of mafic volcanisms and immature clastic influx from Latemore » Proterozoic to post-Early Cambrian/pre-Mississippian time. The duration of active tectonism was clearly longer than can be accounted for by recent models of continental rifting. In both areas, the outboard edges of the crustal highs'' developed during rifting, as outlined by profound stratigraphic changes, correspond to profound structural/metamorphic contrasts developed during subsequent collapse of the margins.« less
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