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Alan K. Cooper

Publications and source records attributed to Alan K. Cooper.

At least 55 records · Page 3Linked to original sources
USGS1985

Episodic rifting of phanerozoic rocks in the Victoria Land basin, Western Ross Sea, Antarctica

Multichannel seismic-reflection data show that the Victoria Land basin, unlike other sedimentary basins in the Ross Sea, includes a rift-depression 15 to 25 kilometers wide that parallels the Transantarctic Mountains and contains up to 12 kilometers of possible Paleozoic to Holocene age sedimentary rocks. An unconformity separates the previously identified Cenozoic sedimentary section from the underlying strata of possible Mesozoic and Paleozoic age. Late Cenozoic volcanic rocks intrude into the entire section along the eastern flank of the basin. The Victoria Land basin is probably part of a more extensive rift system that has been active episodically since Paleozoic time. Inferred rifting and basin subsidence during Mesozoic and Cenozoic time may be associated with regional crustal extension and uplift of the nearby Transantarctic Mountains.

Science↗
USGS1983

Tectonic evolution of Gulf of Anadyr and formation of Anadyr and Navarin basins

New seismic reflectionand refraction data reveal that Anadyr basin is separated from Navarin basin by Anadyr ridge, a southeast-northwest-trending bedrock high that is characterized by high-amplitude, short-wavelength magnetic anomalies. Anadyr ridge may be an offshore extension of the melange belt underlying the Koryak Range. Sonobuoy refraction data indicate that the velocity profile of strata in East Anadyr trough is similar to that in Navarin basin. Structurally, the basins are different.

Siberia↗
USGS1982

Ancient plate boundaries in the Bering Sea region

Plate tectonic models of the Bering Sea suggest that the abyssal Bering Sea Basin is underlain by oceanic crust, a supposition supported by refraction and magnetic data. The oceanic crust is thought to be a remnant of the Kula(?) plate that was isolated within what is now the Bering Sea when the proto-Aleutian arc began to form between the Alaska Peninsula and Kamchatka in late Mesozoic or earliest Tertiary times. Prior to the formation of the Aleutian arc, the Kula(?) plate moved NW, directly underthrusting eastern Siberia; the plate’s eastern edge either obliquely underthrust or slid past the Bering Sea margin along a transform boundary. The Koryak Range in eastern Siberia is composed in part of mélange units that include Palaeozoic and Mesozoic allochthonous blocks juxtaposed within a matrix of Cretaceous sedimentary rocks. Structural trends suggest that these blocks were accreted into the Koryak area from the south along an ancient subduction zone formed by underthrusting of the Kula(?) plate. The base of the Bering Sea continental margin that extends from eastern Siberia to the Alaska Peninsula—the so-called Beringian margin—is underlain by a thick (7–10 km) sedimentary section along the base of the slope. Rocks dredged from the basement exposed farther up the slope (1500–2000 m deep) include shallow-water Upper Jurassic sandstone that is unconformably overlain by shallow-water Eocene to Miocene diatomaceous mudstone. Fauna in the dredge samples indicate that the shelf edge has subsided several kilometres since late Palaeogene time, perhaps in response to the cessation of motion relative to the adjacent oceanic plate and subsequent sediment loading of the oceanic plate. Uplift of the former plate boundary exposed in the Koryak Range occurred principally in late Cenozoic time, and collapse of the adjacent plate boundary, the Beringian margin, began in earliest Tertiary time and has continued to the present. Both tectonic events occurred after the site of active plate collision shifted south to near the present Aleutian Trench. We are uncertain as to why these two ancient, yet adjacent former plate boundaries should behave so differently, i.e. why one area was folded and uplifted while the other was extensionally deformed and subsided, both apparently in response to the cessation of convergent or strike-slip plate motion.

Geological Society, London, Special Publications↗
USGS1981

Multichannel seismic evidence bearing on the origin of Bowers Ridge, Bering Sea

Bowers Ridge is a large, arcuate sub-marine ridge that extends north and west from the Aleutian Ridge and separates the abyssal Aleutian and Bowers Basins in the Bering Sea. Two multichannel seismic-reflection lines recorded in 1976 over Bowers Ridge and the adjacent basins confirm the existence of 8- to 10-km-thick sediment wedges on the north side of Bowers Ridge and at the base of the Bering continental margin. Deformed sediment within the Bowers wedge indicates that subduction of the adjacent ocean crust beneath the ridge probably occurred prior to middle Cenozoic time. Flat-lying reflectors near the bottom of the trench suggest that a bathymetric trough and large ridge existed in Mesozoic time. The major period of underthrusting, subsidence, and in-filling of the trench probably occurred from Mesozoic to early Tertiary time. Small amounts of underthrusting may have continued after the early Tertiary development of the Aleutian Ridge; however, by middle Miocene time, the formerly subaerial Bowers Ridge had subsided below sea level. The multichannel seismic data do not show evidence for a buried spreading center within the eastern Aleutian Basin. Consequently, the sediment wedges (trenches) at both Bowers Ridge and the Bering continental margin are believed to be the consequence of subduction that occurred during the convergence of the ridge and the margin. The large size of Bowers Ridge suggests that a large amount of convergence has occurred since Mesozoic time. If Bowers Ridge was a large feature in Mesozoic time, as suggested by the apparent bathymetric trough, then the ridge may be as old or older than the Aleutian Ridge to which it connects.

GSA Bulletin↗
USGS1981

Early evolution of the Bering Sea by collision of oceanic rises and North Pacific subduction zones

Three major bathymetric features exist in the Bering Sea: Shirshov Ridge, Bowers Ridge, and Umnak Plateau. New refraction data over Umnak Plateau and previous geophysical data across Bowers Ridge indicate that a thickened welt of crustal material is present beneath both features. The crustal structure is transitional between oceanic and continental types. Various models for the origin of these features have been investigated. One that has not been proposed previously assumes that the protostructures of Bowers Ridge and Umnak Plateau could have formed outside of the present Bering Sea. According to this model, before formation of the Aleutian Ridge in late Mesozoic or earliest Tertiary time, these protostructures moved into their present Bering Sea positions. Prior to the arrival of these two structures in the Bering Sea, oceanic crust was subducted along the Bering continental margin connecting Alaska and Siberia. The collision of the Umnak Plateau protostructure with the southeastern edge of the margin may have caused subduction to terminate here and move southward. The new southerly position of subduction beneath the Aleutian Ridge was therefore controlled by late Mesozoic or early Tertiary locations of Umnak Plateau, Bowers Ridge, and possibly, the north-trending Shirshov Ridge farther to the west.

GSA Bulletin↗
USGS1980

Mesozoic and Cenozoic structural trends under southern Bering Sea shelf

Mesozoic rocks exposed near the tip of the Alaska Peninsula form an antiformal structure that flanks the southern side of Bristol Bay basin and that can be traced with geophysical data about 700 km offshore to the vicinity of the Pribilof Islands. Upper Jurassic sandstone and Upper Cretaceous mudstone dredged from the top and flanks of this structure near the islands confirm that Mesozoic rocks extend from the Alaska Peninsula to the Bering sea margin. The southern part of the Bering Sea Shelf is underlain by several large structural basins: St. George, Amak, and Bristol Bay basins. These filled basins encompass an offshore area of about 31,000 sq km; St. George basin contains more than 10 km of strata. Reflection profiles show that the surface of the offshore antiformal structures is an angular unconformity overlain by Cenozoic beds. This unconformity can be traced toward the axes of the adjacent subshelf basins where, as a disconformity, it parallels underlying and overlying strata. Dredge data suggest that the unconformity and disconformity may be as old as middle to Late Cretaceous. The downdip trace of the unconformity in Bristol Bay basin is underlain by reflectors paralleling the contact, a relation suggesting that the basin and perhaps other shelf basins may be underlain by ancient Mesozoic depocenters. The bulk of the thick sections in these basins is, however, thought to be mainly Cenozoic in age. Strata in the basins are cut by high-angle growth faults. The faults commonly offset the seafloor, which implies that basin subsidence and filling continue to the present. Shallow-water diatomaceous mudstone of Eocene and Oligocene age dredged from the continental slope near the Pribilof Islands indicates that collapse of the margin and outer shelf basins began by at least early Tertiary time. In Mesozoic time, the Bering margin between Siberia and the Alaska Peninsula (Beringian margin) may have been a zone of either oblique underthrusting or transform motion between the North American and Pacific lithosphere (Kula plate?). This motion may have rifted the edge of the North American plate, resulting in the formation of a series of elongate basins and ridges paralleling the plate edge. These hypothetical basins may have controlled the location and initial subsidence of Bristol Bay, Amak, and St. George basins. Formation of the Aleutian Island arc in late Mesozoic or earliest Tertiary time presumably terminated plate interaction along the Beringian margin. Sediment loading and subsequent subsidence of the remnant plate within the abyssal Bering Sea may have caused continuing collapse of the Beringian margin in latest Cretaceous and earliest Tertiary time.

AAPG Bulletin↗
USGS1979

Resource report for the deep-water areas of proposed OCS lease sale No. 70, St. George Basin, Alaska

This report summarizes geological and geophysical data from the deep-water (greater than 200m) region of the St. George Basin lease sale area #70. The shallow-water region is discussed in detail by Marlow and others (1979a) in a companion report. The triangular deep-water region lies at the junction of the Bering shelf and the Aleutian Ridge (Fig. 2). This region is bounded on the northeast by the 200 m bathymetric contour that defines the edge of the Bering shelf, on the southeast by a meandering line that lies 3 miles north of the Aleutian Islands, and on the west by the 171°W longitude meridian. Hereafter, this triangular region is referred to as the Umnak Plateau region. The name is derived from a major bathymetric feature, Umnak Plateau, that covers most of the region.

Alaska↗
USGS1978

The Aleutian Basin, Bering Sea a frontier area for hydrocarbon exploration

The Aleutian Basin is the deep water (>3000 m) basin that lies north of the Aleutian Islands adjacent to the Bering Sea continental shelf. The basin, about the size of the state of Texas, is underlain by a 2-9 km-thick flat-lying sequence of mostly Cenozoic sediment and rock that includes diatomaceous silty clay interbedded with turbidities in the upper 1 km. Before 1974, geologic and geophysical investigations in the Aleutian Basin were directed at determining its geologic history; more recently, investigations have also been aimed at assessing the basin's hydrocarbon potential. The four major requirements for hydrocarbon accumulation may be present, namely, structural and stratigraphic traps, source rocks, reservoir beds, and an adequate thermal and sedimentation history. Energy resource investigations by the US Geological Survey indicate that: (1) numerous structural features (gentle folds, diapirs, basement ridges) are present in the central and eastern parts of the basin, (2) acoustic features (VAMP's _ Velocity AMPlitude features) that may be due to trapped gases within the sedimentary section are common (over 350 identified) in the central basin, (3) concentrations of organic gases, primarily methane, in the upper 1-3 m beneath the seafloor are very small, they increase with depth, and they are highest in areas near VAMP's, and (4) both the thermal gradient and the sediment thickness are sufficiently great to allow hydrocarbon maturation at depth, if source rocks are present. Adverse conditions in the Aleutian Basin such as excessive water depths and severe weather pose difficult technical problems for the recovery of hydrocarbons that may be present. The data nevertheless suggest the basin is a promising site for hydrocarbon accumulations and therefore warrants further exploration.

Conference Paper↗
USGS1978

VAMPs—Possible hydrocarbon-bearing structures in Bering Sea Basin: Geologic notes

Narrow (1 to 2 km) subsurface columns of concave reflection horizons are common time-base seismic profiles collected in the Bering Sea basin. The columns of recorded downflexures are thought to be velocity pulldowns and commonly are associated with one or more arched or gently domed high-amplitude reflection horizons about 100 m higher in the section. Inferred from this association is that subsurface deposits characterized by anomalously low acoustic velocity are present. We refer to the velocity-anomaly and reflection-amplitude association as a velocity-amplitude feature, or VAMP, and speculate that VAMPs are deep-seated “bright spots” underlain by a strong velocity pulldown possibly caused by gas-charged deposits.

AAPG Bulletin↗
USGS1976

Plate tectonic model for the evolution of the eastern Bering Sea Basin

The eastern Bering Sea Basin, composed of the Aleutian and Bowers Basins, is flanked to the north by Mesozoic foldbelts that probably represent zones of plate subduction in Mesozoic time. Present plate subduction occurs 400 to 1,000 km farther south, at the Aleutian Trench. North-south magnetic lineations that formed at an oceanic spreading ridge, probably in Mesozoic time (117 to 132 m.y. ago), have been identified in the Aleutian Basin. The orientation and age of those anomalies can be explained by reconstructing Kula-Farallon Pacific plate motions during late Mesozoic–early Tertiary time. In Mesozoic time, subduction of the Kula plate occurred north of the Aleutian Trench near the present location of the Bering Sea continental margin. At about 70 m.y. B.P. (Late Cretaceous), the zone of subduction shifted south to the present location of the Aleutian Trench, thereby trapping a fragment of oceanic plate imprinted with north-south magnetic lineations within the eastern Bering Sea Basin. A stable basin framework has prevailed behind the Aleutian arc since early Tertiary time.

GSA Bulletin↗
USGS1976

Structure and evolution of Bering Sea shelf south of St. Lawrence Island

The virtually featureless Beringian shelf south of St. Lawrence Island is underlain structurally by at least 14 basins. Encompassing a total area of more than 300,000 sq km, most of the basins are either elongate structural sags, grabens, or half (asymmetric) grabens beneath the outer shelf. The regional trend of these basins is northwest, parallel with that of the continental margin. Two of the basins, St. George and Navarin, contain 7 to 10 km of Upper Cretaceous(?) and Cenozoic sedimentary strata. A major divergence in dip of beds in the upper half of the sedimentary section may reflect an abrupt shelf-wide change in the rate of sedimentation and/or subsidence, probably during the Miocene. The outer sub-shelf basement grabens and adjacent ridges (horsts) are bounded by high-angle normal faults that exhibit growth-type structure. St. Matthew basin, an elongate, southwest-trending feature of the inner shelf, lies along the offshore expression of the Kaltag fault of western Alaska. The Kaltag fault, like the Denali fault in southwestern Alaska, does not extend to the outer Bering Sea shelf but ends or turns parallel with the margin within the inner shelf. The inner shelf is underlain by a broad basement high, Nunivak arch, the seaward half of which is characterized by an arcuate belt of high-frequency and high-amplitude magnetic anomalies. This zone of intense magnetic anomalies along the shelf is probably the signature of a Mesozoic magmatic arc that extends from southwestern Alaska to eastern Siberia and consists of Jurassic to Cretaceous plutonic and volcanic rocks. We speculate that this magmatic arc resulted from oblique convergence and subduction in the Mesozoic between the Kula(?) and North American plates along the eastern Beringian margin. Folding and uplift in the area of the present outer shelf occurred contemporaneously with magmatism along the inner shelf. Plate convergence apparently ceased by the end of the Mesozoic or t e beginning of the Cenozoic. Subsequently, the foldbelt underlying the outer shelf was eroded extensively and rifted extensionally to form large, deep basins. On the average, the shelf has subsided more than 1.5 km. Subsidence and sediment burial of the eroded orogen formed the modern Beringian shelf.

AAPG Bulletin↗