Terrace Wedge Research Articles

Crockerland - The Source Area for the Triassic to Middle Jurassic Strata of Northern Axel Heiberg Island, Canadian Arctic Islands

Abstract The main source terrains for the Sverdrup Basin of Arctic Canada lay to the south and east and comprised the extensive Canadian and Greenland cratons which were mainly covered by Devonian siliciclastic strata. From Carboniferous to mid-Jurassic, the basin also received sediment from a northern source area, named Crockerland. The Triassic-Jurassic succession of northernmost Axel Heiberg Island was examined to document the facies and stratigraphic distribution of Crockerland-derived sandstones in the succession. Such sandstones were identified by documenting a southward facies change to offshore siltstone and shale. Northerly-derived sandstones were found to be common in the Lower Triassic, Upper Triassic and lower Middle Jurassic portions of the succession. Their presence in the sandstone-dominant, uppermost Triassic–Lower Jurassic succession was not determined due to difficulties in differentiating northerly-derived sediments from those derived from the south and east. All three Lower Triassic sequences contain sandstones of shallow to offshore shelf origin. Sandstone units are up to 45 m thick and the sandstone facies tract for each sequence is estimated to have been about 100 km wide. Thick intervals of beach to offshore shelf sandstones occur in two Carnian (Upper Triassic) sequences and the sandstones of the upper sequence are truncated northward by a major unconformity. This indicates that in late Carnian the Crockerland shoreline was well south of the northern tip of Axel Heiberg Island. Northerly-derived, Middle Jurassic (Aalenian) sandstones are preserved at only two localities. They are extensively burrowed and are of middle to outer shelf origin. On the basis of the amount, composition and grain size of the sediment derived from Crockerland, it appears that low-lying, sedimentary strata characterized this source terrain. If Triassic to mid-Jurassic strata are preserved below the Cretaceous-Tertiary continental terrace wedge on the inner portion of the continental shelf north and west of Axel Heiberg, the succession may contain shallow shelf to beach sandstones which have the potential to act as reservoirs for petroleum.

Selkirk fan structure, southeastern Canadian Cordillera: Tectonic wedging against an inherited basement ramp

A revised cross section through the Selkirk fan structure provides the basis for a new model for the Middle Jurassic tectonic evolution of the southern Omineca belt, Canadian Cordillera. Palinspastic restoration of this cross section shows that the southwest-verging structures along the west flank of the Selkirk fan structure formed as a result of tectonic wedging of distal North American strata (Clachnacudainn complex) beneath more proximal North American strata, and that the Selkirk fan structure developed outboard from a crustal ramp (Dogtooth high) inherited from Late Proterozoic–early Paleozoic rifting along the western margin of North America. The first episode of Mesozoic deformation in southeastern British Columbia occurred between 187 and 173 Ma and involved the northeastward juxtaposition of the Intermontane superterrane over the outer part of the North American continental terrace wedge. It resulted in deep burial (20–25 km) of the outer margin of North America. A crustal ramp, localized along the western edge of the Late Proterozoic–early Paleozoic Dogtooth high, impeded the northeastward propagation of the orogenic wedge comprising the Intermontane superterrane and the imbricate, underlying northeast-verging thrust sheets of North American supracrustal rocks. Tectonic wedging, involving southwest-verging deformation, occurred within the orogenic wedge, and the resulting crustal thickening established sufficient topography and gravitational potential to drive the propagation of the deformation eastward into the Dogtooth Range and the Rocky Mountains. The southwest-verging structures along the west flank of the Selkirk fan developed between approximately 173 and 168 Ma concurrent with synorogenic extension and ∼10 km of exhumation. The initial subsidence of the foreland basin during Kimmeridgian time (ca. 154 Ma) provides the first indication of tectonic loading and lithospheric flexure of the North American plate. It is interpreted to mark the time at which the orogenic wedge overrode the crustal ramp of the Dogtooth high and advanced onto relatively thick and rigid continental lithosphere. The tectonic model proposed for the Selkirk fan structure illustrates how the configuration of the rifted margin influenced the style of crustal thickening during subsequent compressional deformation.

Nature of the Canadian polar margin northwest of Axel Heiberg Island

Theoretical responses, which have properties comparable to the complex field observations of the 1985 and 1986 offshore reflection experiments of the Frontier Geoscience Program, were derived from sonic log and borehole geological information from the Arctic continental margin. Tests of a specially developed reverberation suppression algorithm, both on the theoretical and field observations, proved quantitatively that this simple but robust technique effectively removes most of the coherent noise patterns from these records. Correlation of spatially generated velocity-traveltime information to the borehole data allowed establishment of the basic stratigraphie sequences offshore, west of Axel Heiberg Island. Five sedimentary units were recognized under the thin water cover. The first, nearest to the surface, represents the Tertiary Arctic Terrace Wedge with an average velocity characteristic of 2750 m/s. It appears to cover most of the Canadian segment of the continental shelf. The next, a relatively thin reflector, has a velocity of 3650 m/s and is comprised of a diversity of Mesozoic to Early Tertiary strata along the known parts of the shelf. The third layer (5180 m/s) is representative of the Lower Mesozoic to Upper Paleozoic rock units. The high velocity, 5840 m/s, fourth layer is revealed by a significant thickness of Lower Paleozoic Franklinian rocks. Regional acoustic properties of these last two strata are rather uniform across the shelf. The surveys mapped a local depocentre with sedimentary rock accumulations slightly over 7000 m. The lowermost strata, having 6400 m/s velocity are interpreted as the crystalline basement. The total sedimentary section discovered by the reflection investigation is somewhat more than either the land to offshore geologic extrapolations or the results of the 1985 refraction survey.

Geometry, evolution, and tectonic framework of the Skeena Fold Belt, north central British Columbia

The Intermontane Belt of the Canadian Cordillera has long been viewed as a passive, relatively rigid block between two metamorphic‐plutonic belts, the Coast Plutonic Complex and the Omineca Belt. However, the Skeena Fold Belt, which spans most of the width of the northern Intermontane Belt, exhibits shortening comparable with that in the Rocky Mountain Fold and Thrust Belt. The Skeena Fold Belt has many features of thin‐skinned fold and thrust belts, such as thrust faults which sole into a detachment, a wide variety of structural styles which depend on rock type, a foreland basin which was cannibalized by continued deformation, a frontal triangle zone, and a hinterland of metamorphic and plutonic rocks (Coast Plutonic Complex). The Skeena Fold Belt thus is comparable with the Rocky Mountain Fold and Thrust Belt, but rather than deforming a continental terrace wedge, it developed in a terrane (Stikinia) which had accreted to North America in the early Mesozoic, and in Jurassic and Cretaceous clastic successions (Bowser Lake and Sustut groups) which overlie Stikinia. Structural and stratigraphic relationships show that the earliest deformation occurred between Oxfordian and Albian time and that the last folds developed in latest Cretaceous or early Tertiary time. As much as 160 km of northeastward shortening in the Skeena Fold Belt was broadly contemporaneous with crustal thickening in the Coast Plutonic Complex and Omineca Belt, with dextral strike‐slip faulting east of the Skeena Fold Belt, and with shortening in the Rocky Mountain Fold and Thrust Belt. Therefore, between latest Jurassic and early Tertiary times, horizontal shortening occured across most of the width of the northern Canadian Cordillera. Concurrent shortening across the Cordillera suggests that a common detachment (or detachments) fed all of these zones as far east as the Rocky Mountain Fold and Thrust Belt.

The Lower Jurassic phosphorites of southeastern British Columbia and terrane accretion to western North America

Sinemurian phosphorites in southeastern British Columbia and southwestern Alberta conform with the "West Coast type" phosphorite depositional model. The model indicates that they were deposited on or near the Early Jurassic western cratonic margin, next to a sea or trough from which cold water upwelled. This suggests that the allochthonous terrane Quesnellia lay well offshore in Sinemurian time. The sea separating Quesnellia from North America was partly floored by oceanic crust ("Eastern Terrane") and partly by a thick sequence of rifted, continental terrace wedge rocks comprising the Purcell Supergroup and overlying Paleozoic sequence. This sequence must have been depressed sufficiently that access of upwelling deep currents to the phosphorite depositional area was not impeded.

The Western Canada sedimentary basin

The Western Canada Sedimentary Basin, a simple northeasterly tapering wedge of sedimentary rocks more than 6 km thick, extends southwest from the Canadian Shield into the Cordilleran foreland thrust belt. Its internal structure and the lateral variations in its shape reflect a long and complex history of development involving a foreland basin that was superimposed on a cratonic platform and continental terrace wedge. This history, which is inextricably linked to the evolution of the Canadian Cordillera, can be outlined succinctly with reference to the unconformity-bounded transgressive-regressive stratigraphic sequences established by Sloss ( Bull. geol. Soc. Am . 74, 93 (1963)), each of which has a distinctive character in Western Canada. The continental terrace wedge was established with the deposition of the Proterozoic Purcell (1500-1350 Ma) and Windermere (850-600 Ma) sequences, but the first record of the platformal phase is the early Palaeozoic transgressive onlap of the early Proterozoic ( > 1750 Ma) crystalline basement by the Sauk sequence. Early Palaeozoic subsidence of the margin of the craton may have been due to cooling of the lithosphere after renewed stretching at the ancient rifted western margin of the Precambrian craton, and to isostatic flexure of the lithosphere under the weight of the sediment that had accumulated at the margin in the oceanward prograding continental terrace wedge. During a subsequent Middle Ordovician to Middle Jurassic phase, the cratonic platform became differentiated into an intersecting network of epeirogenic arches with intervening basins. Development of the basins was as much a result of erosion and uplift of the arches between transgressive-regressive cycles as it was a result of differential subsidence of the basins during the cycles. The cause of the long (> 300 Ma) episode of intermittent epeirogenic movements that produced the basins and arches is a major unsolved problem. The foreland basin developed in two stages, in Middle Jurassic to early Cretaceous and late Cretaceous to Palaeocene time, as a result of collisions between North America and two pieces of a tectonic collage of oceanic terranes that were accreted to its western margin. During these two collisions, the continental terrace wedge, which had accumulated outboard from the rifted margin of the continental craton, was compressed and displaced over the western margin of the craton. Part of the supracrustal cover was scraped off the craton and accreted to the overriding mass to form a wedge of imbricate thrust fault slices that was tectonically prograded over the margin of the continental craton. Isostatic flexure of the continental lithosphere in response to the tectonic loading imposed on it by the displaced continental terrace wedge and the accretionary wedge of thrust slices produced the migrating moat in which the outwash of clastic detritus from the evolving thrust belt was trapped to form the foreland basin.

Evolution of sedimentary basins in the Canadian Arctic

Since middle Proterozoic time, two long-lasting phases affected the Canadian Arctic Archipelago, each forming a different sedimentary basin. The Franklinian Basin, which was floored by continental or quasi-continental crust, received 10 km or more of clastic, carbonate and volcanic rocks from the mid-Proterozoic to Devonian. Internal parts of the basin were deformed, intruded and metamorphosed locally, and external parts were folded and thrust cratonward by compressional episodes of the Ellesmerian Orogeny, which culminated in the late Devonian. This marked the end of a phase, at which time the entire region may have been emergent. The nature of plate interactions that produced Ellesmerian deformation are unknown. The second phase began in the early Carboniferous, when plate movements of the Boreal Rifting Episode created the proto-Canada Basin by left-hand transform motion of a plate along the modern continental margin and the location of the Kaltag Fault of northern Alaska. As a marginal side effect of that motion, the Sverdrup Basin developed as a peri-cratonic incipient rift. From the Carboniferous to late Cretaceous the basin received about 13 km of cratonic-derived clastic detritus. From late Cretaceous to early Tertiary time, the Arctic Archipelago was disrupted by the interference of two plate movements originating in the Arctic and North Atlantic regions. Those events had three main effects: the craton was extended and a graben-filled depression formed in the southeastern part of the archipelago; the eastern and central parts of the Sverdrup Basin were compressed and uplifted (Eurekan Orogeny); and resultant elastics prograded northwestward toward the Canada Basin, to form the Arctic continental terrace wedge.

Mineral Resource Expansion in British Columbia: ABSTRACT

Starting in 1980, British Columbia embarked on a decade of major growth in its mineral industry that is based on expansion of existing mines, development of established reserves, further investigation of known prospects, and widespread grass roots exploration. Production valued at less than $2 billion in 1980 is expected to double before the end of the decade. The evolution of the Cordillera provided a uniquely fertile environment for the formation of mineral deposits. An extensive terrace wedge on the margin of the craton and a growing collage of allochthonous terranes have been repeatedly activated by a sequence of collision, fault translation, and subduction. The result has been a great diversity of deposit types, found in a wide variety of tectonic settings from the I sular Belt to the Rocky Mountains. In the last few years it has become evident that favorable terranes for massive sulfides, porphyry, and shale-hosted lead-zinc deposits are much more extensive than originally believed. Current search concentrates on precious metal deposits or base metal deposits with significant by-product gold and silver. An array of deposit types for the latter is now recognized, including at least four types of bulk gold-silver deposits as well as a variety of vein types. Interest in the search for strategic minerals, not previously mined in the Canadian Cordillera, is also increasing. End_of_Article - Last_Page 960------------

Evolution of a Mixed Carbonate/Terrigenous Platform: Lower Cambrian Continental Terrace Wedge of Mackenzie Mountains, Northwest Territories, Canada: ABSTRACT

Rocks of the Sekwi Formation and underlying siltstones of G.S.C. units 10A and 13 exposed in the western Mackenzie Mountains define a continental terrace wedge that evolved during the Early Cambrian on the northwestern flanks of the North American continent. Growth of the continental terrace wedge can be categorized by three phases of platform up-, out- and in-building. Phase 1--Continuous out- and up-building, and evolution from a ramp to a platform configuration. Growth culminated in middle and upper Nevadella zone rocks with prominent karst surfaces, abundant intraclast pebbles in oolite shoal deposits and a switch in composition of slope and fan deposits from carbonates to terrigenous clastics. Phase 2--Coincided with transgression of the shelf near the Nevadella and onnia-Olenellus zone boundary. Maximum platform out-building occurred at this time and climaxed with subaerial exposure of mid-Bonnia-Olenellus zone rock. Slope deposits changed from carbonate to terrigenous sedimentation. Phase 3--Renewed slow up- and in-building that led to eventual drowning of the platform by latest Bonnia-Olenellus zone time. Sedimentation patterns on this platform are reciprocal. Periods of sea-level highstand are characterized by abundant limestones, whereas periods of sea-level lowstand are marked by terrigenous influxes. These two stages of sea-level stand are highlighted in compositional differences of carbonate breccia deposits. During sea-level highstands, breccias contain predominantly ribbon and nodular limestone clasts. They reflect conditions of early lith fication of the sea floor and non-equilibrium compaction of these deposits with subsequent sliding and downslope transport. In contrast during sea-level lowstands breccias consist preferentially of shelf and shelf-margin derived clasts. End_of_Article - Last_Page 590------------

Geodynamic evolution of the Canadian Cordillera — progress and problems

The present geodynamic pattern of the Canadian Cordillera, the main features of which were probably established in Miocene time, involves a combination of right-hand strike-slip movements on transform faults along the continental margin, and, in the south and extreme north, convergence in subduction zones in which oceanic lithosphere moves beneath the continent, with consequent magmatism along the continental margin. In the southern Canadian Cordillera, geophysical surveys have outlined the subducting slab and the asthenospheric bulge that occurs beneath and behind the magmatic arc. They also show that there is now no root of thickened Precambrian continental crust beneath the tectonically shortened supracrustal strata in the southern parts of the Omineca Crystalline Belt and Rocky Mountain Belt.The Rocky Mountain, Omineca Crystalline, Intermontane, Coast Plutonic, and Insular Belts, the structural and physiographic provinces that dominate the present configuration of the Canadian Cordillera, were established with the initial uplift and the intrusion of granitic rocks in the Omineca Crystalline Belt in Middle and Late Jurassic time and in the Coast Plutonic Complex in Early Cretaceous time, and they dominated patterns of uplift, erosion and deposition through Cretaceous and Paleogene time. Their development may be due to compression with thrust faulting in the eastern Cordillera, and to magmatism that accompanied subduction and to accretion of an exotic terrane, Wrangellia, in the western Cordillera. Major right-lateral strike-slip faulting, which occurred well east of but sub-parallel with the continental margin during Late Cretaceous and Paleogene time, accompanied major tectonic shortening due to thrusting and folding in the Rocky Mountain Belt as well as the main subduction-related (?) magmatism in the Coast Plutonic Complex.The configuration of the western Cordillera prior to late Middle Jurassic time is enigmatic. Late Paleozoic and early Mesozoic volcanogenic strata form a complex collage of volcanic arcs and subduction complexes that was assembled mainly in the Mesozoic. The change in locus of deposition between Upper Triassic and Lower to Middle Jurassic volcanogenic assemblages, and the thrust faulting in the northern Cordillera may record emplacement of another exotic terrane, the Stikine block, in latest Triassic to Middle Jurassic time.The earliest stage in the evolution of the Cordilleran fold belt involved the protracted (1500 to 380 Ma) development of a northeasterly tapering sedimentary wedge that discordantly overlaps Precambrian structures of the cratonic basement. This miogeoclinal wedge may be a continental margin terrace wedge that was prograded into an ocean basin, but it has features that may be more indicative of progradation into a marginal basin in which there was intermittent volcanic activity, than into a stable expanding ocean basin of the Atlantic type.

Has there been an oceanic margin to western North America since Archean time?

Western Canada consists of a series of progressively younger orogenic belts from the Archean Slave province westward to the present continental margin. Each belt was superimposed on the westernmost rim of its predecessor. With no sign of preliminary rifting, each cycle started with deposition of a continental terrace wedge. Each cycle then evolved through various styles of compression orogeny but was never terminated by continental collision. Thus the data indicate that there has been a western continental margin to Canada since Archean time–a margin that has been both passive and active but which was neither generated by rifting nor destroyed by continental collision. The pattern is similar farther to the south for middle Proterozoic to Holocene time, but is less clear for early Proterozoic time.

Miogeoclines (Miogeosynclines in Space and Time): A Reply

Previous articleNext article No AccessDiscussionMiogeoclines (Miogeosynclines in Space and Time): A ReplyRobert S. Dietz and Walter P. SprollRobert S. Dietz Search for more articles by this author and Walter P. Sproll Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 76, Number 1Jan., 1968 Article DOIhttps://doi.org/10.1086/627313 Views: 2Total views on this site Copyright 1968 University of ChicagoPDF download Crossref reports no articles citing this article.

Geomorphic Evolution of Continental Margins: ABSTRACT

Continental margins are important realms of gas and oil accumulation; 20 per cent of United States production already is offshore. It is necessary, therefore, to try to understand the genesis of continental margins and their subsequent geomorphic evolution. The writer suggests that continental margins are young (generally not older than mid-Mesozoic) and may be classified into two types: (1) accretionary margins caused by the collapse of continental rises and the associated accretion of new fold belts to the continents (Pacific-type); and (2) modified rift scars remaining after continental drift (Atlantic type). Most of the margins around the Atlantic and Indian Oceans are of the latter type but the bulge of Africa and the bight of North America are regarded as Pacific ty e. One can expect the two types to develop somewhat differently insofar as sedimentary modification and geomorphic evolution are concerned. They are either sites of mountain building and hence uplift, or are subsiding isostatically and accumulating a capping wedge of sediments and a thick continental-rise prism. These two elements (the terrace wedge and the rise prism) are living examples of the miogeosyncline and the eugeosyncline, respectively. Continental-rise prisms probably are petroliferous, but the petroleum is lost through metamorphism when the rise is collapsed into a eugeosyncline; terrace wedges also are petroliferous, and this petroleum is not lost with time because the presence of sial beneath prevents intense metamorphism. In equating terrace wedges with miogeosynclines, t is to be noted that both have a wedge-shape rather than the form of a syncline. This is regarded as a natural consequence of continental-margin sedimentation. In the search for oil in miogeosynclines, geologists should be aware that they were not closed but one-sided basins open toward the sea. Thus, instead of basin deposits, one finds mainly those sediments deposited during the prograding and retrograding of paralic-zone sediments. End_of_Article - Last_Page 461------------

Collapsing Continental Rises: An Actualistic Concept of Geosynclines and Mountain Building

Conventional geosynclinal theory is not actualistic-modern examples are said not to exist. This deficiency is obviated if the continental terrace sedimentary wedges are modern miogeosynclines and if the subjacent continental-rise sedimentary prisms are modern eugeosynclines, together forming a couplet. Such equating is reasonable once the wave-built terrace concept of continental terrace development is discarded. Instead, the writer assumes construction of the terrace wedge (a miogeosyncline) by prograded wedges of paralic sediments on an isostatically downwarping continental margin. Most sediment, however, is assumed to be carried off the continental terrace to form the continental rise (a eugeosynclinal prism), an apron of turbidity current deposits of graywacke or flysch facies. The accumulation of the mio-eugeosynclinal couplet requires diastrophic quiescence except for isostatic depression. The orogenic or alpine mountain-building cycle is initiated by thrusting probably related to decoupling of a sp...