ALBERT

Description: The southern Canadian Cordillera strata of the Neoproterozoic Windermere Supergroup form an areally extensive outcrop belt of deep-marine sedimentary rocks. Within this generally monotonous pile of siliciclastic and minor carbonate rocks, the Old Fort Point Formation forms a lithologically and geochemically distinctive unit that serves as a key regional stratigraphic marker. New sedimentologic and stratigraphic work demonstrates that it is lithologically distinctive, mappable and correlatable on a regional scale and deserves formal recognition. The Old Fort Point Formation comprises three lithostratigraphic members that have a consistent stratigraphic relationship across the basin and can easily be distinguished from lithofacies in the enveloping strata of the Windermere Supergroup. The basal Temple Lake Member is composed primarily of siltstone to mudstone that grades upward into rhythmically interstratified limestone-siltstone. The middle Geikie Siding Member is a thin, organic-rich mudstone-pelite. The Whitehorn Mountain Member is the uppermost unit and varies locally and regionally in thickness and lithology, including diamictite, breccia to conglomerate, mudstone to siltstone, subarkose, quartzarenite, calcareous arenite, arenaceous limestone, and limestone. This unique lithostratigraphic unit is here formally named the Old Fort Point Formation and other site-specific names should be discontinued. The use of the name Old Fort Point Formation is an attempt to simplify part of a complicated and informally defined stratigraphic nomenclature currently in use for rocks in the Neoproterozoic Windermere Supergroup, southern Canadian Cordillera.

Description: The approximately 608 Ma Old Fort Point Formation (OFP) is a unique, mixed siliciclastic and carbonate stratigraphic marker horizon exposed locally over an area of about 35 000 km 2 within the Neoproterozoic Windermere Supergroup (WSG) of the southern Canadian Cordillera. Paleogeographically, these strata were deposited by a variety of deep-marine processes in slope to basin-floor settings along the continental margin of Laurentia (ancestral North America). The OFP ranges from approximately 60 to 450 m thick and comprises three informal lithostratigraphic members, which stratigraphically upward are the Temple Lake (TLM), Geikie Siding (GSM) and Whitehorn Mountain (WMM) members. The TLM and GSM are generally uniform in thickness and lithofacies and are interpreted to reflect basin-wide, synchronous deposition. The TLM is characteristically composed of variably coloured fine-grained rocks that stratigraphically-upward grade from siltstone to rhythmically bedded limestone-siltstone couplets. These fine-grained strata represent deposition during a major, post-glacial eustatic rise that essentially terminated the supply of coarsegrained siliciclastic sediment into the deep-water part of the Windermere basin. These strata are then gradationally overlain, although over an interval only 〈0.5–2 m thick, by strata of the GSM that stratigraphically-upward become progressively more organic-rich. The GSM is interpreted to have accumulated during terminal transgressive and highstand conditions, likely in anoxic bottom-water below a postulated pycnocline. The youngest member of the OFP is the WMM. The base of the WMM is almost always sharp and locally marked by scours more than 100 m deep. This surface is interpreted to be the result of an abrupt fall of relative sea level related to regional tectonic uplift, which in turn caused widespread mass wasting on the slope, and locally the formation of deeply incised submarine canyons. Sediment that accumulated in the canyons and more distally on the basin floor during the ensuing lowstand and early transgression was composed of remobilized slope sediment mixed with texturally and mineralogically immature clastic sediment derived directly from the hinterland. As transgression proceeded, the shelf eventually became flooded and shut-off the hinterland sediment supply. At the same time (late transgression and/or highstand), however, new, more landward canyons formed, or existing canyon heads eroded headward and reactivated the slope-basin floor transport system. This time, however, sediment was made up of a mixture of texturally and mineralogically mature palimpsest siliciclastic and carbonate shelf sediments. Notwithstanding its unique lithological and geochemical characteristics, strata of the OFP are underlain and overlain by a thick monotonous succession of mostly siliciclastic deep-marine sandstones and mudstones, suggesting that deposition of the OFP represented only a short-term anomaly superimposed on a much longer-term history of physical and geochemical stability throughout the Neoproterozoic deep-water Windermere basin.

Description: 〈p〉Detailed sedimentological and stratigraphic analyses of a 〈i〉c.〈/i〉 1500 m thick, siliciclastic-dominated slope succession in the Neoproterozoic Isaac Formation at the Castle Creek study area (southern Canadian Cordillera) reveals the occurrence of four well-preserved mass-transport complexes (MTCs) composed principally of slide/slump and debris-flow deposits. The stratigraphically lowest of these complexes is about 60 m thick and crops out for 〉2.5 km laterally, consisting of slide and debrite. The slide has an irregular erosive base with ramp-and-flat geometry. This is overlain locally by boulder-sized blocks of slightly to moderately deformed strata, bounded by shear surfaces. The slide is overlain by a debrite that pinches and swells laterally, consisting of matrix-supported conglomerate with common metre-scale clasts of mudstone and coarse-grained sandstone embedded in a mudstone-rich matrix with dispersed, pebble quartz grains. Based on its stratigraphic position at the base of the slope, vertical stacking of slide-debrite, lithological distribution, considerable thickness and lateral extent, this MTC is interpreted to be associated with a major episode of continental slope instability and submarine mass-wasting. The close association between the MTC and underlying/overlying mixed carbonate-siliciclastic strata suggests that sea level most likely exerted a key control on sediment supply, which ultimately led to the emplacement of this MTC.〈/p〉

Description: Detailed sedimentological and stratigraphic analyses of a c. 1500 m thick, siliciclastic-dominated slope succession in the Neoproterozoic Isaac Formation at the Castle Creek study area (southern Canadian Cordillera) reveals the occurrence of four well-preserved mass-transport complexes (MTCs) composed principally of slide/slump and debris-flow deposits. The stratigraphically lowest of these complexes is about 60 m thick and crops out for 〉2.5 km laterally, consisting of slide and debrite. The slide has an irregular erosive base with ramp-and-flat geometry. This is overlain locally by boulder-sized blocks of slightly to moderately deformed strata, bounded by shear surfaces. The slide is overlain by a debrite that pinches and swells laterally, consisting of matrix-supported conglomerate with common metre-scale clasts of mudstone and coarse-grained sandstone embedded in a mudstone-rich matrix with dispersed, pebble quartz grains. Based on its stratigraphic position at the base of the slope, vertical stacking of slide-debrite, lithological distribution, considerable thickness and lateral extent, this MTC is interpreted to be associated with a major episode of continental slope instability and submarine mass-wasting. The close association between the MTC and underlying/overlying mixed carbonate-siliciclastic strata suggests that sea level most likely exerted a key control on sediment supply, which ultimately led to the emplacement of this MTC.

Description: The Potsdam Group is a Cambrian to Lower Ordovician siliciclastic unit that crops out along the southeastern margins of the Ottawa graben. From its base upward, the Potsdam consists of the Ausable, Hannawa Falls, and Keeseville formations. In addition, the Potsdam is subdivided into three allounits: allounit 1 comprises the Ausable and Hannawa Falls, and allounits 2 and 3, respectively, the lower and upper parts of the Keeseville. Allounit 1 records Early to Middle Cambrian syn-rift arkosic fluvial sedimentation (Ausable Formation) with interfingering mudstone, arkose, and dolostone of the marine Altona Member recording transgression of the easternmost part of the Ottawa graben. Rift sedimentation was followed by a Middle Cambrian climate change resulting in local quartzose aeolian sedimentation (Hannawa Falls Formation). Allounit 1 sedimentation termination coincided with latest(?) Middle Cambrian tectonic reactivation of parts of the Ottawa graben. Allounit 2 (lower Keeseville) records mainly Upper Cambrian quartzose fluvial sedimentation, with transgression of the northern Ottawa graben resulting in deposition of mixed carbonate–siliciclastic strata of the marine Rivière Aux Outardes Member. Sedimentation was then terminated by an earliest Ordovician regression and unconformity development. Allounit 3 (upper Keeseville) records diachronous transgression across the Ottawa graben that by the Arenigian culminated in mixed carbonate–siliciclastic, shallow marine sedimentation (Theresa Formation). The contact separating the Potsdam Group and Theresa Formation is conformable, except locally in parts of the northern Ottawa graben where the presence of localized islands and (or) coastal salients resulted in subaerial exposure and erosion of the uppermost Potsdam strata, and accordingly unconformity development.

Description: The tectonic setting of northern Laurentia prior to the opening of the Arctic Ocean is the subject of numerous tectonic models. By better understanding the provenance of detrital zircon in the Canadian Arctic prior to rifting, both the prerift tectonic setting and timing of rifting can be better elucidated. In the Sverdrup Basin, two distinct provenance assemblages are identified from new detrital-zircon U-Pb data from Lower Triassic to Lower Jurassic strata in combination with previously published detrital-zircon data. The first assemblage comprises an age spectrum identical to that of the Devonian clastic wedge in the Canadian Arctic and is termed the recycled source. In contrast, the second assemblage is dominated by a broad spectrum of near syndepositional Permian–Triassic ages derived from north of the basin and is termed the active margin source. Triassic strata of Yukon and Arctic Alaska exhibit a similar dual provenance signature, whereas in northeastern Russia, Chukotka contains only the active margin source. Complementary hafnium isotopic data on Permian–Triassic zircon have Hf values that are consistent with the common evolved crustal signature of the Devonian clastic wedge detrital-zircon grains and Neoproterozoic–Paleozoic basement rocks in the Arctic Alaska–Chukotka microcontinent. Furthermore, newly identified volcanic ash beds throughout the Triassic section from the northern part of the Sverdrup Basin, along with abundant Permian–Triassic detrital zircon, suggest a protracted history of magmatism to the north of the basin. We interpret that these zircons were sourced from a magmatically active region to the north of the Sverdrup Basin, and in the context of a rotational model for opening of Amerasia Basin, this was probably part of a convergent margin fringing northern Laurentia from the northern Cordillera along the outboard edge of Arctic Alaska and Chukotka terranes. In Early Jurassic strata, Permian–Triassic zircons decrease substantially, implying the diminution of the active margin as a sediment source as initial rifting isolated the Permian–Triassic source from the Sverdrup Basin.

Description: : In the deep-marine stratigraphic record classical turbidites are common and when complete comprise five vertically stacked units, which in ascending order are termed the a to e divisions. The b division consists of planar lamination, which in almost all cases is overlain by ripple cross-stratification of the c division. Conspicuously absent in this succession is dune cross-stratification, which for sediments coarser than middle fine sand in a decelerating flow should occur between the planar-stratified and ripple cross-stratified units. Here it is argued that the paucity of dune cross-stratification is the result of the deleterious effect of suspended sediment on dune inception. Specifically, high suspended-sediment concentration, and hence high density, in the bottom part of the parent turbidity current prevents the development of a sufficiently sharply defined interface between the bottom part of the current and the dense, underlying bed-load layer. This results in the absence of the necessary hydrodynamic instability whose waveform structure along the interface would otherwise mould the bed surface into the incipient bed forms from which dunes (and also ripples) grow. The almost exclusive occurrence of ripple cross-stratification above planar lamination suggests that in almost all natural turbidity currents a sufficiently well developed density interface does not become established until flow speed is in the ripple stability field.

Description: A bstract :  Turbidity currents, and other types of submarine sediment density flow, redistribute more sediment across the surface of the Earth than any other sediment flow process, yet their sediment concentration has never been measured directly in the deep ocean. The deposits of these flows are of societal importance as imperfect records of past earthquakes and tsunamogenic landslides and as the reservoir rocks for many deep-water petroleum accumulations. Key future research directions on these flows and their deposits were identified at an informal workshop in September 2013. This contribution summarizes conclusions from that workshop, and engages the wider community in this debate. International efforts are needed for an initiative to monitor and understand a series of test sites where flows occur frequently, which needs coordination to optimize sharing of equipment and interpretation of data. Direct monitoring observations should be combined with cores and seismic data to link flow and deposit character, whilst experimental and numerical models play a key role in understanding field observations. Such an initiative may be timely and feasible, due to recent technological advances in monitoring sensors, moorings, and autonomous data recovery. This is illustrated here by recently collected data from the Squamish River delta, Monterey Canyon, Congo Canyon, and offshore SE Taiwan. A series of other key topics are then highlighted. Theoretical considerations suggest that supercritical flows may often occur on gradients of greater than ~ 0.6°. Trains of up-slope-migrating bedforms have recently been mapped in a wide range of marine and freshwater settings. They may result from repeated hydraulic jumps in supercritical flows, and dense (greater than approximately 10% volume) near-bed layers may need to be invoked to explain transport of heavy (25 to 1,000 kg) blocks. Future work needs to understand how sediment is transported in these bedforms, the internal structure and preservation potential of their deposits, and their use in facies prediction. Turbulence damping may be widespread and commonplace in submarine sediment density flows, particularly as flows decelerate, because it can occur at low (〈 0.1%) volume concentrations. This could have important implications for flow evolution and deposit geometries. Better quantitative constraints are needed on what controls flow capacity and competence, together with improved constraints on bed erosion and sediment resuspension. Recent advances in understanding dilute or mainly saline flows in submarine channels should be extended to explore how flow behavior changes as sediment concentrations increase. The petroleum industry requires predictive models of longer-term channel system behavior and resulting deposit architecture, and for these purposes it is important to distinguish between geomorphic and stratigraphic surfaces in seismic datasets. Validation of models, including against full-scale field data, requires clever experimental design of physical models and targeted field programs.