Sediment Gravity Flow Processes Research Articles

Slope readjustment: A new model for the development of submarine fans and aprons

In a new model proposed for the formation of submarine canyons and submarine fan-aprons, numerical simulations of basin fill are utilized to illustrate simple concepts of slope grading. This modeling suggests that erosional truncation, sediment bypass, and marine onlap of submarine fan-apron complexes are formed in response to changing basin physiography. Two kinds of basin margins are identified. (1) Progradational margins represent the basinward advance of graded depositional profiles formed when diffusive and sediment gravity-flow processes are in equilibrium with sediment supply, basin subsidence, and basin physiography. (2) Erosional margins form when upper slope gradients exceed an equilibrium grade, and are characterized by erosion, slumping, and sediment bypass to lower slope environments via sediment gravity-flow processes. Erosional margins are transformed into progradational margins when the bathymetric escarpment (i.e., an oversteepened margin) is buried by onlapping and aggrading fanapron deposits. Progradational margins can develop bathymetric escarpments and become erosional in response to a rapid rise in relative sea level, structural deformation of basin profiles (e.g., faulting), and/or a transition from carbonate to siliciclastic deposition. Whereas relative falls in sea level play a major role in bringing the source of sediment input to the shelf edge, the development of slope unconformities and onlapping submarine fan-apron systems is controlled primarily by slope- readjustment processes triggered by changing basin physiography.

Patterns and variability in sediment accumulation rates, Fraser River delta foreslope, British Columbia, Canada

Minimum sediment accumulation rates on the Fraser River delta foreslope exhibit a high degree of spatial variation, with accumulation rates ranging from 0.50 to 3.0 cm/yr. Accumulation rates generally increase towards Sand Heads channel, the active foreslope depocenter. Sedimentation rates and patterns and micropaleontological assemblages are interpreted to reflect reintroduction of older sediment from upslope via slumping and sedimentgravity flow processes. Such processes account for the bulk of sedimentation in much of the subaqueous delta. These processes provide a mechanism for sand bypassing of the delta plain and foreslope and for delivery of coarse-grained sediment directly to prodelta and basinal environments.

Facies analysis of the Kopervik sand interval, Kilda Field, Block 16/26, UK North Sea

Abstract The Kilda Field is a large gas condensate accumulation located in UK Block 16/26. The reservoir sands, informally termed Kopervik, are Lower Cretaceous in age and are developed on the southern dip slope of the Fladen Ground Spur. They represent a localized sandy development within the lower Sola and upper Valhall formations, associated with the Austrian orogeny. Sands were carried out into the basin by a variety of sediment gravity flow processes, and were deposited as an apron near the foot of the slope. Their distribution is controlled largely by the nature of the flow and by pre-existing topography. Six main sand facies have been recognized, three of which are high-density turbidite (HDT) types and contain virtually all reservoir quality rock. HDT S 3 sands resulted from suspension collapse. They exhibit porosities around 15%, and permeabilities around 40 md. HDT Laminated sands probably formed from traction sedimentation. They have similar average porosities but reduced permeabilities between 10 and 15 md. Sands of the Marginal facies are of an inferior reservoir quality. The remaining three facies consist of non-reservoir quality sands deposited by liquefaction, debris or low-density turbidite flow. Post-depositional disturbance suggests that deposition took place on an unstable palaeoslope. Subsequent porosity loss within clean sand intervals has been dominantly through compaction.

Geologic Reservoir Model for the Triassic Doig Formation, Northeast British Columbia, Canada

A subsurface investigation of the mid-Triassic Doig formation in northeastern British Columbia documented two main reservoir facies. Both are a product of mass movement and sediment gravity flow processes on a progradational, tectonically active continental shelf margin. Substrate instability was likely a product of sediment loading, perhaps in concert with seismic activity. Sedimentary facies and reservoir parameters were determined from analysis of approximately 150 cores and 900 well logs. Laterally discontinuous Doig sandstones are up to 60 m thick and trend northeasterly within the study area. The main reservoir facies are incised density flow deposits and laterally extensive slump deposits. Reservoir quality within these sands is extremely variable with porosity ranging from less than 5% to 15%. In core, these deposits consist of moderately well sorted, very fine grained sandstones with no vertical grain size variation. The best production to date is in the Buick Creek field with initial flows of 346 BOPD. The slump deposits are thinner and tend to be more elongate parallel to paleoshoreline. These sands were subject to some wave or current reworking. Modern analogs where similar processes and products of deposition are known to occur include the Gulf of Alaska continental shelf and the Frasermore » River Delta slope. Doig sandstones usually are enclosed in fine-grained shelf deposits that provide a good stratigraphic trapping mechanism. Successful development of Doig reservoirs must incorporate geologic modes that assist in understanding the complex and highly variable reservoir quality of sandstones units.« less

Bio- and Lithofacies of Modern Sediments in Knight and Bute Inlets, British Columbia

The high sedimentation rates that occur in fjords make them an ideal setting for studying sediment transfer from proximal first cycle sources. These environments, and their associated sediment transport processes, are good analogues to some small hydrocarbon-bearing basins. Knight and Bute Inlets, on Canada's west coast, are fords that display textural facies of recent sediments and distributions of benthonic foraminifera, thecamoebians and clay-size minerals that reflect freshwater discharge, bottom-water renewal and sediment gravity flow processes. In Knight and Bute, a fiord head prodelta mud facies shaped by fluvial depositional processes is defined by unimodal (5-60) sediments and by low-diversity arenaceous foraminifera assemblages that include a large proportion of displaced freshwater thecamoebian specimens. A comparatively distal basinal mud facies reflects a flocculation process that gives rise to a unimodal (80) sediment. The clay-size fraction of this basinal facies is enriched in quartz and feldspar compared to the prodelta mud. Arenaceous foraminifera predominate in upper and middle fiord basinal mud facies environments but are replaced by calcareous types near fiord mouths because of the seasonal incursion of marine water from the continental shelf The prodelta mud and basinal mud facies interfinger with a gravity flow and overspill transition facies. The grain size distributions of the transition facies includes types having 1.50 or 4-60 modes that reflect a combination offluvial, hemipelagic, and gravity flow processes. Channels cut by the downslope transport of coarse sediment as cohesionless mass flows focus deposition in distal basins. Inherent in this process is sediment bypassing and the passive transport of large numbers of thecamoebians to distal prodelta marine environments. Sediment on sills near the mouth of Knight and Bute Inlets are reworked morainal deposits facies that are presently being modified by tidal currents that also serve to concentrate indigenous calcareous foraminifera populations into foraminiferal lag deposits. INTRODUCTION

Late Pleistocene debris-flow deposits in large glacial lakes in British Columbia and Alaska

The movement of Late Wisconsin (<30,000 B.P.) ice-sheet lobes counter to the direction of regional drainage ponded large lakes (up to 5000 km 2) in both the Upper Fraser River Valley in British Columbia and the Cooper River Valley in Alaska. Along the Fraser River, near Quesnel, glaciolacustrine sediment gravity-flow complexes contain thick (up to 10 m) and laterally extensive (up to 3 km) diamict units containing large rafts of poorly lithified Tertiary rocks. Diamicts offlap and thin away from Tertiary bedrock highs and were emplaced by subaqueous debris flows. These flows probably evolved from slumps and slides associated with the downslope failure of unstable Tertiary bedrock slopes triggered by rising lake levels, a process akin to “first-filling” slope failures around the margins of man-made lakes. In Alaska, along the Copper River Valley, massive and stratified diamicts were also deposited by debris flows and are interbedded with thin-bedded turbidites, poorly sorted gravels and normally graded diamicts. Debris flow deposits show basal grooves, internal compressional structures and rafted clasts that protrude from bed tops. Flows originated by slumping of rapidly deposited and underconsolidated glacial sediments around the basin margins; an important factor in flow generation has been the combination of steep substrate slopes and frequent earthquake shock. Observations in both British Columbia and Alaska stress the significance of sediment gravity-flow processes acting to deposit “glacial” sedimentary successions in large ice-contact lake basins.