Channels with a sinuous planform are common in both continental and deep-marine environments on Earth, and similarly in high-resolution images of the surface of Mars. Whereas common in rivers, continuous lateral channel migration and point-bar deposition appear to be much less common in the deep sea. In the bends of rivers, near-bed flow driving point bar growth results from a cross-flow superelevation of the water surface that sets up a lateral hydrostatic pressure gradient driving an inward-directed flow near the bed. However, in deep-marine systems the surface between the turbidity current and overlying ambient fluid sits well above the channel margins, and therefore precludes a similar lateral superelevation of the current top. Here it is argued that the cross-flow component is related to a density gradient that mimics the effect of the hydrostatic pressure gradient in rivers, and develops as coarse suspended particles that experience little uplift, and therefore negligible overspill, become concentrated along the outer bank. This condition develops best in a two-part suspension made up of a highly concentrated, unstratified basal plug of coarse sediment overlain sharply by a dilute cloud of much finer sediment—a density structure that differs from the more typical upward exponential decrease in density. The abundance of coarse and fine sand, but depletion in the intermediate grain size fraction, is related to transgressive shelf processes and its influence on sediment supplied to the system, and in turn, the flow structure of the current. It is under these seemingly uncommon granulometric conditions that continuous laterally migrating channels, and accordingly, riverine-like point-bar deposition, is most common in the deep sea.

At the Castle Creek study area a superbly exposed continental-slope channel-complex set of the Isaac Formation termed ICC1 crops out. ICC1 is 220 m thick and exposed over 5 km parallel to bedding and consists of four vertically stacked channel complexes—lower channel complex (LC), and three upper channel complexes (termed UC1–UC3). Based on differences in lithological and textural makeup, and stratal trends, two styles of channel fill and stacking pattern are recognized—disorganized stack of bottom-up channel fills (LC, UC1, and UC2), and organized lateral-offset stack of laterally accreting channel fills (UC3). Bottom-up channel fills show an upward and axis-to-margin fining and thinning, and were filled by poorly sorted, density-stratified flows. In contrast, channel fills of UC3 exhibit negligible upward and lateral fining and thinning and were filled by flows enriched in coarse and very coarse sand and were only negligibly stratified in their lower part. Accordingly, the lower, depositionally important part of the flow had a plug-like density profile with limited overspill that caused circulation patterns in channel bends to resemble those in rivers. Notably, the grain-size makeup of late-stage channel fills of UC3 indicates a change in the makeup of the sediment supply, namely grain size and sorting. Note that this systematic upward change in sediment texture and channel type is observed in most (5 of 8) Isaac slope channel-complex sets at the Castle Creek study area. These changes reflect a change from hinterland- to shelf-dominated sediment supply interpreted to be associated with a long-term rise of relative sea level that progressively expanded continental-shelf accommodation and retention of coarse relict and palimpsest sediment. During short-term relative sea-level falls this increasingly voluminous source of coarse, moderately well-sorted siliciclastic and minor carbonate sediment eventually came to dominate the supply of coarse sediment to the shelf edge, and in turn, controlled the character of turbidity currents that remobilized sediment farther down the continental slope and possibly onto the basin floor.

No abstract

Geological and geomechanical heterogeneities exist at multiple scales in fine-grained rocks; however, the complexity of characteristics at the centimeter- to microscale heterogeneities remains poorly understood. In this study, 10 representative samples composed of three centimeter-scale sedimentary fabrics (massive siltstone (F1), stratified siltstone (F2), and bioturbated siltstone (F3)) were analyzed from the Lower Triassic Montney Formation in the Western Canada Sedimentary Basin to describe sedimentological heterogeneity based on sedimentary fabric, compositional, and geomechanical properties. Sedimentary fabric was determined based on grainsize and the distribution of bedforms, which subdivide the facies into four μm- to mm-scale microfacies (massive siltstone (MF1), pinstriped laminated siltstone (MF2), planar- to cross-stratified siltstone (MF3), and bioturbated siltstone (MF4)). Microscale analysis using a scanning electron microscope was used to characterize microfacies and their respective mineralogical makeup (matrix, cement, and framework grains). To quantify heterogeneity, sedimentary fabric was assessed using a CT scan complemented by elemental composition (using X-ray fluorescence), and geomechanical hardness (using Equotip Piccolo Bambino handheld microhardness tool) was collected within a 1 cm by 1 cm grid within each sample. Datasets were compared using a discriminant analysis (DA) to recognize trends between multiple properties and suggest that sedimentary fabric with the highest centimeter-scale aluminum content from XRF (avg. 11%) comprises microfacies that are comparatively matrix-rich consisting of micas, negligible calcite cement, and exhibit the lowest handheld hardness values (<770). Alternatively, sedimentary fabric with a higher elemental calcium component (avg. 18%) comprises microfacies that are matrix-poor, cemented by carbonate (calcite and dolomite) and quartz, and overall exhibit a positive trend with hardness measurement (770–850). Furthermore, to relate the elemental and geomechanical proxies to controls on rock mechanics, natural calcite-filled fractures within the studied core intervals were characterized. Fractures were subdivided into three types—brecciated, bed-parallel, and vertical to subvertical fractures with each type being constrained to a specific sedimentary fabric. Based on centimeter gridding, microscale analysis and the degree of fabric interbedding play a primary role on the variability in mechanical hardness and the geometry and termination of natural fractures. Collectively, this dataset provides insight into the influence that sedimentary fabric and the distribution of elemental composition has on mechanical properties and natural fractures below well log resolution. These findings can be used to better model and predict fine-grained deposit characteristics before undergoing hydraulic stimulation.