Nick Howes

Comparing submarine and fluvial channel kinematics: Implications for stratigraphic architecture

Submarine and fluvial channels exhibit qualitatively similar geomorphic patterns, yet produce very different stratigraphic records. We reconcile these seemingly contradictory observations by focusing on the channel belt scale and quantifying the time-integrated stratigraphic record of the belt as a function of the scale and trajectory of the geomorphic channel, applying the concept of stratigraphic mobility. By comparing 297 submarine and fluvial channel belts from a range of tectonic settings and time intervals, we identify channel kinematics (trajectory) rather than channel morphology (scale) as the primary control on stratigraphic architecture and show that seemingly similar channel forms (in terms of scaling) have the potential to produce markedly different stratigraphy. Submarine channel belt architecture is dominated by vertical accretion (aggradational channel fill deposits), in contrast to fluvial systems that are dominated by lateral accretion (point bar deposits). This difference is best described with the channel belt aspect ratio, which is 9 for submarine systems and 72 for fluvial systems. Differences in channel kinematics and thus stratigraphic architecture between the two environments appear to result from markedly different coupling between channel aggradation and overbank deposition. The methodology and results presented here are also applicable to interpreting channelized stratigraphy on other planets and moons.

High-resolution, millennial-scale patterns of bed compensation on a sand-rich intraslope submarine fan, western Niger Delta slope

Near-seafloor core and seismic reflection-data from the western Niger Delta continental slope document the facies, architecture, and evolution of submarine channel and intraslope submarine fan deposits. The submarine channel enters an 8-km-long by 8-km-wide intraslope basin, where more than 100 m of deposits form an intraslope submarine fan. Lobe deposits in the intraslope submarine fan show no significant downslope trend in sand presence or grain size, indicating that flows were bypassing sediment through the basin. This unique data set indicates that intraslope lobe deposits may have more sand-rich facies near lobe edges than predicted by traditional lobe facies models, and that thickness patterns in intraslope submarine fans do not necessarily correlate with sand presence and/or quality. Core and radiocarbon age data indicate that sand beds southward during the late Pleistocene, resulting in the compensation of at least two lobe elements. The youngest lobe element is well characterized by core data and is sand rich, ∼2 km wide × 6 km long, and >1 m thick and was deposited rapidly over ∼4000 yr, from 18 to 14 ka. Sand beds forming an earlier lobe element were deposited on the northern part of the fan from ca. 25 to 18 ka. Seafloor geomorphology and amplitudes from seismic reflection data confirm the location and age of these two compensating lobe elements. A third compensation event would have shifted sand deposition back to the northern part of the fan, but sediment supply was interrupted by rapid sea-level rise during Meltwater Pulse 1-A at ca. 14 ka, resulting in abandonment of the depositional system.

Facies architecture of submarine channel deposits on the western Niger Delta slope: Implications for grain‐size and density stratification in turbidity currents

High-resolution bathymetry, seismic reflection, and piston core data from a submarine channel on the western Niger Delta slope demonstrate that thick, coarse-grained, amalgamated sands in the channel thalweg/axis transition to thin, fine-grained, bedded sands and muds in the channel margin. Radiocarbon ages indicate that axis and margin deposits are coeval. Core data show that bed thickness, grain size, and deposition rate strongly decrease with increasing height above channel thalweg and/or distance from channel centerline. A 5 times decrease in bed thickness and 1–2 ψ decrease in grain size are evident over a 20 m elevation change (approximately the elevation difference between axis and margin). A simplified in-channel sedimentation model that solves vertical concentration and velocity profiles of turbidity currents accurately reproduces the vertical trends in grain size and bed thickness shown in the core data set. The close match between data and model suggests that the vertical distribution of grain size and bed thickness shown in this study is widely applicable and can be used to predict grain size and facies variation in data-poor areas (e.g., subsurface cores). This study emphasizes that facies models for submarine channel deposits should recognize that grain-size and thickness trends within contemporaneous axis-margin packages require a change in elevation above the thalweg. The transition from thick-bedded, amalgamated, coarser-grained sands to thin-bedded, nonamalgamated, finer-grained successions is primarily a reflection of a change in elevation. Even a relatively small elevation change (e.g., 1 m) is enough to result in a significant change in grain size, bed thickness, and facies.

Analysis Tools to Quantify the Variability in Deltaic Geological Models Using Delft3D Simulation Results

The process of constructing geological models is used on various scales in mining, oil and gas exploration, hydrology as well as in large construction projects. Development of geological models is a complex process consisting of various phases. A large degree of uncertainty is introduced from the interpretation of the data to the construction of the geological model. To arrive at the best approximation of the subsurface, relevant analogues are identified and consulted. Therefore, uncertainties originate from unknown depositional processes, but also from uncertain correlation between the study area and the analogues. We developed a set of tools to quantify the variability in deltaic geological models resulting from these uncertainties. These tools were applied to an ensemble of simulations generated in Delft3D by processed-based forward modelling. We show how a set of analyses can be used to quantify the differences in the resultant delta deposits. Analyses investigated channel networks, topographic profiles and sediment distribution in the delta. The tools make use of the unique advantages of numerical forward models, allowing single variables to be studied at high spacial and temporal resolution.