Zane R. Jobe

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.

Halokinetic effects on submarine channel equilibrium profiles and implications for facies architecture: conceptual model illustrated with a case study from Magnolia Field, Gulf of Mexico

Abstract In Magnolia Field, deepwater sediments were affected during deposition by allochthonous salt. Pleistocene channel systems developed on a salt flank and were initially deeply incised close to the salt but progressively avulsed down the lateral slope, each time with decreasing depth of incision. Following this degradational stage, a lobe developed on top of the channel fills and a large-scale aggradational system developed. A conceptual model of submarine channel development adjacent to active topography has been developed from this dataset. Channels may become deeply entrenched during stages of salt growth, but only where flow frequency and magnitude are sufficient to outpace topographic growth. Where flows are less frequent topographic growth may present a barrier to successive flows, causing avulsion. The large-scale cycles of salt growth and withdrawal commonly recognized in subsurface systems, combined with eustatic sea-level changes, may result in a cyclic style of evolution whereby channels initially become entrenched and/or step away from the growing topography, switching to backfilling as salt growth slows or pauses, followed by a distributive-style as the entire system backsteps. During salt withdrawal the equilibrium profile may become relatively raised and channels may develop an aggradational style. In these settings, significant cross-channel facies asymmetry may result.

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.

Global (latitudinal) variation in submarine channel sinuosity: COMMENT

[Peakall et al. (2012)][1] propose that submarine channel sinuosity correlates with latitude, and conclude that this correlation results from modification of the turbidity flow field by the Coriolis force. However, a closer look at the data and simple physical arguments suggest that slope and the

Volume and recurrence of submarine-fan-building turbidity currents

Submarine fans are archives of Earth‐surface processes and change, recording information about the turbidity currents that construct and sculpt them. The volume and recurrence of turbidity currents are of great interest for geohazard assessment, source‐to‐sink modelling, and hydrocarbon reservoir characterization. Yet, such dynamics are poorly constrained. This study integrates data from four Quaternary submarine fans to reconstruct the volume and recurrence of the formative turbidity currents. Calculated event volumes vary over four orders of magnitude (105 to 109 m3), whereas recurrence intervals vary less, from 50 to 650 years. The calculated turbidity‐current‐event volume magnitudes appear to be related to slope position and basin confinement. Intraslope‐fan deposits have small event volumes (ca 106 m3) while ponded‐fan deposits have very large event volumes (108 to 109 m3). Deposits in non‐ponded, base‐of‐slope environments have intermediate values (107 to 108 m3). Sediment bypass in intraslope settings and flow trapping in ponded basins likely account for these differences. There seems to be no clear relationship between event recurrence and basin confinement. Weak scaling exists between event volume and source‐area characteristics, but sediment storage in fluvial and/or intraslope transfer zones likely complicates these relationships. The methodology and results presented here are also applied to reconstruct the time of deposition of ancient submarine‐fan deposits of the Tanqua Karoo basin, South Africa. The volume and recurrence of submarine‐fan‐building turbidity currents form intermediate values between values measured in submarine canyons and channels (<105 m3 and <101 years) and on abyssal plains (>108 m3 and >103 years), indicating that small, frequent flows originating in submarine canyons often die out prior to reaching the fan, while rare and very large flows mostly bypass the fan and deposit sediment on the abyssal plain. This partitioning of flow volume and recurrence along the submarine sediment‐routing system provides valuable insights for better constraining geohazards, hydrocarbon resources and the completeness of the stratigraphic record.

Controls on submarine channel-modifying processes identified through morphometric scaling relationships

Submarine channels share morphological similarities with rivers, but observations from modern and ancient systems indicate they are formed under processes and controls unique to submarine settings. Morphologic characteristics of channels—e.g., width, depth, slope, and the relationships among them—can constrain interpretations of channel-forming processes. This work uses morphometric scaling relationships extracted from high-resolution seafloor bathymetry to infer connections between morphology and process in submarine channels. Analysis of 36 modern channels in five geographic regions shows that channel widths vary regionally (from <100 m to >10 km wide) but occupy the same range of aspect ratios (~10:1–100:1). This suggests an autogenic control on aspect ratio, perhaps resulting from feedback processes in levee growth and/or bank erosion, and allogenic (e.g., sediment supply, grain size) controls on channel width. Submarine channel aspect ratios tend to decrease with increasing dimensions, while the opposite relationship has been observed for fluvial channels, likely due to opposing relationships between flow discharge and channel distance. Additionally, observation of an apparent lag between channel thalweg and levee responses to gradient changes suggests that thalweg and levee deposition and erosion may be partially decoupled due to the vertical structure of turbidity currents, with thalweg evolution driven by the basal, higher-shear-stress portion of the flow and levee evolution by the dilute upper portion. The data presented here provide a basis for predicting channel metrics in exploration scenarios, in which data coverage may be sparse. This documentation of a diverse suite of channels also captures the range of scales and variability exhibited globally by submarine channel systems, providing context for local studies.

A consistent global approach for morphometric characterisation of subaqueous landslides

Abstract Landslides are common in aquatic settings worldwide, from lakes and coastal environments to the deep sea. Fast-moving, large-volume landslides can potentially trigger destructive tsunamis. Landslides damage and disrupt global communication links and other critical marine infrastructure. Landslide deposits act as foci for localized, but important, deep-seafloor biological communities. Under burial, landslide deposits play an important role in a successful petroleum system. While the broad importance of understanding subaqueous landslide processes is evident, a number of important scientific questions have yet to receive the needed attention. Collecting quantitative data is a critical step to addressing questions surrounding subaqueous landslides. Quantitative metrics of subaqueous landslides are routinely recorded, but which ones, and how they are defined, depends on the end-user focus. Differences in focus can inhibit communication of knowledge between communities, and complicate comparative analysis. This study outlines an approach specifically for consistent measurement of subaqueous landslide morphometrics to be used in the design of a broader, global open-source, peer-curated database. Examples from different settings illustrate how the approach can be applied, as well as the difficulties encountered when analysing different landslides and data types. Standardizing data collection for subaqueous landslides should result in more accurate geohazard predictions and resource estimation.