Zane Jobe

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 strati-graphic 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.

Most submarine canyons are erosive conduits cut deeply into the world’s continental shelves through which sediment is transported from areas of high coastal sediment supply onto large submarine fans. However, many submarine canyons in areas of low sediment supply do not have associated submarine fans and show significantly different morphologies and depositional processes from those of ‘classic’ canyons. Using three-dimensional seismic reflection and core data, this study contrasts these two types of submarine canyons and proposes a bipartite classification scheme.The continental margin of Equatorial Guinea, West Africa during the late Cretaceous was dominated by a classic, erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational rather than erosional, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (>150 m), dilute, turbidity currents. There is little evidence for erosion, mass-wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. During canyon abandonment, fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes.Based on comparison of canyons in the study area, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, highly aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass-wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.

Near-seafloor core and seismic reflection-data from the western Niger Delta continental slope document the facies, architecture, and evolution of submarine channel and intra-slope 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 intra slope 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.

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 , 0.6u. 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

Changes in sediment supply and caliber during the last , 130 ka have resulted in a complex architectural evolution of the Y channel system on the western Niger Delta slope. This evolution consists of four phases, each with documented or inferred changes in sediment supply. Phase 1 flows created wide (1,000 m), low-sinuosity (1.1) channel forms with lateral migration and little to no aggradation. During Phase 2, the Y channel system began to aggrade, creating more narrow (300 m) and sinuous (1.4) channel forms with many meander cutoffs. This system was abandoned at , 130 ka, perhaps related to rapid relative sea-level rise during Marine Isotope Stage (MIS) 5. Phase 3 flows were mud-rich and deposited sediment on the outer bends of the channel form, resulting in the narrowing (to 250 m), straightening (to a sinuosity of 1.22), and aggradation of the Y channel system. Renewed influx of sand into the Y channel system occurred with Phase 4 at , 50 ka, during MIS 3 sea-level fall. The onset of Phase 4 is marked by the initiation of the Y9 tributary channel, which re-established sand deposition in the Y channel system. Flows entering the Y channel from the Y9 channel were underfit, resulting in inner levee deposition that is most prevalent on outer banks, acting to further straighten (1.21) and narrow (to 200 m wide) the Y channel. The inner levees accumulated quickly as the flows sought equilibrium, with deposition rates. 200 cm/ky. Marked by the presence of the last sand bed, abandonment occurred at , 19 ka in the Y channel and , 15 ka in the Y9 channel and is likely related to progressive abandonment due to shelf-edge delta avulsion and/or progressive sea level rise associated with Melt Water Pulse 1-A. The muddy, 5-meter-thick Holocene layer has thickness variations that mimic those seen in the sandy part of Phase 4, suggesting that dilute, muddy flows continue to affect the modern Y channel system. This unique dataset allows us to unequivocally link changes in submarine channel architecture to variations in sediment supply and caliber. Changes in the updip sediment routing system (i.e., the channel ''plumbing'') are shown to have profound implications for submarine channel architecture and reservoir connectivity.

ABSTRACT The modern seafloor and shallow subsurface of the western Niger Delta slope exhibits sinuous submarine channels with multiple phases of evolution. Investigations of these channels using three-dimensional (3D) high-resolution seismic-reflection data, and piston coring offer a rare opportunity to examine a lithologically calibrated submarine channel system. At least three phases of channel development and evolution are related to changes in sediment supply coincident with updip avulsions. The first phase is predominantly incisional and creates a large valley within which the subsequent phases evolve. The second phase creates a wide, low sinuosity channel within the valley. This channel displays fluvial point-bar like accretion and expansion of the meander bends. The third and most recent phase is initiated when drainage from a nearby channel is captured by the valley system. This addition of sediment supply causes a narrowing and downcutting of the channel. This narrowing occurs via deposition along the inner bends of the channel and results in very complex inner levee geometries and architectures, which have been intricately mapped on the 3D seismic data. Piston coring of these inner levees reveal heterogeneous facies, while facies in the channel thalweg consist of coarser grained sands that are often amalgamated. The modern seafloor and grain size distributions from the piston cores were used as inputs for a three-dimensional numerical model (TCSolver). Particularly important was the inputted vertical grain size distributions obtained from the piston coring. Results from simulations of turbidity currents in this lithologically calibrated model indicate a normal sense of helical flow in the channel bends, supporting the observations from the 3D seismic data.

Sedimentological observations, planktonic microfossil data, terrigenous index proxy data, benthic morphogroup analyses, pressure analyses and geochemical fingerprinting were integrated in order to assess the nature of mud-stones in the Cardamom Field in the Auger salt withdrawal minibasin (Gulf of Mexico). Both ponded and slope accommodation have occurred through the salt basin's history resulting in complex stratigraphic architecture of stacked submarine lobes and channels. Continuous core (164 ft) from the Messinian reservoir unit enabled this study to assess the depositional nature of the mud-rich intervals. Four mudstone facies (Mudstones 1–4) and respective depositional settings were interpreted. A modified benthic foraminifera microfacies model, in conjunction with variations recorded in planktonic flora and fauna abundances and reworking, serves as the key reference for the paleoecological interpretations. Of particular interest is the potential of a 41 ft thick (below seismic resolution) intra-reservoir mudstone (Mudstone 3) to act as a baffle/barrier to fluid flow between the lower U Sand and the upper U Sand. Mudstone 3 shows a hemipelagic character containing a diverse benthic community, high abundances of autochthonous planktonic flora and fauna, reduced terrigenous input and extensive bioturbation. Downhole formation pressure data and fluid fingerprinting from the reservoir pay sands (upper and lower U Sands) show significant differences in the calculated pressure gradients and fluid composition. This suggests that most likely the two reservoir pay sands are not in vertical communication. Mudstone 3 paleoenvironmental analyses suggest the lateral extension of a possible baffle/barrier to vertical fluid flow. These results show that benthic morphogroup analyses in mudstones can be a robust method to assess reservoir compartmentalization and consequently, impact field development planning and reservoir modeling.