Ian A. Kane

Submarine transitional flow deposits in the Paleogene Gulf of Mexico

Gravity-driven flows on the seafloor are the largest, yet least well understood, sediment transport agents on Earth. Recent exploration wells in ultradeep basins have revealed the presence of large sandy submarine fan systems of enigmatic facies types, many hundreds of kilometers from paleocoastlines. These sedimentary deposits often defy conventional turbidite or debrite interpretations, having a character suggestive of deposition from flows with transient turbulent-laminar rheologies. In the Wilcox Formation (Gulf of Mexico), inferred transitional flow deposits have distinctive stratigraphic stacking patterns, from fine-grained debrites to coarser grained turbidites. The vertical sequence of beds is here inferred to reflect the longitudinal bed distribution in response to lobe progradation, and demonstrates a transition from well-mixed turbulent flow, to progressively more rheologically stratified flow, and eventually to fully laminar flow. The progressive development of internal rheological boundaries resulted in a high-concentration but fluidal basal layer, and an upper quasi-laminar layer with an overriding sheared dilute turbidity current. The long runout of the flows is linked to their high silt and clay content; it is most likely flow expansion at the channel-lobe transition that drives flow transformation. This process-based model may be applicable to many deep-water settings and provides a framework within which to interpret the stratigraphic and spatial distribution of these complex deposits.

Global (latitudinal) variation in submarine channel sinuosity

Current classifications of submarine channels and fans link channel sinuosity to gradient, and in turn to sediment caliber, with end members being high-sinuosity, low-gradient, fine-grained systems and low-sinuosity, high-gradient, coarse-grained systems. However, the most sinuous modern submarine channels, such as the Amazon, Bengal, Indus, and Zaire, along with ancient sinuous submarine channels, are located in equatorial regions. Here we quantitatively compare slope versus latitude controls on submarine channel sinuosity and show that the latitudinal control is strong, while that of slope is weak. Variation in sinuosity with latitude is shown to occur uniquely in submarine channels; no comparable relationship is observed for terrestrial river channels. Possible causal mechanisms for this latitudinal variation are explored, focusing on the influence of the Coriolis force, flow type, and sediment type. Although climate does not vary straightforwardly with latitude, climatic controls on flow and sediment type may explain some of the latitudinal variation; Coriolis force, however, varies with latitude alone and produces an excellent fit to the observed sinuosity-latitude distribution. Regardless of which control predominates, latitudinal global variation in channel sinuosity should have changed over geologic time. Since deposit architecture and facies are linked directly with sinuosity, submarine channel deposits should also systematically vary in space and time.

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.

Supercritical-flow structures on a Late Carboniferous delta front: Sedimentologic and paleoclimatic significance: COMMENT

Ventra et al. (2015) propose a model of hyperpycnal supercritical flow regime deposition, related to a megamonsoonal climate, for a series of interpreted cyclic steps on a Carboniferous delta-front. The new model has major implications for paleoenvironmental reconstructions of this well-studied area, in terms of climate and geographical setting. We contend that fundamental sedimentological observations do not support their proposed model. Ventra et al. interpret that each cross-bedded ‘set’ is the deposit of a single mega-monsoonal flood event; we propose that each set represents multiple events; e.g., their Set 3 is at least 30 discrete events. Event bed sandstones are represented by variable to normal grading, crude cross-stratification and plane-parallel lamination, and a finer-grained top, which is generally ripple-cross laminated (Fig. 1). These are capped by a very thin, silty to fine-grained sand layer with abundant organic material, representing interflood deposition (Hampson, 1997). This repeated motif is only interrupted where bed tops are eroded. Abundant loading, dewatering and flame structures between beds and rip-up clast horizons support the interpretation of discrete depositional events (Fig. 1). A mega-monsoonal origin cannot be invoked for ~20-cm-thick beds. Ventra et al. state that there is a lack of ripple cross lamination or stratification, aside from some upflow dipping crude laminae. Most bed tops are laminated to crosslaminated (Fig. 1). These are interpreted as current ripples, rather than antidunes or climbing ripple backsets, as they have clearly developed tangential foresets. These ripples suggest paleoflow broadly toward the east, in contrast to the overall southwesterly paleoflow. Eastward paleocurrents typically occur on the updip side of the ‘undulatory beds’ of McCabe (1977) and Hampson (1997), suggesting that the exposed section is oblique to depositional dip, rather than parallel as proposed. We infer that there is a significant lateral, as well as downstream, accretion component to these three-dimensional bedforms. The lack of bioturbation between beds cannot be used as evidence for the short duration of events, as there is, in general, a lack of bioturbational structures in the sandy marine sediments of the British Carboniferous Basins. However, the depositional environment as proposed by Ventra (marine delta-front) is at odds with the existing fluvial models, which are based on detailed mapping and correlations (McCabe, 1977; Hampson, 1997). The large-scale architecture shows a progressive decrease in depositional dip that we interpret to reflect healing of scour-related topography. The depth of the scour (minimum of 13 m) corresponds to that observed in major fluvial confluences and is aligned with Hampson’s (1997) interpretation. The overlying section, which Ventra et al. do not discuss, is represented by many smaller-scale trough cross beds, interpreted to represent local shallowing in a fluvial setting associated with filling of the scour. In this stratigraphic unit, features of a similar geometry but at a smaller scale are convincingly shown to represent supercritical flow conditions (Bijkerk, 2014). In summary, we concur with the observations and interpretations of Hampson (1997) that these beds represent the fill of a major scour, potentially associated with a fluvial confluence, which filled in a stepwise manner, by gravity flow in the deepest parts, with periods of minor erosion of the downslope sides of the bedforms (e.g., Ullah et al. 2015). Figure 1. A: Short sedimentological log (from the base of Ventra’s [2015] ‘Set 2’) showing the general character of event beds. B: Character of event beds and finer interbeds (i) and amalgamation surfaces (a). C: Small dune with tangential foresets (f). D: ripple cross lamination (r) with tangential foresets. E: Loading (L) between beds.

Regional distribution and controls on the development of post-rift turbidite systems: insights from the Paleocene of the eastern North Viking Graben, offshore Norway

Abstract Paleocene deep-water deposits of the Norwegian sector of the North Sea Basin are prospective for oil and gas, although little is known about their sedimentology and distribution, or the controls on their stratigraphic evolution. To help unlock the potential of this poorly explored interval, we integrate 3D seismic reflection, well logs and core data from the eastern North Viking Graben, offshore Norway. We show that thick (up to 80 m), high net to gross (N:G) (up to 90%), sandstone-rich channel-fills and sheet-like, likely lobe deposits occur on the slope–proximal basin floor, forming part of an aerially extensive fan system. Sediment dispersal and the resultant stratigraphic architecture are controlled by slope morphology. Bypass occurred on the northern, passive margin-type slope; whereas, in the south, sediment gravity currents were deflected around, and deep-water sandstones onlap and pinch-out onto an exposed rift-related fault block that generated intra-basin bathymetric relief. Pinchout of deep-water sandstone into mudstone suggests that future exploration should focus on identifying subtle stratigraphic traps on fault block flanks or at the fan fringe. This trapping style contrasts with that encountered in the UK sector of the Northern North Sea, where most Paleocene fields and discoveries are in structural traps related to the flow of Zechstein Supergroup salt.

Deep-water clastic systems in the upper carboniferous (upper mississippian-lower pennsylvanian) shannon basin, western Ireland

The Upper Carboniferous Shannon Basin of western Ireland contains a more-than-2300-m-thick (7540 ft) basin-fill succession, shallowing upward from deep-water to deltaic and incised fluvial deposits. The deep-water basin floor and slope succession is world renowned as an analog for hydrocarbon-bearing deep-water sandstones on several continental margins such as the basins in East and West Africa, South America, the Gulf of Mexico, and not least offshore northwest Europe. The Shannon Basin is frequently visited by both academia and industry for research and training purposes. A series of behind-outcrop research boreholes reveals the subsurface expression of the deep-water rocks and is complemented by seismic-scale cliff exposures. The succession is interpreted as a first-order basin-scale linked sedimentary system. This system can be analyzed using the principles of source-to-sink analysis and leaves the visitor with a complete picture of the basin fill, enhanced by spectacular sedimentological and stratigraphic detail.

Giant submarine landslide triggered by Paleocene mantle plume activity in the North Atlantic

The 290-km-long ‘Halibut Slide’ is the world’s largest epicontinental submarine landslide. Between 64 and 62 Ma, plume-related uplift in the North Atlantic and far-field stresses caused reactivation of major intra-plate faults. This reactivation caused instability of Cretaceous chalk slopes across the North Sea Basin, triggering the Halibut Slide. Megascours, up to 1 km wide, 150 m deep, and 70 km long, indicate slope failure from an intra-shelf high east of mainland Scotland, and subsequent flow down an ~1.1° slope. Megascours were gouged by cuboid chalk blocks, up to 1 km wide and 170 m high, some of which out-ran the main slide body by up to 10 km. The Halibut Slide has a decompacted volume of 1450 km3 and a basal slide surface extending over ~7000 km2. Subsequent clastic sediment input points and dispersal pathways were controlled by the underlying Slide topography for ~10 m.y. The discovery of this major submarine landslide provides new insights into the response of sedimentary systems to regional and deeply rooted tectonic events, and the initiation of long-term sediment routing patterns.