Laterally accreting sinuous channels and their deposits: The Goldilocks of deep-water slope systems

Abstract

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.

Source Rock Characterisation and Petroleum System Modelling a Review of Marginal Marine Deposit in the Permo-Triassic Sydney Basin, Australia

“…It is dominated by wave, river and tidal processes. The depositional settings in marginal marine sediments are delta, beach, barrier island, estuarine, lagoonal and tidal at, various scholarly works in the Sydney basin have shown these signatures (Arnott et al, 2021;Feng et al, 2021;Fielding et al, 2021;Wan et al, 2021), however, the potentials that this holds in terms of hydrocarbon resources remain largely unexplored.…”

Stratigraphic evolution of a spectacularly exposed turbidite channel belt from the Tachrift System (late Tortonian, north‐East Morocco)

“…The laterally extensive nature of the sandprone package of storey-sets A to D suggests that it encompasses a relatively prolonged phase of channel meandering, accompanied by very little aggradation and arguably significant sediment Laterally migrating channels producing laterally extensive LAPs similar to those of storeysets A to D have been widely documented both in outcrop (Elliott, 2000;Abreu et al, 2003;Navarro et al, 2007;Wynn et al, 2007;Li et al, 2018;Arnott et al, 2021) and in the subsurface (Babonneau et al, 2010;Labourdette & Bez, 2010;Janocko et al, 2013;Reimchen et al, 2016), and generally are interpreted to reflect low sediment accommodation regimes or, alternatively, an early phase of channel gradient adjustment due to increasing sediment discharge (Kneller, 2003).…”

“…In meandering turbidite channels ( cf . with laterally accreting sinuous channels of Arnott et al ., 2021), sinuosity increases as a result of channel bend (or meander) expansion (i.e. swing; Peakall et al ., 2000).…”

“…This results in accrection of inner bank apices with formation of point bar‐like deposits which are commonly referred to as lateral accrection packages (LAPs hereafter, Abreu et al ., 2003; Dykstra & Kneller, 2009; Pyles et al ., 2012), and contemporaneous erosion at the outer cut‐bank (Babonneau et al ., 2010). Lateral accrection package deposition is promoted by a near‐bed secondary flow (perpendicular to main flow direction) directed towards the inner bank cell circulation and appears to preferentially develop when turbidity currents are depleted in the intermediate grain‐size fraction (Arnott et al ., 2021).…”

“…Most of the understanding of how turbidite channels evolve has come from several decades of exploration of the seafloor and its subsurface, which has yielded incredibly detailed morphobathymetric and seismic imaging of present‐day and hydrocarbon‐bearing fossil examples (Weimer, 2000; Pettingill & Weimer, 2002; Weimer & Pettingill, 2007). Nonetheless, prediction of sub‐seismic scale heterogeneity (below a few tens of metres) of channelized turbidites still relies on outcrop investigations (Sprague et al ., 2005; Wynn et al ., 2007; Janocko et al ., 2013; García et al ., 2015; Reimchen et al ., 2016; Gong et al ., 2020), which can provide lithological and sedimentary facies calibration down to the scale of component event beds (Elliott, 2000; Navarro et al ., 2007; Kane & Hodgson, 2011; Pyles et al ., 2012; Arnott et al ., 2021).…”