The Stratigraphic Record of Submarine-Channel Evolution

Submarine channels are conduits for sediment-gravity flows that sculpt continental margins as they carry terrigenous sediment to the deep sea (Piper and Normark, 2001). Sediment-gravity flows are mixtures of sediment and water in which the sediment component pulls interstitial water down slope under the influence of gravity (Bagnold, 1962; Middleton and Hampton, 1973). Submarine channels are important components of deep-sea fans, which comprise canyon, channel, levee-and-distal-overbank, and depositionallobe architectural elements (Mutti and Normark, 1987; Normark et al., 1993; Piper and Normark, 2001; Posamentier and Kolla, 2003). Submarine canyons transition to U-shaped, lower-relief channels with levee-and-distal-overbank deposits across the slope and rise of continental margins. Channels can extend across the seafloor for hundreds to thousands of kilometers (Covault et al., 2011; 2012), and their deposits can host significant hydrocarbon resources (Mayall et al., 2006). Submarine-channel evolution is a result of the interaction between the seafloor within and around the channel, and overriding sediment-gravity flows. Sediment-gravity flows have rarely been directly observed in the ocean (Talling et al., 2015). However, recent monitoring data record the hourly to annual interaction between submarine channels and sediment-gravity flows (e.g., Zeng et al., 1991; Xu et al., 2004; Paull et al., 2010; Conway et al., 2012; Cooper et al., 2013; Sumner and Paull, 2014; Talling et al., 2015; Hughes Clarke, 2016). These data underscore the short-term transience of seafloor geomorphology and multi-phase bed reworking, local deposition, and bypass of sediment-gravity flows active during channel initiation, maintenance, and filling (e.g., Covault et al., 2014). Furthermore, insights from monitoring have inspired reinterpretation of outcropping sedimentary rocks (e.g., Fildani et al., 2013; Hubbard et al., 2014; Postma et al., 2014; Bain and Hubbard, 2016; Pemberton et al., 2016). Missing from the short-term record of monitoring is a longer-term perspective, which is afforded by outcropping and subsurface stratigraphic successions (e.g., Deptuck et al., 2003; Hubbard et al., 2014). Here we summarize the facies architecture and stratigraphic evolution of outcropping submarine-channel systems. Many outcropping channel fills exhibit a common facies architecture of thick-bedded sandstone deposited in the deepest segment of the bounding channel surface (i.e., the thalweg) that transitions laterally to thin-bedded heterolithic deposits in ABSTRACT Submarine-channel systems record basin-margin sediment dispersal and can host significant natural resources. We review the facies architecture (i.e., facies heterogeneity and stacking patterns) of outcropping submarine-channel systems, focusing on the Cretaceous Tres Pasos Formation, Magallanes basin, southern Chile. The fundamental building block of submarine-channel systems is the channel-fill architectural element. A channel fill comprises thick-bedded turbidite sandstone deposited in the deepest segment of the bounding channel surface (i.e., the thalweg), which transitions laterally to thin-bedded heterolithic deposits in the margins. Submarine-channel fills stack to form composite channel systems, which commonly exhibit an evolution from early channel incision and lateral migration to late-stage aggradation. The incising-to-aggrading trajectory of a submarine-channel system is likely influenced by adjustments toward an equilibrium gradient that is established and maintained by feedbacks between the slope and overriding sediment-gravity flows. A steep slope will promote swift flows that are erosive; a more gradual gradient will promote sluggish flows that aggrade sediment. A combination of these two processes brings the channel floor closer to an equilibrium gradient. Changes in sediment-gravity-flow properties driven by allogenic controls, such as eustatic sea-level change, have also been linked to the incising-to-aggrading trajectory of channel systems. We illustrate the evolution of channel systems with a surface-based stratigraphic forward model. The model allows us to visualize the three-dimensional (3D) stacking patterns of channel systems, which control heterogeneity and sand body connectivity in channelized hydrocarbon reservoirs. Future research opportunities include the interpretation of stratigraphic products integrated with direct monitoring of turbidity currents, physical experiments, and numerical modeling to understand the 3D facies architecture and stratigraphic evolution of channel systems. The Stratigraphic Record of Submarine-Channel Evolution