McHargue et al Architectural Diversity of Submarine Lobes

Abstract

The most popular model for submarine unconfined lobate deposits has the following attributes: (1) a single feeder channel that delivers sediment, (2) a set of distributary channels present only in the proximal part of the lobate body, and (3) unchannelized tabular deposits present in the middle and distal part of the lobate deposit. This model has become a standard to guide interpretation of outcrop and subsurface examples of submarine lobate deposits. In this contribution, three well imaged subsurface lobate deposits are described that display three markedly different morphologies, all of which differ from the “standard” model. All three lobate examples are buried by less than 150m of muddy sediment and imaged with high resolution 3D reflection seismic data of similar quality and resolution. Distinctively different distributary channel patterns are present in two of the examples, and no distributaries are imaged in a third example. We conclude that if channels are not imaged, it is because channels are not present. The different distributary channel patterns are interpreted to have resulted from different processes: (1) a lobate deposit that is pervasively channelized by many distributaries that have avulsed from numerous nodes is interpreted to result from mud-rich, stratified, turbulent flows ; (2) an absence of distributaries in a lobate deposit is interpreted to result from collapse of mud-poor, turbulent flows remobilized from littoral drift; and (3) a lobate deposit with only a few, long, straight distributaries without avulsions is interpreted to be dominated by debris flows (laminar flows) . Reconciling 3D seismic morphologies with observations of channels, scours, and amalgamation zones in outcrops is problematic. It is concluded from this study that, when characterizing unconfined deep water deposits, multiple models with significant differences in predicted permeability structure should be considered.

Cretaceous continental margin evolution revealed using quantitative seismic geomorphology, offshore northwest Africa

“…Based on these observations, we interpret that the debris-poor lobes were likely deposited predominantly by turbidity current processes and the debris-rich lobes by debris flows (McHargue, Hodgson, & Shelef, 2019). The degree of mixing will be difficult to distinguish without lithological data (Talling, 2013).…”

Evolution of a mixed siliciclastic‐carbonate deep‐marine system on an unstable margin: The Cretaceous of the Eastern Greater Caucasus, Azerbaijan

“…Our field work suggests that these blocks may be associated with a multitude of different types and sizes of submarine landslides and blocks that are below seismic scale (Figures 12 and 14). The presence of carbonate blocks, lobe-morphology and similarity in run-out length and volume to debris dominated lobes documented elsewhere (McHargue et al, 2019) suggests the carbonate-dominated systems were deposited by debris flows. Hence, two stages of mixing occur, first during erosion to form mixed lithology flows (bed-scale strata mixing), and then through deposition of interdigitating systems (lithofacies-scale strata mixing; sensu Chiarella et al, 2017).…”

“…These conglomerates could also represent sediment bypass within lobe axes (Kane, Dykstra, et al, 2009) or channel fill conglomerates (Knaust, Warchol, & Kane, 2014). Their thickness, amalgamated nature, downlapping geometry and association within a FA typical of lobe axis deposition (Soutter et al., 2019; Spychala, Hodgson, Prélat, et al, 2017) suggest they are most likely to represent deposition in the axis of a debris flow dominated lobe (McHargue et al., 2019).…”

“…Amalgamation of event beds suggests parent flows were energetic and capable of eroding sediment and incorporating it into the flow (Lowe, 1982;Stevenson et al, 2015). Amalgamation of conglomerates indicates deposition in an environment characterised by frequent debris flows (Dickie & Hein, 1995;Postma, 1984aPostma, , 1984bSurlyk, 1984), similar to the debris flow dominated lobes described by McHargue, Hodgson, and Shelef (2019).…”