Processes of Sedimentation and Stratigraphic Architecture of Deep-Water Braided Lobe Complexes: The Pliocene Repetto and Pico Formations, Ventura Basin, U.S.A

Abstract

This paper reexamines the Pliocene Repetto and Pico formations, Ventura Basin, California, to evaluate the processes of sedimentation and interpret their environmental evolution within an elongate trough basin. Analysis of stratal architecture, lithofacies distributions, paleotransport directions, and measured sections from select stratigraphic intervals show that the two formations consist of sheet-like units, 20-50 m thick, composed of three main lithofacies associations: 1) structureless to graded gravelly sandstone, 2) thick-to medium-bedded sandstone composed principally of Bouma T a /T abc /Lowe S 3 divisions with thin mudstone layers, and 3) thin-to very thin-bedded sandstone and mudstone composed of Bouma T bcde divisions. At the bed-set level, clear trends in bed thickness cannot be recognized, but the lithofacies associations show fining-upward, coarseningupward, and symmetric stacking patterns, reflecting disorganization and frequently shifting sand and gravel depocenters, which are common trends in braided systems. Additionally, the frequent and shallow erosional features, widespread deposition, and paucity of wedge-shaped bedding geometries and associated laterally equivalent mud-rich units suggest that channel-levee elements were not present. Due to limited lateral and upslope data, the formations may represent up to three alternative depositional environments: 1) sedimentary fill of a channel that is wider than outcrop (greater than 4-5 km wide), 2) frontal lobes that necessitate at least one upslope channel, or 3) lobes emanating from canyon mouths in a loosely confined pattern. However, the flows were not affected by the margins if there existed a wide channel, and there is no evidence for channel-like incision in outcrop or core. Therefore, these units are interpreted as an unconfined network of coarse-grained, braided lobe complexes that formed elongate sedimentary aprons at the bases of canyons feeding the trough basin. This study describes deposits that are likely found in other elongate or high-latitude basins characterized by high rates of coarse-grained or clay-poor sedimentation, and is of interest to those working to understand the range of possibility in deep-water depositional systems.

Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy)

“…The minor thickness of individual amalgamated elements (compared to BTA 1.1 and 1.2) and the low depth-towidth ratios support this interpretation as distributive channels directly incised into a lobe environment (Saller et al, 2008;Rotzien et al, 2014) recording episodes of progradation of the system (Brunt et al, 2013). An alternative interpretation would be that of bed type association 1.3 representing sandy fills of shallow scours (Hofstra et al, 2015;Pemberton et al, 2016;Terlaky et al, 2016) that likewise would imply sediment bypass.…”

“…Campion et al, 2005;Satur et al, 2005;McHargue et al, 2011). This bed type assemblage is interpreted to represent a domain located off-axis with respect to bed type association 1.1, based on reduced overall amalgamation ratio (Romans et al, 2009;Rotzien et al, 2014), increased proportions of structured sandstones, more abundant heterolithic intervals (e.g. Camacho et al, 2002;Grecula et al, 2003;Hubbard et al, 2008;McHargue et al, 2011) and slightly finer grain-size (e.g.…”

“…and the absence of associated heterolithic strata points towards bed type association 1.3 representing shallow channels of low sinuosity (e.g. Crevello et al, 2007;Rotzien et al, 2014). The absence of lateral passages into heterolithic strata furthermore suggests a lack of levee development (Posamentier & Kolla, 2003;Terlaky et al, 2016).…”

“…The typically non-erosive character of the sandy beds is interpreted to reflect the decrease in flow capacity characteristic of distal and marginal lobe environments (e.g. Rotzien et al, 2014).…”

Geometry and dynamics of braided channels and bars under experimental density currents

“…The channels that convey turbidity currents are commonly sinuous and have a single thread (Flood & Damuth, ; Clark et al ., ; Deptuck et al ., ). Apparently multi‐thread channels are rarer on the modern sea floor (Walker, ; Hein & Walker, ; Cronin & Kidd, ; Rotzien et al ., ). Proposed modern examples of braiding occur on submarine fans in the Orinoco (Fig.…”