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

Abstract

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 Ta/Tabc/Lowe S3 divisions with thin mudstone layers, and 3) thin- to very thin-bedded sandstone and mudstone composed of Bouma Tbcde divisions. At the bed-set level, clear trends in bed thickness cannot be recognized, but the lithofacies associations show fining-upward, coarsening-upward, 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.