Deepwater slope channels on active continental margins, Eocene forearc basin, California: architecture and Froude supercritical flow channel fills

Abstract

This dissertation focuses on the southern California active margin continental slopes, and tests the hypothesis that deepwater slope architectures on active continental margins differ from passive margins or foreland basins due to both the structurally-controlled steep gradients, and the narrowness of the shelf platform. These controlling factors allow the high-gradient hinterland to connect directly to the steep deepwater slope and are insusceptible to relative sea-level change. This high-gradient hinterland to deepwater slope margin causes the direct and sustained sediment supply from the river to the continental slope. To test this hypothesis, two main study regions were chosen in the Eocene forearc deepwater basins filled with the Great Valley Sequence Belt, which are the La Jolla Group, exposed north of San Diego, and the Juncal Formation in Frazier Park. Field work in these two regions allowed for a comparison between two types of deepwater slope systems. The La Jolla Group is composed of relatively lower aspect ratio channel complexes, which form a prograding upper slope. The Juncal Formation is composed of relatively higher-aspect ratio channel complexes, which form an intra-slope fan complex with a feeder channel on the middle slope. Both systems are highly channelized, as was hypothesized to be characteristic for active margins. The La Jolla prograding channel complexes demonstrate a highly channelized upper slope environment with considerably lower slope progradation rate, as compared to passive margins or foreland basins. The Juncal slope fan complex demonstrates that the majority of the fan volume is formed by avulsive channel complexes without lobe elements. The channel fills inboth systems span from heterolithic fine-grained channel fills, to sandy to heterolithic channel fill, and sandy fills with basal conglomerates. All the channel fills indicate an abundance of bypass signatures. This is especially remarkable for the heterolithic channel fills, which are different from the existing models for fine-grained channel fills that consider channel abandonment or passive infill. Furthermore, all channel fills are interpreted to be dominated by Froude supercritical flow deposits due to pervasive scour-and-fill structures with backsets. This suggests that the Froude supercritical flow may control the observed highly irregular shape and distribution of the sandstones (e.g., mounded shape and mosaic-like facies distribution) in the channel fills. The studied channel complexes provide an outcrop analog for modern sea-floor upslope migrating scour and fill structures that form crescent-shape bedforms in channels and submarine canyons. This work and the comparison to modern systems suggest that supercritical flow and bypassing turbidity currents are significant processes on active continental margins due to both high sediment supply, and the steep hinterland to continental margin slopes.