Comparison of Sedimentary Processes in Twenty-Two Modern Submarine Canyons Along the Northern California Margin

Abstract

Sedimentary processes and resultant facies in submarine canyons are poorly understood because canyons are bathymetrically complex and sedimentologically heterogeneous environments. Thus it has proven difficult to study submarine canyons using traditional wire-line coring techniques. This study uses a novel vibracoring system deployed using a remotely operated vehicle that enables precise core location (latitude, longitude and altitude above thalweg) and can capture coarse-grained sediments. Whilst previous studies have focused on individual canyons, this study makes use of 127 cores from 22 canyons along a 600 km stretch of the California margin enabling recognition of different canyon types and identification of the processes that control them. Canyons can be divided into two types: coarse-grained canyons and fine-grained canyons. Coarse-grained canyons have a sharply defined thalweg-channel filled with chaotic sands and gravels, episodic movement of these coarse sediments maintains and probably actively erodes the thalweg-channel; finer grained sands and silts drape the canyon walls. Fine-grained canyons have a poorly defined thalweg-channel and are filled with fine-grained sands and silts that anneal preexisting channel topography and drape the canyon walls. Coarse-grained canyons are formed where the canyon head intersects the local littoral cell, whereas fine-grained canyons have heads on the continental shelf. Facies type, grain size and composition (% sand) are strongly constrained by altitude above the thalweg and much less by distance down canyon. Whereas the grain size of fine-grained facies comprising sands and silts fines slightly down canyon, the sands and gravels in the canyon axis show no such fining. This lack of fining may reflect episodic remobilization of material during numerous sediment transport events. Infilling of canyons by frequent small events and flushing by occasional large events has been inferred for many decades. Two of the canyons studied have areas devoid of canyon fill, where bedrock is exposed. High-resolution bathymetry within one of these canyons reveals that scouring of the previous canyon fill, presumably by exceptionally high-energy flow events, has exposed the bedrock. This study reveals the spectrum of processes operating in modern canyons are controlled by the type and volume of sediment available and whether the canyon can tap these supplies, which is strongly dependent on the position of the canyon head.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015