Slope Instability Processes in a Complex Deepwater Environment, Bryant Canyon Area, Northwest Gulf of Mexico
Authors: Tripsanas Efthymios K. , Phaneuf Brett A. , Bryant William R. 
Published: October 2008

Halokinetic processes acting in the Bryant Canyon Area have intensely modified the topography into a series of intra-slope basins, and have led to the genesis of a great variety of slope instability processes. Sediment cores, and seismic records from four basins were studied extensively in order to determine the nature and sedimentological patterns of the sediment failures; most of the slope instabilities appear to occur mainly during low sea-level stands, whereas during the Holocene only a few isolated and small-scale sediment failures developed. Mud-flow and mud/clast dominated debris flow deposits are the most prominent features in the sediment cores. Deposits are characterized by a basal-uniform to shearlaminated zone that represents the deposition of a basal lubrication zone reducing bottom drag of overflowing debris flows allowing them to run as Bingham flows for several kilometers. In some cases the lubricated zones were totally passed over by the overriding debris flows, and resembling the lubricants left behind in the wake of a slug. Convoluted/distorted laminations in debris/mud flows reveal that in a final stage the heads/margins of the debris/mud flows evolved into ‘creeping-like’ masses that resulted in an obstacle to the forward transportation of the flow body, leading to the genesis of successive imbricated zones behind the head of the flows.

Introduction

Debris flows are gravity-driven flows of highly concentrated sediment/water mixtures, displaying diverse properties, behaviors, and depositional characteristics.1 The geometry, shape, morphological and internal structure of debris flow deposits are primarily determined by the type of flow (Bingham vs. inertial grain flow, hydroplaning vs. nonhydroplaning flows, etc.), nature (cohesive/incohesive), concentration and grain-size of the sediment particles, concentration and size of transported clasts, and the topography of the area in which they evolve and are eventually deposited.1,2,3 Deposition by debris flows is caused by "freezing" of the flow when the applied stresses fall below a threshold value (yield strength) that leads to en masse deposition, initiated at the front of the flow.1,4,5,6,7,8,9 However, many researchers have been arguing that based on field observations and experimental data, it is more probable that debris flow deposits result from an amalgamation of deposits in successive surges, occurring in a single flow event.1,2,10,11

Subaireal and submarine debris flows display very different characteristics and flow mechanisms due to their propagation by different means, the later being able to flow much faster and for longer distances than the former.3,12 Although extensive studies have been conducted on recent, submarine debris flow and mass transport deposits through seismic data and sediment core collection and analysis, little is know about their internal structure that can be highly variable even in flows consisting of similar sediments.7,11

This paper presents the results of detailed sedimentological analysis of five Jumbo Piston cores (up to 15 m long), in combination with high-resolution seismic information of the sampled areas, from the Bryant Canyon Area. The main purpose of this study is to provide better insight into the interaction between diapiric tectonics and slope instabilities, and to determine the nature and sedimentological patterns of sediment failures.