Abstract

We develop a method to characterize turbidity current flow conditions using single-event deposits from the Miocene Capistrano Formation near San Clemente , CA. This approach is advantageous because turbidity currents are difficult to observe directly in nature or model accurately in experiments. Lateral and vertical sampling strategies through channel cross-sections provide instantaneous vertical-flow-structure constraints as well as flow-condition information at a single point through time. A simple sediment transport model is used to produce vertical concentration profiles as a function of shear velocity and current thickness that are consistent with grain-size variation measured in these deposits.

Introduction

The goal of this study is to develop a method for characterizing turbidity current flow conditions from particle-size distributions in turbidites using algorithms describing sediment transport. This work was motivated by the fact that, although currents driven by the excess density of sediment in turbulent suspension are responsible for sculpting submarine landscapes and transferring large amounts of sediment from the shelf to deep waters, the physical properties of these flows remain poorly constrained. Direct measurement of sediment concentrations, transport styles, velocities and channel/flow interactions has been limited by the infrequent, unpredictable and often violent nature of these events (e.g. Alexander, 2002 ; Hampton, 1996 ; Khripounoff, 2003 ; Puig, 2003 ; May, 2000 ). For sandy currents in particular, attempts to test theoretical flow models with physical experiments are complicated by our inability to properly scale the dimensions associated with the energetic flows observed in nature (Kostic, 2002 ; Middleton, 1966). As a result, subaqueous gravity flow characteristics are commonly inferred from modern and ancient seafloor topographies and turbidite deposits based on intuition from subaerial flow models (e.g., Canals, 2000 ; Morris, 2000 ; Mulder, 2001 ; Vittori, 2000 ). However, because the densities of the ambient fluids in these settings differ greatly, this intuition can be misleading, as it may not be possible to attribute submarine forms to the same flow processes that are observed on land. The extent to which flow processes can be uniquely ascribed to channel forms could be evaluated by characterizing submarine flow conditions directly from turbidites.

Although submarine and terrestrial channels have similar morphologies, the fact that seawater is roughly 800 times denser than air implies that flows in these environments "feel" preexisting topography in different ways. On land, the high water/air density contrast inhibits the elevation of river height above a channel because a high potential-energy drop is associated with flow onto the floodplain. This concentrates sediment near the base of the channel, leading to deposition of horizontal beds that onlap the channel walls ( Figure 1a ). In addition, the large water/air density contrast suppresses any mixing of the two fluids. In contrast, turbidity currents can concentrate sediment above the channel to produce drape deposits, or they can deposit onlapping beds if the concentration profile is basally charged ( Figure 1b ). As a result, spatial variations in particle size within the deposits are indicative of conditions in the flow. In order to develop quantitative methods for estimating flow characteristics, both drape and onlap turbidites were sampled from a channel cross-section in an exhumed submarine canyon complex near San Clemente , CA ( Figure 2 ). The sample site lies within a 15-m thick, 1.4-km wide outcrop of sandy turbidites, interpreted to be the result of sedimentation onto the continental margin of a forearc basin ( Campion, 2000 ; May, 2000 ). In this paper, we use Rousean criteria for sediment transport style and modeled sediment concentration profiles to determine the range of flow velocities and thicknesses capable of producing these deposits. Although conditions for only two flows are explored here, the method outlined here can be applied to a distribution of deposits in order to determine representative depositions turbidity current characteristics.