Initiation processes and flow evolution of turbidity currents: Implications for the depositional record

Abstract

Interpretations of sea-level change and source-area tectonism from the character of turbidite deposits require knowledge of the sediment source, the flow-initiation process, how the turbidity current evolved during flow, and what flow phases were associated with deposition. The flow responds to shape and size characteristics of both the erosional pathway and the morphology of previous deposits; these, in turn, reflect factors such as basin size, shape, and tectonic activity. The initiation of turbidity currents generally involves either transformation from mass failures or one of a variety of fluid-flow events involving ignitive flow. The variability of these initiating processes with geologic setting is illustrated by analysis of three case studies involving 1) initiation by storm surge on a carbonate-reef slope, 2) a seismically triggered failure of a line source within midslope sediment, and 3) a bedload delta showing both hyperpycnal flow of river bedload and failure of unstable prodelta sediment. General criteria for the initiation of turbidity currents are given. Depositional and erosional features of the turbidite deposits themselves allow recognition of changes in flow processes as the turbidity currents traverse the basin. High-concentration basal flow beneath the main turbulent turbidity current will leave a distinctive depositional record from more energetic flow, particularly in proximal and channel settings. Turbidity currents of different sizes and sediment distribution interact quite differently with local fan morphology. Many large, muddy turbidity currents are much thicker than levee-channel relief; the upper parts of such flows are "stripped off" across levees and can become rechannelized farther downslope. in restricted basins, the basin margin acts like a channel wall for larger flows, thus maintaining competence over long distances. Thinner, sandier flows flush out channel systems, and a large proportion of sediment in some turbidite deposits may be derived from bed erosion. A broad range of local bedforms, especially sediment waves and scours, reflects changes in the character of turbulence and competence within quasi-steady flows. This paper emphasizes the broad range of size, sediment distribution, and depositional processes in turbidity currents that largely reflects the variety of initiation processes. This variability means that caution is needed when interpreting depositional conditions of ancient turbidite sediment with ill-defined morphologic setting.