When is a Turbidity Current not a Turbidity Current? A Question of Mobility

Ben Kneller

The interpretation of deep water sands, always problematic, has recently been thrown into greater confusion by publication of a model that reinforces many of the misconceptions about the way turbidity currents transport and deposit sediment (Shanmugam et al. 1995). It may be comforting to believe that turbidites can be reliably differentiated from debris flows and contourites on the basis of, for example, normal size grading, sharp basal contacts, gradational upper contacts, and Bouma divisions; however, it is also wrong. A consideration of the mechanics of turbidity currents suggests a wide diversity of structures and textures is to be expected in their deposits. The interplay of a relatively small number of variables (principally trigger mechanism, topography a d available grain-size range) produces predictable and highly variable grading, sorting, stream-wise fining, and bedform characteristics. Thus many features recently suggested as characteristic of debris flow (e.g. sharp upper grain-size breaks, floating or rafted mudstone clasts, inverse grading of clasts, moderate to high matrix content) or contourites (e.g. traction structures) are to be expected in turbidites. A framework will be presented that utilises basic fluid mechanics to relate these variables to depositional facies, and explains a wide range of common as well as some exotic turbidite facies. This framework can, for example, be applied to prediction of depositional facies away from the well-bore in situations where the paleotopography can be constrained from seismic.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California