Reverse Flow in Turbidity Currents: The Role of Internal Solitons
- Dorrik A. V. Stow
Published Online: 29 APR 2009
Copyright © 1992 The International Association of Sedimentologists
Deep-Water Turbidite Systems
How to Cite
Pantin, H. M. and Leeder, M. R. (1991) Reverse Flow in Turbidity Currents: The Role of Internal Solitons, in Deep-Water Turbidite Systems (ed D. A. V. Stow), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304473.ch7
Department of Geology, University of Southampton, UK
- Published Online: 29 APR 2009
- Published Print: 11 NOV 1991
Print ISBN: 9780632032624
Online ISBN: 9781444304473
- reverse-flow units;
- thick-bedded calcareous wacke;
- probable nature of primary turbidity flow;
- sediment concentration;
- rate of momentum loss of the flow
Pickering & Hiscott, (1985) have demonstrated amply the presence of reverse-flow units within the thick-bedded calcareous wacke (TCW) beds of the turbiditic Cloridorme Formation (Middle Ordovician, Gaspé Peninsula, Quebec, Canada). These reverse-flow units are underlain and overlain by units which reveal flow in the primary (obverse) direction.
In this paper, a model is proposed for this reverse flow, based on the probable nature of the primary turbidity flow. It appears that the initial flow was highly elongated (thickness h < < length L), with h ≈ 500 m, velocity U ≈ 2 m s−1 and sediment concentration C ≈ 1.25 ‰. The rate of momentum loss of the flow is estimated by means of a useful parameter which we call the ‘drag distance’, symbol dD, defined by
dD = hL/(L.cCd + h.fCd),
where h and L are the thickness and length of the flow, respectively; cCd is a combined drag coefficient representing friction on the bottom and at the upper interface; and fCd is a form-drag coefficient related to the shape and size of the head. dD is the distance travelled by a current of constant h and L, flowing over a horizontal bottom and obeying a quadratic friction law, for an e-fold reduction in velocity.
Simple considerations, confirmed by our own experiments (described in this paper), show that such an elongated turbidity current cannot be reflected as a whole from an adverse slope; when the nose of the current reaches the slope, it forms a hump, which surges backwards and sooner or later breaks up into a series of internal solitons. The latter, probably numbering 4–7, will cause reverse flow at a given point as they pass by, provided that the residual velocity in the tail is not too great. Flow in the original (obverse) direction will be re-established after the passage of the solitons. Quiescent periods in front of, between and behind the solitons, when soliton-associated currents cancelled out the residual obverse flow, would allow the deposition of thin mud-drapes.
Additional flow reversals observed in a few of the TCW beds cannot be explained readily by the re-passage of solitons, since wave breaking at the ends of the basin would cause massive energy loss; internal seiches are the preferred explanation for these later reversals.