Reverse flow in turbidity currents: the role of internal solitons



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%o. 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


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.