ABSTRACT

The sand fraction of turbidites can be divided into three parts: from base upward the graded division (A), lower laminated division (B) and current rippled division (C). Some divisions may be missing in individual beds, which can thus start with either division A, B, or C. For any group of turbidites, a parameter P₁ can be derived, based on the proportion of beds in the group beginning with divisions A, B, and C. P₁ is therefore directly related to the flow regimes of the currents which deposited the beds. The average flow regime for the group of beds is influenced mainly by the distance the currents have flowed across the basin floor, and to a lesser extent by the gradient of the floor and the flow parameters of individual currents. P₁ is therefore la gely a function of proximality, and is expressed as a percentage.

Measurements of 3352 beds from seven long outcrops (representing five basins of different age and tectonic setting) show that as P₁ decreases, that is as the environment becomes more distal, bed thickness decreases exponentially, erosion becomes less common and abundance of parallel laminae and cross-laminae increases. The combined data also allow definition of normal and abnormal turbidity current behavior. In the vertical section, P₁ can be expressed as a moving average, and regular, irregular or cyclic changes in proximality can be demonstrated during basin filling. The Upper Devonian turbidites of Maryland are taken as an example of irregular basin filling.

The characteristics of proximal turbidites are found to be very similar to those of fluxoturbidites. The latter name has been poorly defined, both geologically and hydrodynamically, and it is suggested that it be abandoned.