ABSTRACT

Small (less than 1 cm) elongate mud rip-up clasts within carbonate turbidite sequences were used as paleocurrent indicators in a Jurassic carbonate-slope environment. These slope deposits are exposed on Djebel Bou Kornine in northern Tunisia. The clast lineations were proved to be consistently parallel to flute-cast directions and consistent throughout the thickness of a turbidite bed. Growth faulting in this area during the Jurassic produced localized troughs between adjacent fault-blocks. Paleocurrent directions in these troughs were westerly, at right angles to the northerly slope of the continental margin. This type of lineated mud clast may be a useful paleocurrent indicator in oriented cores of certain turbidite sequences.

CONCLUSIONS

1) The long axes of small (less than 1 cm) mud rip-up clasts were found to be parallel to depositional paleocurrents and are, therefore, excellent indicators of ancient paleocurrent directions.

2) These paleocurrent indicators were used to confirm the existence of a trough between adjacent growth-fault blocks which trended parallel to the basin edge during the middle Jurassic in northern Tunisia.

3) Elongate muddy clast lineations can be valuable paleocurrent indicators to use alone or in conjunction with, for example, flute casts. They may be especially useful in oriented cores of turbidite sequences. Cores should be slabbed parallel to bedding and inspected for small muddy clasts whose axes are elongate and show some orientation.

INTRODUCTION

The turbidite sequences are middle and upper Jurassic in age. They are exposed on Djebel Bou Kornine in northern Tunisia, about 20 km southeast of Tunis (Fig. 1). Bou Kornine records a sequence of carbonates which indicate a submarine slope environment was prevalent during the middle and upper Jurassic (Cossey and Ehrlich, 1978). The fault blocks were downthrown to the north where an open seaway or gulf existed (Nairn et al., 1978). A carbonate platform with shallow water conditions lay to the south on Djebel Zaghouan (Fig. 1) (Mundy, 1976).

Flutes on the bases of the turbidites were rare but were the best source of paleocurrent directions in the sequence. However, abundant fine-grained clasts in the turbidites were observed in the field and seemed to show a uniform orientation

FIG. 1 . Location of Djebels Bou Kornine and Zaghouan in the Jurassic Ranges of Northern Tunisia.

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FIG. 2. [Grey Scale] See caption on page 875.

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on bedding planes (Fig. 2). It was decided to investigate this observation in more detail.

EXPERIMENT TECHNIQUES

The orientations of the long axes of the mud clasts were measured on 14 parallel-to-bedding slabs through one turbidite bed. This was done to determine whether any variation in orientation exists through one turbidite. A north-south orientation at the base of the bed and an east-west orientation at the top could mean that changes in the flow direction occurred during the deposition of the bed. Parkash (1970) found that the orientations of intrabed lineations were not consistent through a turbidite bed. He attributed this to a change in flow direction as the turbidity current waned. For our data, structural corrections were made to convert the measurements back to a horizontal datum. No significant variation in the orientation of the 502 mud clasts was found (Fig. 3). The rose diagram odes all indicate a northwest-southeast orientation.

The mud-clast long-axis lineations were then compared to flute orientations in order to confirm that the long axes were indeed parallel to the indicated paleocurrent directions. Mud-clast long-axis lineations were measured from a lower Oxfordian turbidite bed, 5 meters stratigraphically above the location of some flute casts. Structurally corrected lineation measurements from the mud-clast long-axes were found to coincide with the corrected paleocurrent directions measured from the flute casts (Fig. 4). Flute casts and mud clast lineations were also found to coincide at one other location of the lower Oxfordian sequence (Fig. 5). The result of this experiment is different to a similar one carried out by Parkash (1970). He found that the orientations of graptolites and sole marks were ubparallel and that the long axes of argillite fragments were not reliable paleocurrent indicators.

It was then decided to use this new paleocurrent direction indicator to interpret basinal paleocurrent trends for the Jurassic sequence exposed on Bou Kornine.

JURASSIC PALEOGEOGRAPHY

The use of mud-clast long-axis orientations as paleocurrent indicators confirmed the presence of a trough between two adjacent growth-fault

FIG. 2. Lineated muddy clasts within an upper Bathonian turbidite from Djebel Bou Kornine. A) Cut parallel to bedding. Note elongate small muddy clasts (arrow) on slab surface. B) Cut perpendicular to bedding. The scale bar is 5 cm long.

FIG. 3. Orientations of muddy-clast long-axis lineations from one upper Bathonian turbidite bed. Note the consistent northwest-southeast orientations throughout the bed. The slabs were cut parallel to bedding.

FIG. 4. Comparison of flute cast and muddy clast paleocurrent direction indicators. Muddy-clast long-axis measurements were taken from a bed 5 meters stratigraphically above the flute cast location. Note the close correlation between the two measurements and that one-half of the muddy-clast long-axis lineations can be ignored.

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blocks and trending parallel to the basin edge. Growth faulting existed in the region of Bou Kornine during the middle and upper Jurassic (Cossey and Ehrlich, 1978).

The results of paleocurrent measurements from flute casts, pebble long-axes, and mud-clast longaxes are shown in Figure 5. The data restricted to the area of maximum growth-faulting shows

FIG. 5. Paleocurrent data from the middle Jurassic turbidites of Bou Kornine. Small numbers at the top right of each paleocurrent rose represent number of observations and the letter refers to the source of the measurement. Note the mud clast lineation 90-degree discrepancy in the middle of the upper Bathonian cross-section which is still evident in the Lower Oxfordian. Small X's represent measured section locations.

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an east-west orientation whereas the data elsewhere shows a northwest-southeast or north-south orientation. One-half of the lineations can be ignored, since the data from the flute casts indicates either a westerly or northerly paleocurrent direction. Thus, in the area of most growth-fault activity, the paleocurrent was westerly, but outside this area the paleocurrents were northerly or northwesterly. An obvious explanation is that the turbidites which show westerly paleocurrents were confined to a low between down-to-basin growth faults but others upslope and downslope were unaffected and show paleocurrents which were parallel to the northerly regional paleoslope. This may be an alternative answer to the problem of right-angle discrepancy of turbidite paleocurrents often encountered n ancient turbidite deposits (Potter and Pettijohn, 1963, p. 133 and p. 242-243).