Abstract

A Jurassic submarine mass movement deposit is exposed on the mountain Bou Kornine in N. Tunisia. It is up to 75 m thick and is part of a carbonate slope sequence. Although it displays some debris flow deposit characteristics, it is shown to be the result of several mass movement mechanisms. An early turbulent stage when clasts were mixed in a medium of lime mud was followed by an intermediate stage of decreased turbulence when clasts settled through the matrix and matrix particles sedimented with coucomittant dewatering. During the final stages when matrix competence increased rapidly, clasts were frozen before they bad completely settled. Mass movement downslope produced three vertical and lateral gradational zones: (1) Zone A, the basal 5-10 m, containing mostly clasts of radius between -6 and -8 phi (6.4 to 25.6 cm) in contact, (2) Zone B, a central matrix--supported "coarse tail" graded zone consisting mostly of clasts of radius -4 to -6 phi (1.6 to 6.4 cm), (3) Zone C, an upper zone consisting of matrix-supported clasts of radius less than -4 phi (1.6 cm). Zone B was used as an indicator of lapsed time from the intermediate stage of decreased turbulence to the final stages of increased matrix competence. Calculation of this "resedimentation time" indicates two stages of development: (1) Uniform "freezing" of the basal 30 m of the flow less than 1 minute after decreased turbulence: (2) linear increase in resedimentation time above 30 m, causing the top of the flow to remain mobile for up to 20 minutes. Multistage resedimentation coupled with the gradual withdrawal of large clasts from the system can result in the genesis of a downslope pebbly mudstone facies.

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