BRECCIAS DEPOSITED BY MASS FLOW IN THE BRECCIA NAPPE OF THE FRENCH PRE‐ALPS

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Abstract
In the Lower Breccia Formation (Lower Jurassic), variations in thickness of the breccia sequence, in individual bed thickness, and in size of clasts suggest separate sources to the southwest and north of the study area. Large clasts, poor clast rounding, grading, and restriction of stratification towards the tops of individual beds indicate deposition by mechanisms other than normal marine bottom currents. The deposits are too thick to have been deposited from the bed load of many successive sandy turbidity currents. A mass‐flow origin is therefore favoured. The stratification in the breccias is believed to have formed at a late stage in their deposition. It is considered to be a result of traction of clasts at the tops of beds by fluid entrained by the flow, or turbidity currents associated with the flow.