ABSTRACT

Most investigations that discuss the origin of turbidites state, or intimate, that turbidity currents are surges commonly initiated by catastrophic events related to submarine sliding on slopes. This concept is almost universally accepted by specialists although little “hard” observational data are available. In general theory, failure is triggered by stresses that can result from several factors (high sediment accumulation rate, slope oversteepening, earthquake, subsurface gas generation, etc) which give rise to excess pore pressure and a weakening of the sediment. Liquefaction and decrease in overall shear resistance are usually invoked as causes of sediment mobility. Increased mobility within the slab results in increased disorder of original sediment stratification and structure. Grain-to-grain structure is disrupted during acceleration, causing changes in pore pressure and allowing mixing with water that, in turn, results in further alteration of original structure. Metastable, high sensitivity, cohesionless sediments are most susceptible to mobility and, in turn, transformation from submarine sliding to turbidity current flow.

Continuing sedimentologic studies of spectacularly exposed gres d’Annot deep-sea fan deposits (lowermost Oligocene) in the French Maritime Alps provide clues to resolve the problem of slide-to-turbidite transformation. Most noteworthy are massive channelized strata in upper and mid-fan sectors characterized by a lower slump member that evolves upward to a turbidite. This merging is indicative of probable generation of sediment gravity flows from submarine sliding. A proposed scheme suggests failure on a fan valley wall by submarine sliding that could entrain the flow of sand either from adjacent reaches of valley walls and/or from farther up-axis in the fan valley. In this manner, flow of some of the sand initiated by the sliding event would come to rest at nearly the same time and position as the slump mass near the base of the fan valley wall and in the axis proper. In some cases, the slump mass may bypass the base of the valley wall and, as it accelerates, transformation may be induced with resulting flow of sediment down-axis and basinward.

The above phenomenon has a bearing on hydrocarbon exploration. Modern ocean floor surveys and regional field studies, such as those of the Annot Sandstone Formation, indicate that there is a nonrandom distribution of turbidites with respect to slumps deposited on marine margins and basins. Moreover, massive channelized submarine fan valleys may serve as ideal reservoirs for both oil and gas. Thus the model proposed here, if valid, clearly would be important in helping refine paleogeographic interpretation and in pinpointing potential hydrocarbon-bearing targets in the subsurface.

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