Geometry, internal architecture and evolution of a slope channel in the Oligocene Tertiary Piedmont Basin (Northern Italy)

Deep water “turbidite” channels are relevant sites of sand and gravel deposition in slope to basin settings where they may form hydrocarbon reservoirs. The details of their external shape and internal architecture, as well as their erosional/depositional evolution, are the present-day subject of outcrop-based research, in view of new advances in understanding and forecasting heterogeneity of this kind of reservoirs. Among several outcrop examples of deep-water channels, the Oligocene Millesimo Unit of the Tertiary Piedmont Basin (TPB, Northern Italy) provides good exposures of conglomerate and sandstone bodies that we interpret as the fill of a submarine channel cut into slope mudstones. The preserved exposures of this unit form a belt of lens-shaped bodies, up to 1.0 km wide, 5 km long (along the average palaeocurrent trends) and up to tens of meters thick, whose envelope yields the planform of a bending channel. Several conglomerate-sandstone bodies lay above a major erosional boundary (Fig.1) and are stacked with second-order erosional relations. Generally the individual bodies show fining-upward sequences up to 15 m thick, that are formed by disorganized boulder conglomerates passing to graded-laminated sandstones and thin bedded turbidites. These bodies are laterally stacked by means of downlapping, sigmoidal sets of conglomerates passing to massive sandstone beds then to mudstone-sandstone couplets that characterize their lateral wedging-out. We interpret these individual units as second-order channel fills within the first order one. Palaeocurrent measurements suggest that these dipping units accreted obliquely and radially with respect to the SSE-NNW average trend of the first-order channel. Slumps, rotated beds and boulder-bearing debris flows are frequent both at the base of the first-order channel and at the base of the second order ones. Slumps occur almost everywhere at the base of the entire system, in close association with synsedimentary faults and folds. These features, together with the westward polarity of the stacking patterns, suggest that westward lateral migration and cross-cut relationships were forced by synsedimentary folding and faulting that occurred during the Oligocene relative uplift of the TPB marginal slope. Every tectonic increment determined a lateral shift and entrenchment of the axial channel, resulting in erosion of the faulted and tilted fringe of the previous valley fill and/or of the uplifted slope mudstones. This evolution promoted also the downcurrent migration of the conglomerate-sandstone-mudstone facies tracts. The bend-shape, the internal architecture and the evolution of the Millesimo Unit slope channel were primarily controlled by the mobile slope morphology and by synsedimentary tectonics during the transtensional opening, deepening and widening of this sector of the TPB.