Within high‐density flood flows a prominent mechanism of gravel transport and deposition is by stream‐driven, high‐density traction carpet (with a rheology similar to grain flow). These gravel carpets are envisaged to form the basal portion of a bipartite high‐density flood flow, decoupled from an overlying sand‐ and silt‐laden turbulent flow. Several examples already documented in the literature are reviewed and an additional case from the Lower Old Red Sandstone of southwest Ireland is presented. Two mechanisms of traction carpet initiation are discussed: by rapid entrainment of gravel into suspension on rising stage, followed by settling into the gravel traction carpet at peak and falling stage; and by overconcentration of a ‘normal’, low‐density bedload. Gravel entrainment, suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension. Theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high‐density, sand‐bearing turbulent flood flow above. This tangential shear stress is converted to dispersive pressure, which aids buoyancy and quasi‐static grain‐to‐grain contacts in the support of the clasts within the gravel carpet. The carpet is thought to have a quasi‐plastic rheology but behave much like a viscous fluid at high shear rates. Stream‐driven gravelly traction carpets are expected to produce sheet‐like units of clast‐ to matrix‐supported conglomerate, characterized by a parallel or an a(p)a(i) clast fabric. These units may be ungraded, normally or inversely graded, depending on the rate of shear, the viscosity of the flow and the celerity of deposition.

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New well and seismic data acquired during recent exploration of the SE Nam Con Son Basin, offshore Vietnam, have been evaluated to assess the tectonostratigraphic evolution. The offshore Vietnamese region has evolved in response to the complex relative motions of Indochina, Peninsular Malaysia, Borneo and the East Vietnam/South China Sea during the Cenozoic. On a regional scale these motions have been accommodated by strike-slip fault development, crustal extension and contraction. Rift pulses occurred in the SE Nam Con Son Basin from the Palaeogene to the earliest Late Miocene in response to the interaction of East Vietnam/South China Sea rift propagation and regional transtensional shear to the west of the evolving ocean basin. The structural evolution was complicated by mild contractional deformation during the Middle Miocene which was broadly synchronous with development of the largest inversion structures in the nearby West Natuna Basin. The oldest dated Tertiary rocks in the SE Nam Con Son Basin are fluvio-deltaic sediments of Late Oligocene age which have been penetrated by several wells. Early to Mid-Miocene depositional environments ranged from non-marine to outer shelf, with a predominantly clastic basin-fill. The thickest Lower to Middle Miocene occurs in N-S- to NE-SW-trending half grabens. A regionally recognized truncational unconformity of late Mid-Miocene age has resulted partly from the combined effects of the mild inversion and by the erosion of uplifted footwalls. During the Late Miocene there was rapid and widespread deepening of depositional environments, synchronous with net-extensional fault reactivation on many previously developed rift structures. The stratigraphic response to this increased bathymetry was the growth of isolated carbonate build-ups on pre-existing structural highs, progradation of the palaeo-Mekong delta system and associated deep-marine submarine channel development and erosion. This pulsed structural and stratigraphic evolution has resulted in deposition of source, reservoir and seals, and produced a variety of potential trapping styles.

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