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Sedimentologic and Stratigraphic Evidence for Flow Transformations as Turbidity Currents Encounter Intraslope Minibasins: Implications for Up-Dip Stratigraphic Traps From an Outcrop Analog


Sediment-laden gravity currents are the primary mechanism for delivering coarse-grained material to the deep-sea. These currents traverse deep-sea slopes where gravity and tectonically driven topographic complexity can promote flow transitions and influence sedimentation. Sandy intraslope fans form where local changes in gradient and confinement generate accommodation for deposition and represent attractive hydrocarbon reservoir targets in many basins around the world (e.g., Gulf of Mexico, offshore West Africa). However, relatively few examples of intraslope fans have been identified in outcrop; as a result, the sedimentological and stratigraphic characteristics of these deposits are poorly constrained. We examine an example of an intraslope fan accumulated in a growth-fault-controlled slope minibasin from the Tres Pasos Formation, Patagonia, Chile. The primary objective is to investigate the interaction between flows and synsedimentary extensional features on the seafloor, based on observations of facies distribution and stratigraphic architecture.

We capture the sedimentological features of the intraslope fan deposit through AUV imagery, detailed measured sections, and paleoflow measurements along a depositional dip-oriented (N-S), >750 m outcrop exposure at El Chingue Bluff. At the head of the minibasin, siltstone dominated strata is preserved and characterized by subtle, south-inclined surfaces and thin, discontinuous fine-medium grained sandstone beds that record sediment bypass. Steep-faced scours containing mudstone rip-up clasts and backset bedding as well as more tabular sandstone beds with foreset and backset stratification reflect hydraulic jumps as turbidity currents entered the minibasin. The sandstone-prone basin fill consists of thick-bedded sandstone deposits that change in nature and thickness from North to South along depositional dip (indicated by south-directed paleoflow measurements), thickening rapidly southward from a few to >50 meters. The thickest part of the depocenter consists of Ta-e sandstone beds 10 - 100 cm thick indicating waning flow conditions in a ponded setting. We document and link up-dip to down-dip components of an interpreted flow transition zone that result in thick ponded sandstone units. These findings shed important insight into the onlap character of ponded turbidites and updip stratigraphic trap effectiveness in slope fan reservoirs.