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Multiphase Deep-Water Slope Channel-Lobe Evolution: Cerro Toro Formation, Silla Syncline Area, Magallanes Basin, Chile


In structurally controlled basins, the prediction of reservoir facies in submarine slope channels tend to be rather challenging, due to the effect deeply rooted or active structures have on turbidity currents. In the Santonian to Campanian of the Magallanes Basin, a series of structurally controlled conglomeratic slope channels developed within thin-bedded mud dominated slope environment. The channels form the Lago Sofia Member within a large structure known as the Silla Syncline. Based on measured sections correlation, geological mapping, and photomosaic interpretation, we propose a more accurate geological map of the Syncline, mainly of the Paine C stratigraphy, highlighting architectural elements and channel development. Six channelising phases of development are recognised: 1- initial channel erosion and bypass; with aggradation and confinement by either confining intra-slope topography; or deposition of a contemporaneous master levee; 2- a phase of erosion into the first conglomerates, and bypass, marked by scours and slightly disturbed fine-grained drape; 3- a phase of conglomerate deposition, eroding into the fine-grained deposits, and initial multiphase stepped erosion into the heterolithics and slumps; 4- another relatively quiescent phase with thin-bedded turbidite deposition and some local slumping; 5- a third conglomeratic phase, building up to 50 metres of amalgamated clast-supported conglomerates; 6- a final phase of confined sheet sand deposition, alternating with rare gravelly transitional event deposits, and thin-bedded turbidite deposition, onlapping onto a MTDs from a confining slope or master levee. In this particular channel, two contrasting channel margin styles are recognised: a north-eastern margin with erosion, aggradation and pinch out, in a series of southwards migrating, offset stacked channel storeys; and a south-western margin marked by lateral facies transition, with conglomerates passing laterally into amalgamated sands that pinch out towards the margin as heterolithics. Off-axis and axis facies associations are also described in four different locations, with an increase in channel entrenchment down-dip. Confined (inside) levee, MTDs, confined frontal splays, laterally accreting channel bar forms, and slope deposits are additional architectural elements recognised. This work proposes a more complete story for the deposits of the Paine C channel complex, with a series of discrete events described and interpreted during its evolution.