Multiphase deep water slope channel evolution: Cerro Toro Formation, Silla Syncline, Magallanes Basin, Chile

In the Late Cretaceous of the Magallanes Basin, a series of conglomeratic submarine channel systems developed within an otherwise thin-bedded mud dominated slope environment. The channels make up the Lago Sofia Member of the... [ view full abstract ]

In the Late Cretaceous of the Magallanes Basin, a series of conglomeratic submarine channel systems developed within an otherwise thin-bedded mud dominated slope environment. The channels make up the Lago Sofia Member of the Santonian Cerro Toro Formation within a large structure known as the Silla Syncline. The area has been investigated by several workers in the past; however, due to a catastrophic fire in 2012, the exposures which were previously largely obscured by vegetation are now significantly better exposed and accessible.
Based on geological mapping and detailed measured sections in the syncline over three field seasons since the fire, we propose an improved geological map of the Syncline, highlighting architectural elements and channel development. Six phases of channel development are recognised:
(1) initial channel erosion and bypass; with aggradation and confinement by either deposition of a contemporaneous master levee or confining intra-slope topography and MTDs;
(2) a phase of conglomerate deposition, eroding into the earlier MTDs;
(3) a phase of erosion into the first conglomeratic phase, and bypass, marked by scours and fine-grained drape;
(4) a relatively quiescent phase with thin-bedded turbidite deposition and some local slumping;
(5) a second conglomeratic phase with initial multiphase stepped erosion into the heterolithics and slumps, building up to 50 metres of amalgamated sandstones and pebbly sandstones in the channel-axis;
(6) a final phase of confined sheet sand deposition, alternating with rare conglomeratic debrites, and thin-bedded turbidite deposition, onlapping onto the confining slope or master levee described in (1)
Two contrasting channel margin styles are recognised: a south-eastern channel margin with erosion, aggradation and pinch out, in a series of north-eastwards migrating, offset stacked channel storeys; and a south-western channel 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.