DOWNSTREAM VARIABILITY IN DEEP-WATER CHANNEL DEPOSIT CHARACTER AND ASSOCIATED SEDIMENTARY PROCESSES, CRETACEOUS CERRO TORO FORMATION OUTCROP BELT, MAGALLANES BASIN, CHILE

Conglomeratic deep-water fan-channel deposits of the Cerro Toro Formation are exposed for over 100 km along a north-south trending outcrop belt in the Ultima Esperanza District of southern Chile. Coarse-grained facies accumulated during the Late Cretaceous (Campanian) in the narrow foredeep of the Magallanes Basin, formed in response to uplift of the adjacent Patagonian Andes. Exceptional outcrops at Sierra del Toro and Cerro Mocho, 45 km apart, expose the relationship between channel fill and out-of-channel strata allowing a detailed comparison of facies, stratigraphic architecture, and interpreted sedimentary processes along the length of a single depositional system.

The Sierra del Toro outcrop, located in a more proximal position, is characterized by incision into older, pre-channel deposits at the channel base and thick constructional levee deposits in the upper part of the succession. Here, the channel complex exhibits 180 m of total relief (degradational and constructional) over a lateral distance of approximately 250 m. The entire channel-belt at Sierra del Toro is less than 1,000 m wide. Conversely, the channel complex margin at Cerro Mocho is largely constructional. Total relief of the margin is almost 300 m over a lateral distance of 1 km. The width of the entire channel-belt at Cerro Mocho is at least 4 km.

Amalgamated conglomerate and sandstone beds up to 10 m thick characterize the stratigraphic succession in the proximal setting at Sierra del Toro. Evidence of bypass, including traction-structured sandstone and conglomerate, is pervasive from the channel axis to the margin; thin-bedded turbidites are restricted to sites of overbank deposition. A master-surface defines the boundary between channel and out-of-channel regions and is indicative of confined flow. More distal deposition at Cerro Mocho was dominated by traction processes in the channel belt axis, however, beds towards the margins are typically non-amalgamated and include conglomeratic mudstones and thick- to thin- bedded turbidity-current deposits. Packages of channel deposits 40-60 m thick interfinger with levee deposits at the margin over a lateral distance of more than 500 m. This margin architecture is indicative of channel migration and/or a variation in sediment supply and deposition through the depositional history.