Deep‐water fan‐channel conglomerates of Late Cretaceous age, southern Chile

ABSTRACT The exceptionally well exposed Lago Sofia conglomerate and sandstone lenses in the Upper Cretaceous Cerro Toro Formation of southern Chile are interpreted as the channel and channel margin facies of a deep‐sea fan. The north‐to‐south oriented channels formed on an elongate fan in a narrow retroarc basin between a rising cordillera to the west and the South American craton to the east. The great length of some of the channels (> 120 km) seems to reflect the long duration (> 30 m.y.) and stable nature of the basin. Enclosing the lenses is the fine‐grained Cerro Toro Formation which represents overbank turbidite flows and hemipelagic sedimentation on levee and levee flank areas. Foraminiferal assemblages suggest deposition in 1000‐2000 m of water. Most of the conglomerate has features developed by tractive currents (parallel‐ and cross‐stratified conglomerate). Most is moderately well sorted, imbricated, and has parallel to inclined stratification; large‐scale dunes up to 4 m high are exposed. Typical sediment, gravity flow structures and bedding styles (e.g. pebbly mudstones, graded conglomerate, giant flutes) are not as common in the channel deposits as are tractive features. Tractive features in the gravels apparently were developed by rolling, sliding, and saltation as the bed‐load component of highly turbulent, moderate‐ to low‐density turbidity currents flowing in a confined channel. Graded‐to‐massive conglomerates appear to have been deposited rapidly from fully turbulent flows; diamictites were deposited from debris flows in which fluid viscosity, yield strength, and buoyancy of the fluid were dominant. The three major conglomerate classes recognized do not occur in a systematic manner; vertical and lateral heterogeneity is the rule.