Foreland Basin Axis Migration Documented by Deep-Water Conglomeratic Channel Deposits, Southern Chile

Bernhardt, Anne¹, Zane Jobe¹, Donald R. Lowe¹ (1) Stanford University, Stanford, CA

Excellent exposures of multiple conglomerate filled deep-water channels in the Upper Cretaceous Cerro Toro formation crop out on the Sierra del Toro, southern Chile. These outcrops are interpreted to represent axial channel belt deposits of the narrow, deep-water Magallanes foreland basin. Sedimentation in the Magallanes basin during the Upper Cretacous was controlled largely by a north-south oriented deep-water channel belt. Paleocurrents along this axial channel were directed southward parallel to the advancing Andean fold-and-thrust belt. At least five discrete channel complexes, with vertical thicknesses ranging from 10 to 200 m and widths varying from less than 1 km to greater than 11 km are composed of conglomerate and thick-bedded sandstone. Each channel complex is separated from the next by from about 5 to 150 m of thin-bedded, mud-rich turbidites. The uppermost and lowermost channels are the thickest and widest, while three smaller channel complexes lie between these. The upper channel complex is the largest, with a vertical thickness of ~ 200 m and a width of at least 4 km.

The five channel complexes exposed on the northern face of Sierra del Toro appear to show progressive eastward stacking up section, suggesting that the foreland basin axis migrated progressively to the east during the Late Cretaceous as the Andean thrust front advanced from the west. This type of axis migration is commonly observed during the evolution of foreland basins next to active fold-and-thrust belts.