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.