Depositional history of the Upper Cretaceous Cerro Toro Formation, Silla Syncline, Magallanes Basin, Chile.

摘要

This dissertation combines outcrop based studies of stratigraphy and facies architecture with petrographic and geochemical analyses of mudstone, sandstone and conglomerate to document the depositional history of the Upper Cretaceous Cerro Toro Formation exposed in the Silla Syncline, Magallanes Basin, Chile.; The Cerro Toro Formation in the Silla Syncline consists of more than 1100 m of deep-water clastic sediment composed primarily of fine-grained sandstone and mudstone interpreted as the deposits of low-density turbidity currents. Three thick units of coarse-grained sandstone and conglomerate are intercalated with the fine-grained sediments. These coarse units, informally termed members, represent the fill of a series of deep-water channel complexes each of which can be subdivided into individual channel systems. Six such channel systems, all trending more or less northeast to southwest, have been identified. Analysis of channel margin geometries in one channel system demonstrates that this system evolved through multiple cycles of erosion followed by coarse sediment deposition and capped by mudstone and thin sandstone. Outcrop relationships, stratigraphic correlations, and paleocurrent data indicate that the fine-grained deposits in the Silla Syncline are not levees, but rather represent either periods of late-stage channel filling or deposition from unconfined, low-density fine-grained turbidity currents moving north-south through the Magallanes Basin. The observed cyclicity in channel development probably reflects local controls within the basin, while the large-scale cyclicity from coarse- to fine-grained sedimentation probably reflects external tectonic controls.; Petrographic and geochemical analyses of Cerro Toro Formation mudstone, sandstone and conglomerate indicate derivation from a mixed source consisting of Paleozoic meta-sedimentary terranes, arc-volcanic rocks, and mafic and ultramafic rocks enriched in Sc and Cr. By comparison with the underlying Punta Barrosa Formation, the Cerro Toro reflects derivation from a more fully developed fold-and-thrust belt and appears to have derived less sediment from the Andean arc.; Textural analysis of a conglomeratic deposit showing an upward transition for clast support at the base to matrix support at the top demonstrates that the sand population in the matrix shows normal distribution grading throughout. Although conglomerate clasts are mud-matrix-supported at the top, the upper portion is a grain-supported sandstone deposited from a fully turbulent flow.