Architectural Elements of a Sinuous Deep-Water Basin Axial Channel Complex, Cerro Toro Formation Outcrop Belt, Magallanes Foreland Basin, Chile

Hubbard, Stephen M.¹, Brian W. Romans², Stephan A. Graham² (1) University of Calgary, Calgary, AB (2) Stanford University, Stanford, CA

Bathyal strata of the Cerro Toro Formation in the Cordillera Manuel Señoret, Chile, represent the deposits of a channel belt (4-8 km wide by > 100 km long) that occupied the foredeep of the Magallanes basin during the Upper Cretaceous. Channel deposits comprise a ~ 400 m thick conglomeratic interval (Lago Sofia Member (LSM)) encased in fine-grained units. Facies of the LSM include sandy- and muddy-matrix conglomerate, and massive sandstone beds. Interbedded sandstone and mudstone intervals are present locally, representing inner levee deposits. The channel belt had a sinuous planform architecture, as inferred from outcrop mapping. Laterally adjacent to the LSM are extensive interbedded mudstone and sandstone facies. A levee interpretation for these units is based on several observations: (1) paleocurrent measurements indicate flows diverged (50-100°) once they spilled over the confining channel margin, (2) sandstone beds thin, away from the channel margin, (3) stepped margins and injection of coarse material into surrounding fine-grained units indicate underlying sediment was not well indurated when scoured, and (4) the presence of sedimentary features common to levees (e.g., slumps and climbing ripples).

The tectonic setting and foredeep architecture influenced channel deposition. The confining influence of the basin slopes that paralleled the channel belt, and the likelihood that numerous conduits fed into the basin along the length of the active Andean fold-thrust belt to the west, suggest that proximal-distal relationships observed from large channels in passive margin settings are not necessarily applicable to axial channels in elongate depocenters.