High-Relief Slope Clinoform Development: Insights from Outcrop, Magallanes Basin, Chile

Abstract

Abstract The Cretaceous–Paleogene Tres Pasos and Dorotea formations of the Magallanes Basin, Chile record the filling of a deep-water foreland setting. Slope clinoforms with at least 700–900 m relief (compacted) prograded southward along the foredeep axis, which was oriented parallel to the adjacent Patagonian Andes. Fluvial- and wave-influenced deltaic deposits of the Dorotea Formation represent the upper, flat portions of the sigmoidal slope profiles. The paleo-shelf edge is estimated where shelf sandstones pinch-out basinward. Mudstone, siltstone, and a notable paucity of sandstone characterize upper slope strata. Further down-slope, conduits are evidenced by sedimentary bodies associated with cross-stratified or normally graded sandy conglomerate and local mudstone rip-up clasts, interpreted to indicate that considerable sediment bypassed the slope. Turbiditic sandstones and mass-transport deposits of the Tres Pasos Formation characterize the lower to base of slope setting. Numerous examples of slope clinoforms have been recognized in the rock record, with the majority characterized by 200–500 m of estimated paleo-relief. Higher relief examples include those mapped in outcrop from the Magallanes Basin documented here, and comparable clinoforms from the subsurface Cretaceous Brookian succession of the North Slope, Alaska. In the Magallanes Basin, numerous factors contributed to the development of high-relief clinoforms, including generation of substantial basin margin relief, the absence of mobile substrata, adequate sediment supply, and the elongate basin shape. The slope that built and maintained the relatively smooth clinoform profile was narrow, and thus, sediment that was transported across the shelf was focused as it passed into deeper water. In general, the development of slope clinoforms, including high-relief examples like those of the Magallanes Basin, is facilitated when the rate of sediment input onto the slope is higher than the rate at which rugose slope topography is generated from mass wasting or substrata remobilization.