The Influence of Mass-Transport-Deposit Surface Topography on the Evolution of Turbidite Architecture: The Sierra Contreras, Tres Pasos Formation (Cretaceous), Southern Chile

Abstract

Abstract The Upper Cretaceous Tres Pasos Formation, exposed on the Sierra Contreras, southern Chile, provides an exceptional opportunity to document well-exposed depositional relationships between fine-grained mass-transport deposits (MTDs) and overlying turbiditic sandstone deposits in an ancient deep-water slope system. The lateral continuity of the reservoir-scale sandstone-rich facies at this outcrop is controlled principally by surface topography intrinsic to the MTDs, where overlying, conformable sandstone beds pinch out and lap onto the relative topographic highs of the MTD upper surfaces. Turbidite architecture evolves to more laterally continuous, sheet-like deposits as a result of depositional smoothing of MTD topographic relief and diminished confinement. An MTD surface-topography model containing a hierarchy of three fundamental tiers of MTD upper surface topography is identified at the Sierra Contreras and is defined by the maximum values of the horizontal (x) and the vertical (y) dimensions of the topography relative to local elevation. Each tier of the hierarchy is distinguished from the next by approximately one order of magnitude difference in both of the two dimensions. Tier 1 MTD surface topography is up to several meters in magnitude in the x and y dimensions and creates local pockets of ponded sandstone. Tier 2 MTD surface topography is 10 meters to several tens of meters in the x dimension and several meters to several tens of meters in the y dimension. This scale of topography can laterally compartmentalize discrete packages of sandstone. The largest scale of MTD surface topography, Tier 3, is several hundred meters in both the x and y dimensions and can laterally divide (in at least two dimensions) kilometer-scale sediment-gravity-flow conduits, significantly compartmentalizing sandstone deposits. The cyclic packaging of sandstone and MTDs observed at the outcrop suggest the depositional slope profile was adjusting between graded (constructional) and out-of-grade (degradational) conditions in this overall prograding slope system. The MTD surface-topography hierarchy presented here is applicable at a range of scales in analogous MTD-dominated deep-water slope environments, from relatively high-resolution ancient outcrops to lower-resolution seismic-reflection-based studies. Whilst the tiers are successively separated by an order of magnitude in size, there is range within the hierarchy with respect to the topographic dimensions facilitating a more flexible comparison of geographically separated MTDs.