Stratigraphic Analysis of Cretaceous Slope Channels: Tres Pasos Formation, Sierra Dorotea, Southern Chile

Andrea Fildani¹ and Stephen Hubbard^2
1Chevron, San Ramon, CA
²Department of Geoscience, University of Calgary, Calgary, AB, Canada

The Cretaceous Tres Pasos Formation is interpreted as a continental slope depositional system because of sedimentary body architectures, facies associations, and stratigraphic position within the Magallanes foreland basin sedimentary fill. The continuous outcrop belt (~120 km) offers the exceptional opportunity to study the architecture and sand distribution of a seismic-scale slope succession. Detailed field analyses and outcrop mapping along the basin axis reveal evidence of various slope depositional processes in a series of outcrops 25-50 km north of the town of Puerto Natales. Laterally continuous outcrops provide the detailed dimensional and geometric data required for generating geologic models and identify key stratigraphic uncertainties.

Mass-transport deposits, thin-bedded siltstone- and shale- dominated units proportionally dominate the stratigraphic succession. However, a series of at least ten coarse-grained channel elements displaying a variety of architectures bear the most significance to reservoir exploration and exploitation models. The channels coalesce down-system (along the strike of the outcrop belt), resulting in an amalgamated stack of sandstone packages with a collective thickness of >300 m. Individual channel-complexes range from 250-450 m in width, with aspect ratios ranging from 8-23. The internal architecture of channels is complicated by erosional scours, mudstone drapes and emplacement of fine-grained mass-transport deposits.

Up-slope pinch-out of channel sands is indicative of sediment back-filling during waning depositional episodes. Channels were sculpted by early by-pass phases, locally preserved in proximal, up-slope areas, as mudstone conglomerate lags in otherwise mud-filled channelform sedimentary bodies. A basinward stratigraphic stepping of the channel bodies is related to the outward building, or progradation of the slope.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas