Facies and Architectural Complexity Within and Among Outcropping Slope-Channel Complexes, Upper Cretaceous Tres Pasos Formation, Sierra de los Baguales, Southern Chile

Brian W. Romans and Stephen M. Hubbard
Stanford University, Stanford, CA

The Upper Cretaceous Tres Pasos Formation represents the uppermost portion of a 7,000 m-thick deep-water succession that records sedimentation in the narrow foredeep of the Magallanes Basin in southern Chile. Local paleocurrent data and regional context suggest these strata record the southward progradation of a deep-marine slope. Exceptional outcrops of the Tres Pasos Formation at Sierra de los Baguales reveal a 500-700 m-thick succession of clastic deep-water deposits along a 5 km transect parallel to depositional dip. Multiple discrete sandstone-rich packages (30-100 m thick) separated by mudstone-rich intervals of comparable thickness are laterally continuous and apparently tabular at this seismic scale. The facies distribution and bedding architecture within a package, at the reservoir-scale, exhibit a high degree of internal complexity. Individual beds and bedsets are laterally discontinuous over several to tens of meters. The degree of amalgamation is generally higher at the base of the sandstone-rich packages whereas lateral offset stacking of beds or bedsets is more characteristic upwards. Lateral changes in amalgamation in the basal portion of sandstone packages are attributed to areas of increased scouring and channelization.

The seismic-scale sandstone packages are composite features recording multiple episodes of bypass and accumulation within slope channel complexes. The mudstone-rich intervals are dominated by debris flow and slump deposits recording periods of mass wasting, slope readjustment, and little to no coarse-grained sedimentation. Comparison of internal facies and architectural complexity among the sandstone packages documents an overall increase in bypass upwards through the stratigraphy.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005