Sedimentologic Processes and Stratigraphic Record of Linked Delta-Continental Slope Progradation, Magallanes Basin, Chile

Brian W. Romans¹, Jacob A. Covault¹, Stephen M. Hubbard², and Andrea Fildani^3
1Dept. of Geological & Environmental Sciences, Stanford University, 450 Serra Mall, Bldg 320, Stanford, CA, 94305 USA (e-mail: [email protected])
²Dept. of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4 Canada
³Chevron Energy Technology Company, San Ramon, CA 94582 USA

The Upper Cretaceous Tres Pasos and Dorotea formations of the Magallanes Basin in southern Chile record axial foredeep filling as a result of basinward accretion of a large-scale delta-slope system. Inherited tectonic relief from a precursor back-arc basin created a shelf-to-basin profile comparable in bathymetric relief to continental margin systems. A > 1500 m thick section exposed at Cerro Divisadero and Cerro Escondido reveals a stratigraphic succession of sand-rich turbidite units on the base-of-slope to lower slope (600 m) overlain by a section of shale-rich strata of upper slope affinity (600 m), capped by a complex package of prograding delta-front and deltaic facies (300 m thick).

The seismic-scale outcrop of slope strata (Tres Pasos Formation) at Cerro Divisadero is characterized by four 20-70 m thick sandstone-rich bodies, separated by mudstone-rich intervals of comparable thickness. Sedimentary body geometry, lithofacies distribution, internal bedding architecture, sandstone content, and degree of amalgamation are analyzed in detail across a continuous 2.5 km long transect parallel to slightly oblique to depositional dip. Basinward accretion of these strata is manifested as a vertical stacking pattern exhibiting increasing amounts of sediment bypass and degrees of confinement reflecting increasing slope gradients. Lithofacies analysis and detailed stratigraphy of the shallower-water facies exposed upsection at Cerro Escondido (Dorotea Formation) reveal an alternation of delta-front turbidite and storm wave-base (i.e., hummocky cross-stratified sandstone) successions reflecting higher-frequency cycles of shelf-edge construction within the overall progradational trend. The sedimentological character and stratigraphic context of slope vs. delta-front turbidites illustrates the importance of gravity-flow deposits at variable water depths and from variable initiation mechanisms in delta-system evolution.

AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina