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 Fildani3
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