Stratigraphic evolution of an outcropping continental slope system, Tres Pasos Formation at Cerro Divisadero, Chile


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    Present address: Chevron Energy Technology Company, 6001 Bollinger Canyon Rd., San Ramon, CA 94583, USA.


Depositional slope systems along continental margins contain a record of sediment transfer from shallow-water to deep-water environments and represent an important area for natural resource exploration. However, well-preserved outcrops of large-scale depositional slopes with seismic-scale exposures and tectonically intact stratigraphy are uncommon. Outcrop characterization of smaller-scale depositional slope systems (i.e. < 700 m of undecompacted shelf-to-basin relief) has led to increased understanding of stratigraphic packaging of prograding slopes. Detailed stacking patterns of facies and sedimentary body architecture for larger-scale slope systems, however, remain understudied. The Cretaceous Tres Pasos Formation of the Magallanes Basin, southern Chile, presents a unique opportunity to evaluate the stratigraphic evolution of such a slope system from an outcrop perspective. Inherited tectonic relief from a precursor oceanic basin phase created shelf-to-basin bathymetry comparable with continental margin systems (∼1000 m). Sedimentological and architectural data from the Tres Pasos Formation at Cerro Divisadero reveal a record of continental margin-scale depositional slope progradation and aggradation. Slope progradation is manifested as a vertical pattern exhibiting increasing amounts of sediment bypass upwards, which is interpreted as reflecting increasing gradient conditions. The well-exposed, seismic-scale outcrop is characterized by four 20 to 70 m thick sandstone-rich successions, separated by mudstone-rich intervals of comparable thickness (40 to 90 m). Sedimentary body geometry, facies distribution, internal bedding architecture, sandstone richness and degree of amalgamation were analysed in detail across a continuous 2·5 km long transect parallel to depositional dip. Deposition in the lower section (Units 1 and 2) was dominated by poorly channellized to unconfined sand-laden flows and accumulation of mud-rich mass transport deposits, which is interpreted as representing a base of slope to lower slope setting. Evidence for channellization and indicators of bypass of coarse-grained turbidity currents are more common in the upper part of the > 600 m thick succession (Units 3 and 4), which is interpreted as reflecting increased gradient conditions as the system accreted basinward.