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Sedimentology, Stratigraphic Architecture, and Ichnology of Gravity-Flow Deposits Partially Ponded in a Growth-Fault-Controlled Slope Minibasin, Tres Pasos Formation (Cretaceous), Southern Chile

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.; present address: ChevronTexaco Energy Technology Company, 6001 Bollinger Canyon Rd., San Ramon, California 94583, U.S.A.
² Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.; stevehub{at}pangea.stanford.edu

A recent resurgence in the study of slope deposits is strongly related to the complexity inherent with the depositional setting, and the continued prospectivity for hydrocarbons in associated units. Intraslope basins are particularly intriguing to explorationists, because they provide accommodation space for coarse-grained facies in areas often characterized by otherwise muddy deposits. An outstanding outcrop of a growth-fault-controlled slope minibasin and its sandy fill are exposed in Cretaceous strata of southern Chile.

A turbiditic sandstone package (TSP) 60 m thick sharply overlies fine-grained deposits of the Cerro Toro Formation, and defines the base of the Tres Pasos Formation in the study area, 65 km north of Puerto Natales, Chile. Individual sedimentation units are rarely amalgamated, and are tabular for at least hundreds of meters, suggesting deposition in a relatively unconfined setting. Trace-fossil assemblages are grouped into the Zoophycos or Cruziana ichnofacies, and are consistent with slope deposition rather than deposition on the basin floor.

Beds at the base of the TSP are truncated by a normal fault at the southern end of the outcrop, and lap out towards the north onto the tilted hanging wall. Stratal thickening and thinning across the fault, and unfaulted overlying deposits, suggest that the fault was active during sand deposition (a growth fault), and that it created accommodation space for the partially ponded TSP. This, and other growth faults in the underlying strata, suggest deposition in an intraslope minibasin setting. The TSP is overlain by ~700 m of amalgamated slump, slide, and debris-flow units, suggesting that deposition immediately preceded rapid slope progradation.

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