Outcrop Example of Intrastratal Slope Deformation Controlled by Depositional Architecture, Tres Pasos Formation, Magallanes Basin, Chile

Abstract

Sediments deposited on slopes have been shown to deform in a dominantly down-slope direction driven by the force of gravity after deposition and burial. Multidirectional gravitational spreading and contemporaneous deformation can occur over a broad range of timescales and depths from shallow-subsurface intrastratal faulting and folding to complete remobilization of material at the seafloor. Submarine slope deformation and failure has important implications for various aspects of petroleum exploration and development including: 1) evaluating potential hazards for seafloor infrastructure, 2) reservoir characterization and prediction in slope strata, and 3) interpretation of discontinuous or chaotic seismic facies associated with slope deformation. While instances of intrastratal slope deformation (e.g. creep or slip) have been documented at the microscopic and seismic scales there is a paucity of examples at the scale of centimeters to 10's of meters (reservoir scales). We present an outcrop example of syndepositional to synburial intrastratal slope deformation controlled by the distribution of turbidite deposits. Deep-marine slope deposits of the Cretaceous Tres Pasos Formation in the Magallanes Basin are exceptionally well exposed along a 3.5 km transect of the Rio Zamora at Cerro Mirador in southern Chile. Facies include: 1) thick-bedded sandstone facies (TSF; 25–200 cm beds), 2) thinly interbedded sandstone and mudstone facies (SMF; 5–30 cm beds), and 3) mudstone-prone facies (MPF; 1–10 cm beds). Facies transition significantly at the meter scale both laterally and vertically due to relative position within a composite succession of compensationally stacked, low aspect ratio sandstone-dominated bodies. Localized zones of cm- to m-scale faulting and subsidiary rotation and folding of discrete beds are bracketed by undeformed intervals that are present dominantly in SMF facies. Fault and fracture planes are parallel to downslope paleoflow orientation. The resulting interpretation is downslope creep deformation with localized slip planes (décollements) in fine-grained interbeds of MPF. The distribution of SMF and/or MPF is interpreted to be a key control on the locations of slip and associated deformation. These results highlight important dynamics and feedbacks in slope environments between depositional architecture, intrastratal deformation, and seafloor topography.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015