Multi-scale Comparison of Proximal Lobe and Channel-Fill Deposits on a Prograding Deep-Marine Slope, Tres Pasos Formation, Cerro Divisadero, Chile

Romans, Brian W.¹, Stephen M. Hubbard² (1) Stanford University, Stanford, CA (2) University of Calgary, Calgary, AB

Depositional slopes are typically characterized by local, yet significant variations in gradient due to accumulation of mud-rich mass transport complex deposits and/or sandy turbidite beds. Thus, slope successions commonly comprise a large variety of sandstone bodies in close paleogeographic and stratigraphic proximity. A comparison of the geometries and internal heterogeneities of proximal lobe and channel-fill deposits is made through a multi-scale (reservoir to seismic) analysis of sandstone-rich slope bodies of the Upper Cretaceous Tres Pasos Formation, southern Chile. The proximal lobe complex has a high net-to-gross and degree of sandstone amalgamation, and is tabular to slightly wedge-shaped at seismic scale (1-2 km lateral distance). The overlying channel-fill complex is markedly more heterogeneous with respect to facies distribution and internal sedimentary body architecture. The channel-fill complex is notably more lenticular, varying in thickness from 8 to 28 m from margin to axis. At a larger scale, four discrete sand-rich intervals record a systematic upward increase in channelization that reflects the progradation of the depositional slope system.

The Tres Pasos Formation (1,500-2,000 m thick) consists of delta-fed slope deposits that filled the Magallanes foredeep longitudinally, along the basin axis. Inherited topographic relief associated with a predecessor back-arc basin may have exerted a primary influence on the bathymetric relief from shelf to basin floor during the Late Cretaceous. The result was a paleogeographic condition comparable to passive margin settings and, thus, this study has implications towards the global understanding of continental slope processes and deposits.