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Stratigraphic Architecture of Bypass-Dominated Slope Channel Deposits, Tres Pasos Formation, Chilean Patagonia

Abstract

The stratigraphic record of submarine slope channel-levee systems is widely observed to include an early phase dominated by channel degradation and lateral migration, followed by a late phase dominated by aggradation. However, this record is preferentially biased toward late stage aggradational channel deposits. Conceptual models supported by outcrop and seismic reflection data sets predict that during phases of erosional degradation or lateral migration deposits are predisposed to cannibalization and erosional truncation, resulting in fragmented preservation. Despite this, associated units are often compared to fluvial scroll bars. It remains unclear, however, how similar the formative processes between these submarine and subaerial channels actually are. In order to elucidate fundamental properties of high mobility submarine channels, we present outcrop examples of bypass-dominated and laterally stepping slope channel fills from the Tres Pasos Formation, Magallanes Basin, southern Chile. The Late Cretaceous Magallanes Basin is an elongate retroarc foreland that formed during uplift of the Patagonian fold-thrust belt. The Tres Pasos Formation records the last stage of deep-water sedimentation in the basin. Bypass-dominated slope channel fills in the Tres Pasos Formation are characterized by abundant basal mudstone intraclast and shell-hash conglomerate lags and lenses separated by innumerable erosion surfaces. These basal channel fill deposits and surfaces are mantled by laminated mudstone and heterolithic interbeds resulting in notably sandstone-poor channel fills. Detailed field observations reveal: (1) distinct stratigraphic channelform bodies with internal evidence for protracted processes including erosion, sediment bypass and deposition; and (2) discrete lateral shifts in the spatial position between successive stratigraphic channelforms. These results indicate that mobile channels act as fairways for multiple high-energy flow events and that lateral migration is facilitated through punctuated steps of geomorphic channels rather than gradual lateral migration.