Cut, fill, preserve: toward an integrated basin model for bedrock paleovalleys in orogenic settings

The term “paleovalley” is widely applied in the literature to describe fluvial channel-related erosion surfaces and their fills. Paleovalleys (commonly, incised valleys) are most widely discussed in the context of sequence stratigraphy and low-lying coastal plain/shelf environments. In stark contrast, we apply the term “bedrock paleovalleys” to describe ancient river valleys incised into orogenic terrains due to long-term surface uplift, perhaps exacerbated by base level and/or climatic forcings. Long-term preservation of such features makes them important basin records for regions dominated by erosional processes.

We explore unique characteristics of bedrock paleovalleys in an effort to formalize a basin model. We synthesize observations from Cenozoic paleovalleys that were incised into the Mesozoic framework of the North American Cordillera and subsequently filled by sedimentary and/or volcanic strata. We incorporate modern geomorphic data obtained from the Central Andes of Bolivia, the classic analogue for the Mesozoic North American Cordillera. We suggest that bedrock paleovalley successions record a unique, three-part geologic history: 1) Bounding erosion surfaces represent prolonged, focused erosion into an orogenic terrain, likely during active tectonism and associated surface uplift. The network of surfaces defines initial conditions for basin-filling, demarcating areas of potential sediment accommodation (modern and ancient examples indicate widths ≥10 km and depths ≥1 km). 2) Bedrock paleovalley fills record a transition from erosion/sediment bypass to deposition. Controls on paleovalley filling may include changes in local base level, climatic change, dynamic subsidence, orogenic collapse, and/or magmatism. 3) Preservation requires that they are protected from subsequent erosion. This may be facilitated through shielding by younger volcanic rocks and/or tectonic dissection, partial preservation, and exposure. Some ancient examples also suggest that well-defined bedrock paleovalley profiles may simply persist in parallel, subtly modified forms for tens of millions of years.