Paleofloods or Arroyo Cycles? Reconciling Alluvial Records in the Greater Buckskin Drainage, Colorado Plateau

For nearly a century, workers have reconstructed arroyo cutting and filling cycles in the U.S. Southwest via chronostratigraphic studies of exposed alluvium in broad alluvial valleys. Conversely, paleoflood hydrologists have produced flood records by studying slackwater deposits preserved in the numerous bedrock canyons of the region. At some point, as a stream transitions from an alluvial reach to a constricted bedrock reach, these contrasting approaches to interpreting similar-aged alluvium must be reconciled.

We hypothesize that in a single drainage, slackwater flood deposits are non-correlated with valley fills, implying that arroyos are cut in alluvial reaches by the same large floods that build the slackwater deposits in constricted reaches downstream. We are testing this hypothesis in the greater Buckskin drainage of the Paria River basin, where broad alluvial valleys drain into a constricted bedrock reach. Previous workers have studied either the slackwater deposits in the constricted reach or the stratigraphy of the alluvial reach; however, existing age-control is inadequate for reconciliation. The present study employs a focused chronostratigraphic approach in both reaches, supported by an OSL-based geochronology.

Early results from the Coyote/Wire Pass fork of the Buckskin drainage indicate that its alluvial history is more complex than previously interpreted. OSL ages indicate that much of the stored sediment in the alluvial reach is late Pleistocene and early Holocene in age, while the sediment stored in the slot canyon downstream is primarily late Holocene in age. Constricted-reach deposits are also thicker-bedded and sedimentologically distinct from those upstream. It is clear that in a single drainage, valley geometry plays a key role in determining depositional processes and preservation potential of stored sediment in any given reach.