Architecture of buried bluff lines: A record of the incising ancestral Rio Grande and Abo Arroyo from the Pleistocene to historical times

Abstract

Approximately 6 km east of the mouth of Abo Arroyo on the active floodplain of the Rio Grande, New Mexico, a 400-m exposure in the arroyo wall shows five preserved bluffs with buttress unconformities between Abo Arroyo and ancestral Rio Grande-derived sediments. The sources of sediments are identified by sediment color, clast types and bedding fabric. Ancestral Rio Grande sediments are pale, moderately well-sorted, and, where exposed, have basal straths immediately overlain by very well-rounded quartzite-bearing gravels and cobbles. Abo Arroyo sediments are red, show east-to-west inclined bedding in some places, are poorly sorted, and, in coarse facies, have subangular clasts from the catchment area. The oldest sediments, which Bluff 1 cross-cuts, are approximately 2.2 Ma, and the most recent sediments, younger than Bluff 5, have been deposited in the last century. The depositional history between Bluff 1 and Bluff 5 is difficult to interpret because of lack of age control, but cross-cutting relationships indicate the following. The oldest sediments in Bluff 1 were deposited during the aggrading period of the ancestral Rio Grande that ended at 800 ka. During the following 800 ky, the ancestral Rio Grande cut Bluffs 1-4. After cutting each bluff, the ancestral Rio Grande aggraded slightly and deposited floodplain sediments. The floodplain and distal Abo Arroyo fan deposits are interbedded, and grade upward into coarser Abo Arroyo pebbly sands. Bluff 1 was not exposed long enough to develop a wedge of colluvium before being buried. Bluffs 2 and 3 were buried by colluvium. Between Bluffs 2 and 3, it appears that there are continuous ancestral Rio Grande floodplain deposits, Abo Arroyo muds and steepening-upward fan deposits. Bluff 3 formed west of an older natural levee. Bluff 4 does not have colluvium and was rapidly buried by Abo Arroyo deposits. Bluff 5 was formed during recent (historical) incision by Abo Arroyo. This sequence shows the complex stratigraphic heterogeneity along the margin of axial river systems. It is difficult, if not impossible, to predict the subsurface depth to good aquifers in these sediments, given the intercalation of marginal tributary fans and streams with axial river bluff lines. 429