Pliocene and Quaternary history of the Rio Grande, the axial river of the southern Rio Grande rift, New Mexico, USA

The history of the Rio Grande, the axial river of the southern Rio Grande rift, New Mexico, may be divided into an aggradational phase (∼ 5 to ∼ 0.8 Ma), during which time about 100 m of fluvial sediment was deposited in eight basins, and a degradational phase (∼ 0.8 Ma to present), when the river alternately incised and partially backfilled its basins. Reversal magnetostratigraphy, dated volcanic rocks, and vertebrate biostratigraphy allow correlation of sediment of the aggradational phase with the Gilbert, Gauss, and Matuyama Chrons, while the degradational phase primarily encompasses the Brunhes Chron. Tectonics was the primary control on river behaviour during the aggradational phase. In half grabens, multistorey channel sands were deposited in narrow belts near the footwall uplifts, in response to tilt of the basin floor towards the border fault and smaller footwall catchments and their alluvial fans compared to those of the hanging-wall mountains. In contrast, the river traversed almost the entire width of full grabens, depositing interbedded channel and floodplain sediment with moderately mature palaeosols. Condensed sections dominated by floodplain sediment with very mature palaeosols developed in basins largely abandoned by the river due to preferential tilting of the basin floor, lateral growth or renewal of fault activity, or alternating flow of the river between adjacent basins. The degradational phase was dominated by net basin erosion related to glacial-climate cycles, despite continued activity on many of the basin-bounding faults.