Pliocene and Quaternary history of the Rio Grande, White Rock Canyon and vicinity, New Mexico

Reneau, S; Dethier, D

Surface and subsurface evidence shows that the elevation of the Rio Grande in the vicinity of White Rock Canyon fluctuated greatly during the Pliocene and teh Quarternary, influenced by volcanism, tectonism, and climatic changes. After the first appearance of distinctive quartzite-rich fluvial deposits at ca. 4-5 Ma, the Rio Grande incised 250m or more into rift-filing sediments of teh Santa Fe Group. The Rio Grande then aggraded 300m by about 2.4 Ma, coinciding with eruptiojn of repeated damming of the river by basaltic lavas of the Cerros del Rio volcanic field. Simultaneous deposition of thick Puye Formation fanglomerates to the north may have been aided by blockage of the valley by the lavas and the resultant rise in local base level. By early Pleistocene the Rio Grande had incised a narrow canyon up ot 180m deep through the western Cerros del Rio field, which was filled first by the Otowi Member of the Bandelier Tuff at 1.61 Ma and then by the Tshirege Member at 1.22 Ma. Both deposits undoubtedly dammed the river, and erosion of the spillways may have generated catastrophic outburst flooding. Notably, the low spot on the 1.22 Ms dam east of the paleocanyon, forcing thr Rio Grande to cut a new channel through 200m of basaltic rocks to reach its former grade. The resistant basalt probably produced a persistent knickpoint and a local control for base level upriver. Extensisve slumps developed in northern White Rock Canyon after the river had incised into mechanically weak sediments beneath basalts, probably beginning in mid-Pleistocene time. Reactivation of slumps in wetter periods in the late Pleistocene repeatedly dammed the river, producing lakes up to 60m deep and 25km long. By the early Holocene the Rio Grande was within 5m of its present grade, and the river has remained near its modern level through the Holocene.

Pliocene and Quaternary history of the Rio Grande, White Rock Canyon and vicinity, New Mexico

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