Abstract

Interbedded lavas and sedimentary rocks in the Guadiana Valley are divisible into six units. The oldest consists of gravel which caps remnants of a pediment. An ancient canyon of the Rio Mezquital, entrenched about 200 feet below this pediment, was filled with basalt, and the basalt in turn overlapped by gravelly alluvium of the Guadiana formation.

The Guadiana formation contains fossils of pre-Wisconsin Pleistocene age. After this alluvium accumulated, it was eroded and deeply weathered, first under dry and later under humid conditions, as indicated by superposed pedocal and pedalfer soils.

Following renewed eruptions of basalt, the Guadiana formation was covered in most places by mimic silty alluvium of the Pueblito formation. Both the lava, as indicated by its freshness, and the Pueblito, as indicated by its fossils, are of Recent age. A mumic zone at the top of the Pueblito contains hearths and artifacts identified with the Southwestern Desert Cultures, which were best developed between 6000 and 500 B.C.

Gullies cut into the Pueblito formation were later filled with sand and gravel. Prior to the cutting of the present arroyos, a thin blanket of silt, which contains Indian pottery and relics of Spanish and Mexican occupation, was spread over the lower parts of the Guadiana Valley. Some of the pottery corresponds with a Chalchihuites Culture occupancy dated circa A.D. 500–1350.

Tentatively the Guadiana formation, Pueblito formation, and the pottery-bearing silt are correlated respectively with the Lower Becerra, Totolzingo, and Nochebuena units of the Valley of Mexico, and with the Neville, Calamity, and Kokernot formations of western Texas. Units older than the Guadiana are included in the Quaternary because they are younger than an erosional surface of probable Pleistocene age developed upon the Sierra Madre Occidental.

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