Types of Pediments in the San Pedro Valley Region, Arizona

Abstract

1954of Pacific Coast Geographers17 TYPES OF PEDIMENTS IN THE SAN PEDRO VALLEY REGION, ARIZONA Tuan Yi-fu University of California, Berkeley The word pediment, as first defined by McGee (1) and as used in this paper, refers to an exposed bedrock surface at the foot of the mountain front, or a rock surface covered by a layer of alluvium no thicker than the depth of the stream scour. In igneous rocks of coarse texture, the pediment may be covered only by its own regolith — i.e, the material weathered in place rather than transported alluvium. Under this definition, which excludes all postulates of an extension of the surface beneath the alluvium, the pediment is only a fragmentai feature of the arid landscape in the American Southwest in contrast to the ubiquity of alluvial fans, bahadas, and bevelled piedmont surfaces or "peripediments". There are at least two major types of pediments in the San Pedro Valley region of Arizona: 1. The narrow rock benches or embayments which fringe a broad mountain mass, and are separated from the alluvial deposits of the basin by a high-angle fault; 2. The extensively bevelled bedrock surfaces out of which the mountains appear to rise as mere residuals. The¦Santa Catalina and Huachuca pediments belong to the first type, whereas the south Dragoon and Johnny Lyon Hills pediment belong to the second. The Santa Catalina Pediments The northwest and south fronts of the Santa Catalina Mountains show narrow pediments because they have had little time to develop since the latest major uplift along the normal faults in late Tertiary or early Quaternary time (?). The morphology of these pediments can be described best in the context of their adjoining mountain fronts. (Figure 1) The south front of the Santa Catalina Mountains is composed of gneiss. It is much ravaged by erosion, and has a series of parallel canyons separated by bare, sharp-crested ridges. The northwest front is divisible into two morphologic units along the Romero and Montrose canyons. The belt occupied by these canyons marks the transitional zone between gneiss of the southern forerange and an intrusive igneous rock of distinctly granitic texture in the northern portion of the northwest front, where it is known as the Samaniego Ridge. Canyons and sharp crests are absent in the Samaniego Ridge which is rounded at the top and has long, smooth, concave slopes leading down to the foot of the mountain. It is along this granite ridge that amphitheatre-like, incipient pediments have formed. (2) Wide pediments generally do not occur along a deeply dissected mountain front, and the southern forerange off the Santa Catalina Mountains is no exception. Here, large alluvial fans appear to emerge directly from the mouths 18 Yearbook of The Association Vol. 16 Figure 1. Diagram of the northwest corner of the Santa Catalina mountains. SW SANTA CATALINJA PEDIMENT BENCH NE Pediment Bench Fan A (Mesa) Fan B Fan C Fan B Gneiss . Conglomerate beds (Tilted and deformed) Figure 2. Diagrammatic cross-section of the south front of the Santa Catalina mountains showing the relationship between the pediment benches and the alluvial fans. (Modified from Blissenbach ) 1954of Pacific Coast Geographers19 of the canyons, and the steep mountain wall seems to be in sharp contact with the aggradational slope of the fans. But this simple arrangement is deceptive and fades upon closer scrutiny. The fans have been much eroded. Blissenbach recognizes three fan levels, the highest and oldest remnants of which are isolated knolls or mesas. (3) The highest fan mesa, when projected toward the mountain front and across the fault that separates the gneiss from the alluvium, correspond in elevation to the narrow pediment benches developed on gneiss, whereas a more recent fan (fan B) still covers much of the lower gneissic surface. It is outside the scope and purpose of this paper to trace the history of the uplift of the mountain front along the fault, and the relation of the succesive fans to the pediment benches. Figure 2 summarizes a possible interpretation in diagrammatic form. The rock benches are at most only a few hundred feet wide. They have suffered much from erosion, and are best preserved in the intercanyon areas...