The Geomorphic and Paleoclimatic Significance of Alluvial Deposits in Southern Arizona

Alluvial deposits in southern Arizona are conveniently grouped into four major classes for purposes of geomorphic-climatic interpretation. The oldest group consists of (1) great thicknesses of highly deformed, varied, continental sediments and volcanic rocks deposited under conditions of crustal instability and a subhumid climate prior to the major events of the Basin and Range orogeny; (2) moderately tilted and faulted, coarse, postorogenic fanglomerates deposited during rapid erosion of uplifted mountain blocks with steep slopes, under a hot, semiarid climate. A second group consists of locally deformed, fine-grained lake beds and correlative deposits, deposited from lower Pliocene to middle Pleistocene under generally mild climates in response to damming of the ancestral drainage. A third group consists of very coarse, unde-formed fan deposits that were the product of rapid mechanical weathering during the cold Illinoian and Wisconsin glaciopluvial stages. The fourth group consists of fine-grained, denega beds, deposited from 500 B.C. to about A.D 1800 in almost every watercourse under a climate very much like today's. The change from fine-grained deposition of the lake episode to fan deposition indicates that the early Pleistocene pluvial stages were more humid than today, but not particularly cold; mechanical weathering was subordinate to chemical weathering. During the much colder Illinoian and Wisconsin stages only the low desert ranges escaped intensive frost action. A mature, leached, red-brown soil separates Illinoian from Wisconsin deposits over a wide area of southern Arizona. Fan gradients were determined by the ruggedness of the parent basins. Fans are thicker for basins with higher summit elevations and northeast orientation. The empirical equation $$\gamma^{0}= 7.30(H/\sqrt{A})^{0.87}$$ relates gradient to relative relief for both late Pleistocene fans and modern channels. In the headwaters cienega surfaces on fine deposits generally have steeper gradients than the modern bouldery arroyos trenching them. During their formation, the cienegas were zones of very high hydraulic roughness covered by dense stands of tall grass. The pattern of cienega deposition and entrenchment in second-order basins is controlled by mean hill-slope inclination and by the nature of the material upon which the basin is developed.