Two active alluvial fans in the tectonically quiescent region of southern Arizona are composed of discontinuous ephemeral stream systems characterized by alternating channelized reaches and sheetflood zones. Channel erosion and overland flow on inactive portions of the fans form new channel reaches with the capacity to divert active flow paths. The distribution and importance of sheetflooding on the fans is strongly controlled by local drainage-basin characteristics and base-level controls. Sheetflooding predominates where sandy bank sediments lead to the development of wide shallow channels, but channelization occurs at the fantoe in areas of long-term base-level lowering. The influence of climate on alluvial-fan formation has often been emphasized in previous studies, but is of only limited importance on active alluvial fans in Arizona.
The White Tank Mountains are a mid-Tertiary metamorphic core complex located 65 kilometers west of Phoenix. Bedrock is composed of Proterozoic metamorphic and plutonic rocks, two late Cretaceous to early Tertiary plutons, and less abundant mid-Tertiary plutonic, volcanic, and sedimentary rocks. The range is surrounded by Quaternary deposits and surfaces that range in age from Recent to Early Pleistocene. The oldest rocks are Early Proterozoic metamorphic rocks, consisting of banded gneiss and biotite schist (Xm), amphibolite (Xmm), and a pod of ultramafic rocks (Xu) and gabbro (Xug). These rocks are intruded by two foliated Proterozoic plutons, a tonalite (Xt) to the south and a granodiorite-granite (Xgd) to the north. The northern part of the range exposes a quartzofeldspathic gneiss (Xmf), in part composed of thin sills of pegmatite and granite probably related to the granodiorite-granite pluton. In the south, the tonalite invades the metamorphic rocks as sheetlike intrusions concordant to foliation in both rock types and locally takes on a gneissic appearance. Several generations of Proterozoic metamorphic fabric are documented, including a main northeast-trending, high-grade gneissic foliation (S1). This fabric is present in both Proterozoic plutons and is folded and overprinted by a later sillimanite-grade Proterozoic mylonitic fabric (S2) that is concentrated in an east-dipping, normal shear zone (Reynolds and DeWitt, 1991; Alter, 1994; Wood, 1998). Two distinct plutons of probable Late Cretaceous to early Tertiary age intrude the Proterozoic rocks. To the northwest is an undeformed, medium-grained granodiorite with locally associated dioritic and granitic phases. The eastern flank of the range is composed of the light-colored White Tank Granite, which forms a large silllike pluton and numerous smaller sills in the metamorphic rocks and tonalite. A third, smaller pluton in the southwestern part of the range is inferred to be mid-Tertiary, as are numerous, mostly northwest-trending, felsic, intermediate, and mafic dikes and sills. Tilted mid-Tertiary volcanic and sedimentary rocks are preserved above a brittle detachment fault along the western edge of the range. Mid-Tertiary mylonitic fabrics, with a characteristic northeast-southwest-trending, streaky lineation are locally present in the Proterozoic rocks, White Tank Granite, and some mid-Tertiary dikes and sills. These fabrics are inferred to have formed as the White Tanks core complex, which is in the lower plate of the detachment fault, was brought to the surface from underneath the Union Hills to the northeast (Rehrig and Reynolds, 1980; Brittingham, 1985; Reynolds and Dewitt, 1991; Reynolds and Lister, 1987; Kruger and others, 1998). Younger, northwest-trending, high-angle faults, with some lateral displacement, cut across the range. Quaternary alluvium around the range is divided into older, middle, and younger deposits and surfaces that document a complex history of erosion and aggradation. Younger deposits are lower in elevation, are less dissected by erosion, and have less developed caliche, desert varnish, and desert pavement than higher, older Quaternary deposits (Field and Pearthree, 1991).
The distribution of depositional facies on eight Holocene alluvial fans of varying dimensions is used to evaluate prehistoric Hohokam agricultural settlement patterns. Two facies are recognized: channel gravelly sand facies and overbank silty sand facies. No debris flow deposits occur. The channel facies is characterized by relatively well sorted stratified sands and gravels with common heavy mineral laminations. Overbank facies deposits are massive and very poorly sorted due to heavy bioturbation. Lithostratigraphic profiles from backhoe trenches and sediment size analysis document headward migration of depositional facies which results in fining upward sequences. Each sequence is a channel fan lobe with an underlying coarse grained channel sand which fines to overbank silty sands. Lateral and vertical variations in facies distributions show that depositional processes are affected by drainage basin area (fan size) and distance from fan head. Gravelly channel sands dominate at the headward portions of the fan and are more pervasive on large fans; overbank silty sands are ubiquitous at fan toes and approach closer to the fan head of smaller alluvial fans. When depositional facies are considered as records of water flow over an alluvial surface, the farming potential of each fan can be analyzed. Depositional models of alluvialmore » fan sedimentation provide the basis for understanding Hohokam settlement patterns on active alluvial surfaces.« less
Abstract Geoarchaeological investigations of the alluvial piedmont or bajada emanating from the Tortolita Mountains, Arizona, show that the distribution of Hohokam sites apparent from the surface is complete and undisturbed by geological processes. Late Holocene geomorphic processes and their resultant deposits on the bajada affected the location of prehistoric Hohokam agricultural settlements. Hohokam settlements were commonly situated on small alluvial fans dominated by sheetwash processes and deposition of fine‐grained alluvium, where floodwater farming could be easily pursued with little modification of the natural surface. Large alluvial fans, dominated by channel processes and coarse‐grained alluvium were avoided by the Hohokam farmer because these areas were less suitable for floodwater farming.
Assessment of the character of flood hazards and the extent of flood-prone areas on the piedmonts of Arizona is an increasingly important concern to floodplain managers as urban areas continue to expand. Piedmonts are the low-relief, gently sloping plains between mountain ranges and the streams or playas that occupy the lowest portions of the valleys. Proper management of flood hazards on piedmonts is important because much of southern, central, and western Arizona is composed of piedmonts; they comprise most of the developable land around Phoenix and other rapidly expanding population centers of the State. Management of flood hazards in Arizona and elsewhere in the western United States is complicated because portions of many piedmonts are composed of active alluvial fans. During floods, these fans are subject to widespread inundation and local high velocity flow, and substantial changes in channel patterns may occur. Development that proceeds on piedmonts without regard to the locations of active alluvial fans is likely to place people and property at risk during large floods. Report and 4 map sheets.
These nine maps depict the general ages and distribution of Quaternary geomorphic surfaces and associated alluvium of the northern Avra Valley-Desert Peak (AVDP) area, Pinal and Pima counties, southern Arizona. The area mapped includes portions of the piedmonts surrounding the Silver Bell, Picacho, and Tortolita mountains, and the basin flats between them. The southern margin of the study area is approximately 40 km northwest of downtown Tucson. The AVDP area is one of many broad valleys in the Basin and Range physiographic province of southern Arizona. The Basin and Range province in the vicinity of the study area is characterized by relatively small mountain ranges with fairly low relief separated by wide, gently sloping piedmonts and basin bottom river drainages. The major drainages flowing through study area are the Santa Cruz River, Brawley Wash, and McClellan Wash. By indicating the age and physical characteristics of surficial alluvial deposits, these maps provide a basis for evaluating the Quaternary geologic history of the area and assessing potential geologic hazards. Alluvial deposits differentiated for these maps are assigned to Quaternary and Upper Tertiary age categories primarily on the basis of the estimated age of cessation of major deposition on geomorphic surfaces associated with the deposits. Relative topographic positions of each surface, drainage patterns, surface characteristics, and degree of soil development associated with the surface are the principal criteria used to assess surface age. Six ages of surfaces are differentiated in these maps. Map units are further subdivided into piedmont and basin axis units. The characteristics of each map unit are described below. The estimated ages of the units are inferred by correlation with similar surfaces and soils dated elsewhere in the southwestern United States (Gile and others, 1981; Bull, 1991; Menges and McFadden, 1981). Generalized lithologies of exposed bedrock are also depicted on the maps. (11 pages)
