Why we map, #3: Upper Gila River and Safford area

The third area proposed for mapping under the next round of funding for the STATEMAP program is the upper Gila River region in eastern Arizona. AZGS geologists Jon Spencer and Phil Pearthree made the case to the USGS on why this is important to the National Cooperative Geologic Mapping Program. New mapping was proposed around the town of Safford because of Quaternary faulting, opening of a new and very large copper mine in a belt of similar copper deposits, associated development in the Safford area, and geomorphic and hydrologic features along the Gila River and tributaries. Phil will lead our mapping effort in this area.

The upper Gila River area in southeastern Arizona was identified as a long-term priority for new mapping by the Arizona Geologic Mapping Advisory Committee (GMAC) in 2009. Although no STATEMAP mapping has been done in this area, the GMAC gave it high priority largely because of environmental and water resource issues in Gila and San Simon Valleys and because of expected increasing development and population growth associated with recent opening of the Safford mine in the Lone Star mining district. Critical geologic issues affecting this area include groundwater availability for agriculture, urban development, and mining, water quality, surface water / groundwater interactions, seismic hazards, potentially unstable substrate, the nature and timing of development of the through-going Gila River in this area, and several issues surrounding the geology of mineral deposits and environmental geology of mine development and closure.

Location and Geologic Setting

The proposed map area covers a swath across Gila Valley and northernmost San Simon Valley, including some limited bedrock areas on the basin flanks. It is located in the southern Basin and Range province on the margin of the Transition Zone. Bounding mountain ranges are the Gila Mountains to the northeast and the high Pinaleño Mountains (up to 10,720 feet) to the southwest. The valley axes are dominated by the Gila River, a large, perennial river that heads in New Mexico and drains a significant fraction of southeastern Arizona and southwestern New Mexico, and San Simon River, a large ephemeral tributary that enters the valley from the south. The Gila River has flooded numerous times historically, resulting in widespread inundation and substantial channel change and bank erosion (Burkham, 1972; Klawon, 2003). The Gila River has downcut through the Quaternary, resulting in the formation of prominent stream terraces and extensive exposures of basin-fill deposits. Valley piedmonts contain beautiful, stair-stepped suites of alluvial deposits and associated alluvial surfaces ranging in age from Holocene to early Pleistocene (Menges and McFadden, 1981); these relict alluvial fans and terraces developed in response Gila River downcutting and climate changes. Most of the proposed map area is underlain by thousands of feet of middle to late Cenozoic sediment, possibly including evaporites (Richard et al., 2007). Quaternary normal faults along the west side of San Simon Valley separate the Pinaleño Mountains, a horst of Proterozoic crystalline rocks, from the deep, valley-filling, clastic sediments that make up the hanging wall. Other Quaternary faults have been identified closer to the center of the valley (Machette et al., 1986).

Purpose and Justification

The proposed mapping area is in Graham County, which has grown from a population of 14,000 in 1960 to approximately 39,000 in 2008. Population growth is due to a number of factors, including the area’s scenic character and diverse economic activity including agriculture, mining and small businesses. The basin aquifer beneath Gila Valley is utilized by agricultural interests, mines, and the towns of Safford, Thatcher, and Pima. Water-table drawdown has been less of an issue in Gila Valley than in many other basins in Arizona, in part because of recharge from the Gila River (average 300,000 acre-feet/year), but understanding flow paths and recharge rates in the valley will become increasingly important as development continues. Geologic mapping will assist in identification of geologic resources such as aggregate that will aid future development in Gila Valley, and it will provide critical data to avoid flooding and erosion hazards and potential unstable substrate issues as that development occurs.

The primary purpose of mapping is to characterize and accurately depict alluvial deposits and surfaces, identify faults, characterize and map exposed basin deposits, and map bedrock and surficial deposits along the highly mineralized southwest side of the Gila Mountains. (1) Detailed surficial geologic mapping will allow better characterization of potential for large earthquakes in this area. Quaternary faults have been mapped in the area on a reconnaissance basis, but need careful evaluation of deposits and geomorphic features that may reveal their age of most recent movement. (2) More detailed depiction of surficial and basin deposits and faults will allow for development of more realistic groundwater flow models, which will serve the interests of property owners in obtaining long-term water supplies and will provide a better foundation for decisions that affect water use and riparian and aquatic habitats along the Gila River. In addition, these models may improve understanding of moderate-temperature geothermal systems that exist in the valley (Stone and Witcher, 1982). (3) Detailed surficial geologic mapping will assist in identification of potentially flood-prone areas on piedmonts and along the axial rivers. (4) Fine-grained deposits exposed extensively across much of the valley may pose potential hazards due to shrink-swell or hydrocompaction, and gypsum dissolution may also pose hazards. (5) Mapping of Gila River deposits of various ages will assist in the identification of potential aggregate resources. (6) Little research has been done into the timing of initial development and the subsequent evolution of the Gila River, but the proposed map area has the potential so shed considerable light on this subject through better dating of deposits that pre-date and post-date development of the through-going river. (7) The proposed map area is downslope from mines and mine facilities under development in the Lone Star mining district. Proposed mapping will contribute to understanding the regional hydrologic setting of open-pit mines and facilitate mine-complex design that minimizes adverse hydrologic consequences.

Quaternary faulting and earthquake hazards. The Pinaleño Mountains, which form the footwall to the normal-fault zone at the southwest margin of the Safford basin, included mylonitic crystalline rocks at the foot of the range that formed down-dip from a normal fault or fault zone during regional Oligo-Miocene extension (Naruk, 1987; Long et al., 1995). Quaternary normal faulting appears to represent the modern incarnation of this faulting, with continued exhumation of a metamorphic core complex adjacent to a listric normal fault and a deep sedimentary basin (Kruger et al., 1995). Several Quaternary fault zones have been identified in or near the foot of the range (Machette et al., 1986; Houser et al., 2004), but none have been mapped in detail. New detailed surficial geologic mapping will document faulted and unfaulted alluvial deposits and thus provide age constraints on the Quaternary faulting history, long-term slip rates, and age of most-recent rupture. In addition, the fault zones themselves will be mapped in much greater detail, which will better define their length. This will be useful in estimating the magnitudes of paleoearthquakes on these faults.

Basin geology and groundwater flow models. Basin geology exerts controls on groundwater recharge and groundwater circulation systems that feed moderately hot springs at several locations in the map area (Stone and Witcher, 1982). Groundwater in the Gila Valley is undoubtedly recharged through several mechanisms, including infiltration along the Gila River and infiltration of local runoff on piedmonts. Due to orographic effects, the high parts of the Pinaleño Mountains receive upwards of 60 cm of average annual precipitation, whereas the adjacent valleys receive 25‐30 cm/yr. Some of this montane precipitation is transferred to the adjacent valley via moderately large, east‐flowing drainages, and some water may flow into the basin via fractures in bedrock. A number of springs with water as warm as 46 °C are found within 10 km east of the mountain front. Stone and Witcher (1982) suggested that these warm springs result from recharge in coarse alluvial fan deposits along the mountain front, circulation into the basin to sufficient depth to heat the water, and confinement of groundwater in sand and gravel beds between finer‐grained beds. The higher altitude of the recharge input along the mountain front would provide the hydrostatic head to drive the system. Detailed mapping of alluvium of various ages will delineate areas of young deposits associated with montane drainages and the river system, where most recharge occurs.

Flood hazards and debris flows. Flood hazards in the area exist along major drainages and their tributaries throughout the proposed map area (e.g., Klawon, 2001). Lateral bank erosion during floods is a significant hazard along major rivers and desert washes, and is especially likely along banks formed in weakly cohesive Holocene terrace deposits. Mapping of active and abandoned channels and Holocene terraces along these streams defines the corridor that is most likely to be subjected to flooding or bank erosion. Flash floods generated by thunderstorms can cover broad portions of piedmonts with sheet wash in areas of active alluvial fans. Such floods are infrequent and of short duration, but are potentially devastating to homes because of the extent of inundation and the potential for developing new channels. Areas that are covered by young deposits are, or have recently been, part of active fluvial systems, whereas areas covered by Pleistocene deposits have not been subject to significant inundation for at least 10,000 years. Thus, the surficial mapping component of our mapping effort will outline areas where flooding along major drainages may occur as well as potential alluvial-fan flooding areas on piedmonts.
Soil and substrate problems. Unstable soil clays and near-surface compaction of some sediment types due to wetting or loading can cause cracking or destruction of overlying buildings. In addition, poorly compacted, low-density, fine-grained alluvial deposits have been the source of piping problems in desert soils. Fine-grained basin-fill deposits and Quaternary deposits reworked from basin deposits are candidates for these kinds of problems. Gypsum content is fairly high in some of the fine-grained deposits, and this may make them even more prone to subsidence or collapse due to dissolution if they are wetted. Mapping of surficial deposits generally outlines surficial geologic units that may be characterized by problem soils, and provides a template with which to evaluate the spatial distribution of existing and potential substrate problems.

Aggregate resources. Large quantities of aggregate are required for concrete and asphalt that are essential to housing and infrastructure development. All major aggregate operations in central and southern Arizona are located along major regional drainage systems like the Gila River, where gravel has been transported some distance and contains a wide mix of rock types. No large aggregate operations currently exist in the proposed map area, but large-scale deposits associated with the Gila River are likely to be present. Sand and gravel production has become controversial in Arizona because expanding suburbs encroach on quarry operations, and new residents complain of associated dust and noise. If locations of potential resources are identified before development occurs in an area, they can be factored into management decisions regarding land use.

Development of the Gila River. The Gila River developed sometime after the late Miocene through a series of formerly closed basins, including Gila Valley. Detailed analysis of the Duncan Valley, the next basin upstream, indicates that it was a closed basin in the late Miocene (Reid and Buffler, 2002). Major incision of the valley began after the basin was integrated downstream. The situation in Gila Valley was probably similar, but the age of drainage integration is poorly constrained. Very high and old alluvial fan remnants on the north side of the Pinaleno Mountains may record the level of maximum valley filling, prior to the beginning of Gila River downcutting. Several prominent Gila River terraces record the downcutting history, and the 640 ka Lava Creek B tephra has been found in one of the lower Pleistocene Gila River terrace gravels (Houser et al., 2004), implying that substantial river downcutting occurred in the early Pleistocene. Better dating of basin deposits that predate river development, the beautiful suite of terraces on the north side of the Gila River, and the highest preserved alluvial fan surfaces on the valley flanks has the potential to substantially improve our understanding of the timing and nature of river development in this area.

Metallic mineral deposits. A string of copper deposits northeast of Safford that make up the Lone Star mining district (Langton and Williams, 1982) form one of the largest copper districts in the world (11th on the list compiled by Cooke et al. [2005], 4th in North America). Open-pit mining began in 2007 and is expected to continue for decades.

Development of multiple, large open-pit mines will have long-term consequences for groundwater hydrology. A standard plan in closing an open-pit mine in the arid Southwest is to leave a pit lake that will evaporate, thus producing a cone-of-depression in the water table. The cone-of-depression will draw in sulfate-bearing groundwater and associated cations and prevent flow of contaminants off-site. Proper understanding of groundwater behavior is necessary to implement procedures that will minimize groundwater contamination and evaporative loss from pit lakes after mine abandonment. While proposed mapping is not directly in the area of mining, it covers the river valley that is downslope from the mining district. Proposed mapping will improve understanding of regional hydrology and may influence plans for mining-facility development, environmental remediation, and mine abandonment.