The Tucson Basin comprises a portion of the upper Santa Cruz Basin, a valley draining to the northwest and bordered by narrow, rugged mountain ranges. The Tucson Basin's northeastern boundary is formed by the Santa Catalina and Rincon Mountains, and its western boundary is formed by the Tucson Mountains (Upper Santa Cruz Basin Mines Task Force, 1979). Tucson is centrally located within the 1,000 square mile Tucson Basin and has an elevation of 2,300 feet. A geologic barrier near the town of Cortaro provides a reasonable dividing line between the ground waters of the upper and lower Santa Cruz Basins (Burkham, 1970). The Rincon Mountains reach a maximum elevation of 8,666 feet while the Tucson Mountains reach only 4,687 feet.
Rincon Mountain District and Coronado National Forest encompass the Rincon Mountains. The Rincons are composed of folded and foliated banded gneiss, schist, and granite of Precambrian age, draped over at their base by Tertiary and younger alluvial and colluvial deposits. Often the colluvium forms a thin cover over pediment surfaces. The TMD and adjoining Tucson Mountain Park cover most of the Tucson Mountains which are dominated by Permian and Cretaceous limestone, arkose, red beds, and Tertiary intrusives and volcanics. Quaternary gravels are also present and cover most of the pediments (Streitz, 1962). The lower flanks of the Tucson and Rincon Mountains are covered by terrace deposits or other alluvium ranging from 100 feet thick in RMD to 400 feet thick in TMD (National Park Service, 1995).
The Tucson Basin is filled with the fluvial, lacustrine, and debris flow deposits derived from the erosion of the surrounding mountains and ranges farther up the drainage. The center of the basin is a dissected graben structure where massive accumulations of fine-grained sediments and evaporites are present (Leake and Hanson, 1987). Alluvial fan deposits occur along the perimeter of the basin, while river channel and flood plain deposits are common in the center of the basin and make up the larger proportion of the fill. The basin deposits are typically Tertiary and Quaternary in age, and may be as much as 8,000 feet thick (Streitz, 1962). Geologic factors controlled the formation of the valley fill and determined the textural and structural relationships of the basin sediments. The characteristics of the sediments in turn control the occurrence and movement of ground water in the basin (Kidwai, 1957).
Standard soil surveys were completed for both districts in 1987 by the Soil Conservation Service. Soils are typically shallow, coarsely textured, and well drained on the mountain slopes. Soils on the bajada are alluvial and contain distinct areas of sandy or rocky soils with equally distinct plant associations (National Park Service, 1995). These soils lend themselves to rapid recharge, although recharge normally does not occur outside stream channels because antecedent soil moisture is usually very low, evaporation is very high, and rainfall amounts are insufficient to push the wetted front to ground water depths. An impermeable layer of caliche frequently forms at this depth, which can limit plant establishment and growth. Because alluvium within stream channels, fans, and bajadas are very permeable, streams spread out and rapidly lose flow as they leave the steep mountain gradients and enter the alluvial flats (Osterkamp, 1973a).
MINING AND MINERAL DEVELOPMENT
Both districts of Saguaro National Park have had some mining activity. However, mining was never widespread or intensive in RMD (Clemensen, 1987). Small scale lime kiln operations occurred between the 1880s and 1910s. The Loma Verde Mine reached 350 feet in 1902, but was not included on a list of mines just five years later. In the mid-1930s, the Civilian Conservation Corps filled in 30 prospect holes in RMD.
Mining activity was more pronounced in TMD, where there are 150 mine shafts with associated tailings piles. Most of the abandoned mines are within designated wilderness areas. The two largest operations, the Gould and Mile Wide mines, have quite large tailings piles and some yellow staining in stream channels up to 1/4 mile below the piles. However, because the streams in TMD are only ephemeral, there is no possibility of impacting aquatic communities with mine land runoff. Although there is very little vegetation on these large refuse piles, vegetation below the piles appears to be little affected by runoff.
Ground water contamination by leaching from the tailings piles is unlikely because the rainfall/evaporation ratio is low and diffuse recharge provides little input to the underlying aquifers. The conclusion drawn by the Upper Santa Cruz Basin Mine Task Force (1979) was that ground water contamination from mining activities could not be detected in either the Tucson or Avra Valley basins, even from large active mines. The only potential for contaminant migration is during periods of intense thunderstorms when surface runoff could be generated from the tailings piles. During these events dilution would be high, and metals or other contaminants would be carried from the park down to the adjoining Avra Valley or Tucson Basin before significant infiltration could occur.
The 1994 boundary expansion of TMD encompassed the Old Yuma Mine Area, which includes a valid lode claim, a valid placer claim, and a disputed lode claim. Management of these claims transferred from BLM to NPS when the 1994 expansion bill was approved. Patent is being sought on the valid lode claim, and the claimant has submitted a plan of operations for the disputed lode claim. An approved plan of operations must be in place before any activity can occur on either claim. The placer claim consists of a refuse pile and two catchment basins which were built to process the refuse with cyanide leaching. There has been no indication to date that the claimant intends to pursue the placer claim.
Burkham, D.E. 1970. Depletion of Stream Flow by Infiltration in the Main Channels of the Tucson Basin , Southeastern Arizona . U.S. Geological Survey Water Supply Paper 1939-B, U.S. Government Printing Office. Washington D.C.
Clemensen, A.B. 1987. Cattle, Copper, and Cactus: The History of Saguaro National Monument , Arizona . Historic Resource Study, National Park Service. Denver, CO. 271 pp.
Kidwai, Z. U. 1957. The Relationship of Ground Water to Alluvium in the Tucson Area, Arizona . M.S. thesis, University of Arizona . Tucson , AZ. 69 pp.
Leake , S.A. , and R.T. Hanson. 1987. Distribution and Movement of Trichloroethylene in Ground Water in the Tucson Area, Arizona . U.S. Geological Survey. Water Resources Investigation Report 86-4313. 40 pp.
National Park Service. 1995. Statement for Management. Saguaro National Park . Tucson , AZ , 33 pp.
Osterkamp, W.R. 1973a. Ground Water Recharge in the Tucson Area, Arizona . U.S. Geological Survey. Reston , Virginia . Map I-844-E. 1 map
Streitz, R. 1962. Subsurface Stratigraphy and Hydrology of the Rillito Creek-Tanque Verde Wash Area, Tucson , Arizona . M.S. thesis, University of Arizona . Tucson , AZ. 60 pp. 1 sheet.
Upper Santa Cruz Basin Mines Task Force. 1979. Upper Santa Cruz Ground Water Quality Baseline Report. Pima Association of Governments, 208 Program. Tucson , AZ. 71 pp.
The Struggle For Survival Begins
The saguaro begins its life as a shiny black seed no bigger than a period on this page. But what it lacks in size it more than makes up for in numbers. One saguaro produces tens of thousands of seeds in one year, and as many as 40 million in a lifetime of 175 to 200 years. But from the start the odds against survival are great. Out of all the seeds that one saguaro produces in its life, probably only one will live to adulthood.
Seeds and young saguaros have the best chance for survival if they are "cared for" by nurse trees such as paloverde and mesquite. Saguaro seedlings that grow under these sheltering plants are shaded from the desert's intense sunlight blanketed from winter cold, and hidden from rodents, birds, and other animals that eat them. Rocks provide similar protection for young saguaros. Saguaros do best on bajadas - gently sloping outwash plains at the foot of desert mountains.
Growth of a Green Giant
A saguaro's growth is extremely slow. Growth occurs in spurts, with most of it taking place in the summer rainy season each year. By the end of a year the saguaro seedling may measure only 14 inch. After 15 years, the saguaro may be barely one foot tall. By 50 years the saguaro can be as tall as seven feet. After about 75 years it may sprout its first branches, or "arms". The branches begin as prickly balls, then extend out and upward. This is also the stage of life when the saguaro begins to flower and produce fruit and seed.
By 100 years, the saguaro may have reached 25 feet. Saguaros that live 150 years or more attain the grandest sizes, towering as high as 50 feet and weighing 8 tons-and sometimes more-dwarfing every other living thing in the desert. These are the largest cacti in the Urged States. Their huge bulk is supported by a strong but flexible cylinder-shaped framework of long woody ribs.
Death . . . and Rebirth
Saguaros may die of old age, but they also die of other causes. Animals eat the seeds and seedlings, lightning and winds kill large saguaros, and severe droughts weaken and kill all ages. The saguaro is vulnerable during every stage of its life.
Where there is a balance of life and death, saguaro forests thrive. But in some forests in Saguaro National Monument deaths have greatly outnumbered the growth of new young saguaros.
- What has caused the decline in these areas?
- Biologists believe killing freezes are the number one cause of saguaro deaths in the park. The saguaros here are at the extreme northern and eastern edge of their range, where the coldest winter temperatures most often occur.
- Man, too, has played a part in the decline. Livestock grazing, which continued from the 1880s until 1958, devastated some forests. Many seedlings were killed outright by trampling or were unable to find suitable places to grow because the ground had been compacted and nurse trees killed.
Today, with grazing eliminated, recovery appears to be underway in several areas, where thousands of young saguaros have taken hold and are thriving. Still, natural forces, vandalism, and cactus rustling-the theft of saguaros for use in landscaping-continue to take a toll on the park's saguaro forests.
The General park map handed out at the visitor center is available on the park's map webpage.For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.
A geology photo album has not been prepared for this park.For information on other photo collections featuring National Park geology, please see the Image Sources page.
Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.
Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
9" x 10.75", paperback, 550 pages, full color throughout
The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!
Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.
Information about the park's research program is available on the park's research webpage.
For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.
The NPS maintains a searchable data base of research needs that have been identified by parks.
A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.
NPS Geology and Soils PartnersAssociation of American State Geologists
Geological Society of America
Natural Resource Conservation Service - Soils
U.S. Geological Survey
Currently, we do not have a listing for any park-specific geology education programs or activities.
General information about the park's education and intrepretive programs is available on the park's education webpage.For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.