6.2.2.b Geological Resources Discussion

The city of Surprise is located within the Salt River Valley, exhibiting very little topographical change in the range of 0-2 percent slope (Map 6.2A). A two percent slope indicates that for every 100 feet horizontal distance, the land rises or drops two feet. The existing slope drains to the southeast. A slope of two percent allows virtually unrestricted development for agricultural, rural, or urban land uses.

Geologically Surprise lies within the Basin and Range province (Fig 6.2A). Tectonic stretching of the North American continental crust during the last 25 million years has yielded a broad rift called the Basin and Range province, so named because topographically, it consists of long linear mountain ranges separated from one another by flat plains (basins) of alluvium. The range in the general area of Surprise contains Proterozoic metamorphic, and igneous rocks and the aquifers here are formed of volcanic and carbonate rocks and unconsolidated to consolidated basin-fill deposits. The basin fill deposits form the most productive aquifers and are generally in individual alluvial basins that are drained internally and separated by low mountains. The geology of the area plays a large role in the soils, drainage patterns, and other physiographic features of the region.

Surprise itself is located on top of an alluvial valley and connected to the Salt River Valley to the east. The alluvium is composed of three quaternary sedimentary deposits that are nearly 1,200 feet deep and contain significant deposits of gypsum and calcite. The upper alluvial unit is composed of relatively coarse grained unconsolidated materials and extends
to a depth of 800 feet. The middle fine grain unit is composed of finer grained material that extends to a depth of 1,050 feet. The lower conglomerate unit contains mostly consolidated, relatively coarse grained deposits. The characteristics of these strata create a very solid building foundation as well as holding and purifying the underground water aquifer.

Figure 6.2A

Source: Basin and Range Aquifer, Ground Water Atlas of the United States, USGS, 1995 accessed at http://capp.water.usgs.gov/gwa/ch_b/B-text2.html 

Figure 6.2B

Source: Basin and Range Aquifer, Ground Water Atlas of the United States, USGS, 1995 accessed at http://capp.water.usgs.gov/gwa/ch_b/B-text2.html

Figure 6.2C

Figure 1 Diagrammatic profile of a valley in the Basin-Range province. This illustrates the conventional concept of valley fill with coarse material adjacent to mountains grading into fine alluvium and lake sediments near the middle of the valley.

Due in part to the characteristics of the sedimentary strata underlying Surprise, the region is geologically inactive with respect to faults. A seismic risk map places Maricopa County in Zone 2, which can receive moderate earthquake damage, although no record currently exists. The closest linear earth fissures, which act as drains for overland waterflow have been identified in Section 25, Township 3 North, Range 3 West, which is located to the south of the Surprise Planning Area.

One of the major geologic features in the general area is the White Tank Mountains. The White Tank Mountains form one of the several metamorphic core complexes in central Arizona. Two major types of rocks are found in the mountain range; 1.7-1.6 billion years old Proterozoic metamorphic rocks and a Tertiary or Cretaceous age granitic intrusion.