GEOLOGY AND SOILS 



Geology 



Geologically, the Wasatch Mountains east 

 of Farmington are very complex; they show 

 evidence of folding, major and minor faulting, 

 uplift, and multiple erosion cycles. Bell 

 (1952) describes the area as ". . . essentially a 

 composite series of north-northwest trending 

 fault blocks bounded by normal faults." The 

 two study watersheds lie between the Wasatch 

 Fault's main crest (which overlooks Great Salt 

 Lake) and a subsidiary crest which forms the 

 northeastern boundary of these watersheds. 



The underlying bedrock is a complex series 

 of igneous, metamorphic, and sedimentary 

 rocks. Scattered remnants of a coarse tertiary 

 conglomerate (Knight formation) occur in the 

 lower portions of the area and also to the 

 west of the watershed boundary. These rocks 

 consist of a series of interbedded, reddish- 

 colored shales, siltstones, sandstones, and con- 

 glomerates. The most prevalent rock types are 

 metamorphic gneiss and schistose families in- 

 truded by relatively narrow bands of pegma- 

 tites. Migmatites (composite gneiss) and 

 greenstone schists are also abundant. Finally, 

 outcroppings of Pre-Cambrian quartz are ap- 

 parent on the ridge crests. Some of the more 

 common mineral constituents include: feld- 

 spars (dominantly plagioclase), quartz, mus- 

 covite, chlorite, biotite, epidote, and 

 actinolite. 



Seismic Survey 



A shallow-depth seismic survey was com- 

 pleted for the study areas using an MD-3 seis- 

 mograph (Soiltest, Inc.). The survey consisted 

 of 226 systematically located transects, each 

 120 to 150 feet in length, which provided 

 subsurface information to a depth of 40 to 50 

 feet. 



Careful correlation of the seismic data with 

 the information obtained from test holes and 

 other known surface and subsurface charac- 

 teristics provides a wealth of geologic and 

 hydrologic information such as: thickness and 

 depth of subsurface layers, hardness, weather- 

 ing, stratification, fracturing, faulting, and dip 

 angle of strata. 



An isopach map of subsurface depths (page 

 48) was prepared to aid in planning a water- 

 shed treatment that would offer the greatest 

 potential for increasing water yields. This map 

 indicates the depth of low velocity (700 to 

 1,600 feet per second (f.p.s.), loosely consoli- 

 dated surface material. This surface layer is 

 generally quite deep on both watersheds, with 

 only small scattered areas less than 5 feet 

 deep in the West Branch. 



The isopach map indicates that the depth 

 of the low-velocity, surface-soil layer along 

 the eastern ridge ranges from 5 to 10 feet and 

 occasionally down to 15 feet in the saddles. 

 In this case, information from the surface 

 soils is lost because of their very shallow 

 depths and the very similar velocities of the 

 soil material and the dry, strongly weathered 

 and fractured underlying gneiss. From the 

 standpoint of water movement and storage, 

 this soil-rock complex may be considered 

 quite deep. 



It was not possible to calculate second lay- 

 er soil depths for more than a quarter of the 

 transect lines; thus, there are probably too 

 few depths to plot an accurate isopach map of 

 the second layer. A plot of equal velocity 

 lines and available depth information is shown 

 on page 49. With few exceptions, the second 

 layer is greater than 20 feet deep and is com- 

 posed of fairly low velocity material in the 

 3,000 to 5,000 f.p.s. range. This layer is either 

 wet or compacted alluvial material in the val- 

 ley bottoms or it is deeply weathered gneiss 

 and schistose parent material on the ridges 



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