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BRECCIA-PIPE- AND PALEOKARST-SUSCEPTIBILITY MAP FOR THE ST. GEORGE–HURRICANE METROPOLITAN AREA

by

DISCUSSION

USING THIS MAP

MAP LIMITATIONS

SYMBOLS

Municipality boundary

Interstate Highway

State Highway

Major local surface street

Other road

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1 0 1 20.5Miles

1 0 1 20.5Kilometers

Scale 1:24,000

Approximate meandeclination, 2007

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Map Location

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Bedrock units that are known to contain breccia pipes and/or paleokarst features. These units include the Toroweap and KaibabFormations, and the Timpoweap and Virgin Limestone Members of the Moenkopi Formation.

Southwestern Utah is host to numerous large solution-collapse breccia pipes that have formed chiefly by dissolution of limestone. The breccia pipesare rubble-filled vertical tubes that form and project to the surface as overlying strata collapse into buried Redwall Limestone karst caverns, whichare present only in the deep subsurface in southwestern Utah. A second zone of smaller paleokarst features exists at the contact between theHarrisburg Member of the Kaibab Formation and the overlying Moenkopi Formation. These paleokarst features formed in both limestone andgypsum, and are at or close to the ground surface in some areas of southwestern Utah. Historic sinkholes, likely associated with this zone ofpaleokarst, have opened in or adjacent to the Virgin River and La Verkin Creek, and in some cases have intercepted all or part of the flow of thosestreams. Most breccia pipes and paleokarst features in the St. George – Hurricane metropolitan area are no longer active due to the region’s aridclimate and deep water table. However, with the addition of water from irrigation, onsite wastewater disposal systems, canals and reservoirs, orother causes, or construction above existing caverns, these relict features may reactivate locally resulting in subsidence or collapse and damage tostructures, transportation and utility corridors, and reservoirs. Breccia pipes and paleokarst features also provide highly permeable pathways to thesubsurface and are of concern for wastewater disposal and ground-water pollution. Because breccia pipes and paleokarst features rarely if evercause rapid, catastrophic property damage or are a threat to life safety, for purposes of this study, breccia pipes and paleokarst features areconsidered adverse construction conditions and not geologic hazards.

For additional information about breccia pipes and paleokarst features in the St. George – Hurricane metropolitan area, refer to the Problem-Soil-and-Rock text document in this report.

The Breccia-Pipe-and-Paleokarst-Susceptibility Map shows the location of bedrock units in the St. George – Hurricane metropolitan area typically associatedwith breccia pipes and paleokarst features. The map is intended for general planning purposes to indicate where breccia pipes and paleokarst conditions mayexist and special studies may be required. The UGS recommends performing a site-specific geotechnical foundation/geologic-hazards study for alldevelopment at all locations in the study area. Site-specific studies can resolve uncertainties inherent in generalized mapping and help ensure safety byidentifying the need for special foundation designs or mitigation techniques. The presence and severity of breccia pipes and paleokarst features along withother adverse construction conditions and geologic hazards should be addressed in these investigations. If breccia pipes or paleokarst features are present at asite, appropriate design recommendations should be provided.

The Breccia-Pipe-and-Paleokarst-Susceptibility Map is based on limited geologic and geotechnical data; site-specific studies are required to produce moredetailed geotechnical information. The map also depends on the quality of those data, which varies throughout the study area. The boundaries of thesusceptibility category are approximate and subject to change with additional information. Small areas of localized susceptibility may exist throughout the studyarea, but their identification is precluded because of limitations of map scale. Additionally, gypsum-bearing units in the study area are locally covered by a thinveneer of unconsolidated deposits. Such areas may be susceptible to sinkhole reactivation or collapse (for example Big Round Valley [Milligan, 2000] south ofBloomington); however, because subsurface information is generally unavailable, those areas are not identified on this map. This map is not intended for use atscales other than the published scale, and is designed for use in general planning to indicate the need for site-specific studies.

!. Known paleokarst or breccia pipe feature

William R. Lund, Tyler R. Knudsen, Garrett S. Vice, and Lucas M. Shaw2008

PLATE 9

MITIGATIONAlthough potentially costly when not recognized and properly accommodated in project design and construction, problems associated with breccia pipes andpaleokarst features rarely are life threatening. As with most adverse construction conditions, early recognition and avoidance is the most effective way tomitigate potential problems. However, breccia pipes and paleokarst terrain susceptible to subsidence and sinkhole formation are widespread in some areas ofthe St. George – Hurricane metropolitan area, and avoidance may not be a viable or cost-effective mitigation option, particularly for transportation and utilitycorridors and large reservoirs. Mitigation techniques include installing inverted aggregate filters, transit-mix plugs, various types of grouting, dynamiccompaction, special foundation designs such as piles, and careful drainage design to keep water away from karst features (Fischer and McWhorter, 2006).

Although this product represents the work of professional scientists, the Utah Department of Natural Resources,Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use.The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstancesfor any direct, indirect, special, incidental, or consequential damages with respect to claims by users of thisproduct.

For use at 1:24,000 scale only. The Utah Geological Survey does not guarantee accuracy or completeness ofdata.

Basemap consists of National Agricultural Imagery Programnatural color aerial photography.

Universal Transverse Mercator Projection, zone 12.North American Datum 1983.

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113 22' 30" W113 30' W113 37' 30" W R 15 W R 14 W

T 42 ST 41 S

T 41 S

T 40 S

R 14 W R 13 W

R 13 W R 12 W

37 7' 30" N

37 15' N

R 17 W R 16 W

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T 41 ST 40 S

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37 15' N

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37 N

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113 30' W

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UTAH GEOLOGICAL SURVEYa division of

Utah Department of Natural Resources

Plate 9

Utah Geological Survey Special Study 127

Geologic Hazards and Adverse Construction Conditions,

St. George−Hurricane Metropolitan Area, Washington County, Utah

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