GEOLOGIC CONSTRAINTS ON GROUND-WATER FLOW PATHS TO LOCOMOTIVE SPRINGS, NORTHERN GREAT SALT LAKE, UTAH

This study combines geologic mapping, gravity and magnetic modeling, and hydrogeologic data to define the ground-water flow path to Locomotive Springs at the north end of Great Salt Lake. Flow at the six Locomotive Springs has decreased by 70-100% since 1970, causing ecologically important wetlands in the outflow area to shrink significantly. Locomotive Springs are at the SE end of Curlew Valley, a 3,100-km2, Y-shaped surface-drainage basin in the Utah-Idaho Basin and Range. Bounding ranges consist of limestone and sandstone of the Permian-Pennsylvanian Oquirrh Group. Basin fill includes Miocene alluvial and lacustrine rocks, Tertiary to Quaternary basalt, deposits of Pleistocene Lake Bonneville, and Quaternary alluvium. Basin fill is the principal aquifer in the Curlew Valley ground-water system. Flow paths and distribution of confined and unconfined zones are complex due to facies variations and interlayering of sediment and basalt. Locomotive Springs issue from the margin of a Quaternary basalt shield, and are supplied by two regional flow systems that originate in the northern two arms of the "Y" and are separated by a concealed bedrock high. The structure and hydrogeologic role of the bedrock high, the extent and thickness of the basalt and basin fill, and basin-bounding and intrabasin faults influence flow to the springs. Previous workers suggested that the NE flow system is the principal contributor to the springs, that ground water is stagnant within basalt in the southern arm, and that flow is along the Hansel Mountains normal fault on the SE valley margin. A NW-striking fault was proposed to form a ground-water barrier within the basin fill, diverting the water SE to the Hansel Mts normal fault. Modeling of new gravity data and existing aeromagnetic data reveals an abrupt southward thinning of basin fill in the NE flow system, at a jog in the Hansel Mts range front. This structure likely originated as a thrust-displacement transfer zone during the Jurassic-early Tertiary Sevier orogeny, and was reactivated during Tertiary Basin and Range extension. It may intersect a now-unused reservoir, causing leakage. The lost water may have flowed along the NW-striking fault to the Hansel Mts fault, then SW to Locomotive Springs. Some of the decreased spring flow may, therefore, have resulted from disuse of the reservoir, consistent with the ideas of previous workers.