VOLUME 41 NUMBER 1

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Multiple phases of Tertiary extension and synextensional deposition of the Miocene–Pliocene Salt Lake Formation in an evolving supradetachment basin, Malad Range, southeast Idaho, U.S.A.

Sean P. Long^1,*, Paul K. Link¹, Susanne U. Janecke², Michael E. Perkins³ and C. Mark Fanning⁴

¹ Department of Geosciences, Idaho State University, Pocatello, ID 83209, U.S.A.
² Department of Geology, Utah State University, Logan, UT 84322, U.S.A.
³ Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, U.S.A.
⁴ Research School of Earth Sciences, The Australian National University, Canberra ACT, 0200, Australia

^* Correspondence should be addressed to: longsean@isu.edu

The extensional history of the Malad and Bannock ranges in southeast Idaho and northern Utah involves multiple phases of Tertiary normal faulting and synextensional deposition. Detailed geologic mapping, structural and stratigraphic analyses, and geochronologic data from this study elucidate previously defined deformational events in the region, and define two new extensional episodes.

The largest-magnitude extension took place along low-angle normal faults of the 10–4-Ma Bannock detachment system, with concurrent sedimentation of the Miocene–Pliocene Salt Lake Formation in a regionally continuous supradetachment basin. This basin developed by 10.2 Ma, and was preceded by two phases of smaller-magnitude, aerially restricted normal faulting and sedimentation. In the Henderson Creek quadrangle of the southern Malad Range, the earliest event involved 8% north–south extension during deposition of the Paleocene–Eocene Wasatch (?) Formation. This conglomerate unit was deposited in an asymmetric, south-tilted half-graben bounded on the south by the syntectonic Willow Spring normal fault.

The next pre-detachment extensional event produced a north-striking, east-tilted half-graben in which the Middle to Late Miocene Skyline Member of the Salt Lake Formation (11.9–10.2 Ma) was deposited as an ash-rich alluvial fan. This half-graben is bounded on the east and south by the syntectonic Red Knoll and Spring Trail normal faults. Detrital zircon age data from a tuffaceous sandstone bed in the Skyline Member suggest incorporation of reworked zircons from the 12.5–15-Ma Owhyee-Humboldt volcanic field in southwest Idaho with 10.3-Ma glass.

The inception of the regional Bannock detachment system is recorded by the breakup and 16% west-southwest extension of its hanging wall by a set of north- to north–northwest-striking Late Miocene normal faults. These faults were associated with syntectonic deposition of the Cache Valley Member of the Salt Lake Formation (10.2– <9.2 Ma) in a regional-scale lake system. During deposition, the northeast-dipping Steel Canyon normal fault accommodated uplift of an intrabasinal horst that shed a wedge of Third Creek Member conglomerate eastward into the lake system between 10.0 and <9.2 Ma. The Third Creek Member interfingers with a Cache Valley Member lake-margin tufa-bearing facies, which changes eastward into a deeper-water, micritic limestone-bearing facies.

The most recent extensional event in the Henderson Creek quadrangle involved 9% east–west extension on the north-striking Pliocene–Quaternary Wasatch fault. A segment boundary of the Wasatch fault, consisting of a 2.5–3.5-km-wide relay ramp that formed between two right-stepping en echelon segments, lies just north of the Idaho-Utah border. In addition, a broad, north- to north–northwest-trending antiformal zone of extensional folds is present in the south half of the study area and is interpreted as a double-rollover anticline that formed progressively in Late Miocene and Pliocene–Quaternary time above two oppositely dipping listric normal faults.

Key Words: Bannock detachment system • Basin and Range province • extensional fold • Malad Range • Salt Lake Formation • SHRIMP geochronology • southeast Idaho • tephra correlation • Wasatch fault • Wasatch Formation

Application of remote-sensing and ground-truth techniques in determining the effects of coalbed-methane discharge waters on soils and vegetation

David C. Micale¹ and Ronald W. Marrs^1,*

¹ University of Wyoming, Department of Geology and Geophysics, Dept. 3006, 1000 E. University Ave., Laramie, WY 82071-2000, U.S.A.

^* Correspondence should be addressed to: rwmarrs@uwyo.edu

The current coalbed methane (CBM) play in the Powder River Basin of northeast Wyoming and southeast Montana is the most active in the United States. Large volumes of water generated during methane production present environmental concerns because the water typically contains high levels of sodium and is generally discharged at the surface, possibly resulting in the alteration of soil properties and changes to plant biomass. This study utilizes Landsat Enhanced Thematic Mapper (ETM) data and field observations for change analysis in a region of CBM production along Spotted Horse Creek in northeast Wyoming. Hypotheses tested in this study are: (1) plant biomass increases with the addition of saline and/or sodic CBM waters; and (2) CBM waters increase soil salinity, sodicity, and pH. Results indicate the application of CBM waters at the Spotted Horse Creek study site has initially increased plant biomass in areas of CBM activity. However, the CBM waters are very high in salinity and sodicity. Soil chemistry is affected such that pH, salinity, and sodicity levels have changed to the point of making the soils slightly sodic. Presently, these levels are within the tolerance limits for most of the plants in the region but may continue to increase with the continued discharge of CBM waters.

Key Words: CBM • coalbed methane • ground truth • Landsat • Powder River Basin • remote sensing • salinity • sodicity • soils • vegetation biomass • Wyoming

⁴⁰Ar/³⁹Ar dates from alkaline intrusions in the northern Crazy Mountains, Montana Implications for the timing and duration of alkaline magmatism in the central Montana alkalic province

Stephen S. Harlan

Department of Environmental Sciences and Policy, George Mason University, Fairfax, VA 22030-4444, U.S.A.

e-mail: sharlan@gmu.edu

Intrusive rocks in the Crazy Mountains, Montana, consist of numerous stocks, dike swarms, laccoliths and/or sills of strongly alkaline to subalkaline affinity that have intruded and metamorphosed Tertiary sedimentary strata of the Crazy Mountains Basin. The subalkaline rocks form stocks, sills, and radiating dikes and are located primarily in the southern Crazy Mountains (e.g., Big Timber stock, Loco Mountain stock). With the exception of the Ibex Mountain sill (?), the alkaline rocks are restricted to the northern Crazy Mountains. New ⁴⁰Ar/³⁹Ar dates are reported for alkaline rocks of the northern Crazy Mountains, with results ranging between 50.61 Ma and 50.03 Ma. Five dates from the strongly alkaline nepheline and mafic nepheline syenites of the Ibex Mountain sill (?), Robinson anticline intrusive complex, and Comb Creek stock (?) and dike swarm give tightly clustered dates suggesting that they were emplaced during a restricted time interval at 50.1 Ma. The dates from the alkaline rocks of the northern Crazy Mountains are slightly older than those previously reported from the subalkaline Big Timber stock in the southern Crazy Mountains (i.e., 49.3–49.2 Ma, biotite ⁴⁰Ar/³⁹Ar) (du Bray and Harlan, 1996). However, the limited span of dates (i.e., 50.6–49.2 Ma) and the geographic proximity between the alkaline and subalkaline rocks indicate that the magmas represented by these different geochemical groups were closely associated in both time and space. Furthermore, all the igneous rocks in the Crazy Mountains were emplaced in a narrow time interval of 1–2 m.y. On a regional scale, the 51–49-Ma age span from the Crazy Mountains is similar to that of most of the igneous centers of the central Montana alkalic province and is coeval with the peak of widespread volcanism in the Absaroka-Gallatin volcanic field immediately to the south of the Crazy Mountains Basin.

Key Words: Absaroka-Gallatin volcanic field • alkaline magmatism • Cenozoic • central Montana alkalic province • Crazy Mountains • Cordilleran orogenic belt • disturbed belt • geochronology • intrusive structures • magmatism • Montana • tectonics

A. J. Eardley, profile of a geologist

M. Dane Picard

Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, U.S.A.

email: picard@earth.utah.edu

Key Words: History of geology • structural geology • biography • Utah • Earth history • teaching • Wasatch Mountains