Abstract

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.

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