MULTIPLE PHASES OF TERTIARY EXTENSION AND SYNEXTENSIONAL DEPOSITION IN AN EVOLVING SUPRADETACHMENT BASIN, MALAD RANGE, SOUTHEAST IDAHO

The extensional history of the Henderson Creek 7.5' quadrangle, located in the Malad Range of SE Idaho, involves multiple phases of Tertiary normal faulting and synextensional deposition. The largest-magnitude extension took place from ~10-4 Ma along low-angle normal faults of the top-to-the-WSW Bannock detachment system, with concurrent sedimentation of the Miocene-Pliocene Salt Lake Formation in its hanging wall. This was preceded by a) ~8% N-S extension along E-striking faults, during deposition of the Eocene(?) Wasatch Formation, a conglomeratic unit that was deposited in a S-tilted half-graben, and b) >11.9->10.2 Ma development of a N-striking, E-tilted half-graben in the Malad Range, in which the late Middle to Late Miocene Skyline member of the Salt Lake Formation was deposited as an ash-rich alluvial fan. Detrital zircon data suggest incorporation into the Skyline member of sand from an ~11 Ma E-flowing river system, which drained the 12.5 to 15 Ma Owyhee-Humboldt volcanic field in southwest Idaho. In this study, thirteen new tephra correlations and one new detrital U-Pb SHRIMP zircon analysis date units of the Salt Lake Formation.

The inception of the Bannock detachment system is recorded by widespread deposition of the lacustrine Cache Valley Member (~10.2-<9.2 Ma) of the Salt Lake Formation in a large saline lake system. A NE-dipping antithetic fault uplifted an intrabasinal block that shed a wedge of conglomerate eastward into the lake between ~10.0-<9.2 Ma. This unit interfingers with lake-margin tufa-bearing deposits, which change eastward into deeper-water micritic limestones and tuffaceous rocks. A set of N to N-NW-striking Late Miocene normal faults extended the hanging wall ~16% after deposition of these lake beds.

The youngest event in the study area involved ~9% E-W extension on Pliocene-Quaternary normal faults. The boundary between the Malad City and Clarkston Mountain segments of the Wasatch fault is exposed just N of the Idaho-Utah border, and is a 2-3 kilometer-wide relay ramp between two right-stepping en echelon segments. In addition, a broad, N to N-NW-trending antiformal zone of extensional folds is present, 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.