Cenozoic Structural and Stratigraphic Evolution of the Southeastern Salmon Basin, East-Central Idaho

The southeastern Salmon basin (SB) of east-central Idaho is a complex east-dipping half graben containing four unconformity-bounded sequences of Tertiary volcanic, alluvial fan, lacustrine, and fluvial deposits. From oldest to youngest these are the Challis volcanic group, sedimentary rocks of Tendoy (new name), sedimentary rocks of Sacajaweja (new name), and Quaternary-Tertiary deposits . The thick sequence of volcanic rocks was deposited in a southeast-trending paleovalley . New mapping, cross-cutting relationships, 40Ar/39Ar age determinations, and angular unconformities show that the SB has experienced at least four episodes of extension, and that it lies in the core of an Eocene to Oligocene rift zone. T he largest episodes of extension in the SB were the second and third episodes of extension. These were responsible for the deposition of the sedimentary rocks of Tendoy and sedimentary rocks of Sacajaweja, respectively. Episode 2 occurred along the west southwest-dipping Agency Yearian fault, and can be attributed to gravitational collapse of the Sevier fold and thrust belt between the late middle Eocene and Oligocene (?). Several southwest-dipping low-angle normal faults were active during the third episode and may have been active into early Miocene time. The third episode of extension reflects continued gravitational collapse. Also active during episode 3 was the Salmon basin detachment fault, which is interpreted as a regional detachment based on its lateral extent and low dip angle (11°). Extensional folds are common in the southeastern Salmon basin and represent at least two generations of folds. Interference between northnortheast and southeast-trending folds locally produced dome and basin features. The folds are typically open to gentle, and have a maximum fold height of 2.2 km. Most of the folds are fault-bend folds, and some are associated with growth strata. The development of the Tendoy anticline and Pattee Creek syncline in the hanging wall of the Agency-Yearian fault produced two depositional basins during the deposition of the sedimentary rocks of Tendoy. Facies patterns and megabreccia deposits in the subbasins indicate that there was considerable topography along the margins of the subbasins