EOCENE TO RECENT NORMAL FAULTING AND SYNTECTONIC SEDIMENTATION, HENDERSON CREEK QUADRANGLE, SOUTHEAST IDAHO

New geologic mapping (supported by EdMap) in the Henderson Creek 7.5’ quadrangle in Oneida County, Southeast Idaho, has identified three Tertiary fault sets, each with associated synextensional deposits, which include the Eocene Wasatch Formation, the Miocene Salt Lake Formation, and Quaternary deposits. The study area lies within the hangingwall of the Miocene Bannock detachment system.

The Wasatch Formation consists of red pebble conglomerate overlain by quartzite-boulder beds, with Ordovician Swan Peak quartzite blocks up to 6 meters in diameter. The formation thickens southward toward an east-west striking fault set. This suggests synextensional deposition in an east-striking Eocene half-graben.

The synextensional Miocene Salt Lake Formation (~11.9 Ma to <9.3 Ma based on tephrachronology) is associated with a north-striking fault set, and occupies two subbasins in the hangingwall of the Bannock detachment system (Janecke et al, 2003, Rocky Mountain SEPM Cenozoic Systems Volume). The fault-controlled depocenters and nature of the basin fill changed dramatically as the underlying detachment system evolved. The basal Skyline member (~11.9-10 Ma) was deposited as an alluvial fan in an isolated half-graben in the north half of the map area. A second basal conglomerate (>10.2 Ma) was deposited in a separate shallow basin within the south half of the map area. The lacustrine Cache Valley Member (~10.2-9.3 Ma) was deposited in a widespread, shallow lake, bounded on the south and west by the synextensional Steel Canyon fault. An upper conglomerate (~9.7-9.3 Ma) interfingers with the Cache Valley Member, and represents coarse deposits proximal to the active fault.

The third fault set consists of north-striking, Pliocene to Recent, Basin-and-Range faults, including the Wasatch Fault, which makes a 3-5 kilometer-wide right step at a segment boundary just north of the Idaho-Utah border. Extensional folds within the Salt Lake Formation are interpetreted as results of movement along listric faults of this set. A 7 kilometer-long, north-trending extensional anticline is due to a double rollover above oppositely-dipping listric normal faults. Crestal collapse produced a complex array of discontinuous normal faults in the eastern half of the quadrangle.