GEOLOGIC MAP OF THE CENTRAL AND LOWER BIG CREEK DRAINAGE, CENTRAL IDAHO

Geologic mapping along much of the Big Creek drainage by the Idaho Geological Survey has identified previously unrecognized major structures and deposits that have important implications for the Cenozoic eruptive and mineralization history of central Idaho. Sporadic mapping from backpacking camps in this remote area commenced in 1995 and continued until 2010, resulting in a more detailed map than the previous reconnaissance-level studies.

The area is underlain by Mesoproterozoic metasedimentary rocks and Neoproterozoic metasedimentary and volcanic rocks. These are near-vertical to overturned and face to the SW. These strata are intruded by Neoproterozoic diorite-gabbro-syenite stocks, Cretaceous Idaho batholith-related intrusions, and bi-modal Eocene Challis hypabyssal rocks. Two ash-flow equivalents of the Eocene intrusive rocks are present locally where down-dropped due to collapse of the Thunder Mountain cauldron complex (TMCC) following eruption of the uppermost tuff. One of these faults may have controlled mineralization at the Thunder Mountain district SW of the study area.

Collapse of the TMCC was accommodated by structural displacement of the rocks within the cauldron into three structural blocks with contrasting structural styles. The middle block, the Big Creek graben, experienced the greatest amount of vertical displacement along a northern cauldron-bounding normal fault and two NE-SW trending boundary normal faults (the Cave Creek and Cow Creek faults) and contains relatively flat-lying Challis volcanic rocks. The block west of the Big Creek graben contains volcanic rocks that have not been tilted and that also displaced along an E-W striking cauldron-bounding normal fault. The block east of the Big Creek graben dropped by tilting into the evacuated magma chamber beneath the TMCC rather than along a cauldron-bounding normal fault.

Lahar deposits are present at one locality beneath the lowermost Eocene ash-flow tuff at the present water level of Big Creek, and, along with patches of volcanic rocks resting on the Precambrian metasedimentary rocks that underlie most of the Big Creek drainage, demonstrate that an Eocene paleo-canyon with orientation very similar to the present canyon existed prior to eruption of the tuff, at least from Acorn Creek to Cave Creek.