NEW INTERPRETATIONS OF STRUCTURES IN THE COLUMBIA RIVER BASALT GROUP OF THE CLEARWATER EMBAYMENT, IDAHO

The Clearwater embayment is located along the eastern margin of the Columbia Plateau where a large area (64 km²) of the Columbia River Basalt Group (CRBG) is exposed.  Several major basins, and a few folds and reverse faults have previously been noted, although most workers described the area as essentially horizontal CRBG flows with steep normal faults and minor folds forming the basin boundaries.  In the past decade, mapping has identified additional basins and determined that folding is more common than faulting.  Several linear structures, each previously described or illustrated as normal faults, were examined and shown to be primarily asymmetrical folds.  Over 40 asymmetrical folds with minor topographic expression have been mapped that are typically low-amplitude, non-cylindrical folds with narrow anticlinal ridges separated by broad, essentially flat or gently folded synclinal valleys.  Where faulting is present, movement has been determined to be reverse or strike slip.  Larger folds that are significant topographic features include the Limekiln structure or Waha front, Lewiston Hill, and White Bird Hill.  The Waha front is a 35-km long northeast-trending monocline that forms the southern boundary of the Lewiston basin.  On the southwestern end, the monocline contains strike-slip faults both parallel and oblique to its axis.  On the northeastern end, the monocline has only minor associated faults.  The Lewiston Hill borders the Lewiston basin on the north and is primarily an east-west trending asymmetrical anticline with a steep southern limb and associated reverse faults.  The White Bird Hill in the southern part of the embayment is a large asymmetrical anticline, which has a steep northwestern limb near Grangeville that changes symmetry to a steep southeastern limb near White Bird.  On White Bird grade the anticline is crossed by a strike-slip fault that produces over 50 meters of apparent vertical offset.  These three features are similar in overall morphology to large Yakima-style folds in west-central Washington.  Additional mapping in the embayment is needed, but enough work has been completed to show that the structural setting for the CRBG is not adequately portrayed in the literature.