Abstract This one-day field trip of geologic and historical significance goes from Washtucna, Washington, through Palouse Falls State Park, Lyons Ferry State Park, and Starbuck, and ends in the Tucannon River valley. At Palouse Falls, it is readily apparent why Native Americans crafted stories about the origins of this spectacular area and why geologic debates regarding the role of Pleistocene glacial Lake Missoula floods during the formation of this natural wonderland have been centered here. This field trip focuses on structural geology and the Palouse Falls fracture zone, Columbia River Basalt Group stratigraphy at the falls, and subsequent erosion by glacial outburst floods. Discussion of the falls will include human history and the formation of Palouse Falls State Park. The main stop at Palouse Falls will explore the stratigraphy of the Columbia River Basalt Group, Vantage Member, loess islands, fracture zones, and human history dating back at least 12,000 yr. Driving south through Lyons Ferry State Park and the Tucannon Valley, we will discuss topics ranging from the Palouse Indians to sheep herding and from clastic dikes to terracettes.

Mélanges represent a significant part of the Miocene Nabae accretionary complex. Such mélanges show sheath folds with D1 axial plane pressure-solution cleavage, whereas the coherent unit shows asymmetric folding with D1 slaty cleavage. In addition, the mélanges are characterized by D1 asymmetric shearing, which includes both thrust and right-lateral-sense components, in contrast to D1 pure shear that characterizes the coherent unit. Thus, this tectonic style acted on the climax of prism development can be referred to as a tectonic mélange. However, because the D1 shear displacement is almost negligible, D0 normal faults and basaltic dikes, operated when matrix sediments were not consolidated, are not disrupted. Oceanic materials such as basalt and chert cannot be incorporated into terrigenous matrix, given the small displacement associated with the D1 shearing. Exotic blocks of chert and sandstone show D minus 1 (D–1) cleavage, which is apparent in the older, probably Eocene, accretionary prism. When this prism was exhumed, it supplied debris to the Miocene trench, and then underwent additional D1 deformation, which included the above asymmetric shearing. This sedimentary and two-way-street tectonic process was recycled within the prism as the latter developed. Thus, as the block-in-matrix fabric was originally sedimentary and labeled D0, the tectonic mélange process that forms block-in-matrix fabric is only conjectural for the Nabae complex. Also it is suggested that these deformations are not progressive nor distinctive for each other.

A complex sequence of deformation produced the major central and southern trends of the Appalachian fold-and-thrust belt in the Roanoke recess, Virginia. The incipient recess first experienced the Appalachian-wide stress field, then shifted to far-field effects from incremental counterclockwise rotation of the shortening direction, which resulted in the Central Appalachian fold belt, and then shifted to incremental clockwise rotation, which produced the Southern Appalachian fold-and-thrust belt. We analyzed joints, veins, normal and reverse faults, stylolites, and paleoseismites from Mississippian strata at the structural front of the Southern Appalachian fold-and-thrust belt, and the adjacent Appalachian Plateau west of the recess. We distinguished seven deformational events using orientations, intersection relationships, fault-slip directions, and mineralization histories. Five of these sets represent late Paleozoic deformational events (A1–A5), with shortening directions that show an evolving Alleghanian fold-and-thrust belt in the recess. A1 (shortening trend 085°–265°) is consistent with the previously determined Appalachian-wide stress field and incipient layer-parallel shortening strain in middle Mississippian carbonates. A2 (trend 145°–325°) is a newly recognized event, herein called the Princeton event, which is consistent with dominant orientations of previously determined layer-parallel shortening strain, clastic dikes in Upper Mississippian strata, and stylolites. These far-field effects may mark high-angle basement faulting associated with development of the foredeep bulge during incipient thrusting along the Pulaski thrust system far in the hinterland. A3 (trend 120°–300°) corresponds to initiation of the major Central Appalachian deformation, which resulted in fold-and-thrust belt structures such as the North Mountain fault and Wills Mountain anticline, while A4 (trend 160°–340°) is associated with Southern Appalachian (e.g., St. Clair thrust and Glen Lyn footwall-syncline) structures. A5 (trend 010°–190°) represents late Alleghanian deformation of the Glen Lyn syncline, likely associated with blind thrusting coeval with emplacement of the nearby Pine Mountain thrust sheet. Two post-Alleghanian fracture sets, PA1 (joint trend 150°–330°) and PA2 (joint trend 060°–240°), are orthogonal; PA2 is younger. These joint sets are associated with strike-slip and normal faults that are compatible with some fault-plane solutions from the nearby Giles County seismic zone.

We used ground penetrating radar (GPR) data to help determine the spatial distribution and the subsurface geometry of clastic dikes at the Hanford Site. This information will help to improve the understanding of the hydrological role of these ubiquitous clastic dikes at the Hanford Site. We collected 100 MHz ground penetrating radar (GPR) 3-D surface reflection data at two sites, the 216-S-16 (S-16) Pond and the Army Loop Road sites, and 2-D reflection data along a 6.9 km linear transect (Traverse site) near the Army Loop Road site. The dikes are distinguished in the GPR data by a strongly attenuated zone, disruptions in the continuity of reflections, and diffractions where reflections are disrupted. In general, the data quality is better at the Army Loop Road and Traverse sites than at the S-16 Pond site, probably due to the presence of cobbles at the S-16 Pond site. A high-moisture, fine-grained unit probably causes the strong reflections at the Army Loop Road site and the Traverse survey site. The signal penetration varies between 5 and 12 m below the land surface.