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Paleoseismic Trenches in Eastern Washington

Locations of paleoseismic trenches and Quaternary faults in Eastern Washington. Trenches excavated across fault scarps, often visible in lidar images or aerial photographs, provide geologists with clues about recent movements and targets for trenching. Most paleoseismic investigation trenches are a few meters deep and a few tens of meters long. A large trenche can easily cost a hundred thousand dollars and require heavy equipment and several weeks of staff time to construct. Trenches are refilled once the information preserved in their vertical walls is logged (gridded, sketched, photographed). All of the trench sites shown on the map above are on basalt-cored Yakima Fold Belt structures except the Gate Creek (Mt. Hood Fault Zone) and Spencer Canyon in the North Cascades. The Buroker trench is located just off the map a few kilometers southeast of Walla Walla. The Spencer Canyon trench at Entiat, not shown, is located 60 km north of Kittitas Valley. Finley Quarry is a quarry high wall exposure, not a trench. The Wallula trench is reviewed in detail in a different post: Base map by Lidke and others (2003).

Infrastructure at risk.

Fault studies in Eastern Washington are driven by the need to protect hydroelectric dams, highways, and nuclear waste-related facilities at the Hanford Site. Figure credit: Seattle Times/Terry Tolan.

Trench through Lind Coulee Fault West at O'Sullivan Dam.

Sand-filled fractures exposed in the Lind Coulee trenches post-date faulting (GEI/West & Shaffer, 1988, Plate 13). No indication is given whether they are sheeted (e.g., Touchet-type dikes). They are sourced in an old flood bed, descend into fractured basalt, and parallel nearby shear planes. The "dikes" intrude the shear zone which predates undeformed sediments above (lacustrine silt, a younger flood deposit, loess). Unlikely the features are products of liquefaction. Sheeted dikes intrude an older shear zone in basalt at Gable Mountain (see trench log above). Loess-filled fractures (passive, gravity infill) appear to be quite young, formed sometime after deposition of youngest flood bed and the development of its soil. Paleosols are shown with a vertically striped pattern. More on Lind Coulee Fault and local stratigraphy here:

Spencer Canyon Trench near Entiat, WA (1872 Chelan/North Cascades earthquake).

No liquefaction features were observed in the Spencer Canyon Trench #2 according to USGS logs (Sherrod and others, 2015; Brocher and others, 2017; Brocher and others, 2018; Sherrod and others, 2021). Vertically-elongate structures shown in the figure above are sediment-filled root casts.

According to the Olympia-based Washington Standard, shaking caused by the 1872 quake was "insignificant" in Puget Sound, but near-apocalyptic east of the Cascades. Funny how rural folks on the East Side are depicted,

It appears that our earthquake experience…although it awakened considerable interest in the future state, was insignificant compared to that of our neighbors east of the mountains, who were forced to believe at the time that the end of all things sublunary had indeed come.

A letter from Klikitat county says that the earthquake...was very violent in that vicinity, but did no damage. The writer…formerly of this county, gives a very amusing account of the conduct of [our reporter] Mr. Shazer at the time. Greatly excited he sprung from bed, and ran out to his chicken-coop, and soon returned with the gratifying information that the chickens were all safe! Earthquakes will never injure such men. - Washington Standard, 11 January 1873

Finley Quarry at Horse Heaven Hills (Wallula Fault Zone).

No clastic dikes or liquefaction features were found in the quarry exposure through a portion of the Wallula Fault Zone south of Pasco, WA. The exposure was initally investigated in the late 1970s by Foundation Sciences, Inc. Consultants (Foundation Sciences, 1980). Several other field geologists were involved. Miocene basalts, a Miocene sedimentary interbed, and colluvium that pre-dates Missoula flooding are faulted. More on Finley Quarry geology here:

Smyrna Bench paleoseismic log for Trench No. 1, north flank Saddle Mountains.

No clastic dikes or liquefaction evidence found in loess, colluvium, or paleosols that overlie sheared basalt. Sketch from Bingham et al. (1970, Plate 4) redrawn by me in 2022.

Buroker Fault paleoseismic trench log, Walla Walla Valley.

No liquefaction evidence or clastic dikes found atop this minor fault located in the Russell Creek valley southeast of Walla Walla, WA. The fault was investigated in the late 1970s by Shannon & Wilson Consultants for Washington Public Power Supply System (WPPSS). This is one of seven faults they evaluated in the field. The Buroker Fault offsets Miocene basalt and oxidized Pleistocene loess. Fault offset of the tan Holocene loess (<11,000 years) is unlikely. Elevation of the site (1315', 400m) is above the maximum level of Lake Lewis (~366m).

Smyrna Bench paleoseismic log for Trench No. 2, north flank Saddle Mountains.

Conspicuous vertical features are loess-filled tension cracks (Bingham and others, 1970 Plate 6). They are not vertically-laminated clastic dikes nor products of liquefaction, but features formed by passive (gravity) infill of vertical openings formed by sub-horizontal block sliding (mass wasting) in the Ringold Fm. Block sliding at Smyrna Bench is a local phenomenon, a product of its peculiar geology. Shear displacement is not associated with the loess-filled cracks, only tensile opening (Mode I). According to the trenching project geologist John Bingham, "In both trenches [3N and 3S], the [Ringold] fanglomerate is broken by the separation cracks similar to those in trench 2. Some of these are filled with loess; others contain fragments derived from the walls of the crack. Several of the cracks show some stratigraphic offset, but no gouge zones or slickensides were found." I have labeled units in red text for clarity.

Wenas Creek trench logs. A scarp in the Wenas Creek Valley was identified from lidar imagery. In 2009, USGS excavated two trenches, located 5 km apart, in alluvial fan deposits. Trenching revealed several small offsets in bedrock, young sediments, and soils. No evidence of liquefaction was found and no clastic dikes we noted. The red vertical features are small shear zones (gouge) mostly in the bedrock. PDF of complete report is available online:

Gable Mountain Fault paleoseismic log for Trench #3 East Wall at Hanford Site, WA.

No liquefaction evidence. A few clastic dikes found in other trenches nearby clearly originate in Missoula flood deposits and intrude downward into fractured basalt.

Gable Mountain Trench G-2. Sketch of trench wall containing a Touceht-type clastic dike (Bingham and others, 1970). In the diagram below, I've attempted to unravel the deformation history and clarify the timing of dike injection using crosscutting relationships and 5 time-slice snapshots. According to the geologists who logged the trench, both the older blue and younger pink dikes post-date faulting; it exploits the weakness created by the fault.

According to the Nuclear Regulatory Commission geologist on site (Justus, 21 Nov 1980), great care was taken in preparing trenches at Gable Mountain for viewing, Observations in the trenches were jointly made by staff from Golder Associates, Rockwell Hanford, USGS, and HRC. Features in the trenches were clearly marked by colored flags, walls had been gratifyingly cleaned, and walls were shored with obvious great care. General observations of the faults were as follows: apparent reverse slip sense, displacement of glacial floodwater deposits of up to several inches, apparent spatial continuity of faults and fault zones in basalt with faults in overlying sediments, at least one fault has splays and anastomosing segments, fault gouge in basalts are several feet thick in places, clastic dikes occur within and across fault zones, clastic dikes and basalts may be slickensided, at least one clastic dike was offset by a fault, apparent normal faults of less than about one inch displacement are associated with the reverse faults.

Philip S. Justus, the NRC geologist summarizes his observations following inspection of several open trenches at Gable Mountain (Justus, 1980). Field notes pertinent to clastic dikes are provided below. Key take aways are a.) the dikes at Gable Mountain are typical of those found throughout the region, b.) the dikes are sourced in flood deposits and descend into fractured basalt, c.) the dikes are not themselves fault-generated structures, but exploit bedrock faults (weaknesses), and d.) minor faulting offsets some dikes.

  • Flood deposits on Gable Mountain bear a close resemblance to typical Missoula flood deposits found elsewhere.

  • Two distinct cycles of [Ice Age flood] deposition are present on the north side of Gable Mtn; possibly three on south side.

  • Clastic dikes on Gable Mountain are similar in lithology and fabric to those found elsewhere in the Pasco Basin.

  • Clastic dikes associated with each overlying cycle are found in Trenches CD-8, G-2, and G-3.

  • The youngest clastic dikes originate from the base of the coarse upper unit of flood deposits which is bounded at the top by St. Helens S ash as found in Trenches CD-4 and G-1.

  • Clastic dikes in Trenches CD-5 and G-3 are displaced by shearing on the fault plane (CD-5) and in the hanging wall (G-3).

  • In Trench G-3 displacements in the flood deposits appear to post date the youngest clastic dike.

  • Shears, possibly associated with the thrust fault, appear to cross and slightly displace clastic dikes in the footwall in an area of Trench CD-6.

  • Clastic dikes along fault plane in Trench CD-6 have slickensides surfaces with stipe parallel to the dip of the fault.

  • Oriented slickensides in clastic dikes parallel to slickensides in gouge on fault breccia (Trench CD-5).

  • Wherever fine-grained material is present along fault plane, slickensides are present.

Undeforming the deformation. Structural reconstruction of a feature identified as a clastic dike (blue body) in the log of Gable Mountain Trench G2. The older, unsheeted "dike" does not have a clear source. The feature ostensibly intrudes a fracture in Floodbed A. Floodbed A is thickened in the fold hinge, consistent with oblique thrusting. The second episode of faulting segments the main body of the blue dike and folds the thinner, now detached portion (oblique faulting). Two episodes of movement are indicated prior to Time 5. Faulting does not offset the upper two floodbeds. Time 5 shows the depsition of two glacial outburst flood gravels. A sheeted clastic dike descends from the younger floodbed into brecciated material along the older fault. Both dikes appear to intrude weaknesses and neither seems to have formed because of faulting.


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