Rulo Site - Walla Walla Valley, WA

Rulo is a Palouse outcrop with a twist. The Rulo site is a large, double-sided roadcut located a few miles northwest of Walla Walla along Sudbury Road. Roadcuts and nearby outcrops expose what at first appears to be the same-old, same-old innards of a Palouse loess hill, but upon closer inspection proves to be much more interesting. Faulted Miocene basalt bedrock is overlain by locally-derived gravel and an unusual micaceous, fossiliferous fluvial deposit of Pliocene age. Above, lie a number of cross-lapping beds of windblown silt - Pleistocene Palouse loess in its classic form, overprinted by calcic paleosols and interfingered with sandy flood rhythmites deposited by both Missoula floods and floods far older. Elevation of the site is ~285m, well below the maximum level of Lake Lewis (366m). Distinct horizons were chewed through by innumerable cicada during the Pleistocene. Waves of cicada thrived beneath a long-lived sagebrush plant community and left behind a clear record of their presence in thousands of backfilled burrows. Several whispy tephras can be seen in the stack, few of which have their age well constrained. Four generations of sheeted clastic dikes, each truncated by local erosional surfaces, cut conspicuous paths through the high exposure. A tumbleweed-choked gully just west of the road reveals a slug of reworked and over-thickened ash from the Holocene Mount Mazama eruption. Perhaps most remarkable thing about Rulo is that the entire Miocene to Holocene vertical succession occupies just a few 10s of meters and lacks a major unconformity. The landscape has not changed all that much in the past few million years here along the northern margin of Walla Walla Valley, where the Palouse Hills meet the Channeled Scabland.

West side of roadcut.

The Rulo five. Five unconformity-bounded units recognized at Rulo by Bader et al. (2016, Fig. 3). Note how clastic dikes truncate at erosional surfaces - a phenomenon I've documented at numerous other outcrops in the Channeled Scablands.

Rulo from the top of the west cut.

Easy access. The roadside exposure accommodates both standers and climbers. Good views are had from the road shoulder to those of the former group, while clearer details are had by more intrepid geologists willing to kick a few steps and eventually slide back down (with varying amounts of grace).

Burrow and dike. An old clastic dike with its top truncated by younger sediments shoulders up to a rodent burrow, on right. The dike, the surrounding sediment, and the burrow all show signs of weathering.

Rodent burrow in old loess.

Rare find. Cicada burrow cast inside one of the sheets in a clastic dike. Many others are found nearby. Is it a clast - a constituent of the dike fill or did the critter burrow in after the dike formed, treating it as it would any other sediment?

Local unconformities separate units. Four generations of sheeted Touchet-type clastic dikes are associated with packages of sediment at Rulo. The dike sets are truncated by erosional surfaces (a,b,c,d), highlighting the fact diking was repetitive. Note the dikes never crosscut younger horizons, only older ones; they are not liquefaction features. Sheeted clastic dikes in Eastern Washington are found exclusively within the margins of scabland floodways. My sketch condenses the stratigraphy described by Bader et al. (2016) into a fieldbook-friendly column.

Water-worked. Channel fills containing pebbles and small cobbles hint at flashy, seasonal flows in wet swales that typically carried silt. Weakly-developed to non-existent bedforms are par for the course in windblown silts reworked and redeposited by flowing water. Silt-pebble diamicts.

Thrust fault. A small thrust displaces Miocene basalt and an overlying gravel containing locally-derived basaltic clasts. A thin, mica-bearing sandstone unit, likely quite old, lies just above and is also involved in the faulting. The reddened, silty material above remains undeformed. The fault formed at least a few thousand years ago, prior to deposition of most if not all of the Pleistocene section.

Basaltic gravel and micaceous stuff. Angular to subrounded basalt clasts comprise a gravel (stratified stream deposit) laid down atop the weathered surface of a Miocene basalt flow. The clasts were certainly derived from one or more basalt flows nearby and likely from this flow - once loose detritus and colluvium picked up and transported a ways by flowing water. Its basal surface may have been steepened by tilting. Also here is a micaceous, crossbedded fluvial deposit. Not common, but sometimes found deep in Palouse hills. These deposits are never very thick out here, but describe a sluggish sidestream network in the southern Palouse/WW Valley/Pasco Basin/Umatilla Basin associated with the ancestral Columbia River lowland during its wandering days (<3 Ma). Any mica-bearing sediment found at Rulo dates to Ringold time (late Miocene-Pliocene) and derives either from the Okanogan to the north, the Umatilla-Dalles-John Day trough to the southwest, or Idaho to the east.

Cemented dike cuts gravel. A calcite-cemented clastic dike with a very fine-grained fill descends a short distance through a basaltic gravel. Don't remove it. Identical dikes are found a Winans Rd and elsewhere.

Whitman College contributions. We understand the geology at Rulo in large part due to the work of Professors Pat Spencer, Nick Bader, and their students from Whitman College (Bader et al., 2016).

Rip-ups not nodules. Rip-ups of calcrete in a weakly-stratified, silt-sand diamict attest to reworking of loess hills (hillslope deposits) by flowing water (channels). It can be difficult to pick these units out at first, but once you see these pebbly diamicts, you cannot unsee them. Where found, always note your elevation and compare it with the local maximum flood stage for Missoula floods. In many locations, deposits like these represent the transition between wind-swept Palouse uplands and Scabland valleys below. Its here where deposits from two different geomorphic domains interfinger.

Cementation and time. Calcrete thoroughly cements (overprints) densely cicada-burrowed loess.

Bioturbation. Delicate details preserved in many fossil cicada casts suggest bioturbation decreased markedly in recent millennia.

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Truncated top. A sheeted dike with two branches extends downward through silty, oxidized, slightly-cemented sediment, its top truncated by an erosional surface and silty-sandy material. Oxidation overprints both dike and host sediment, indicating both are old and that they aged together at least for some period of time. Sediments below the erosional surface are many tens to a few hundred thousand years old. Sediments above the surface are mostly late Pleistocene age. Pencil for scale.

Surface. Another sheeted dike pair nearby is similarly truncated at its top by erosion. Note hints of water transport in overlying units. This surface and two distinct paleosols appear in several exposures along the northern margin of the Walla Walla Valley.

Mazama. Thickened white Mazama ash atop dark, silty Holocene alluvium are exposed in brushy gully to west.