Flood-deformed & Landslide-deformed Glaciolacustrine Deposits at Priest River, ID
April 18, 2020
The Ice Age floods inundated the southern portion of the Priest River Valley in northern Idaho, though this is rarely mentioned in the literature on the Channeled Scabland. I recently excavated a few windows into the rhythmically-bedded strata exposed in landslide scarps in order to get a better look. Breckenridge and Garwood (2019) map the deposits as glaciolacustrine (sediments deposited into lakes by glacial meltwater), but it appears the lake bed muds are interrupted by a number of sand beds. I'm thinking these are outburst flood sands, based in part on the repeated deformation associated with them and in part on their rhythmic character. They resemble lake-flood rhythmites to the west in the Columbia River Valley (Glacial Lake Columbia or modern Lake Roosevelt).
Its amazing what you can find when you step off the road and hog out some outcrop. I climbed 100' from my truck and just started in, discovering this. Several stratified rip-up clasts each up to a meter in length and composed of unconsolidated rippled, oxidized sand surrounded by massive (deformed) green-gray mudstone. Flat-lying ripple sands below. Flat-lying lacustrine muds above. Lots of soft sed def between. Never reported in any article or guidebook. One hour's time. Peninsula Road. #fieldgeology
Field sketch of outcrop shown in photo above. Deformation appears to be more prevalent in lower in the section, where beds are thicker and stronger lithologic/hydrologic contrasts have developed over time, forming slide planes.
Repeated, laterally-continuous zones of soft sediment deformation (large flame structures, load casts, rip-up clasts) occur in association with sand beds. Deformation is always bounded above and below by flat-lying lacustrine bedding. Glacial Priest Lake strata periodically interrupted by Missoula Floods. Flat beds indicate resumption of lake bottom deposition. Exposure is 3m wide and 1.75m high.
Varved lake sediments regularly disrupted by flood-deposited sand beds. "Stingy" and "generous" counts on varved intervals were made (i.e., Atwater's method for Sanpoil Valley). The southern portion of Priest River Valley (a proglacial lake basin) received waters from the Missoula Floods according to mapping by Breckenridge and Garwood (2019) and field evidence like this. I believe Priest River Valley contains the northeasternmost rhythmites produced by floods flowing out of western Montana.
Spectacular folds in glaciolacustrine muds deformed by landsliding. Exposure is along a branch of the Priest River. Visible from Hwy 57, accessed from Peninsula Rd. Flat-lying beds above and below the 3m-thick deformed zone. Not reported previously. 10 minute hike from roadway. #leaveyourvehicle
Another look at the "rhythmite section" preserved along Priest River. I counted about a dozen repeated intervals (varves and sands) here and about 10 more in exposures above this one. At an even higher location, I counted 6 repeated varved interval and bed pairs in a quick 3m-high excavation that lacked SSD altogether.
In many places (textbooks for example), geologists would call a conspicuously deformed horizon like this a "seismite". Hair would burst into flames and there would be much running about, "Call the newspaper! Call my editor!". Here (and perhaps there) such a call would be presumptuous. Soft sed def is the norm in this glaciolacustrine sequence and others like it (lake deposits in mountainous terrain). Rarely does such deformation signify a large-magnitude seismic event. #gobacktoseattle
Meter-scale folds in lacustrine muds. Ancient landslide.
A map by Breckenridge (1989, Figure 4) identifies a "rhythmite section" (triangle symbol) northeast of Priest River, ID which is probably that near Peninsula Road. Its unclear whether he ever described this section. I've not found notes. I don't think he published a description, anyway. Maybe Dean knows.
This map by Breckenridge (1989, Fig. 2) is an update of an earlier map by Richmond (1986). It shows the configuration of the Cordilleran Ice Sheet margin at Last Glacial Maximum. The town of Priest River is not shown, but it is located due east of Newport, just across the WA/ID border.
Map of post-basalt drainage pattern by Savage (1965, Fig. 21) in the vicinity of Priest River, ID. The stippled pattern shows "approximate boundaries of ponding and frequent flooding (Tertiary Lake Rathdrum)". I'm not sure if Lake Rathdrum is something geologists have since recognized, especially if its Tertiary age (2 to 66 Ma). Sediments in the area are pretty much all Pleistocene, but, hey, it was the '60s and Savage was a wild man.
Map showing glaciation of Bonner County, ID by Savage (1965, Fig. 15). According to the map, the Late Wisconsin Priest River Lobe covered most of the Priest River Valley (middle left). The ice margin was later refined by Breckenridge and Garwood (2019).
Savage, C.N. (aka "Wildman"), 1965, Geologic history of Pend Oreille Lake region in north Idaho, Idaho Bureau of Mines and Geology Pamphet 134, 18 pgs. + figures.
Breckenridge, R.M. (editor), 1989, Glacial Lake Missoula and the Channeled Scabland, 28th International Geological Congress Field Trip Guidebook T310, 72 pgs.
Breckenridge, R.M.; Garwood, D.L., 2019, Glacial geologic map of north Idaho, Ice Age Floods Institute Coeur Du Deluge Section/Keokee Press