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 (Savage, 1965). The Priest River joins the larger Pend Orielle River at the town of Priest River, ID. I recently excavated a few windows into the rhythmically-bedded strata exposed in landslide scarps of the valley in order to get a better look. Breckenridge and Garwood (2019) map these deposits as glaciolacustrine (sediments deposited into lakes by glacial meltwater), but it appears the lake bed muds are regularly interrupted by a number of thin sand beds. The rhythmic sands represent outburst floods invading a proglacial lake. Nearly identical flood rhythmites (turbidietes) are described to the west in the Upper Columbia River Valley (Glacial Lake Columbia or modern Lake Roosevelt) by Atwater (1986) and Hanson and Clague (2016).
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 what you see in the photo above. Several large rip-up clasts each up to a meter in length and composed of unconsolidated rippled, oxidized sand are surrounded by massive (deformed) green-gray mudstone. Flat-lying ripple sands occur below the deformed zone. Flat-lying lacustrine muds above. Lots of soft sed def between. Spectacular, but never reported in any article or guidebook. One hour's time. Peninsula Road. #fieldgeology
Field sketch of the excavation shown in photo above. Deformation appears to be more prevalent 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, including large flame structures, load casts, rip-up clasts, occur in association with sand bed deposition. Deformation is always bounded above and below by flat-lying lacustrine bedding. Interpretation: Glacial Priest Lake strata were periodically interrupted by Missoula Floods. Flat beds indicate resumption of lake bottom deposition after flooding ceased. 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 across the valley from Hwy 57, but accessed by hiking downhill from Peninsula Rd pullouts. Flat-lying beds occur 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 here and about 10 more in exposures above this one (varves + sand bed = one interval). At an even higher location, I counted 6 varves-sand bed pairs in a quick 3m-high excavation that lacked SSD altogether. So, around 30 floodbeds are exposed along Peninsula Road.
In many places (textbooks for example), this conspicuously deformed horizon would be called a "seismite". Hair would burst into flames and there would be much running about, "Call the newspaper! Call my editor! Tweet this out! The Big One is coming!". Here (and perhaps elsewhere) such an interpretation would be presumptuous. Soft sed def is the norm in glaciolacustrine sequences near mountain topography and megaflood channels. Sometimes such deformation signifies a large-magnitude earthquake. Many times, however, its just boring old gravity doing its thing. #gobacktoseattle #sadfaceallthewayhome
Meter-scale folds in lacustrine muds. Interpretation: Local landslide at the dynamic margin of the ice sheet, possibly in response to erosion by floods. Probably not an earthquake.
An excellent 1924 photo taken by by O.P. Jenkins of Pleistocene-age clastic dikes intruding rippled glaciofluvial/glaciolacustrine sediments of Glacial Lake Pend Orielle, which filled the southern end of the Purcell Trench during the last Ice Age. The photo's caption reads, "Metaline District, differently weathered sand dikes, with limey concentrations around roots. Sand of old Pend Oreille Lake in roadcut several hundred feet above Metaline Falls, but not at top of bench". The town of Metaline Falls, WA is located 80km NW of Priest River. Photo source: Washington Geological Survey Photo Archives (#554).
Another O.P. Jenkins photo of the same outcrop with caption, "Metaline District, sand dikes in ripple structure, sand of Old Pend Orielle Lake". Photo source: Washington Geological Survey Photo Archives (#553).
O.P. Jenkins photo of the same outcrop as previous photo with caption, "On road uphill from town of Metaline Falls toward the road which turns off for Sullivan Lake. Ripple marks in sand of Old Lake Pend Orielle-6 inch ruler in picture". Photo source: Washington Geological Survey Photo Archives (#551).
A map by Breckenridge (1989, Figure 4) identifies a "rhythmite section" (triangle symbol) northeast of Priest River, ID which is probably one along 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).
Atwater, B.F., 1986, Pleistocene Glacial-Lake Deposits of the Sanpoil River Valley, Northeastern Washington, U.S. Geological Survey Bulletin 1661
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
Hanson, M.A.; Clague, J.J., 2016, Record of glacial Lake Missoulafloods in glacial Lake Columbia, Washington, Quaternary Science Reviews, 133, p. 62-76
Savage, C.N., 1965, Geologic history of Pend Oreille Lake region in north Idaho, Idaho Bureau of Mines and Geology Pamphet 134, 18 pgs. + figures.