Flood and Slide Deformed Deposits at Priest River, ID

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 (turbidites) 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).

Discovery in progress.

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. Paved road. Boots, hoe, brush. #fieldgeology

Field sketch of the excavation shown in photo above. Spectacular, but not previously reported in any article or guidebook. The deformation appears to be slide related. Deformation is more prevalent lower in the section, where the sand beds are thicker and the overburden is greater. Excavated exposure is 250cm wide x 160cm high.

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.

Rip-up clasts of mudstone in sand.

Varved lake sediments are repeatedly 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 and significant soft sediment deformation produced by floods flowing out of western Montana.

A varve-sand interval.

Richard Waitt investigated deposits in the Priest River Valley (Waitt, 1984), noting "16 or more" flood beds separated by varved intervals that correlate to similar lake-flood successions preserved along Latah Creek near Spokane, in the Sanpoil Valley, and along the Upper Columbia River upstream (east) of Grand Coulee,

Glacial Priest Lake, dammed in the Priest River valley by a tongue of the Purcell trench lobe of the Cordilleran ice sheet, generally accumulated varved mud; the varved mud is sharply interrupted by 14 sand beds deposited by upvalley-running currents. The sand beds are texturally and structurally similar to slackwater sediment in valleys in southern Washington that were backflooded by outbursts from glacial Lake Missoula. Beds of varved mud also accumulated in glacial Lake Spokane (or Columbia?) in Latah Creek valley and elsewhere in northeastern Washington; the mud beds were disrupted, in places violently, during emplacement of each of 16 or more thick flood-gravel beds. This history corroborates evidence from southern Washington that only one graded bed is deposited per flood, refuting a conventional idea that many beds accumulated per flood. The total number of such floodlaid beds in stratigraphic succession near Spokane is at least 28. The mud beds between most of the floodlaid beds in these valleys each consist of between 20 and 55 silt-to-clay varves. Lacustrine environments in northern Idaho and Washington therefore persisted for two to six decades between regularly recurring, colossal floods from glacial Lake Missoula.

E.H. Walker's outcrop near Priest River, ID (Walker, 1967). The caption for his Figure 2 reads, "Glacial-lake in the Priest River Valley, 3 miles north of the town of Priest River, Idaho. The dark layers are wet beds of sand and the light layers are mainly varved clay coated with dry overwash."

Richard Waitt revisited Walker's outcrop (Waitt, 1985). The caption for his Figure 13 reads, "Beds of varved clay (dark bands) alternating with flood-laid beds of sand (light colored) in Priest River valley, northern Idaho. Eight flood-laid beds are visible, but entire section exposes 14 such beds. Details of section discussed by E.H. Walker (1967) and Waitt (1984)."

West Branch Priest River Outcrop. Spectacular folds in glaciolacustrine muds deformed by landsliding. Exposure is along West Branch Priest River, visible through openings in the trees across the valley from Hwy 57, but accessible from Peninsula Rd by hiking for 10 minutes downhill from large pullouts (private land, ask permission, cross a short wall and a couple barb wire fences). Flat-lying lacustrine beds occur above and below the 3m-thick deformed zone shown in the photo above. Exposure not reported previously.

Another look at the "rhythmite section" along West Branch Priest River (stratigraphically lower than Walker's outcrop) that may be the same indicated by Breckenridge (1989, Fig. 4). I counted about a dozen repeated intervals here and about 10 more in exposures above this one (varves + sand bed = one interval). At what is possibly an even higher location, I counted 6 varve-sand intervals in a 3m-high excavation that lacked SSD altogether. So, around 30(?) lake-flood rhythmites are exposed along Peninsula Road, about twice as many found by Waitt (1984).

West Branch Priest River Outcrop. In many places (textbooks for example), this conspicuously deformed horizon would be called a "seismite". Upon seeing this deformation, paleoseismologists liberated from their offices, would observe their hair catching fire and there would be much running about, "Call the newspaper! Call my agent! Tweet this out! The Big One is coming! Where's my phone? I need a latte." However, here (and elsewhere) that interpretation would be presumptuous. Soft sed def is the norm in sediments deposited along the former ice margin - in glaciolacustrine sequences deposited in mountainous topography, in kame terrace country, and along megaflood channels. Sometimes this kind of deformation signifies a large-magnitude earthquake. Other times, its just Mean Old Mr. Gravity doing his thing.

#gobacktoseattle #sadfaceallthewayhome

West Branch Priest River Outcrop. Meter-scale folds in lacustrine muds. Interpretation: Local landslide at the dynamic margin of the former ice sheet, possibly in response to erosional cutting by floods. Probably not an earthquake.

Metaline Falls Outcrop. An excellent 1924 photo taken by by O.P. Jenkins (USGS) of Pleistocene-age clastic dikes intruding rippled glaciofluvial/glaciolacustrine sediments of Glacial Lake Pend Orielle at Metaline Falls, WA some 80km NW of Priest River, ID. The lake used to occupy 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 [CaCO3 cement] around roots. Sand of old Pend Oreille Lake in roadcut several hundred feet above Metaline Falls, but not at top of bench". Photo source: Washington Geological Survey Photo Archives (#554).

Metaline Falls Outcrop. Another O.P. Jenkins/USGS 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).

Metaline Falls Outcrop. 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).

Its easy to lose track of what's come before in the Missoula floods game, especially when a lot of the older information was captured in paper field guides and other semi-formal publications. I include these maps for that reason.

Locate the town of Priest River, ID on the map above. Roy Breckenridge drew it (Breckenridge, 1989, Figure 4). He identifies a "rhythmite section" (triangle symbol) northeast of Priest River, ID which is probably along Peninsula Road or the West Branch. Its unclear whether he ever formally described this section. I've not found anything yet and I don't believe he published much on the area except for the 2019 map. 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).

Excerpt of Richard Waitt's map of the Priest Lake area from an FOP field guide (Waitt, 1983, Figure 3; Waitt 1985, Figure 13). His site #1 from 1985, the same as Walker's site on the Peninsula Rd, is circled in red.


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.

Waitt, R.B., 1983, Tens of successive, colossal Missoula floods at north and east margins of Channeled Scabland, Friends of the Pleistocene Rocky Mountain Cell field guide/USGS Open-file Report 83-671, 29 pgs.

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