SEDIMENT ANALYSIS OF A LARGE, ARCUATE-SHAPED, SAND BAR DEPOSITED IN THE NORTHERN CHENEY-PALOUSE SCABLAND TRACT BY A CATASTROPHIC GLACIAL OUTBURST FLOOD APPROXIMATELY 13,000 YEARS AGO

A late Wisconsin glacial outburst flood deposited a large tractive bar east of Deep Creek within the northern terminus of the Cheney-Palouse scabland tract to the south of the Spokane River drainage basin. This arcuate-shaped deposit composed primarily of sand is herein named the Deep Creek Bar (DCB) and covers about 35 km² of the West Plains of Spokane County in and around Fairchild Air Force Base. Outcrop mapping, supplemented by well log descriptions, shows that the sand bar reaches 50m in thickness where it filled in a portion of the pre-flood Deep Creek drainage basin.

Three kms west of Fairchild is a 20 meter thick exposure in a sand pit at the leading edge of DCB which gives a three-dimensional view of a delta-like sequence composed of rhythmically bedded, west dipping foresets underlain and overlain by subhorizontal beds. Giant current ripples formed on the thinner portion of the DCB, mostly on the inner east side. No tephra layers were seen within the DCB, but water laid Mount St. Helens set-S tephra layers (12.9 KBP) occur in a thin gravel layer overlying basalt just a few kms to the west of the advancing sand deposit. The Mazama tephra (7.7KBP) layer and a few meters of burrowed eolian soil overlying the DCB are exposed in an abandoned sand pit two kms north of Fairchild AFB.

Lab analysis, coupled with field observations of the sediment including loess clasts and small granitic fragments found within the rhythmites of the DCB, show that the sand and gravel was locally derived. No evidence of subaerial exposure was seen within the sand bar deposit between the basalt floor surface and the overlying Mazama tephra layer, suggesting that DCB was formed during one or more pulses of a single southwest directed glacial outburst flood event. The giant current ripple field developed on the east side of the DCB is aligned in a northwest direction and was formed by the flood backwash returning to the Spokane River basin. The lowest elevation of the sand bar is at 700 m and the highest erosional flood level on a nearby butte is at 800 m, suggesting that the flood waters at their height reached 100 m locally.