NEW COSMOGENIC 10BE SURFACE EXPOSURE AGES FOR THE PURCELL TRENCH LOBE OF THE CORDILLERAN ICE SHEET IN IDAHO

During the late Pleistocene, the Purcell Trench lobe (PTL) of the Cordilleran Ice Sheet dammed the Clark Fork River, creating glacial Lake Missoula. Repeated failure of the Clark Fork ice dam produced multiple catastrophic floods. Timing of the flood events has been based largely on dating of distal flood deposits correlated with glacial Lake Missoula. Relatively little attention has been given to the history of the Clark Fork ice dam itself. Here we present new cosmogenic ¹⁰Be exposure ages that constrain the age of the PTL at a near maximal position in the area of the ice dam.

The ages are from Happy Fork Gap on a prominent lateral moraine on the southern flanks of the Selkirk Mountains about 12 km west of Sandpoint, Idaho. The moraine was deposited by the PTL at an elevation of 1020 to 1370 m as it moved south into the Lake Pend Oreille basin and west down the Pend Oreille River valley. A contiguous tongue of ice crossed through Happy Fork Gap into the Priest River valley and formed the Blue Lake end moraine (Richmond, 1986, Quat Sci Rev 5, 135-137). Many researchers have considered this ice extent as a near last glacial maximum.

We chose boulders on the highest recognizable preserved moraine of the PTL. Minimum heights of sampled boulders range from 1.3 to 3.3 m. Samples were sawn from top-center of boulders with excellent control of sample thickness. Samples were analyzed at PRIME Lab and ages computed with CRONUS exposure age calculator v. 2.2 with St scaling and standard 07KNSTD. Ages are corrected for a blank averaging 3.70±0.14 x 10^{4 10}Be atoms. No corrections were made for erosion or snow/soil cover. Exposure ages have an error weighted mean of 14.1 +/- 0.6 ka.

Our ages for a last major position of the PTL in the ice dam area are similar to those for young flood deposits in the Channeled Scabland of Washington and Oregon. They also provide additional evidence for stratigraphic correlation of proximal glacial Lake Missoula flood deposits in Idaho with distal sections in the Channeled Scabland.

Sample	LAT	LONG	Elev (m)	10Be (at/g)	Error (%)	Age (ka)	Error (ka)
06CO1	48.30981	-116.704	1372	187164	2.9	13.37	1.19
06CO2	48.30978	-116.721	1295	196700	2.3	14.87	1.30
06CO3	48.30645	-116.717	1216	171778	2.9	13.83	1.24
06CO4	48.29911	-116.762	1052	161537	5.0	14.88	1.47
06CO5	48.30302	-116.753	1021	146500	5.0	14.03	1.38