THE PARDEE -- CHAMBERLIN ERRATIC: A QUATERNARY ENIGMA

A ±15 kg glacially polished and transported erratic was recovered from the surface of a high shoreline beach ridge of glacial Lake Missoula in Missoula, Montana, USA. The boulder surface has compression features and glacial meltwater polish typical of englacial erratics with a long or multi-cycle transport history. It may have been carried by the primary ice stream in the Rocky Mountain Trench from eastern British Columbia into northern Idaho or northwestern Montana to calve off in a large enough iceberg to be transported by wind 100 or more km southeastward to be stranded on a high ice-dammed lake shoreline.

The lithology of the erratic appears to resemble impact regolith breccias that are believed to be of lunar origin. The matrix of the breccia is a fine-grained red quartz and plagioclase(?) arkosic metasilt and the sharply angular matrix-supported breccia clasts are white and dark grey quartzite(?). No sedimentary structure is evident. No petrographic or chemical analysis has been done. The full remarkable transport story is yet to be deduced from the boulder’s surface features. All clasts appear highly shocked. Oxidation and/or reduction patterns similar to some Mid-Protrozoic Belt Supergroup quartzites that exist in the possible glacial source areas would even permit origin hypotheses that allowed remobilization of a meteorite or adjacent deposits that fell during Precambrian time. The matrix-support of the breccia resembles a fall-back impact deposit like those at Sudbury, Ontario and like those classed as lunar impact deposits.

Enough meteorite fragments have been collected in Antarctica and the Arabian Peninsula in the last 20 years to establish a group of characteristics that identify lunar impact ejectia. Although this erratic was found in the 1970’s, it has been necessary to wait until today to present this object for peer criticism and review. Glacial or ice-rafted transport is certain. The rest of the story is yet to be deduced using richly creative multiple working hypotheses. This poster session presents the erratic itself and its geologic context. It is named in honor of Joseph Pardee who was willing to present himself to peer ridicule with the hypothesis of a giant ice-dammed lake, and to T. C. Chamberlin who presented Quaternary and planetary scientists with tools for evaluating controversial theories.