ABSTRACT

A carbon-banded siltite interval within the Precambrian Prichard and Aldridge Formations of the Belt-Purcell Supergroup correlates across a 3,000 square mile area from northern Idaho and Montana into southeastern British Columbia. The banded interval consists of a 13-foot sequence of light and dark gray bands 1 mm to 2 cm thick which can be matched, band-for-band, from outcrop to outcrop throughout the entire area. A "type" section of the banded interval was cored and photographed to scale. The photo of a distinctive six-inch sequence of bands was then matched to outcrops of the banded interval from one location to another.

Adjacent light and dark bands of the marker interval both consist of uniform, well-sorted quartz and feldspar silt, with disseminated biotite, muscovite, and pyrrhotite. Dark bands differ from the light ones in that they contain amorphous carbon particles and clusters of monazite grains.

The uppermost 6,000 feet of the Prichard and Aldridge Formations can be subdivided into five informal units A, B, C, D, and E. Units A and D are thick, uniform turbidite quartzites, B is interlaminated quartzite and siltite, and C is even-bedded carbonaceous siltite. Within the lower part of unit C is the 13-foot banded sequence that can be correlated on a band-for-band scale. The uppermost unit E, is cross-bedded and rippled argillite.

The banded interval and the rest of unit C probably represent subaqueous deposition below wave base in a stagnant basin. Fluctuating widespread surface "blooms" of primitive planktonic algae may have formed the carbon and monazite-rich bands of the marker interval. The well-sorted silt matrix was perhaps derived from dust storms settling out over the Precambrian basin.