Abstract

Strata of the Brigham Group in the Paris-Putnam plate of the southeastern Idaho thrust belt span the Late Proterozoic-Cambrian boundary and consist of quartzose sandstone with subordinate pebble conglomerate and siltstone. The Brigham Group is overlain by fossiliferous Cambrian carbonate units that represent the transition from siliciclastic to carbonate deposition in the Cordilleran miogeocline. The top of the Brigham is placed at different stratigraphic horizons depending on the presence of mappable Cambrian carbonate.

The Brigham Group contains four stratigraphic sequences bounded by regional disconformities. The lowest sequence (Papoose Creek Formation and most of the overlying Caddy Canyon Quartzite) is dominantly marine, which inferred tidal and shoreface deposits at the base, and open marine strata in the middle. The upper part of this sequence contains shoreface and braided fluvial strata.

The first sequence is overlain disconformably by offshore subwave base marine strata of the upper Caddy Canyon Quartzite and Inkom Formation. This second sequence is entirely marine and is composed dominantly of siltstone. This siltstone is cut by inferred submarine channels filled with conglomerates composed both of extrabasinal quartzite pebbles and intrabasinal siltstone rip-up clasts.

The third sequence comprises the Mutual Formation, an entirely braided fluvial and lacustrine unit.

The fourth sequence locally overlies the Mutual Formation with erosional unconformity and consists of dominantly marine strata of the Camelback Mountain Quartzite, Gibson Jack Formation, Windy Pass Argillite, and Sedgwick Peak Quartzite. The base of this sequence contains channels up to 50 m (164 ft) thick cut into the underlying Mutual Formation and filled with chaotic conglomerates. These are inferred to be the deposits of submarine debris flows during regional transgression.

This preliminary synthesis suggests the Brigham is a post-rift deposit, as indicated by regional persistence of facies, paleocurrents, and quartzose petrology.