Abstract

The Mississippian Copper Basin Formation of south-central Idaho represents a submarine fan complex associated with the Late Devonian to Early Mississippian Antler orogeny. Several newly discovered stratigraphic relations suggest that the formation accumulated in a hybrid loaded and faulted basin, rather than a simple foreland basin with a western load produced by an Antler thrust plate.

Contrary to previous descriptions, a major thrust fault does not duplicate the Copper Basin Formation. Abrupt along-strike and up-section changes in lithology, previously mapped as segments of the regionally extensive Mesozoic Glide Mountain thrust fault, are reinterpreted to be locally sheared stratigraphic contacts. Some segments of the Glide Mountain thrust are remapped as Eocene gently to moderately dipping normal faults with top to northwest displacement, and some are Mesozoic thrust faults with less than 1 km of displacement.

Several source terranes for the Copper Basin Formation (reworked cratonal, chert-rich outer continental margin, and distal carbonate bank) are suggested by siliciclastic and calciclastic conglomeratic turbidites with distinct provenance signatures. This, along with observed on-strike facies changes, indicates that the Copper Basin Formation accumulated in a small, tectonically active basin fed by several sediment transport systems. Basin bottom relief (possibly caused by syndepositional faulting) may have led to intrabasinal sediment reworking.

The Lower Mississippian (upper Kinderhookian) part of the Copper Basin Formation contains up to 5180 m (decompacted thickness) of mainly sub-wave base turbidites (Little Copper Member through member of Muldoon Canyon) that accumulated in 5 m.y. This suggests a sedimentation rate of 1036 m/m.y. and a tectonic subsidence rate of 300 m/m.y. These are higher than rates of most foreland basins, but comparable with observed rates of pull-apart and transtensional basins.

The lower part of the Copper Basin Formation thins northward along depositional strike from 5180 m to 3500 m (decompacted thicknesses) in 30 km, and eastward across strike from 5180 m to 117 m in 68 km (palinspastically restored). Such basin parameters for a simple loaded basin would require crustal rigidity between 10²² and 10²³ Nm depending on the shape and size of the load; expected crustal rigidity values for the subjacent stable Early Proterozoic continental crust range between 10²³ and 10²⁴ Nm.

Two possible models that incorporate previously documented late Paleozoic compressional and extensional structures in the Pioneer Mountains are proposed for the origin of the Copper Basin Formation trough:

1) A sinistral transpressional/transtensional orogenic system may have caused syn-depositional late Kinderhookian faulting and abrupt basin subsidence;

2) Local normal faults may have been superimposed onto the regionally thrust-loaded Copper Basin Formation trough.