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Abstract: Sequence Stratigraphic Analysis of the Mississippian Copper Basin Formation, South-Central Idaho

Paul Karl Link, Ian Warren

The Mississippian Copper Basin Formation in the Pioneer Mountains, south-central Idaho, demonstrates tectonically driven 3rd order stratigraphic sequences.

The 4200 m lower Copper Basin Formation (late Kinderhookian) contains at least two siliciclastic fan systems, (Little Copper and Scorpion Mountain-Muldoon Canyon members). These lowstand/transgressive systems tract strata are separated by highstand calciclastic turbidites and proximal carbonate platform debris of the Drummond Mine Limestone Member. Rapid Kinderhookian subsidence of the Roberts Mountain tectonic phase was caused by latest Devonian thrust(?) loading to the west and included local normal faulting. Subsidence outpaced eustatic changes.

In contrast, the less pronounced Late Mississippian Christina Peak tectonic phase produced a basin influenced by eustatic changes. A low-angle disconformity (type 1 sequence boundary, 3rd order), possibly spanning much of Osagean and Meramecian time, separates middle to outer fan Muldoon Canyon member turbidites from overlying lowstand wedge proximal fan-delta conglomerates of the Brockie Lake member (1100+ m).

Above the Brockie Lake conglomerate, a transgressive surface marks the base of a transgressive systems tract characterized by mudrocks with Nereites ichnofacies and conglomerate-filled channels of the lower Iron Bog Creek member (200 m). They formed on a sub wave-base prodelta during late Meramacian to early Chesterian relative sea level rise. Channels may represent higher order sea level falls, or may be part of an autocyclic shifting slope-fan system. The highstand systems tract is marked by prograding storm-influenced lower shoreface sandstones and mudrocks of the upper Iron Bog Creek member (175 m). These strata contain Cruziana ichnofacies. The sands may reflect several 4th or 5th order sea level falls.

The uppermost Iron Bog Creek member (75+ m) contains highstand carbonaceous shale and fore-bank bioclastic packstone overlain abruptly by lowstand boulder conglomerate. This sequence boundary is perhaps related to the latest Chesterian 2nd order eustatic sea level fall.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada