GEOLOGIC MAPPING AND STRATIGRAPHIC CORRELATION OF NEOPROTEROZOIC TO LOWER ORDOVICIAN STRATA, CENTRAL LOST RIVER AND SOUTHERN LEMHI RANGES, EAST-CENTRAL IDAHO

In southeastern Idaho, thick quartzite and overlying carbonate are Neoproterozoic and lower Paleozoic in age and record rifting of Rodinia. In contrast, in east-central Idaho, correlative rocks are absent where Middle Ordovician Kinnikinic Quartzite lies on tilted Mesoproterozoic Belt Supergroup along the Lemhi Arch unconformity. Preliminary work on the Wilbert, Tyler Peak, and Summerhouse formations suggested that these rocks may define the southern and western margins of the Lemhi arch in the Lemhi and Lost River ranges; however, the ages and regional stratigraphic context of these rocks are poorly known.

Based upon prior reconnaissance mapping, we identified several localities within the Lost River Range for field mapping, measurement of continuous stratigraphic sections, and sampling for detrital zircons (DZ). This included field mapping west of Leatherman Peak and measurement and DZ sampling at Elkhorn Creek and Merriam Lake. Integrating field and DZ results from several localities results in a regional consistency in stratigraphy and DZ provenance and indicates a Neoproterozoic to Ordovician age designation (upper part of the Sauk sequence). At the base of the studied section, the 120-950 m thick Wilbert Formation contains a 1.0-1.2 Ga detrital zircon population derived from the Grenville province, and a 680-650 Ma population likely derived from the rift related Big Creek plutonic belt. Above the base of the 150-260 m thick Tyler Peak Formation, the Grenville age DZ population disappears, and a dominant 1780 Ma population indicates a (proximal?) Paleoproterozoic magmatic provenance. The overlying lower Summerhouse Formation contains a 500-480 Ma population (likely derived from the Beaverhead plutonic suite); the 1780 Ma population is absent.

Sedimentary structures and trace fossils indicate that the siliciclastic Wilbert and Tyler Peak formations are dominantly marine, however rip-up clasts and scoured surfaces suggest that some intervals within the Wilbert are fluvial. The Upper Cambrian and Ordovician Summerhouse Formation, deposited in a marine environment, is primarily siliciclastic and usually calcareous with interbeds of dolomite. The Middle Ordovician Kinnikinic Quartzite unconformably overlies the Summerhouse Formation and is the base of the Tippecanoe Sequence.