Page 13

Rocks of the Eastern Idaho Mountains:

Late Proterozoic Sedimentary Rocks
The oldest rocks in the southeast Idaho thrust belt belong to the Pocatello Formation (750 to 700 million years old), which is exposed in the Bannock Range from the Pocatello area south through Oxford Mountain and almost to the Utah border. The Pocatello Formation contains two unique rock types, metamorphosed basaltic lava or greenstone, and dark-brown or gray rock known as diamictite, which contains clasts of older rocks, ranging from pebbles to boulders in size, floating in a matrix of sand and clay. Some of this diamictite was deposited as glacial-marine till.

Fossil of intact head of colonial coral Favosites, Permian Snaky Canyon Formation, east side of Lost River Range, (July,1990).
The Brigham Group overlies the Pocatello Formation, and contains mainly sandstone or quartzite. The Brigham Group is exposed in much of the Portneuf, Bannock, and Bear River Ranges. The Wilbert Formation in the southern Beaverhead and Lemhi Ranges is correlative. The sediments which now make up the Brigham Group were deposited from about 700 to about 530 million years ago, during the time when complex life forms were developing. However, the sandstones of the Brigham Group were inhospitable places for latest Proterozoic and early Cambrian life to leave fossilized remains, and so the Brigham is largely devoid of fossils. The Brigham (named for Brigham City, which was named for, of course, Brigham Young) was deposited in shallow oceans and wide alluvial plains near the shorelines of those oceans.

Some of the micaceous quartzite or "Oakley stone" found on the flanks of the Albion Range south of Burley is probably correlative to parts of the Brigham Group. The distinctive bright green Elba Quartzite, now quarried at Park Valley, Utah, is older, of Middle Proterozoic age. It has been metamorphosed so that the chromium rich clay minerals have recrystallized to green, shiny plates of mica.

(above) Diamictite, or pebbly mudstone, of the Scout Mountain Member of the Pocatello Formation north of Portneuf Narrows. Note quartzite boulder above head of hammer. These rocks were deposited by glacial marine ice sheets about 750 million years ago when North America was attached to Antarctica or Australia, and Idaho was apparently in a low latitude, (July, 1979).
Overturned syncline in Cambrian and Late Proterozoic Camelback Mountain Quartzite, Brigham Group, in Mill Creek, south of North Putnam Mountain, Fort Hall Indian Reservation, (September, 1986).

Paleozoic Limestones and Sandstones
A thick sequence of carbonate rocks (limestone and dolomite), of early Paleozoic age (530 to 300 million years) lies above the Brigham Group and is present in most of the mountains of eastern Idaho. These rocks were deposited on broad carbonate platforms in warm seas off the western edge of the paleo-North American continent, settings not dissimilar to today's Bahama Banks or Great Barrier Reef. In many places the Paleozoic limestones contain invertebrate fossils, including trilobites, bryozoans, brachiopods, corals, and molluscs, which allow determination of their age and the precise conditions under which they were deposited.

Surface streams are rare in the limestone country. The limestone cliffs are highly porous and most of the water sinks into the subsurface. In places extensive cavern systems exist, as at Minnetonka Cave west of St. Charles. Large springs form where the subsurface water (groundwater) intersects the land surface. Such springs supply important parts of the municipal water supplies of southeast Idaho. They are easily contaminated and their purity must be guarded carefully.

Thin units of fine-grained sandstones or quartzite are present within this stack of Paleozoic limestones in the Swan Peak Quartzite of Ordovician age and the Wells Formation of Pennsylvanian age. These sandstones represent times when wedges of quartz sand, eroded from distant mountain ranges, prograded over the carbonate platforms.

Also present in some limestone units are gray and black chert beds and nodules, which weather to distinctive pebbles of hard flint. This chert formed from dissolved skeletons of silica-secreting invertebrates, which also existed in the warm shallow seas.

Some of the limestone has been altered to the crystalline carbonate rock dolomite. In this process original fossils are generally destroyed and the result is a light or dark gray rock that preserves bedding structures but little of the fine detail found in limestone.

Mesozoic Continental Strata
Rocks of Mesozoic age in the southeast Idaho thrust belt include thick limestones of Triassic and Jurassic age and even thicker sandstones and shales of Cretaceous age. They were deposited before and during the thrust deformation, in basins which subsided because they were loaded down by the weight of advancing thrust sheets. These rocks are exposed on Highway 89 east of Montpelier, and on the Tincup highway east of Wayan, but the most spectacular place to see Mesozoic rocks is Raymond Canyon, Wyoming, just south and east of Geneva, Idaho.

Soft Cenozoic Rocks in the Valleys
The rocks that partially fill the valleys of southeast Idaho differ in fundamental ways from those that underlie the mountains. First, the valley fill is predominantly young, less than 10 million years old, and contains the imprint of volcanic activity associated with the Snake River Plain. There are large quantities of tuff, a distinctive white sedimentary rock made of collapsed shards of glass and derived from clouds of ash given off by volcanic eruptions on the Snake River Plain. Other parts of the valley fill are conglomerate, often with a red matrix. These coarse-grained rocks were carried by streams from adjacent mountains and deposited as the valleys were lowered along range-bounding faults. Lacustrine limestones also make up parts of the valley fill.