Region 5: The Basin and Range

The mountain ranges of southeastern Idaho contain thick sections of early Paleozoic marine sedimentary rocks, mostly sandstones and limestones, deposited during the Cambrian and Ordovician periods. Early Cambrian rocks (variously assigned to the Brigham, Camelback Mountain, and Gibson Jack formations) contain Skolithos and other Cambrian trace fossils (see Figure 3.14), as well as occasional trilobites. Sponge-like archaeocyathids are present in shale and limestone formations. The Ordovician Swan Peak Quartzite contains abundant large trace fossils, as well as trilobites and brachiopods.

Archaeocyathids

Archaeocyathids were the first important animal reef builders, originating in the early Cambrian. These vase- shaped organisms had carbonate skeletons and are generally believed to be sponges. They went extinct in the late Cambrian, but were very diverse. Archeocyathids are often easiest to recognize in limestones by their distinctive cross-sections.

Archaeocyathids are found in lower Cambrian rocks in northern California and southern Oregon. Their vase-shaped calcite skeletons commonly reached lengths of 5 to 20 centimeters (2 to 8 inches).

Archaeocyathids are found in lower Cambrian rocks in northern California and southern Oregon. Their vase-shaped calcite skeletons commonly reached lengths of 5 to 20 centimeters (2 to 8 inches).

The most abundant Paleozoic rocks in this region are early Carboniferous (Mississippian) marine sediments consisting mostly of carbonates, but also some sandstones and shales. Mississippian limestones (known variously as the Madison, Mission Canyon, or Lodgepole formations) contain abundant horn corals, tabulate corals, and spiriferid brachiopods (Figure 3.61), and represent a warm, clear-water carbonate environment. The late Carboniferous (Pennsylvanian) is represented by the same marine sediments, but both the variety and quantity of fossils are inferior to those found in Mississippian strata. Nonetheless, fossil algae, foraminifera, bryozoa, brachiopods, crinoids, and corals have been reported from sediments of this age.

Figure 3.61: Paleozoic brachiopods of southeastern Idaho. A) <em class='sp'>Macropotamorhyncus insolitus</em>. B) <em class='sp'>Prospira albapinensis</em>. Both about 2 - 4 centimeters (1 - 1.5 inches) wide.

Figure 3.61: Paleozoic brachiopods of southeastern Idaho. A) Macropotamorhyncus insolitus. B) Prospira albapinensis. Both about 2 - 4 centimeters (1 - 1.5 inches) wide.

Southeastern Idaho also contains a thick sequence of early Triassic marine strata, and several horizons (especially the Thaynes and Dinwoody formations) contain biostratigraphically significant ammonoids. Overlying Jurassic rocks, especially the Twin Creek Limestone, contain well-preserved ammonoids and bivalves.

See Chapter 6: Glaciers to learn more about glacial lakes that formed during the last ice age.

Sand and gravel deposits in southeastern Idaho that were laid down in association with glacial Lake Bonneville have yielded fossils of Pleistocene bison (see Figure 3.47), camels, muskoxen, and horses.