Geologic history of Latah County, Idaho

Latah County is located on the eastern edge of the Columbia River Plateau, near the western slope of the Rocky Mountains. Major geologic units consist of Precambrian metasediments and metamorphics of the Belt Supergroup, Cretaceous-Tertiary intrusions and metamorphics of the Idaho Batholith, Miocene volcanics and sediments of the Columbia River Basalt Group and Pleistocene loess of the Palouse Formation. Lesser units include Cretaceous syenite near Potlatch, Eocene rhyolite and breccias generally restricted to the Deary-Bovill area, and sedimentary interbeds associated with the Columbia River Basalt Group in the Troy-Bovill area.

Precambrian rocks of the Belt Supergroup generally exhibit features associated with low-grade regional metamorphism, and consist mostly of quartzite, argillite, and siltite. Outcrops contain relict cross-beds, ripple marks, bedding laminations, and other sedimentary features. However, at numerous localities, the Belt rocks are metamorphosed to phyllite, schist, and gneiss and the sedimentary features are destroyed. These Precambrian units, along with Mesozoic intrusions, form most of the topographic highs in the Moscow area. Exceptions could be Kamiak and associated buttes. Kamiak Butte consists of coarse-grained, recrystallized quartzite. Savage (1973) considered the Kamiak quartzite to be a Precambrian, coarse-grained equivalent of Belt Supergroup rocks. However, Hooper and Webster (1982) tentatively correlated the quartzite in the Moscow-Pullman area with similar Cambrian quartzite of northeast Washington.

Intrusive rocks in the Moscow area belong primarily to the Cretaceous-Tertiary aged Idaho Batholith and commonly range in composition from quartz monzonite to granite and quartz diorite. Local outcrops of tonalite, gabbro, and syenite are common. The Palouse Range, which rims Moscow to the north and northeast, consists of undifferentiated Idaho Batholith rocks. Bald Butte, three miles southwest of Moscow, consists of quartz-rich tonalite devoid of alkali feldspar (Hooper and Webster, 1982).

Accompanying and following emplacement of the Idaho Batholith was a period of extensive erosion. Near Moscow, the pre-basalt relief was considerable, whereas to the west the relief was less and a broad, flat plain may have existed. The lower areas were filled by the basalt that erupted from vents during an 11 million year period in the Miocene between 17 million and 6 million years ago (Hooper, 1982). These flows belong to the Columbia River Basalt Group which filled local canyons and flowed primarily westward into a rapidly subsiding basin that existed over the present Tri-Cities area. Along the edges of the plateau, lava dams formed across canyons; lake sedimentation occurred, and preservation of plant fossils was common. The fossils indicate that the climate was slightly warmer and more humid during the extrusion of the lava flows.

Beneath Moscow, the Columbia River Basalt Group is approximately 1,300 feet thick and represents a filled paleo-valley between the Palouse Range to the north and Paradise Ridge to the south. In general, the rocks beneath Moscow consist of three thick sediment sequences alternating with the basalt flows.

At least 95 percent of the enormous volume of Columbia River Basalt accumulated in the first 3.5 million years (Hooper, 1982), and after this eruptive event the major drainages of the nearby Snake and Clearwater began to re-establish themselves. In addition to influence from continuing sporadic volcanism, the river courses were also influenced by deformation. In the Moscow area, this deformation is best expressed along the Snake and Clearwater Rivers in the Lewiston vicinity, where two major east-west structures dominate. One structure, the Lewiston Basin, is approximately 21 miles long; and its axis is located just south of Lewiston and Clarkston, paralleling the Lewiston Basin in a complex, asymmetric and faulted east-west anticline, referred to as the Lewiston structure and was mapped in detail by Camp (1976). The basalts directly beneath Moscow are considered to be primarily horizontal with a slight westerly dip.

Pleistocene loess of the Palouse Formation mantles the bedrock in the Moscow area. The sources for this wind-blown soil have been debated a great deal in recent years.

Approximately 6,700 years ago, Mt. Mazama, one of the Cascade volcanoes, erupted; and ash was deposited over much of the northwest. The center of the volcano was Crater Lake, Oregon, and the ash that was deposited is referred to as the Mazama Ash. There is evidence of earlier ash deposits in this area associated with other explosive Cascade volcanoes, but none are as extensive as the Mazama Ash. Most recently, Mt. St. Helens, in 1980, was responsible for the deposition of a very thin layer of ash in the Moscow area.