Geologic History of the Main Salmon River, Idaho
Compiled by Tim Gunderson
The geologic history of the Main Salmon began at least 1.4 billion years ago and still continues today. New landslides, side stream blowouts, and rapids formation continues to change the character of the river corridor, yet metamorphic rock billions of years old still exists. Geology involves numerous processes, many of which are exhibited in the Main Salmon Canyon.
During Precambrian and Paleozoic time shallow seas were advancing and retreating over Idaho, depositing various sizes of sediment. Some of this sedimentary rock was later metamorphosed into the schist, gneiss, and quartzite of the Precambrian Yellowjacket Formation (1.4 Ga) and other metasediments.
Mesozoic time was a period of mountain building with the initiation of a subduction zone off the west coast. Large reservoirs of magma formed deep underground, later cooling and crystallizing into the Cretaceous Idaho Batholith granite (100 Ma). Heat and pressure from this intrusion also metamorphosed existing rock.
The subduction zone was flattening some at the beginning of Tertiary time. Magma was still being formed but the relaxation of the crust produced numerous rifts and fissures in Central Idaho thus allowing the eruption of the Eocene Challis Volcanics. The magma that did not erupt cooled underground as the Casto Pluton (50 Ma).
Extensive faulting, rapid uplift of central Idaho, and the inevitable downward cutting of water have shaped the Main Salmon canyon during Quaternary time. The Main is a youthful river, as evidenced by its V-shaped canyon, steep gradient, and steep tributaries. It probably began incising about 2 million years ago and often follows fault lines. More recently, Pleistocene glacial episodes and mass wasting have had an effect on the shape of the canyon.
Heat and pressure (probably from early subduction and burial) changed existing sedimentary rock into the metamorphic rocks schist, gneiss, and quartzite. The river cuts through two Precambrian metamorphic rock units. 1) From Corn Creek to Big Squaw Creek and 2) From Big Mallard Creek to Teepee Creek. There are also a few small outcrops of Precambrian granite. All are assumed to be about 1.4 Ga.
Schist – heat and pressure causes mica grains to align the in the same direction, called “foliation.” May split easily and looks shiny. Original rock was probably mudstone.
Gneiss – more heat and pressure segregates the minerals into alternating light and dark “bands.” Original rock was probably mudstone or old granite.
Quartzite – heat and pressure from all directions recrystallizes sandstone into quartzite. It is a very hard rock and is resistant to weathering.
The Idaho Batholith is a huge expanse of granite encompassing most of central Idaho and is responsible for the rugged topography of the area (see maps p.5,6,7). It is approximately 200 miles long (N to S) and 75 miles wide (E to W). Its age is from 100 to 75 Ma. The batholith is hourglass-shaped with the Salmon River Arch splitting it into roughly into two lobes. The Arch is a swath of Precambrian schist and gneiss (see Yellowjacket Formation), which the Main Salmon follows for most of its east-to-west course. The granite crystallized slowly, 8 to 10 miles underground, producing interlocking crystals visible to the naked eye making identification somewhat easier. Common Idaho Batholith granite minerals are:
quartz – generally white, clear or gray colored, translucent (lets light through).
feldspar – solid white or pink color. There are two common varieties, plagioclase and orthoclase, which are hard to differentiate, except if it’s pink, it’s orthoclase.
mica – thin silver (muscovite) or black (biotite) flakes. Often sparkles on sandy beaches.
hornblende – black needle-like crystals.
To geologists, the term “granite” can be a general term, including many variations of that rock. For laymen on the river the term granite is OK, but more specifically the granites of the Idaho Batholith on the Main Salmon are granodiorite and quartz diorite. The difference between them is hard to detect for the casual geologist.
granite – salt-and-pepper texture, ~30% quartz, orthoclase > plagioclase, ~10% micas.
granodiorite – gray color, ~30% quartz, plagioclase > orthoclase, ~10% micas. quartz diorite – 50-50 color, 5-20%
quartz, plagioclase > orthoclase, hornblende
Large volumes of extrusive (cooled above ground) volcanic rock erupted through fissures and calderas along the Trans-Challis Fault Zone between 51 and 44 Ma (see maps p.7). The rifts probably formed from extension of the crust due to the relaxation of the subduction zone off the west coast. The river does not flow through the Challis Volcanics but a small band can be found between Bargamin and Big Mallard Creeks on the north side of the river. Some volcanic rocks may have flushed down side streams and may be found in the riverbed. Crystals are smaller than those of granite, often not visible with the naked eye. Possible Challis Volcanic rocks will be river rocks, not outcrops, and might include:
andesite – generally brown, gray, or red with small crystals.
dacite porphyry – large light colored lath-like minerals in a dark groundmass, extrusive equivalent of granodiorite.
In summary, the basic rock types in the wilderness section of the Main Salmon River are:
|Corn Creek to Gunbarrel Rapid
Gunbarrel Rapid to below Devils Teeth Rapid
Below Devils Teeth Rapid to Big Squaw Creek
Big Squaw Creek to Arctic Creek
Arctic Creek to Hot Springs Creek
Hot Springs Creek to Bargamin Creek
Bargamin Creek to Big Mallard Creek
Big Mallard Creek to Blowout Creek
Blowout Creek to Teepee Creek
Teepee Creek to Basin Creek
Basin Creek to Vinegar Creek
|Precambrian Yellowjacket Formation
Precambrian Yellowjacket Formation
Cretaceous Idaho Batholith
Precambrian granite and Idaho Batholith
Precambrian Yellowjacket Formation
Cretaceous Idaho Batholith
Contacts. Places where two different in situ rocks touch one another are called contacts. There are several places in the canyon where the Idaho Batholith and metamorphic rocks are in contact. At Corey bar, for instance the Idaho batholith granite is in contact with gneiss, schist and the Precambrian granite.
Jointing. Joints or fractures may form in granite as a result of the release of overlying pressure. As material is eroded off the top, the cooling granite moves slowly toward the surface in a process called isostacy. As long as the granite is deep there is confining pressure on it, but as it nears the surface the pressure decreases and the granite expands, causing fractures. Look for both vertical and horizontal fractures in the granite along the river.
Dikes. If magma is present below the jointed granitic body described previously, it may inject into the joint creating a dike. In the Main Salmon canyon, if a dike is dark colored it is basalt, if it is light colored it is granite. Many times you will see smal crystals along the edges where it cooled quickly and larger crystals in the center where it cooled more slowly. The dike may also slightly metamorphose the edge of the country rock. Small dikes are called veins.
River Mechanics and Erosion
Downcutting. Water, especially sediment-laden water, has great erosive power. Tons of sediment particles are moved downriver every year. The velocity of the water controls the size of particle (called competence) the river can move. In the spring the river will be able to move larger particles than later in the summer. Short flood periods may be able to move cobbles and boulders. Rivers always try to erode to base level, ultimately sea level, but for the Main Salmon base level is the Snake River. Continued uplift of the mountains increases the gradient of the river and forces it to continue downcutting. This process oversteepens slopes resulting in mass wasting into the riverbed, thus contributing more sediment to the process.
Mass Wasting. All loose sediment on a slope has a certain angle at which it will remain stable, called the angle of repose. Larger particles such as broken blocks of rock can maintain a steeper angle of repose than say sand or soil. Talus slopes form when rocks break off an outcrop or cliff and accumulate in a steep pile. If the angle of repose is exceeded some type of slope failure will result. The river hastens this process for all particle sizes however, especially by undercutting banks. Common Main Salmon mass wasting forms are slump, rock falls, rockslides, landslides, mudflows, or debris flows. Solid rock is more stable than loose sediment and rock but metamorphic rock is probably more prone to give way than granite because of its foliated nature, depending on the way the planes of foliation are oriented with regard to gravity. Vegetation helps hold slopes in place.
Rapids. Rapids are formed in one of three ways generally on the Main Salmon. Rock ledges that are resistant to erosion form ledge drops like Killum Rapid. Boulder fields like Rainer, Bailey, and Hancock are common rapids formed by debris flows from adjacent side streams. Constriction rapids, like Mallard, form when the river is pinched from both sides, often by rockslides, forcing water to the middle of the river. The gradient has much to do with the difficulty of rapids.
Bars. Gravel and sand bars (beaches) are common on the Main Salmon, especially on the lower end where the gradient is less. Both result from stream velocity capable of transporting a particular particle and a decrease in velocity whereby the particle will drop out of solution. Sediment is often eroded from the outside of bends and deposited on the inside of bends (point bars), behind a rocky point, or wherever the velocity of the water has slowed.
Terraces. The Main Salmon had to cut its way to its present level from the tops of the tallest peaks you can see. Therefore it left evidence of its passage in the way of rounded river cobble terraces high above the present river channel. If you camp on one of the benches above the river you are probably on one of the old terraces. See the chart at right for estimates of ages. Look for long gravel bars parallel to the river at places like Nixon Bar, Yellowpine, and Mackay Bar. (Meyer).
Hot Springs and Mineral Deposits
Hot Springs. Not all rain and winter snow ends up in the river. Some of it percolates into the soil to help recharge the local aquifer system and feed springs all year long. Ultimately springs are what keep a river flowing into the fall season. In a highly fractured area, such as Central Idaho, water easily finds its way underground where it makes contact with the still-hot granite. Hot water easily dissolves minerals like gold, silver, and sulfur. Since hot water rises, it quickly finds its way back to the surface through the fractures and provides us with natural hot tubs such as Barth Hot Springs.
Mineral Deposits. Hot water dissolves precious minerals like gold easier than cold water. That mineral-laden water flows up through fissures and is deposited on the walls of the fracture. Old time miners would look for quartz veins and often find gold with it. Some small gold can be found along the Main Salmon although getting rich was a tough proposition. Panning in side streams would probably be more rewarding than the river itself. Between Big Squaw Creek and Smith Gulch several fluorspar prospects can be found a short way up the hill. Fluorspar is used to make hydrofluoric acid but this find never proved to be economical to mine and transport.