8.4: Sedimentary Products of Rifting in the Basin and Range

Sediments in Rift Basins

As continental rifting progresses, uplifted footwall blocks are eroded, and clastic debris is shed into adjacent valleys. This debris can be used to reconstruct the history of extension and basin formation. Depending on how far it travels into the rift basin, this sediment will be poorly or well-sorted.

Video \(\PageIndex{1}\): Basin and Range: Extension, Erosion, Sedimentation

This silent animation illustrates the way the way that sediment fill basins develop under basin and range extension. Pay attention to how the tectonic processes described earlier in this chapter interact with erosional processes to form sequences of sedimentary deposits. Take note of where the sedimentary sequences are thickest and be sure you can explain why.

Sediment that does not travel far, or for very long tends to be compositionally and texturally immature. Alluvial fans are examples of structures built of immature sediment; they are built of poorly-sorted gravel and flank the uplifted regions, smoothing out the highest relief areas. These fan deposits emerge from the canyons of uplifted ranges and either stand alone as distinct alluvial fans at the base of mountains (Figure \(\PageIndex{1}\) or merge along range fronts.

Figure \(\PageIndex{1}\): Alluvial fans, viewed from the air, are built up at the foot of the Black Mountains. Within each fan, material spills out onto Badwater Basin, forming a perfect fan shape with a radial drainage pattern on the surface. Alluvial fans like this form at the mouths of every canyon in Death Valley. Generally, the coarsest sizes lie near the canyon mouth and the finest lie at the distal edges of the fan. "Badwater Fans" by USGS, is in the public domain.

Finer grained sediment gets carried further into the valley and creates deposits of arkosic sand (a poorly sorted sand that contains rock fragments, feldspars and other easily weathered minerals). These deposits accumulate great thicknesses as the basin continues to drop down. If the basin is part of a horst and graben, the sediment thicknesses are symmetric as is represented in Figure \(\PageIndex{2}\) where two sets of inwardly dipping normal faults bound a down-dropped basin. Within the basin, the sediment that is shed from the ranges is coarse in grainsize near the faults, but becomes finer toward the center of the basin. If the basin is created by a half-graben, the sediment thickness is greater adjacent to the primary normal faults. The Owens Valley, for example, is a half-graben basin that contains roughly 3219 m (2 miles) of sediment in its western half, closest to the major normal fault.

Figure \(\PageIndex{2}\): A cross-section of the sediment filling a horst and graben valley with coarse sediment adjacent to horsts and finer sediments toward the valley center. Crustal extension caused the Basin and Range Province to rift, creating horsts, such as Snake Range, and grabens, such as Spring Valley, shown in the image. The grabens filled with sediment eroded from the adjacent horsts. Similar structures are developed in California. "Spring Valley" by NPS/Trista L. Thornberry-Ehrlich, is in the public domain.

Lakes often form in the lowest regions of these basins, towards their centers and furthest from the rift escarpment. When they are situated in dry areas, like the Basin and Range Province, these lakes grow salty as they evaporate and are important sources of evaporite minerals (refer to the Chapter on the Mojave desert to learn more about these types of deposits).

Dry Lakes of the Basin and Range Province

The climate of the Basin and Range region has not always been as dry and warm as it is today. During the last glacial stage, which peaked at around 20,000 years ago, there was an extensive interconnected lake system (refer to the chapter on the Mojave Desert to learn more about this ancient system of lakes). For example, the dry lake in Badwater Basin is the last remnant of the much larger, Pleistocene Lake Manly, which had depths as great as 600 ft!. This is quite a change, since Badwater today is mostly dry.

Both Mono Lake and Owens Dry Lake have experienced the same climate impacts as those that changed Lake Manly into Badwater. These lakes faced additional depletion and reduction due to the diversion of their tributaries by the City of Los Angeles. This diversion began in 1913, and has been the subject of political turmoil and angst ever since. Motivated by the development of Los Angeles, representatives of the Los Angeles Department of Water and Power purchased land and water rights all throughout the the valleys east of the Sierra Nevada Range. Diversion of water to feed Los Angeles has greatly changed the environment of these valleys and impacted the surrounding communities. California's Water discusses this situation in more detail.

Mono Lake, which once drained to other lakes in the region, is now a closed lake with no outlets and an average depth of 48 m (149 ft). Changes in lake level can be tracked by “strandlines”, or ancient shorelines of the lake. These features are like rings in a bathtub, providing evidence of the ancient water levels and the reduction in water depth in this area. These "Bathtub rings" can be seen easily from the air, appearing like thin lines parallel to the lake shores like those shown in Figure \(\PageIndex{3}\) around Mono Lake. Because today’s Mono Lake occupies an arid climate and has no outlet streams, evaporation has produced very high salinity and alkalinity (pH = 10). A series of calcium-rich freshwater springs around the perimeter of the lake interact with these alkaline waters, causing the precipitation of tufa, a freshwater form of limestone. The calcium carbonate towers created by this process are also a marker of past lake levels, since they can only form in the water. The tufa towers that today rise above the water were formed when the lake levels were higher (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{3}\): Photo of ancient shorelines surrounding Mono Lake. Mono Lake strand lines mark ancient lake levels parallel the perimeter of the lake. "Mono Lake and the Sierra Nevada at the western edge of the Basin and Range" by Marli Miller, used with permission.

Figure \(\PageIndex{4}\): Tufa towers on the shores of Mono Lake. The towers mark the past highest lake levels. These towers are now “dead”. "Tufa towers of the Mono Lake" by CatalpaSpirit is licensed under CC BY-SA 4.0.

Video \(\PageIndex{2}\): The Geologic Oddity in California; Mono Lake Tufas

If you would like to learn more about Mono Lake's tufa towers, this video provides further detail about their formation and shows additional views of the towers.

Another example of the impact of water diversion is found in Owens Lake, which was originally a perennial lake that held water continuously and periodically overflowed to the south over the past 800,000 years. During the late 1800’s and early 1900’s the lake was as deep as 15 m (49 ft.); steamboats hauled ore across the lake from mines in the Inyo Range. The city of Los Angeles began diverting water in 1913, and by 1926 Owens Lake was dry.

Due to the diversion, large areas of very fine lake bed sediments were exposed to winds of the Owens valley; when these winds whip up, large clouds of dust develop, obscuring the view at lake levels and higher (Figure \(\PageIndex{5}\)). Residents tolerated this development, calling the clouds “Keeler Fog”, after the town of Keeler on the east of the lake, which was strongly impacted by the dust. However, by the latter half of the 20th century, the impact became more severe and the dust cloud was found to be the largest single source of PM10 dust (aerosol particles smaller than 10 microns in aerodynamic diameter) in the United States. As a result of legal action taken by the residents of the region, Los Angeles is now required to contain and prevent dust emissions through actions such as shallow flooding, managed vegetation, gravel and tillage.

Legal action taken by valley residents has forced the city of Los Angeles to partially restore these lakes and to limit diversion. Nature has helped mitigate the problem as well: in March 2023, climate change induced atmospheric rivers brought an unusually large amount of precipitation to California (5-13 inches), breaching the Los Angeles Department of Water and Power’s aqueduct and filling Owens Lake.

Figure \(\PageIndex{5}\): Blowing Alkali Dust at Owens Lake. Dry lake deposits are whipped up by winds and creates white clouds above the lake. "Blowing alkali dust Owens Lake" by Eeekster (Richard Ellis) is licensed under CC BY 3.0.

Query \(\PageIndex{1}\)

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