8.6: The Oldest Rocks in the Basin and Range

Before Rifting

The uplift of older rocks in the footwalls of normal faults in the Basin and Range Province provide a peek into the geologic environment during the Late Proterozoic-Early Paleozoic period when most of what is now California did not yet exist. Well before the onset of extension leading to the formation of the basins and ranges of this region, what is now the Basin and Range Province of eastern California was part of the ancient, or passive continental margin that developed following the rifting of the supercontinent Rodinia.

Passive Margin Basics

A passive continental margin is formed where the edge of a continent meets oceanic crust within a tectonic plate. An example of a modern passive continental margin is found along the east coast of the United States, where a broad continental shelf extends beyond the current coastline. This continental shelf preserves the geologic record of changes to this region following the rifting of Pangea.

These margins are “passive” because they lack the tectonic activity seen at active continental margins, such as subduction zones or continental collision zones. A key feature found at a passive margin is a broad continental shelf, which is the submarine continuation of the continent (Figure \(\PageIndex{1}\)). This shelf has a shallow slope and is the site of continuous sedimentation and subsidence dating back to the time of rifting (and before). They are typically covered by a mix of terrestrially-derived sediments and marine sediments. The specific types of sediments can vary depending on factors such as the geological history of the region, the climate, the proximity to tectonic boundaries, and the input of sediment from nearby rivers and other sources. Sediment types may include terrestrially derived gravel, sand and mud, or marine muds and limestones (including shell matter and reef material), as well as layers of evaporite deposits. These sediments may drape over faulted basement rocks whose structures originate with the initial rifting of the continent!

Because the continental shelf is generally less than 200 m deep, it can also preserve the history of global sea level change: these regions may be extensively flooded during times of sea level rise and partially exposed during glacial periods. Each of these situations brings a sedimentary signature with it. For example, as sea level lowers, these regions may preserve intertidal mud flats and sandy beach deposits, as well as shallow water evaporite minerals. As sea level rises, these near-shore deposits may be “drowned” and covered with deeper water deposits (mud and clay).

Figure \(\PageIndex{1}\): A diagrammatic passive margin resembling what western North America looked like. Continental crust that has been rifted is broken up by west-dipping listric normal faults. It is overlain by thick ocean sediments. Note that this diagram is very vertically exaggerated to emphasize these different features. This work by Allison Jones, a derivative of the original, is in the public domain.

The Paleozoic Passive Margin of the Western US

Geologists have determined that the Paleozoic rocks of the Basin and Range represent an ancient passive margin along an ancient coastline of a stable craton that resided close to sea level at low latitudes. In these areas, sedimentary rocks that are marine in origin, such as siltstones, limestones, dolomites and quartzites are common. Geologists have been able to demonstrate that the distinctive units in one area can be correlated with units in other areas; as a result, geologists have been able to reconstruct the geometry of the ancient continental shelf. They’ve even been able to determine the direction that sediment was transported and have been able to estimate the depth of water in these areas; in some areas, fossils and sedimentary structures suggest that water was only a few meters deep (Figure \(\PageIndex{1}\))!

Figure \(\PageIndex{1}\): Paleogeography of the Basin and Range of California at the beginning of the Paleozoic era. The region that corresponds to the Basin and Range is contained in the black dashed triangular zone; this zone sat offshore at the edge of the continental shelf transition to deeper water. The black circle in the southeast corner of the triangle indicates the approximate location of Paleozoic rocks continental shelf rocks now exposed in Death Valley. The white circle to the northwest (and still within the triangle) indicates the approximate location of the deep water sedimentary facies represented by Paleozoic rocks in the White-Inyo Range. The region that will become the future Colorado Plateau is contained within the red dashed line to the east, straddling a shallow inland sea to the west and a coastal area to the east. The cross-sectional sketch of a generic passive margin represents the geological cross section projected along the west-northwest/east-southweast striking black solid line. Arrows show the approximate location of the Death Valley and White Mountains facies projected onto this cross section as described in the text. This work by Martha House, is licensed under public domain /derivative of original work by NPS.

An example of this correlation is shown between the Late Proterozoic-Cambrian rock layers in the White-Inyo Mountains, on the east side of the Owens Valley and those of the Death Valley region to the east (Figure \(\PageIndex{2}\)). The rocks of the Death Valley region are thought to have been deposited closer to the craton because they contain more debris that is shed from the continent (such as sand), so the stratigraphic column in the figure is shown above this region of a schematic cross section of a passive margin. On the other hand, the sedimentary units in the White Mountains region appear to be thicker and to have more deep-water features, such as more shale and mudstone, therefore, the stratigraphic column illustrating these units is shown above the deep-water portion of the passive margin cross section. Correlations between these units are shown in the diagram as well. These correlations are described below starting with the White Mountains units:

• Middle Cambrian Units
  • Bonanza King Dolomite (is found in both areas)
  • Monola Formation (corresponds to upper Carrara Formation in Death Valley area)
• Lower Cambrian Units
  • Mule Springs Limestone (corresponds to middle Carrara Formation in Death Valley)
  • Saline Valley Formation (corresponds to Zabriskie Quartzite in Death Valley area)
  • Harkless Formation (corresponds to upper Wood Canyon Formation in Death Valley)
  • Poleta Formation (corresponds to middle Wood Canyon Formation in Death Valley)
  • Campito Formation ((corresponds to lower Wood Canyon Formation and Upper Stirling Quartzite in Death Valley)
  • Deep Spring Formation and Upper Reed Dolomite (corresponds to middle Stirling Quartzite in Death Valley)
• Upper Proterozoic (Late Proterozoic)
  • middle and lower Reed Dolomite (corresponds to middle Stirling Quartzite in Death Valley)
  • Wyman formation, base is not exposed (corresponds to lower Sirling Quartzite, entire Johnnie Formation, Noonday Dolormite and possibly underlying Pahrump group in Death valley)

Reconstructions of these units creates a sketch of the pattern of sediments above the continental shelf at the time of deposition.

Figure \(\PageIndex{2}\): Simplified stratigraphic columns for Early Paleozoic stratigraphy in the White Mountains and Death Valley region. Upper panel shows suggested correlations between the regions after Nelson (1981) and Hall (1991). Abbreviations are as follows: Fm = formation; Ls = limestone; dolo = dolomite; Quartzite = quartzite; Spr = Spring; Pk = Peak. Lower panel shows the approximate location of these columns relative to a developing passive margin. White Mountains facies are more distal shelf units, while Death Valley facies represent proximal shelf sequences and rift deposits. This work after Nelson (1981) & Hall (1991) by Martha House, is licensed under CC-BY-SA-NC.

Query \(\PageIndex{1}\)

Continental Rifting and a Snowball Earth

An interesting feature of these very old units is found at the base of the Death Valley sequence, where the sedimentary rocks unconformably overlie very old basement rocks that have been dated at 1.7 billion years. The rocks directly above them are called the Pahrump group. This tilted group of weakly metamorphosed sedimentary rocks contains the sedimentary record for rifting of the ancient Rodinia supercontinent, as well as the likely evidence for ancient “snowball earth” glaciations in which low-latitude regions all around the Earth were fully glaciated!

The “Snowball Earth” glaciations were a series of ice ages during the Neoproterozoic era. These ice ages were thought to have been so profound that perhaps the entire surface of the planet froze over, all the way from the poles to the equator. At roughly the same time, the supercontinent Rodinia was rifting apart. The region that would become eastern California was part of the rifted zone that would become the passive margin discussed above. As the edge of the rifted continent is faulted and subsides, fault-bounded basin fill with seawater. Poorly sorted sediments shed from the rift flanks, as well as from marine mass movements called continental shelf are deposited in these areas(see the discussion in the Coast Ranges chapter).

So how do geologists come to the conclusion that rifting was happening at this time, in this place? And what is the evidence for the glaciation here? A key unit that is part of this story is found in the Pahrump Group. A unit called the Kingston Peak Formation, which has been dated at approximately 700 -750 Ma, contains mafic volcanic rocks of the type that form in rift environments, along with diamictite units (a diamictite is a type of poorly sorted sedimentary rock containing terrigenous sediments; Figure \(\PageIndex{3}\)). This unit is similar to others of the same age that are found throughout the west, and the mapped geometries of these units indicate that they were deposited in narrow bays or rift zones along the developing passive margin.

So what about the glacial evidence? In this region, the dominant evidence is, interestingly, also the diamictite, which contains features called “dropstones”, which are fine-grained, thin-bedded or laminated deep water deposits of clay and silt, into which drop much larger particles, such as pebbles, cobbles, or boulders (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): A dropstone in Proterozoic diamictite of Kingston Peak Formation, SE California. A dropstone cobble is surrounded by layered mudstones and sandstones of the Kingston Peak formation. The layers below are deformed slightly suggesting this rock dropped into soft sediment. Layers above drape over it. "Glacial Dropstone" by Marli Miller is used with permission.

The dropstones pierce into the soft pre-existing sedimentary layers, disrupting their continuity. The layers of clay and silt indicate that the depositional environment was low energy, so it is somewhat strange to see cobbles and boulders, which require high energy to be transported. Geologists believe that that these dropstones in the Kingston Peak Formation are glacial in origin. The idea is that when outlet glaciers from the Snowball Earth ice sheets reached the sea, they calved off icebergs full of gravel, sand, cobbles, and silt. These icebergs floated out to deep water, melted, and let their load of sediment fall through the water below.

Query \(\PageIndex{2}\)

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