6.2: Proterozoic Eon

Proterozoic History

While the Earth is 4.5 billion years old, there are no rocks exposed anywhere that are that old, let alone in California, so we will skip over the first 3 billion years of Earth history and start in the Proterozoic Eon with the supercontinent Rodinia, which existed before the most recent supercontinent, Pangea. Geologists have used multiple lines of evidence to reconstruct the position of the continents in the past, one possible reconstruction of Rodinia places the portion of Laurentia that will eventually be California, adjacent to Australia or Antarctica (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): One proposed reconstruction of the supercontinent Rodina at 750 Ma. Outline of California in red superimposed to orient the viewer. ("Rodinia" by Steven Skinner, a derivative from the original work, is licensed under CC BY-NC 4.0.

Rifting of Rodinia and a Passive Time in California

At approximately 750 Ma, the supercontinent Rodinia began to break apart. A continental rift developed that began to separate Laurentia from East Antarctica and Australia.

Until about 250 Ma, California was a passive margin during which time little to no tectonic activity was taking place. Instead, thick sequences of sediments were accumulating in the ocean basins that developed from rifting (Figure \(\PageIndex{2}\). Over time these thick sediments would eventually compact, become lithified into sedimentary rock, and undergo metamorphism. They would ultimately become California's oldest exposed rocks and the continental basement. (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{2}\): 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. "Passive Margin" by Allison Jones, a derivative of the original, is in the public domain.

If we look at the geologic map of California, we can see that Precambrian rocks are only exposed in the south and eastern parts of the state (Figure 6.2.2). This limited exposure of the oldest rocks is due to the fact that most of California’s geology is made of rocks that are much younger and were added to the North American Plate in the Phanerozoic Eon. The Precambrian rocks in the southeast corner of the state are plutonic and high-grade metamorphic rocks that formed kilometers deep in the Earth. They are the continental basement rocks that were part of the Rodinian supercontinent.

Figure \(\PageIndex{3}\): Simplified Geologic map of California highlighting the distribution of Precambrian rocks. "Precambrian Rocks" by Steven Skinner, a derivative of the original work, is licensed under CC BY-NC 4.0.

There are also Precambrian rocks exposed in the ranges to the east of the Sierra Nevada. The White-Inyo Mountains as well as the ranges around Death Valley contain a thick sequence of metamorphic rocks, however, the level of metamorphism is relatively low grade such that the original lithology can be identified through the metamorphic overprint. Before metamorphism, these rocks formed a thick and laterally expansive package of sediments. Around 750 million years ago, as the supercontinent Rodina started to break apart, Australia and Antarctica were where California exists now. As rifting progressed and pulled Antarctica away from Laurentia, a sedimentary basin developed. The early stages of this rifting process would look something like modern-day East Africa (Figure \(\PageIndex{4}\)). The process of rifting will create fault bounded basins that accumulate coarse clastic sediments as well as volcanic deposits. As the rift developed further extension and subsidence would lead to a marine incursion, or spreading of the sea over a land area (\(\PageIndex{5}\)).

Figure \(\PageIndex{4}\): Rifting of a continental plate. Stretching of the continental lithosphere results in faulting of the upper crust and thinning of the lower crust that allows for upwelling of the asthenosphere. This work by MIT OpenCourseWare is licensed under CC BY-NC-SA 4.0

Figure \(\PageIndex{5}\): Rifting of a continental plate. Thinning of the continental lithosphere results in low elevations that allow sea water to cover the continent. This work by MIT OpenCourseWare is licensed under CC BY-NC-SA 4.0

As the continental rift continued to evolve, the plate boundary shifted from faulting and stretching of continental crust to the creation of new oceanic crust through decompression melting. This margin would have resembled the present-day gulf of California, a narrow sea bounded by continental blocks on either side and a central spreading ridge producing basaltic ocean floor (Figure \(\PageIndex{6}\)). As the ocean basin continued to expand, the margin would have looked like the present-day East Coast of the United States: that is a passive margin where no tectonic deformation is occurring. Upon this rifted margin, a great thickness of Precambrian marine sediments were deposited. Measured stratigraphic columns of eastern California show the sandstones, shales, and carbonate rocks that were deposited in this marine basin and how the thickness of these deposits changes from the shelf in the east to the deeper basin in the west. The deposition of these marine sediments continued into the Paleozoic era. This thick sequence of sedimentary deposits on a passive margin is sometimes referred to as a miogeocline.

Figure \(\PageIndex{6}\): Rifting of a continental plate will eventually result in the formation of new oceanic crust. This work by MIT OpenCourseWare is licensed under CC BY-NC-SA 4.0

You can read more about this stage of California’s geologic past in the chapters on the Basin and Range Province and the Mojave Desert Province where these oldest rocks are still preserved today.

References

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3. Li, Z. X., Bogdanova, S. V., Collins, A. S., Davidson, A., De Waele, B., Ernst, R. E., . . . Vernikovsky, V. (2008). Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research, 160(1-2), 179-210. https://doi.org/10.1016/j.precamres.2007.04.021
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