6.3: Paleozoic Era (540 – 250 Ma)

Paleozoic History

The Paleozoic rocks in California continue the record of passive margin sedimentation. The thick deposits of shallow marine clastic and chemical sedimentary rocks continued to accumulate in the early Paleozoic as the region continued to slowly sink below sea level in response to rifting.

Antler Orogeny

Early in the Paleozoic, the western side of North America was a relatively quiet passive margin. However things began to change in the Devonian to early Mississippian. In the region that is now northern Nevada, thrust sheets were emplaced over the passive margin sediments. This event, known as the Antler Orogeny, marks a change from a tectonically quiet passive margin to an active margin with volcanic rocks and oceanic sediments that are thrust onto the continent. The package of rocks that were emplaced onto the continent are referred to as the Roberts Mountains allochthon or the Roberts Mountain Thrust. An allochthon (Figure \(\PageIndex{1}\)) is a package of older rocks that have been thrust above younger rocks. It may seem like these events recorded in Nevada are beyond the scope of a tectonic history of California, but political boundaries did not exist when these rocks formed, and geologic processes pay little attention to the lines we draw on maps.

The rocks displaced along the Roberts Mountains Thrust signal a change at the plate boundary: the western boundary of North America had gone from an active rift, to a passive margin accumulating marine sediments, and was now experiencing compression. Emplacement of the Roberts Mountains allochthon required a compressive force, most likely generated by a convergent margin somewhere offshore to the west.

Figure \(\PageIndex{1}\): Illustration of a thrust system. Allochthonous crust is indicated in dark gray and has been thrust overtop of autochton material. These units are out of sequence. "Thrust system en.jpg" by Woudloper via Wikimedia Commons is in the public domain.

Sonoma Orogeny

Following the Antler orogenic event, another compressional event, the Sonoma orogeny, added a package of exotic rocks, the Golconda allochthon, to North America. The rocks of the Golconda allochthon represent the sediments that were once on the floor of an ocean that has since disappeared. The Paleozoic rocks of the marine Golconda Allochthon suggest that not only was compression occurring, but subduction was also happening to the west. There are multiple instances of these types of marine sedimentary rocks, originating both nearby and far traveled, that were added to North America at this time. The exact geometries of the Sonoma orogeny are even less well understood than those of the Nevadan.

When imagining what this paleoenvironment looked like, geologists use the western Pacific as a modern analog. The western Pacific is characterized by numerous subduction zones with associated volcanic island arcs as well as marine basins that are collecting sediment from a continental source in one direction and volcanic island arcs in the other (Figure \(\PageIndex{2}\)). If we could wind the tectonic clock forward and watch these ocean basins close, the resulting geology would resemble what geologists have mapped out as the Paleozoic geology in the western US.

Figure \(\PageIndex{2}\): Tectonic map of the western pacific. "Tectonic plates boundaries" by Eric Gaba, a derivative from the original, is licensed under CC BY-SA 2.5.

If we look at the reconstructed trend of the Paleozoic coastline of western North America, there is a striking feature, namely that the rocks indicating the location of the coastline do not continue very far into California. This truncation of the miogeoclinal passive margin sediments and the accreted terranes is an important feature for the next chapter in the tectonic history of California. Geologists hypothesize a left-lateral fault, the California-Coahuila transform, displaced these rocks and truncated the Laurentian shoreline (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Truncation of the miogeoclinal margin and the offset Caborca Block by the California-Coahuila transform (red line). "This work" by Skinner is licensed under CC BY-NC 4.0

References

1. Burchfiel, B. C., & Davis, G. (1975). Nature and controls of Cordilleran orogenesis, western United States: Extensions of an earlier synthesis. American Journal of Science, 275(A).
2. Dickinson, W. R. (2004). Evolution of the North American Cordillera. Annual Review of Earth and Planetary Sciences, 32(1), 13-45. https://doi.org/10.1146/annurev.eart....101802.120257
3. Saleeby, J. (2011). Geochemical mapping of the Kings-Kaweah ophiolite belt, California—Evidence for progressive mélange formation in a large offset transform-subduction initiation environment. GSA Special Papers, 480, 31-73. https://doi.org/10.1130/2011.2480(02)