6: A Brief Geologic History of California
- Page ID
- 20339
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Introduction
Geologists are historians that have a story to tell that is incredibly broad in both space and time. Geologists are also detectives that must search for the evidence and clues to the tectonic past. Often, subsequent geologic events obscure the evidence we are looking for in many ways: rocks are buried and no longer exposed at the surface, rocks are metamorphosed and may no longer look like the original rocks or have the same chemistry, tectonic processes can transport rocks great distances so they are no longer in the same place where they formed, subduction zones can return entire ocean basins to the mantle well beyond our view. While these physical and chemical changes obscure the original properties of the rocks, they are part of the story of how the rocks came to be where we see them today. Geologists are trained to identify these changes to rocks and what they mean about the environment the rock was formed in. Geologists then put these environments and events in chronological order based on cross cutting relationships or absolute ages.
In this chapter we will look at a broad overview of how the tectonic plates interacting along the western margin of North America have changed through time. We will also look at time slices of the geologic map of California to see how the geologic environment has changed through time and understand the evidence used to unravel the geologic past.
Table 6.1 summarizes the major geologic events that shaped California into what we see today. Like many geologic timescales, this table is organized with the oldest events at the bottom and youngest events at the top. This chapter presents a chronologic account of these major geologic events with each of the following pages detailing a different geologic era. This chapter acts as the temporal link between the various geological provinces presented in the following chapters. For more detailed information on the geology of a specific region, the reader should consult the relevant geomorphic province chapter.
| Era | Period | Epoch | Geologic Events |
|---|---|---|---|
| Cenozoic (66 Ma - present) | Quaternary (Q) | Holocene | Last glacial maximum ~20,000 years ago |
| Neogene (N) | Pleistocene (Pe) | Cascade volcanoes develop | |
| Miocene (Mi) | Basin and Range extension | ||
| ~30 Ma: Spreading center is subducted and the San Andreas Fault begins to develop | |||
| Paleogene (Pg) | Eocene (E) | Laramide Orogeny ends | |
| Mesozoic (250 - 66 Ma) | Cretaceous (K) | Laramide Orogeny begins | |
| Sevier Orogeny | |||
| Jurassic (J) | Nevadan Orogeny | ||
| Breakup of Pangaea begins | |||
| Sonoma Orogeny | |||
| Subduction of the Farallon plate | |||
| Paleozoic (540 - 250 Ma) | Mississippian (M) | Antler Orogeny | |
| California was mainly a passive margin | |||
| Proterozoic (2,500 - 540 Ma) | Supercontinent rifted apart | ||
| Abundant carbonate rocks | |||
| Oldest rocks in California form |
- Describe the evidence used to recreate the past tectonic environments in California.
- Describe the tectonic setting of the Precambrian, Paleozoic, Mesozoic, and Cenozoic stages of development of California geology.
- Identify modern tectonic settings that we can use as analogs to understand the processes that created the geology of California.