11.1: A Complicated Land

A Complex Geological Story

There’s more complexity in the Coast Ranges than can be described in one book, much less this one brief chapter, but one place to start is to understand that, along most of their length, the Coast Ranges are the place where the North American plate and the Pacific plate meet. This one seemingly-simple statement explains a lot about the Coast Ranges. But fully understanding what this means takes some unpacking.

The Coast Ranges were first formally mapped in 1895 by Professor Andrew Cowper Lawson of UC Berkeley, a distinguished mineralogist known today to geology students as the guy with the beard of unusual and splendid magnificence. The hirsute Lawson understood the importance of field work–indeed, he founded the first systematic field geology course in the west–and during his explorations of the Coast Ranges he accidentally discovered the San Andreas fault in a linear valley south of San Francisco. At the time Lawson had no idea that the San Andreas’s offsets were anything more than a local disruption. That changed in 1906, and Lawson headed a massive research effort to examine this fault that had ruptured such a great distance and so catastrophically tore the state apart. However, Lawson came from a time before plate tectonic theory; in 1906 he could describe what happened geologically, but not why.

Box \(\PageIndex{1}\): A mineral's namesake

The mineral Lawsonite is named after Professor Andrew Lawson by two of his graduate students. The mineral is typical of blueschist facies metamorphic rocks and was first described in 1895 for occurrences on Ring Mountain on the tiburon peninsula, Marin County, California.

Box Figure \(\PageIndex{1}\): Sample of the pastel pink mineral lawsonite surrounded by mica schist. "Lawsonite" by Rob Lavinsky via Wikimedia Commons is licensed under CC BY-SA 3.0.

Plate Tectonics: Unlocking the Mystery

Though Lawson would not live to see the advent and acceptance of plate tectonic theory in the late 1960s, today we know that comprehending the California Coast Ranges is impossible without plate tectonic theory. Just as biology prior to Darwin’s theory of evolution has been described as mere butterfly collecting–assembling organisms without understanding how they came to be–geology before plate tectonics was conceptually hobbled and unable to explain regions such as the Coast Ranges.

Why do we find pillow basalts–lava that can only form underwater–in contact with metamorphic rocks such as eclogites that have been subjected to brutal extremes of heat and pressure, and both of these right next to unaltered sandstones? If all this formed in place, when these eclogites were metamorphosed, why weren’t the pillow basalts and the sandstones also metamorphosed? These sorts of Coast Range juxtapositions make no sense–until one understands that tectonics has brought all these disparate pieces together from far away.

There are also complications of time and order.

In certain places in the world–Grand Canyon, for example–rock formations obediently accede with geologist’s wishes by forming one atop the other, with the occasional fault or fold distorting otherwise relatively flat sedimentary units. It is possible to stand on the South Rim of Grand Canyon and look down to the river and see the Tapeats Sandstone neatly lying atop even older Vishnu Schist, and the Bright Angel Shale atop the Tapeats, and the Muav Limestone atop the Bright Angel, on and on and on, right up to the younger Kaibab Limestone on which visitors stand. In such situations, we can easily understand that the layers are in order, with the oldest at the bottom and the youngest at the top.

California just isn’t like that.

How do you explain how old a rock assemblage is when most of it formed far from California, then was brought in by tectonics to collide with and stick onto California, a process that altered and metamorphosed the rocks? What’s the proper age of the assemblage? When it first formed in its original state? When it moved and accreted onto California? When metamorphism so transformed the original rocks that they are unrecognizable? There is not an easy answer here.

Further complicating the age of rock groupings, several of the major groups date from the same time range. Geology uses a fancy word for this: “coeval,” meaning forming at the same time. Because of the devilish difficulties coeval rocks pose for interpretation, we might as well just relabel them as “evil.”

Distinguishing the Northern and Southern Coast Ranges

To understand the distinctions between the northern and southern Coast Ranges, closely examine Figure \(\PageIndex{1}\):

Figure \(\PageIndex{1}\): The California Coast Ranges are a series of mountains that lie to the west of the Central (Great) Valley near the coast. The Coast Ranges are primarily composed of Franciscan Complex rocks, with minor serpentinite, granitic igneous rocks, and pre-Cambrian rocks. It is cut by the San Andreas and associated faults. "Geologic Map of the Coast Ranges" by Emily Wright, a derivative of the original, is licensed under CC BY-NC. Access a detailed description.

While there is not a sharp demarcation in this map, we can roughly say the San Francisco Bay Area divides the north from the south. Furthermore, the geologic character of the northern region is quite distinct from the south, as indicated by the generalized geologic units legend.

Northern Coast Ranges

In Figure \(\PageIndex{1}\), the dominant color of the northern Coast Ranges is green, which the legend identifies as “Franciscan complex sedimentary and metamorphic rocks.” This isn’t quite true nor complete; for example, there are also significant igneous rocks, as well as a host of younger igneous and sedimentary rocks interspersed within the Franciscan in pockets too small to be mapped separately on a map of this scale. Moreover, one should not imagine that the northern regions are monolithically and uniformly one type of rock; when you see a map such as this, with broad swaths of single color, the discerning student should understand that this indicates not uniformity, but rather an overgeneralization of areas lacking detailed mapping.

When you actually go to the Franciscan and observe the outcrops, you find not blank uniformity, but fantastic variety, with rock exposures whose ages swing wildly from time period to time period, and whose rock types change abruptly from igneous to metamorphic to sedimentary and back again, in some places all changing radically within a few meters of each other. It’s a jumble, but could with sufficient research be parsed into more detail than shown on this map. Part of the lack of detail for this region is the result of the map scale; it’s a big zone and if all details were included, the map would be unreadable. But even if you were to find the most detailed geologic maps of this area, you’d still find huge swaths of green dismissed as “Franciscan”. By contrast, much more detail has been mapped in the southern Coast Ranges, and even in the sparsely populated Eel River Valley, one of the few non-Franciscan regions of the northern Coast Ranges. One reason for this is that there isn’t much petroleum in the Franciscan, and hence there has been less economic incentive for intense exploration and mapping.

Southern Coast Ranges

The Southern California Coast Ranges contain petroleum deposits, and a lot of other things. A close examination of Figure \(\PageIndex{2}\) shows that the Franciscan is plentiful in the south as well as the north. Moreover, we find significant areas of light brown “Neogene and Paleogene sedimentary rocks” (this is where oil is found).

There is also a lot more dramatic faulting in the south compared to the north. The San Andreas fault falls right in the middle of southern region, but it is accompanied by a myriad of accomplices, fracturing and slicing rock and creating serious headaches for any geologist trying to envision these pieces back when they were together (the word for this kind of tectonic reconstruction is palinspastic, a term that doesn’t help matters much).

One very unusual thing going on in the south is the pink areas of the generalized geologic map, which the legend calls “Granitic igneous rocks mainly of Mesozoic age.” That’s very strange–we normally associate granitic rocks as having formed very deep in the crust, cooling slowly, and unlikely to be seen at the surface. It would take great upheavals to bring granitic rock from the depths of its formation to the surface where geology students can pick at it with rock hammers.

Yet there are multiple places in California where there is abundant granitic rock, most notably the Sierra Nevada [link]. Are these pieces in the southern Coast Ranges chunks of the Sierra Nevada? It seems crazy, but there is a plausible hypothesis for bringing Sierran rocks all along the coast as far as Marin, as will be detailed later when discussing the Salinian Block.

First, let’s start with the oldest of the major rock units in these ranges: the Coast Range Ophiolite.