11.2: North and South

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.

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.