Point Lobos Marinos Reserve: Pre-trip Exercise
Before our field trip to Point Lobos, we need to know a bit about the geologic history of coastal California and the difference in geology between the so called Salinian Block and the majority of the Coast Ranges Franciscan Formation.
First, go to the following url and read a brief geologic history of the Point Lobos Reserve (http://pt-lobos.parks.state.ca.us/nathis/Geology.htm#anchor15261). Make sure that you also read about the four different rock units found at Point Lobos. We will only map the three older units (Santa Lucia Granodiorite, Carmelo Formation, and Marine Terrace Deposits).
After you complete this exercise as homework, we will have a quiz on this material on the lecture before our Point Lobos trip.
This unit is part of the Mesozoic magmatic arc that stretched from British Columbia to Baja California (this on our last exam). This convergent margin resulted in forming the Sierra Nevada Batholith of which the Santa Lucia Granodiorite is a displaced part. Read the following excerpt from Debra Harden’s book California Geology, 2004.
The Salinian Block is an exotic, tectonostratigraphic terrane. This means that the block originated somewhere else and has been tectonically transported to its current position in the California Coast Ranges. The core of the Salinian Block is a magmatic arc represented by the Santa Lucia Granodiorite at Point Lobos. This means that the basement rocks formed in a convergent margin under high temperatures, a batholith. However, most of the California Coast Ranges have basement rocks of the Franciscan Formation. The Franciscan Formation formed in a subduction zone under high pressure and low temperature. Very different than the adjacent Salinian Block. So, the Salinian Block is an exotic terrane and was transported north along the San Andreas Transform.
The Salinian Block is an area in the Coast Ranges underlain by granitic and high-grade metamorphic rocks that is bound on the east by the San Andreas Fault and on the west by the Sur-Nacimiento Fault zone. The basement of the Salinian Block, which represents a CONTINENTAL MAGMATIC ARC, contrasts sharply with those of the Franciscan Formation that lies both to the east and west. (Recall, the Franciscan Formation has oceanic affinities – e.g. contains fragments of oceanic crust and not granites are present. The rocks of the Franciscan Formation were deposited in a trench at a convergent boundary, subducted, and metamorphosed at HIGH PRESSURE and relative low temperature.) Rocks of the Salinian Block originated in the area of the present Mojave Desert region, between the southern Sierra Nevada and the Peninsular Ranges batholiths, and they have been displaced at least 315 km (180 mi) northward by movement on the San Andreas Fault.