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Stop 1, Marin Headlands—The Setting

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Stop 1 is in the Marin Headlands at 37°49.789´N, 122°28.999´W, at the first turnout on Conzelman Road. (more below)
Red chert displayed to perfection
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Roadcuts throughout the headlands display red chert that formed way out in the deep Pacific over a stretch of 100 million years during the Jurassic and Cretaceous (200–100 Ma). In all that time, only 80 meters of rock accumulated. It was carried to the forearc wedge and scraped off in chunks that stacked against each other, last one on the bottom (imagine cars in a pileup, each successive one pushing underneath the rear of the car in front). Each chunk is called a nappe, and about a half-dozen nappes are mapped in the headlands here. The tops of some nappes have sandstone lying on top of the chert, signifying the coarser sediment that began to show up as the chert neared land. The bottoms of some nappes are basalt, bits of the underlying crust created long ago at a spreading ridge.

Chert is one of four major rock types making up the subduction-related Franciscan Complex; the others are basalt, graywacke and serpentinite. All of them occur on the oceanic plate, the outer part of the subduction zone, and were pushed into the forearc wedge. We will see these repeatedly during the trip.

Day 1, Coast Range: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Day 2, Sierra Nevada: 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18

Day 3, Sierra Nevada: 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28

Day 4, Coast Range: 29 | 30 | 31 | 32 | 33

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