The first stop of the day was at San Luis Reservoir (what was left of it) to observe exposures of the sedimentary Great Valley Sequence.
1 Our first stop of the day was at the San Luis Reservoir, the largest off-river reservoir in the country. We were here to check out the Panoche Formation conglomorates and sandstones.
2 These epic conglomerates were deposited in the deep forearc basin of what is now California's central valley. 80 million years ago, the area was on the shoals of a volcanic arc complex not unlike the modern Cascades and Andes mountains. The cobbles are eroded bits of the volcanoes that were transferred by river to brink of a huge sea canyon on the continental shelf. The submarine canyons eventually had underwater landslides called turbidity currents that deposited the mix of boulders and sandstones on the ocean floor. In some places the sediments are over 25,000ft deep!
3 Note pen for scale. These cobbles are made up of the extrusive volcanic rock andesite (R) and the intrusive igneous granitic rock (L). There is also a pre-existing conglomerate in the bunch that were cemented in a sandstone matrix.
4 Quite the day at San Luis Reservoir. The drought was definitely noticeable.
5 Conglomerate on the right and steeply-dipping sandstones and shales on the left. Gee, I wonder what might have caused this to happen?
6 Fault trace and offset in the rocks. Pen for scale
7 A beautiful granitic cobble in the conglomerate. The large mineral crystals indicate slow cooling rates that can only happen underground. This was a piece of the solidified magma chamber from the ancestral sierra volcanoes!
8 Just a cool conglomerate within a conglomerate.
9 Kids, this is what a coarse-grained ingneous rock looks like.
10 Another beautiful example of the granitic rocks in the conglomerate.
11 Our next brief stop was just a few miles down the road, but thanks to the complex faulting in California we were now standing on the remnants of the ancient subduction zone trench that fed the volcanoes that would eventually make the modern Sierra Nevadas. The rock in the picture, called a blueschist knocker, is part of the jumbled and thrashed rocks that were scraped-off the subducting plate over 160 million years ago. Blueschist can only form at a depth of 20 miles beneath the surface, but no one has a solid explanation of how the rocks managed to get back to the surface.
