The Rocks of Leona Quarry

A few days ago I visited the former Leona Quarry, now slowly filling with townhomes. The view was great.

The rock slope on the north side looks rugged but is heavily engineered with many setbacks and retaining walls. But I was here to look at the rocks. There’s no trespassing on the high slopes, which is just as well because there is little bedrock to be seen. The whole hill is mapped as quartz keratophyre at the very bottom of the Great Valley sequence, of Jurassic age (no younger than 140 million years). It’s lava flows from an oceanic volcanic arc, like modern-day Japan or the Philippines, that have been heavily altered. It’s typically light colored with strong rust stains and full of fractures. You’ll see several different examples below.

Some outcrops show bits of less-altered gray lava, although this example is still full of veins.

This small boulder displays slickensides. The white mineralization includes quartz as well as carbonate minerals, sometimes layers of each in the same vein.

It’s pretty sterile, but life is taking hold amid the stones.

I took a couple hand specimens of the hardest, most pristine looking rocks. Even these have a greenish tinge from metamorphism. This specimen has faint banding in it, probably metamorphic rather than from flowing of the lava.

This kind of rock is almost impossible to study without using thin sections and a petrographic microscope. I don’t know what I would call it if the map didn’t say—probably just “altered lava.” Quartz keratophyre is specifically a metamorphosed trachyte, a highly alkaline high-silica lava. That’s meaningful to petrologists in a way that would put most of us to sleep if they were to tell us about it. I don’t mean that unkindly. But I’d hazard a guess that here it reflects some input from subducted continental sediments as well as the usual melted seafloor.

Rock from the Leona Quarry appears all around town in rustic walls, but I suspect that nearly all of its output went to crushed stone.

9 Responses to “The Rocks of Leona Quarry”

  1. Roland De Wolk Says:

    roger that. thx!

  2. Andrew Says:

    If you’re talking about the Leona Quarry, call it a metavolcanic rock or a metatuff. Tell ’em I told you.

  3. Roland De Wolk Says:

    meaning it is technically incorrect to call this handsome red rock at the quarry (and beneath my home) as rhyolite OR a volcanic basalt? what name can i accurately call the reddish rock?

  4. Andrew Says:

    Not at all. Those are very different rock types. Basalt (low silica) makes up the Moraga Formation lavas exposed at Sibley. Rhyolite (high silica) makes up the bouldering grounds in the Berkeley hill parks. The Leona Quarry rocks are of intermediate composition.

  5. Roland De Wolk Says:

    is it correct to call it rhyolite and a volcanic basalt?

  6. Andrew Says:

    Yes, I see I posted contradictory statements. The unit is mapped as the basal part of the Great Valley, but the rocks are volcanic and can just as easily be considered part of the ophiolite. Russ Graymer, in his notes to the Oakland geologic map (dated 2000), says that these appear to be arc rocks, of intermediate and silicic composition, laid down on top of the ophiolitic rocks (which would be basaltic or ultramafic). The fault between it and the younger Knoxville Formation is of indeterminate nature, so the stratigraphy is no help. If I recall correctly, it was Shervais in 2008 who put this unit in the Coast Range Ophiolite. It’s a judgment call among the experts.

    Thanks for the chance to clarify.

  7. Diodogo Says:

    The rocks of Leona Quarry belong to the Coast Range Ophiolite, not the Great Valley sequence. The Leona rocks are lower plate. They’re part of the Pacific (and/or Farallon?) global tectonic plate that was subducted, thrust, under the North American plate during Cretaceous and Paleogene time, roughly 146 to 23 million years ago. By exposure to the high temperatures of the Earth’s mantle, the Leona ocean-floor volcanic rocks were metamorphosed. (In contrast, the Great Valley sequence, as part of the upper plate, relatively speaking, stayed near the Earth’s surface and avoided metamorphism.) Also, the great amount of subduction-zone thrusting and shearing, on the order of 1,000 miles, produced the slickensides seen in (at least one of) the photos of the Leona rocks.

  8. Renate Lellep Fernandez Says:

    Is Indian Rock of North Berkeley of similar geological stuff in the “arc”? I remember it as a kid climbing around on the Rock the stuff looking just like salt and pepper granite. I don’t remember the many variations as pictured of quartz keratophyre.

  9. Andrew Says:

    I’ve been doing some reading about the Coast Range Ophiolite, of which the Leona Quarry rocks are a part. The most recent work I’ve seen considers this rock not a lava, but a highly compressed and altered tuff, that is, the ash and rubble erupted from volcanoes. So the little flat features in the last photo would be the remnants of lapilli: pebble-size particles, like bits of pumice or lava, that are now almost erased. It’s thought to be about 150 million years old, unlike the far younger volcanic rocks of Sibley volcano.

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