Archive for the ‘Oakland sediment and soil’ Category

The Merritt Sand

22 August 2016

merritt-terrace-madison

Downtown Oakland sits on an unusual bit of geology — a large dune field mapped as the Merritt Sand. San Francisco is famous for its sand dunes, of course, and Points Reyes and Año Nuevo have some too, but the dune fields of Oakland and Alameda are the only ones within the bay. Here they are, labeled “Qds” on USGS Map OF-00-444, which shows the young (Quaternary) deposits of the Bay area.

dunesandmap

They’re just like the dunes in San Francisco. They formed during the ice ages, when the shoreline was out near the Farallon Islands, the Bay was totally dry and the Sierra was full of permanent glaciers (on not quite the same schedule as the great continental ice caps). The rivers carried huge amounts of fresh-ground rock dust from the glaciers to the Bay and beyond, and the Pacific beaches of the time must have been formidable. Think of the brisk summer days we have when the sea wind is being sucked into the Central Valley, and now multiply that. Those winds blew all that sand here.

Brian Atwater of the U.S. Geological Survey mapped ice-age deposits in the Delta, and in 1982 he summarized the overall picture as “a stage on which three related and repetitious plays are presented simultaneously. In one play, wetlands and flood plains appear and expand as tidwater invades from the west, then become sites of erosion after the tidewater retreats. In another play, glacially eroded detritus from the Sierra Nevada builds alluvial fans and, reworked by wind, creates extensive sand dunes. In the third, little-understood play, streams draining the Sierra Nevada foothills and Coast Ranges episodically build alluvial fans. Spanish- and English-speaking persons enter during a major incursion of tidewater and find most of the stage covered with tules.”

Our dunes may sit higher than the buried dunes of the Delta because the conditions that built them were more stable here. There was always a wind gap at the Golden Gate and always lots of sand available on the other side.

In detail, the Merritt Sand (Qms) reaches the edge of Lake Merritt.

merritt-lake-geomap

It differs from the marine terrace deposits (Qmt) that I’ve described before. It consists of very fine sand, with no clay. It’s also higher and less flat. Apparently the original, undisturbed surface featured yardangs — elongated ridges of sand running in the direction of the wind — whereas the dunes of Alameda were the more typical arc shapes known as barchans. All of that is obliterated today.

You can see the edge of the Merritt Sand platform from across the lake where the streets rise abruptly away from the shore. Snow Park is probably the least disturbed exposure.

merritt-terrace-snowpark

Another exposure stands out between Jackson and Madison streets, although it probably has also been excavated.

Merritt terrace at 160-17th St

It’s the back yard of an apartment building at 160 17th Street. The view is nice from there.

merritt-terrace-view

That same agreeable elevation attracted Oakland’s early elite, who put up a row of mansions overlooking the lake. Of those great homes, only the Camron-Stanford House survives.

Oakland alluvium

1 August 2016

Qhaf-1

When someone opens up the ground in Oakland, no matter where, I think it’s interesting. This construction site on Telegraph Avenue between 29th and 30th Streets exposes alluvium, the stream-laid sediment that once supported productive farmland throughout Oakland’s flats. Mapped as “alluvial fan and fluvial deposits (Holocene)” or unit Qhaf on the geologic map, it covers more area than any other geologic unit.

The uppermost part, the brown stuff that the excavators have turned over in curls, is rich in organic matter and clay. A little deeper it turns tan as the organic matter thins out. It’s dense and firm, good ground for building.

The nearer you get to the bay, the finer grained this material gets — more clay, less sand and gravel. Streams have carried it down from their canyons in the hills over the last few hundred thousand years, pushing back the sea. And by “streams” I mean floods. The clear trickling streams we know are actually asleep. Floods are the one day in a thousand when streams awaken, picking up and carrying alluvium from place to place.

Occasionally the streams themselves jump their tracks. If you visualize the land in super-fast motion over geologically recent time, our streams whip back and forth over the coastal plain like firehoses out of control, winnowing the alluvium again and again. From the hills outward they build up low, cone-shaped piles of sediment called alluvial fans. Downstream, these coalesce into an alluvial plain.

The “h” in “Qhaf” refers to the Holocene time period. The Holocene (“fully new” in scientific Greek) began when the latest pulse of the ice ages ended, about 12,000 years ago. It’s been a mostly pleasant time. Many geologists argue, with good reason, that the Holocene has given way to a new permanent state of wrenching climatic changes. Because the natural balance of climate is strongly influenced by human activities, they argue, the climate system today is a writhing firehose we may be able to control. They propose to call our new era Anthropocene time.

Clay outcrop in Horseshoe Canyon

25 January 2016

The gorge of Horseshoe Creek, in Leona Heights Park, is unusually grand for its size. Its rugged rocks, mostly Leona “rhyolite,” are pretty homogeneous though.

horseshoeclay-1

So when a spot along the stream caught my eye with its color — reddish red and bluish gray — I went off the trail and checked it out. Notice that the surface is cut into the hillside.

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This streamside lump looked just like concrete. But there was a lot of it, in different states of preservation and age, so I took it as a natural deposit.

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It was hard, but a little higher up I was surprised to find soft material. Not just soft, but pure clay.

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The high-silica lava and volcanic ash that makes up the Leona should weather into kaolinite (white china clay), especially under acidic conditions. We have that combination in parts of the Leona that are rich in pyrite. This mineral, with the formula FeS2, reacts with air to form iron oxyhydroxides and sulfuric acid (here’s a brief treatment).

There may be a pod of rock here with a different texture or composition from its surroundings, which might account for the purity of the clay. But I don’t actually know how pure the clay is. The way to tell would be nibbling it. Maybe on my next visit.

I think that a gradient in pH, plus interactions with air and surface water, explains the transition from gray to white to red clay as you go from depth to the surface.

horseshoeclay-5

Without a lab, there’s not much I can say about it, although geologists with more experience probably know this stuff cold. If so, speak up. There was another piece of evidence at the scene, though: a bit of leaking “yellowboy” from the floor of the streambed.

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It means there’s a little bit of acid drainage here, not up to the level of the ex-sulfur mine just south of here. More like a geologically slow bit of natural acid drainage. It will be interesting to watch this spot during this wet winter.

A peek inside the Fan on Piedmont Avenue

19 February 2015

Construction is going on at the lot formerly occupied by a well-behaved motorcycle club, at 4225 Piedmont Avenue by the Kona Club. What caught my eye is that it offered a clean cut into the stuff that constitutes lobe 2 of the Fan.

I’ve referred to the Fan often over the years, but I haven’t formally introduced it. Here it is on the Oakland geologic map.

the-Fan

It’s a former alluvial fan that was last active during the Pleistocene, which has been dissected by several younger streams. There’s nothing else quite like it in the East Bay, and I think of it as the Fan with a capital F. I divide it into eight separate lobes. Lobe 2 has two separate parts, Pill Hill and Montgomery/Thermal hill. Anyway, I keep an eye on it because it’s rarely exposed. Only excavations and a few stream banks display it.

Here’s what it looks like from a distance.

piedmont4225-1

There’s indistinct bedding that slopes down to the left. The material is gravely clayey sand that’s quite firm and well behaved. Here’s a closeup of a gravely layer; the stones are large pebble size (about 50-60 millimeters) and represent the Franciscan rocks just uphill in the Piedmont block.

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Farther over, the wall of the excavation has been carved with a backhoe, and the clayey matrix is so strong that most of the stones have been cut in two, even the tough black argillite.

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This is alluvium—sediment carried and laid down by streams. The rock clasts are rounded, showing that they’ve been carried in a stream for some distance, although most of the rocks are sandstone that doesn’t endure long. The hardest chert pebbles are still pretty rugged.

Down on the ground was this very typical Franciscan chert boulder, shattered by the builders after enduring for more than a hundred million years.

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The lot will become a nice set of dwellings. The builders are blogging about the job, complete with cool drone shots.