Archive for the ‘sausal creek watershed’ Category

Oakland Conglomerate II

12 March 2009

Here are some of the samples I collected from the Oakland Conglomerate last month.

oakland conglomerate

This rock is utterly and thoroughly brown, deeply brown with iron oxides. You can’t scrub it off—it’s brown all the way down. That reflects two facts: the matrix is iron-rich volcanic material and it’s been pervasively shattered and flushed with fluids under oxidizing conditions to let the iron out. The clasts—the bigger stones in the conglomerate—are beautifully rounded, presumably in a vigorous river or beach. But the setting of Oakland at this time, in the Cretaceous, was way offshore in a shallow sea. It appears that all this coarse material was carried offshore in underwater landslides and laid down in a deep series of sandstones and conglomerate beds.

oakland conglomerate

Long after the conglomerate was laid down and lithified, the plate boundaries changed and the San Andreas fault system (including the Hayward fault) splintered coastal California and wrenched it northward, one earthquake at a time, for tens of millions of years. The forces of that time have crunched nearly every stone in the Oakland Conglomerate. Yet at the time these rocks were still deeply buried, and chemical action and pressure cemented the stones back together. The clasts are delicate, but intact. It’s impressive to imagine the force that shattered these hard stones like so many soda crackers. The next thing I want to do is open some of them with my hammer and see what the original lithologies are.

The Oakland Conglomerate

7 February 2009

oakland conglomerate

The Oakland Conglomerate extends from Montclair, a little north of Snake Road, south along the whole southern end of Skyline Boulevard and beyond Lake Chabot to the northern outskirts of Castro Valley. It’s part of the Great Valley Complex, a huge sedimentary pile spilled off the ancient Sierra/Klamath ranges in Late Cretaceous time, specifically the Turonian and/or Cenomanian ages (about 90 to 100 million years ago). This may be its northernmost exposure, on a fire road across from the Shepherd Canyon fire station where a basketball court has been cut out of the hillside. I brought home some samples and hope to have fun with them soon.

oakland conglomerate

What’s cool about this rock unit is that the big clasts are almost all cracked or shattered or dinged up. That doesn’t happen to these potato-sized chunks of quartzite and granite in riverbeds or the seashore, where the stones were originally shaped. Researchers at Cal State East Bay have argued that they were damaged by thousands of earthquakes on the Hayward fault as they lay buried some 5 kilometers down.

This rock unit is actually mostly sandstone, especially the farther south you go. It crops out all the way down to the Alum Rock area.

Shale concretion

2 October 2008

shale concretion

Just up the hill from the Paleocene sandstone I was looking at last week, on Woodrow Drive, all sign of bedrock disappears. But I kept looking and eventually spotted a dark pod of rock—a concretion. I banged it open and this is what I saw.

The piece in the middle shows the dark outer surface, probably a thin layer of iron or manganese minerals. To its right and above it is the core of the concretion. Its outer surface was pure clay that stained my fingers and got all over my pants. But the definitive field test is to nibble it. Pure clay, as found in shale, is not gritty but smooth as chocolate against the teeth. A mudstone or siltstone is definitely gritty. (Mudstone is a sedimentary rock with a mixture of particle sizes, clay and silt and sand.) And of course, sandstone you just don’t bite.

I don’t know much about concretions, really. Some have an obvious fossil or foreign stone in the center, which releases or attracts elements that create shells of harder minerals around them. In others, I suppose the central object dissolved away completely into the outer layers (this one had none). They can be little things or a meter or more in size. Bowling Ball Beach, up in Sonoma County, is full of them.

Anyway, this concretion told me that the rock along this part of Woodrow Drive is really shale, even though it’s hidden. Shale turns back to clay so readily that you don’t get good exposures of it except in a fresh roadcut. The rock here is mapped generally as Eocene mudstone, Eocene meaning about 55 to 35 million years old.

Fossil hunting

25 September 2008

There aren’t many fossils to be found in Oakland. Maybe microfossils—there must be lots of those in the young hilltop rock units. But on the geologic map is a little body of rock up in Montclair that is supposed to have “well-preserved coral fossils” of Paleocene age (that’s 65 to 55 million years old). Recently I checked it out.

outcrop

The rock unit has no formal name; it’s mapped simply as “unnamed glauconitic sandstone” and extends in a belt from Shepherd Canyon north over the ridge to Snake Road. This outcrop is at the intersection of Paso Robles Road and Shepherd Canyon Road. I examined it carefully and found nothing but massive fine sandstone, with almost no bedding and no sign of fossils of any kind.

outcrop

I traversed all the roads in the area where this rock unit was mapped, and the same sandstone was everywhere I looked. At this outcrop on Paso Robles Road, weathering and lighting combined to bring out some subtle bedding planes, but again no sign of fossils.

outcrop

At the north end of the belt, across Snake Road, is Armour Drive. Here is a large landslide scar that has broken Armour Drive in two, and there was plenty of loose rock for me to apply my rock hammer and take home a hand sample. This kind of rock is all I could find, period. But the unit is said to be “coarse-grained, green, glauconite-rich, lithic sandstone” interbedded with “hard, fine-grained, mica-bearing quartz sandstone.” That’s this stuff.

Somewhere in this belt is hiding a bunch of green, gravelly rock with coral fossils in it, and I haven’t looked hard enough to see it. On the other hand, geologic mapping is an imperfect art, and much of the hills has been mapped on the basis of aerial photos with limited work on the ground. Because I work exclusively on the ground, I’m in a position to do better. Maybe the fossils are only along one edge; maybe they belong to a separate subunit that needs to be mapped more precisely. Maybe someone made a mistake. So far, it’s a puzzle.

Lowland slides

7 April 2008

As Darby commented on an earlier post, you don’t need to be in the hills to find hazards. A good fraction of the residential landslides in Oakland get mentioned in the Tribune, and they happen downslope, too, mainly along the creeks. Last week my buddy Jef visited a notorious example along Wallace Street, downvalley from Highland Hospital. The one above, on McKillop Street in the Fruitvale neighborhood, made the news throughout 2006, and I prepared a rudimentary case study of it for my About.com site.

The clues are plain, both on the map and on the ground. Look below: Just a few steps from that pitiful collapsed house is this view toward the hills. It’s pretty; it’s worth a little extra on a home’s selling price. But why is there a big empty space in the middle of one of Oakland’s older neighborhoods? A fire might clear a lot of land, but people would rebuild. Why does McKillop Street have two parts, one here and a stub at the far end of the park? Why isn’t there a nice bluff along Sausal Creek like there is elsewhere?

Sausal Creek in flood

4 January 2008

dimondflood.jpg

With the heaviest rains I’ve seen in years, I checked out Dimond Canyon today to assess the power of the stream in it, Sausal Creek. The water was brown and impressive. It looked about waist-deep at most. I don’t know how this stream cut the canyon, which is a gorge more than 50 meters deep with stone walls. But I have a theory involving stream capture and movement on the Hayward fault, just upstream from the gorge. At various times, the fault has pulled the canyon past different watersheds. Perhaps lakes lay upstream, or landslides formed dams, that collected enough water to give the canyon a good downcutting once in a while. I hypothesize that the stream’s watershed was once quite a bit larger, perhaps even the valley now occupied by Chabot Reservoir. But the timing has to work.

There are at least two other gorges in the Oakland foothills that appear oversized to me: the upper reaches of Cemetery Creek, along Moraga Road, and the canyon of Peralta Creek in Redwood Heights, best seen from Rettig Avenue north of 35th Avenue.

It is recorded that the early loggers who stripped the redwoods out of the Oakland hills used to float their logs down Sausal Creek to the bay. All I can say is, there must have been a lot more water in the hills back in the 1850s, because even today’s deluge couldn’t have done that.

BTW see the Friends of Sausal Creek site.


Follow

Get every new post delivered to your Inbox.

Join 4,096 other followers