Archive for the ‘Oakland sandstone/shale’ Category

The Paleocene blob revisited

15 February 2021

It may seem to readers like I’ve been out in the field during the past year, but in fact I’ve been holding back since the lockdown last March, taking my own advice. Last week, on the verge of receiving the Covid vaccine, I decided to formally resume geologizing, and start by giving a small patch of rocks a new, more thorough inspection.

It was thirteen years ago when I first reconnoitered this odd little area of rocks, shown on the geologic map as unit Ta, “unnamed glauconitic sandstone (Paleocene).” It’s described as “coarse-grained, green, glauconite-rich, lithic sandstone with well-preserved coral fossils. Locally interbedded with gray mudstone and hard, fine-grained, mica-bearing quartz sandstone.”

Here’s a closeup in Google Maps with the outline of the “Ta” unit. It manifests as a ridge-forming substrate that is undermined by an active landslide scar (part of which is the notorious Snake Road/Armour Drive landslide) on its northwest end.

In a systematic approach, I sought out the three places marked on the map with strike-and-dip symbols. (I used this same strategy a couple years ago with the overlying Eocene mudstone unit.)

The northernmost site, at the stub end of Armour Drive, is hopeless; it’s been thoroughly disrupted by the Snake Road landslide, and the fortress houses being built on the scar will disturb it more as the owners landscape their grounds. There were no good exposures at all, let alone one showing beds dipping 80 degrees south. But this is what some of the rock looks like: a dark siltstone with a greenish tinge and a bit of clay in it.

The middle locality was where my hopes were highest — an aborted foundation pit on Saroni Drive where the “well-preserved coral fossils” had been documented. In fact, I had asked Russ Graymer, compiler of the geologic map, about this pit. That was in 2009, which by his account was 14 years after he’d visited it (or a good 25 years ago today). He replied that his notes from the site were as follows: “The rock here is massive, black, coarse-grained, glauconitic sandstone and pebbly sandstone. There are many fossils here, including pecten, coral (Paleocene?), shark teeth, and snail. There is also pink-brown siltstone and brown mudstone.”

All I can say is I wish I’d been here 25 years ago.

I gave the site a thorough look, without hammering anything as is my practice. I saw no pebbly mudstone, not even any coarse-grained sand. I noted clayey siltstone and silty shale, hard here and soft there, with fine to massive bedding. On the lefthand side the shale beds were vertical, with the upper side to the east. Nothing that I could possibly interpret as overturned beds with a 60-degree dip.

Elsewhere the rocks had no reliable bedding. Down in front were some crumbling mudstone boulders. One of them had some vague fossil-like shapes that fizzed in acid, but the eyes can be fooled and our rocks commonly have some lime in them. It’s not always meaningful, though I always check for it.

You may wonder how this rock unit was determined to be of Paleocene age, unique in Oakland. As I recall our conversation, Graymer was accompanied that day by Earl Brabb, who said the corals reminded him of Paleocene corals he knew from the Santa Cruz Mountains. In fact I wrote Brabb for more detail and he replied with the location of the roadcut he had in mind. But I never got over there, the email was lost, and Earl Brabb died a few years later. Now I would never gainsay Brabb’s judgment — he was a top-tier field geologist — but that’s the main line of evidence behind this age assignment.

I wish he had been with me at the third site. It’s under a power-line tower north of a bend in Balboa Drive and consists of thin-bedded siltstone, nicely tilted. This spot, at least, is still good.

The roadcut on Balboa Drive was where I hit paydirt. Bedding surfaces were exposed that included sole marks. These occur on the underside of beds, and they indicate that here the rocks are overturned, contrary to what the map shows.

And in the gutter of the curve, buffed by errant car tires, were a couple of these round, laminated objects nestled in situ among the siltstone beds. They responded to acid, indicating that the laminations included calcite. And the rocks nearby displayed a fine vein of solid calcite about 4 millimeters thick.

I would peg these as some sort of fossil, but Earl Brabb might well have said they were just like the Paleocene corals he knew from the Santa Cruz Mountains. The setting could have been a cold seep, such as are known elsewhere in the Great Valley Sequence.

The rocks of the Oakland Hills are poorly organized and poorly exposed, and hence not really well mapped. They’ve been overturned and broken and shuffled around. Whenever I try to make sense of them I doubt my senses; that’s the way the Earth just is here. A geologic map is as much an exercise in imagination as in observation. The pros are certainly better mappers than I am, but they aren’t superhuman and their work can be interrogated; the rocks can speak differently with each visit. The outline on the map, as far as I can tell by checking around its edges, is fairly correct — you’ll notice that every line is dashed, meaning it’s inferred, not firmly nailed down.

The “Ta” rock unit hasn’t revealed itself to me as a coarse-grained green lithic sandstone, more like a fine-grained sorta greenish lithic siltstone. Geologists train themselves and have tools to specify rock colors, but to me green is always suspect; our woods favor mosses and algae, and our weathering environment favors rusty colors.

The rock here is definitely something other than the Redwood Canyon Formation to the south and the Eocene mudstone to the north. It’s a little piece of somewhere different.

Mapping rocks never ends

1 February 2021

A few days ago I took part in the latest monthly meeting of my local geological society — we do it via Zoom these days — and our speaker, Christie Rowe of McGill University, reported on three research projects her grad students are doing in the Bay area, specifically the Franciscan Complex. The Franciscan is a scramble of different rocks that has challenged geologists since they first came to California.

Fifty years ago Stanford’s Gary Ernst recognized that the Franciscan represents the mess of material that gathers around a subduction zone, where oceanic crust (a now-extinct neighbor of the Pacific plate, in our case) slides beneath continental crust (the North America plate). So now we know what it is — the tectonic equivalent of the dirt in a bulldozer’s blade — and prompted by that knowledge we can try to unscramble the mixed-up pieces and learn what they might tell us about California’s geologic history or what happens in subduction zones.

Rowe is a Marin County native who’s been working since her PhD days on the latter problem, in the Franciscan rocks of her home ground. Specifically, she’s been looking for preserved bits of ancient earthquake faults. Normally these are buried deep underground, but they’re important because subduction-related earthquakes, so-called megathrust events, are the largest on the planet. Think of the 2011 Tohoku earthquake in Japan, the magnitude-9 monster whose tenth anniversary is coming up on 11 March. The Marin Headlands are full of them, broken in pieces.

Rare bits of the Franciscan have survived being subducted deeper than 25 kilometers and then returned to the surface, without totally wiping out what happened to them down there. The work requires dogged persistence. You have to look hard to find these “high-grade blocks” in the first place, then put your face close to them, magnifiers out, detect signs of slippage, then bring samples to the lab and determine what that slippage means — whether it happened on the way down, on the way up or afterward as the San Andreas fault system wrenched it all sideways.

Heart Rock, at Jenner Beach up the Sonoma coast, is small enough to fit inside a living room. One of Rowe’s grad students is mapping it at centimeter scale, spending a master’s thesis worth of effort on this one outcrop looking at rocks like this.

Seeing all this during Rowe’s talk took my mind, among other places, out to the rocks of Shepherd Canyon and Redwood Peak. The last person to give those strata a PhD-level scrutiny, using all available tools of the time, was a Berkeley grad student named Jim Case around 1960. Yes, 1960, a time when researchers were stuck in a mental framework of now-forgotten concepts and plate tectonics was still years in the future, when optical microscopes, brass seives, fossil correlations and test-tube chemistry were the best tools we had.

Case got his PhD, demonstrating that he’d mastered these tools as well as the literature, but he didn’t accomplish much more than correct a couple of ideas from earlier studies, establish a few new rock units on the map and tentatively correlate them with other units scattered around the East Bay. He put his little brick into the Wall of Science, then went on to a long research and teaching career doing other things.

Since then, other distinguished geologists have been over this territory. Case collaborated with Dorothy Radbruch of the USGS, a sharp and able field geologist. And in the late 1990s when Russ Graymer was putting together the East Bay geologic map that I rely on, he tramped the area with the late Earl Brabb and was ably advised by the late David Jones. Each of these workers found new things and revised their predecessors’ achievements. It always paid to reinspect the rocks. Nevertheless, none of them pulled out all the stops and pioneered a new in-depth reassessment of this interesting area.

We could do much better today. Every tool has advanced. The jigsaw puzzle of ancient California is far enough finished that any piece, if studied closely enough, can be placed on the table near — or even exactly on — its correct position and joined to other pieces. This would be more satisfying than what Case could accomplish in his time. We just need another grad student to take it on, another local who has imprinted on his or her home ground.

Mapping never ends, and geologic mapping always improves. New bits of rock are being exposed all the time. Fresh eyes see new things, and persistence furthers.

In search of McAdam’s quarry

3 August 2020

Alexander McAdam (1854?-1920s) was a minor character in Oakland’s history who left a highly visible mark in our cityscape. A Canadian farmboy who was orphaned at a young age, he came to California after apprenticing as a wheelwright, and after eight years he saved enough money to buy a farm “at the head of Thirteenth avenue,” according to a short biography by James Guinn in 1907. “He was successful in this occupation, but in the meantime had discovered a sandstone quarry on his property. Upon the sale of his farm he acquired considerable financial returns. Stone from it has been used in many of the largest buildings of Oakland, among them being the Unitarian Church, the last buildings of the deaf and dumb asylum, numerous retaining walls, and for many other purposes.”

This caught my eye because I have long thought that Oakland’s rocks were exclusively used as crushed stone. Yet here in the First Unitarian Church, ashlar blocks of genuine Oakland sandstone form the dignified cladding of this important cultural monument and civic institution, built in the early 1890s under the energetic leadership of a leading Progressive of his time, Rev. Charles Wendte.

Rev. Wendte oversaw the building project from his home across the street. The stone cladding was the costliest item in the project, and he singled it out in his memoirs: “Our employment of stone led to vexatious complications. Quarrymen were unable to deliver this material in sufficient quantities, workmen struck for higher pay in handling it. Contracts were broken or remade.”

I had to track down this stone somehow. The documentary clues are slim, and any signs of the quarry appear to be lost. But first, there is the stone itself.

It’s a fairly sound stone of an even consistency with a warm grayish-brown color and massive (i.e., absent) bedding. The block serving as a lintel over the doorway probably broke during the 1906 earthquake, when most of the cladding along Castro Street and the top of the tower collapsed. (The tower was rebuilt without any stone, a smart move.)

A closer look shows that the stone actually varies (although some of that may be substitute stone from another source, as Wendte’s wording suggests), and that a century of exposure has caused a fair amount of spalling. No wonder there were quality problems during construction.

A still closer look reveals it as a medium-grained wacke (“wacky”): a sandstone with grains no larger than a millimeter and a large component of minerals that are not quartz. The black grains are mostly biotite mica; without a microscope I’m limited in what more I can say.

It’s familiar to me. It’s not the Franciscan sandstone produced by the dozen or so quarries in and around Piedmont. I can rule that out categorically. It’s from the high hills on the far side of the Hayward fault.

All of this is consistent with the documentary evidence placing the source in Montclair. The “head of Thirteenth avenue” is where Park Boulevard, the former 13th Avenue in Brooklyn Township, meets Mountain Boulevard. It’s the intersection at the bottom of this excerpt from the 1897 topo map.

To orient (or disorient) you, here’s the same area today.

The “XII Report of the State Mineralogist,” published in 1894, said the following about McAdam’s quarry: “It is in Medos Cañon, back of Piedmont, and is a small quarry, producing sandstone for rubble and ashler [sic]. It is not worked regularly.” The official who wrote that description, a busy guy on a quick visit to cover the whole county, wrote down “Medos Cañon” when someone said “Medau’s canyon,” meaning the valley of present-day central Montclair where the dairy farm of John H. Medau once lay. I believe that if the site had been in Shepherd Canyon, his informant would have said so as that name was in wide use at the time.

All of this means that the quarry could have been a good exposure of the Redwood Canyon Formation, a wacke of Late Cretaceous age, that forms part of the east side of Montclair’s valley along the Hayward Fault. It’s the unit marked “Kr” on the geologic map, below. The lithological description of the unit, and the composition data from Jim Case’s 1963 Ph.D. thesis, are close enough to the stone in the church.

But also likely is the Shepard Creek Formation (Ksc) and even the Oakland Conglomerate (Ko), when you consider that the units are only subtly different, variable in composition and not well mapped despite the best efforts of competent geologists.

In any case, I had a good time visiting these rock units along the Montclair Railroad Trail the other day. There’s a lovely outcrop of the Redwood Canyon Formation above the trail along the route of the recently upgraded powerline, southwest of the word “grade” on the map. That warty weathered surface, reminiscent of the Incredible Hulk’s hide, is one of this unit’s distinctive features.

But the rock there’s not a good match.

Neither is the rock in the landslide at the upper end of the trail.

And just for good measure, here’s a chunk of sandstone from the Oakland Conglomerate. The material is coarser and wacke-er, but again under the 10X hand lens it’s not like the church’s stone.

Nowhere in this area, in many years of visits, have I seen a body of rock big enough and sound enough to support a quarry capable of producing ashlars — not on this side of the Hayward fault. The nearest quarry site is down Park Boulevard where the Zion Lutheran Church sits today, the former Heyland/Diamond Cañon/Bates & Borland quarry on the side of Dimond Canyon. But that produced crushed Franciscan sandstone, something quite unlike McAdam’s stone.

I can only conclude that McAdam found a lucky hillock on his farm and made the most of it, one that’s been obliterated during the waves of development since 1890. And the site of his farm is, as we say, poorly constrained. Even his life dates are fuzzy. But his accomplishments include making a profit from farming, acquiring a large home in Temescal, serving two terms on the City Council in the nineteen-oughts, and equipping an important building with a handsome exterior (despite the vexation he caused Rev. Wendte). I can’t confirm when he died or where he’s buried, so this building surely is his monument.

While I was researching this post, the papers covered a lovely story about how archeologists used advanced geochemistry to pin down the source of Stonehenge’s biggest stones, a peculiar sandstone known in Britain as sarsen. The New York Times version was my favorite writeup, and the hardcore details are in Science Advances in an open-access paper.

The Eocene mudstone, part 2: Shepherd and Thornhill Canyons

29 April 2019

Part of exploring Oakland’s geology (and writing the book on it) is digging deeper, ever deeper. Two posts ago I dug into the unsung body of Eocene-age mudstone in the high hills, doing a systematic survey of its mapped extent, and had to stop halfway. Since then I’ve surveyed the other half, and it still feels like I’ve just begun. But so be it.

The ideal is to learn all of the significant outcrops. That would take a trip down every road and byway, and I’ve done that once already just for reconnaissance, not to pinpoint outcrops. Because life is finite, this time I figured out a shortcut based on the geologic map, where significant outcrops are ready-mapped.

The outcrops in unit “Tes,” the Eocene mudstone, are marked by those little symbols: a line with a tick sticking from the midpoint, labeled with a number. Each symbol tells you the orientation of the rock beds at that spot. The long line shows their strike — the direction the beds would align if you shaved the ground level — and the tick signifies their dip direction — the downhill direction of the beds. The number is the angle, in degrees, at which the beds slope in the dip direction.

For my purposes, all I wanted was the location, which is right where the tick is. I plotted those locations on a street map and set off to visit each one.

Before we start, this is an interesting image. It shows that the terrain where unit Tes is mapped is stronger, more resistant to erosion, than the Redwood Canyon Formation (Kr) to its south and the Sobrante Formation (Tsm) to its north.

This survey will go from east to west, the same way I walked it. The first outcrop, on the rim of Shepherd Canyon at Skyline, labeled “53,” was too far to hike so I skipped it. So we’ll start down in the canyon at the one on Woodrow Drive. I’ve shown you this one before; it’s where I found that cool concretion back in 2008. Supposedly the beds there are vertical, with the original upper side facing south (the black ball on the symbol means that there are indications of the original top and bottom of the beds). You can’t tell that from the outcrop, because it’s pure shale and the rock is so degraded, but there are still concretions weathering out. According to the map, then, we would be looking at the top surface of that concretion.

Around the corner on Paso Robles Drive is this exposure. It matches the map symbol in displaying overturned beds with a 65-degree dip. If you flip it over in your mind, you can see that a layer of fine-grained sand spilled over a muddy seafloor, and the flat surface is its underside.

The next symbol, the one marked “70,” is on Saroni Drive just east of Sayre Drive, but there’s no rock visible there today. It appeared to me that a new house has been built on the spot, or maybe the outcrop is in a back yard and is inaccessible. But farther west on Saroni, right at the edge of the “Tes” belt, some of the rock is exposed: a clean siltstone with the typical blonde color.

Now we cross the crest of Colton Boulevard and enter Thornhill Canyon. The next outcrop, on the east side of Armour Drive, is a roadcut exposing shale that has degraded since it was mapped. But you can still see the bedding’s steep leftward (northward) tilt, along with some near-vertical jointing.

The outcrop just west of Aspinwall Road is on a large vacant lot that used to be accessible (I recall visiting it during a walk led by Dennis Evanosky a few years back), but is now fenced off. Too bad. On the uphill side of Aspinwall is an exposure of clean siltstone, but its orientation is unreliable — these might be loose boulders, not living bedrock. Typically a geologic mapper measures strike and dip at several spots using a special compass/clinometer, often called a Brunton after the most highly regarded manufacturer. I have one, but a smartphone app does almost as well.

Crossing to the north wall of Thornhill Canyon, a steep climb up Beauforest Drive gets you to Valley View Road. The roadcut where the symbol labeled “80” sits is all mossed over. The Eocene mudstone prefers to support vegetation rather than crop out, and until some maniac cleans off the overgrowth or the hillside collapses, whichever comes first, this exposure is retired.

Two more exposures to go. The first of these is farther down Valley View, right next to the uppermost leg of the Upper Merriewood Stairs. It’s a good one, displaying a dip of 56 degrees east just like the map says.

Once you make it up the stairs, the rest of the walk is real pleasant, up to Broadway Terrace and across to the end of little-traveled Virgo Road. Unfortunately, there’s no sign of an outcrop there — either it’s covered with grass at this time of year, or a new house has obliterated it, or I’m just blind. But if you poke around, the views are wonderful. So that’s some consolation.

Getting back home from here is left as an exercise for the reader.

With all that work, I managed to confirm just three of the nine outcrops in this part of the map. Should I, and future mappers, accept the rest of these measurements if they can’t be confirmed? Should we accept them now? One approach to this conundrum is to consider previous geologic maps. I have four of them, and none of them agree. Some of the outcrops on this map also appear, with the same numbers, in the county geologic map of 1996, but that’s because the same guy, Russ Graymer, prepared them both. He measured just two or three outcrops that also appeared in two maps from the 1960s, and his numbers didn’t match theirs. The earliest map, published in 1914, might as well show a different planet. (See how it showed Knowland Park in this post from 2015.)

So I guess the upshot is that every generation of geologists learns the landscape anew, and by extension, that includes me. The certainty of a geologic map is always provisional and subject to correction, or at least to change. It can be disconcerting to realize that geologic knowledge is not necessarily cumulative, authority may not be authoritative, and rocks are not that firm a foundation.