Archive for the ‘Oakland fossils’ 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.

Upper Arroyo Viejo: My first Oakland fossil

23 May 2016

Two weeks ago I wrote about the upper part of Arroyo Viejo, in Knowland Park, and said that I hadn’t walked the whole section exposed along the stream. Soon afterward I returned there and did the deed. I had a special goal of locating fossils in the Knoxville Formation.

This time, instead of following deer trails along the hillside, I went up the streambed, starting at the conglomerate outcrop I showed in that post. This is a closeup.

KJk-traverse1

It was pretty easy going for the most part. There have been no recent landslides into the creek, and in general you can see that the creek has been cutting down leaving tree roots–and bedrock–exposed. Most Oakland creeks aren’t this vigorous.

KJk-traverse2

Most of the section is coarse-grained rocks, sandstone and conglomerate. Here’s an outcrop that exposes conglomerate on the left and sandstone on the right, all steeply tilted.

KJk-traverse3

Researchers from both the US Geological Survey and UC Berkeley have found and collected fossils near here. (In fact I think they were all the same person, Jim Case, who did his Ph.D. work here in the early 1960s and then expanded it for the USGS in Bulletin 1251-J.) Many rocks have microfossils in them, which are useful for experts but not very thrilling for the average amateur geologist. What caught my interest was that these were macrofossils, the remains of regular everyday-sized organisms.

To find macrofossils, you want shale. There were chunks of it in the streambed, but outcrops were scarce.

KJk-traverse4

The fossils were reported as being shellfish of the genus Buchia and unspecified belemnites. Buchia was a group of chunky, oblong bivalves much like clams that lived during Jurassic and Cretaceous time. Belemnites were squidlike creatures with internal shells that lived around the same time. You’ll see polished specimens in any rock shop.

I didn’t come prepared for fossil hunting, with the right hammers and chisels and so forth; I just wanted to see if I could spot some on the ground. Rocks like this, I gave a searching inspection.

KJktraverse-5

Then, there it was in the middle of the streambed–a belemnite mold.

KJk-belemnite

A mold is the space that once held a fossil. Here the stream managed to dissolve the calcite shell and pluck it out of the rock. So this isn’t much of a fossil, and belemnites aren’t useful for zeroing in on the age of a rock the way Buchia is, either. But it’s proof of concept. I was as thrilled as when I was a kid grubbing brachiopods out of the Devonian shales of upstate New York, once upon a time.

The fossil treasury of UCMP

18 May 2015

Last month, as part of Cal Day, they were giving backstage tours at the UC Museum of Paleontology. The day is a mob scene, but on the last tour of the afternoon I had a fine time being shown around by two of the museum’s scientists.

The collection is vast, the world’s largest university collection. Only part of it is in the Valley Life Science building. There’s also a building in Richmond, and there are great accumulations of tar-seep fossils (from La Brea and McKittrick) housed in Sather Tower—the Campanile—because they smelled too much to keep with the others.

The collection at Valley Life Sciences is housed in huge cabinets that slide along tracks to open enough space to pull out the drawers. Many of those drawers are full of microfossils.

UCMPmicros

These are a huge part of California’s heritage. It was a thorough knowledge of microfossils in the rocks of the Central Valley and Southern California that informed and guided our petroleum industry. California remains America’s fourth-largest oil and gas producing state thanks to this scientific-industrial heritage.

But naturally, the museum had some of their photogenic specimens out. This Triceratops horn, with slices sawn off to expose its interior, was a privilege to hold. Unfortunately, the Bay area has no dinosaur fossils at all, although a few marine vertebrates from Mesozoic times have been found.

triceratop-horn

This ammonoid shell was interesting because it has a beautiful set of tooth marks on it, courtesy of a mosasaur. In the corner of the photo is a drawing of the specimen, as published in a scholarly paper.

mosa-chomp

Mosasaur fossils, to my knowledge, are not known from California either. Not yet, anyway.

And this thing sitting on the floor gave me joy. It’s a stump from the world’s first forest, dug up in rocks of Middle Devonian age in upstate New York.

psaronius-erianus

The inscription reads, “Psaronius Erianus Dawson / [Hamilton Group] Tree Fern / GILBOA, Schoharie Co. NY.” The Gilboa forest was first excavated in the 1870s. When I was on About.com, I featured Gilboa in my list of geo-attractions of New York state, and I hope you’ll seek it out if you’re ever there.

This specimen was undoubtedly part of an exchange of fossils between UC Berkeley and the State Museum of New York, a traditional way for paleontologists to enrich each other’s collections. It was surely a source of pride for Joseph Le Conte, the geologist who was the University of California’s first president. His fossil collection was the nucleus of today’s museum.

Wood fossil

22 March 2013

I was lucky enough to examine this large specimen of fossil wood that I was told came from the hills of San Leandro.

SLfossilwood

People have told me about and showed me pieces of fossil wood from the East Bay hills before. I’m no expert on the subject, and I haven’t done a lot of fossil hunting in Oakland. But we have young terrestrial rocks around here in addition to the abundant marine rocks—primarily the Orinda Formation, of Miocene age. I assume this came from there.

People have told me about fossil wood in Wildcat Canyon, but that’s regional park land where collecting is forbidden. This specimen, I was told, was from EBMUD land, where collecting is not expressly forbidden, although I assume that vertebrate fossils (animal bones) are protected. I have purchased an EBMUD permit and have been looking forward to using it for responsible geologizing.