Oakland Geology

This post looks past the Tertiary Period into the previous slug of geologic time, the Mesozoic Era.

To geologists, our oldest rocks are not that old. To us a million years, as I said in Deep Oakland, is like the tick of a clock, and while Oakland rocks go back about 165 ticks there are rocks on Earth about twenty-five times older. Oakland is typical of the whole Bay area in its relative youth.

Oakland’s oldest rocks — the Leona volcanics and the Franciscan melange — have no fossils worth speaking of. Our oldest rocks that could preserve fossils belong to the Great Valley Sequence, a huge thick sheet of sandstone and mudstone that underlies the Central Valley and forms most of the ranges along its west side. Bits of it were torn off much later and moved around as the tectonics changed and the Coast Range rose, ending up in the Oakland Hills. They represent deep-sea environments, where underwater landslides carried sand and gravel far offshore and wiped out any bones or shells they might have had.

The oldest rocks in the Great Valley Sequence are mapped as the Knoxville Formation. They aren’t very tightly dated. The most meaningful fossil in the Knoxville for dating purposes is an elegant clamlike bivalve a couple inches long named Buchia piochii, first described in 1864.

^{Views of Buchia piochii from the Paskenta area, California, from Zacharov & Rogov (2020)}

A recent study in the western Sacramento Valley found B. piochii in rocks ranging from the late Tithonian to the Berriasian Age, between about 150 and 142 million years ago — the latest Jurassic and earliest Cretaceous. Another later species in these rocks, Buchia crassicollis, is somewhat younger, around 130 Ma.

Buchia shells were collected from the Knoxville in the Sequoyah area in the early 1960s, but I haven’t seen one. The only fossil I’ve managed to find in the Knoxville Formation was in that same area, this belemnite mold in the streambed of upper Arroyo Viejo:

Belemnites were squidlike organisms whose fossils are so widespread they aren’t useful for dating purposes.

The next younger set of rocks in the sequence, the Joaquin Miller Formation, has yielded a few ammonite fossils that indicate a much later date, around 100 Ma. Y’all know ammonites, those flat-coiled shellfish that went extinct at the same time as the dinosaurs, 66 Ma.

^{A teeny ammonite I got at a rock shop. Ammonites could be bigger than a meter.}

The next two overlying rock units, the Oakland Conglomerate and Shephard Creek Formation, are barren of fossils. Above them lies the Redwood Canyon Formation, which has yielded one piece of the uncoiled ammonite Baculites consistent with an age in the 70 Ma range. Needless to say, these rocks are poor hunting grounds.

The primary research on these fossils was done in the early 1960s. The people who did that research were lucky, in that the hills were a lot easier to study in those days. The spread of forests and houses since then has fenced off a lot of territory, and the passage of six decades has softened and crumbled the bedrock exposures where they made their collections.

Then there’s what I might call the passage of scientific time, which affects how we interpret published data. The odds that researchers will dig out the old specimens and re-evaluate them are slim. Still, I keep my antennae out for new findings.

What would be great is more fossils. The best place to look is in fresh excavations that expose good bedrock, and possibly in new landslide scars.

Oakland’s Pleistocene Fossils

Oakland’s Tertiary Fossils

My last post was about Oakland’s youngest fossils, which are unmineralized bones found in Pleistocene sediments. Our older ones are traditional fossils — remains encased in rock. They naturally fall into two different segments of geologic time, the Tertiary period (this post) and the Mesozoic Era (next post).

“Tertiary” is an old, unofficial name for the time between the dinosaurs’ demise and the beginning of the ice ages — the archaic part of the Age of Mammals. Geologists find it useful in the American West, where rocks of that age range (66 to 2.6 million years ago) are bountifully exposed. In print, scientists use the formal time units that make up the Tertiary, the Paleogene and Neogene Periods. Along with the younger Quaternary Period, those time units make up the Cenozoic Era.

Oakland has several packets of Tertiary rocks. The Paleogene Period (66–23 Ma) consists of three epochs, and first two are represented in the high hills. Paleocene (66–56 Ma) rocks are a small patch of strata in Shepherd Canyon containing squat little coral fossils, not much to look at where I’ve seen them.

Eocene (56–34 Ma) rocks are represented by a long unnamed strip of mudstone outcrops in Shepherd Canyon and Redwood Regional Park with no visible fossils, except small shells in occasional fossiliferous scraps like this.

Both of these rocks were laid down in the old California, when it was a classic subduction zone much like the current one in the Pacific Ocean off South America. They began as sand and mud deposited far offshore, and they have no bones of land animals, just remains of deep-sea life. They’re also full of microfossils, which I don’t get into in this blog but which are usually the key to assigning ages to sedimentary rocks.

Most of our Tertiary rocks are younger, of Neogene age. Specifically, they date from the Miocene Epoch (23–5 Ma), the middle epoch of the Neogene (between the Oligocene and Pliocene). They are three different packets of Miocene rocks, each formed in separate places but brought together as tectonic disruptions continued to break up the previous subduction zone. Even though these rock units are stacked alongside each other, the boundaries between them are faults and they represent independent environments.

Let me say right up front that I haven’t seen a single fossil in these rocks. Fossils disintegrate quickly, like the rocks themselves, once they’re exposed in roadcuts or in natural outcrops. However, a lot of fossils were recovered from fresh material deep underground during the construction of the Caldecott Tunnel’s four bores. The UC Museum of Paleontology covers that topic in detail.

The oldest of the three Miocene packets is the mudstone traditionally assigned to the Sobrante Formation, whose original sediment was laid down on a continental shelf at least 15 million years ago. The fossils it yielded were mostly sea life: scattered shark teeth, fish scales and bones, small shells and other unidentifiable bits, plus a few leaves and stems of land plants that washed out to sea in those days.

^{Small shell from the Sobrante Formation. UCMP record}

Next is the dramatic striped chert of the Claremont Shale, dating from 15–14 Ma and representing an environment deeper than 500 meters. Just a few shells and fish scales were found in it.

^{Yoldia submontereyensis from the Claremont Shale. UCMP record}

The youngest unit, the Orinda Formation, dates from about 12 to 10 Ma and represents a land environment. The sand and pebbles in it came from a physical setting somewhat like the present East Bay flats, where rising hills are drained by vigorous streams dumping coarse gravel and sand into basins alongside the hills. (The rocks that made up those hills have since been carried north by plate tectonics into Sonoma County.)

Fossils in the Orinda Formation include bones of many small and large extinct mammals. Some are related to modern kinds (camels, horses, tortoises, mice) and others that have no counterpart today (oreodonts etc).

^{Tooth of an oreodont from the Orinda Formation. These animals were piglike browsers.}

The Orinda also yielded many leaves of the plants they lived on.

^{Miocene elm leaf. UCMP record}

Overlying the Orinda is the volcanic Moraga Formation, which is mostly lava, but there are sedimentary beds there too, dating from the long periods between eruptions. And just over the city line is the still younger Siesta Formation, also from a land environment. Both units include some limestone laid down in freshwater lakes — rock like this.

Various researchers from UC Berkeley have found fossils of ancient beaver, rabbit, mastodon, camel, antelope and horse species in these rocks. That takes us up to about 8 million years ago.

A few years ago I offered some advice on responsible fossil collecting for KQED Science Quest.

The word “tertiary” means “third.” The geologic time period after the Tertiary is named the Quaternary (“fourth”), and it neatly encompasses the last 2.6 million years of cyclic ice ages. People with a little scientific latin may wonder why there aren’t Secondary and Primary time periods. In fact, geologists did use those names during the earliest period of their science, but not since the last two centuries. We quickly learned that the rocks aren’t that systematic. But these two names from geology’s simplistic infancy linger in our daily speech like fossils.

Next up, our fossils from the Mesozoic rocks.

For a place with such a wide variety of rocks, Oakland is not a great hunting ground for fossils, at least not for amateurs. But you never know. I’ll get into the rocks in subsequent posts, but today let’s check out the youngest slice of time, recorded in our sediments.

The lowlands near the Bay have ice age fossils, buried in the sand and gravel and clay — alluvium — that constantly wash down from the rising Oakland Hills. They date from late in the Pleistocene epoch, just a few hundred thousand years old at most. They’re actual, unmineralized bones, not petrified.

California was crawling with large mammals until very recently in geologic time. It was an epic scene every bit as impressive as anything in Africa. Bones of these creatures, almost always separated and broken, have been found in alluvium in Oakland and all over the East Bay. Bison legbones. Teeth of archaic horses and camels. Mastodon molars, football-sized, made to chew twigs and bark and leaves. Fangs of the formidable short-faced bear. It’s odd to me, though I’m no expert, that we have no examples of the sabertooth cat, Smilodon californicus, marquee species of the late Pleistocene. This guy.

^{Smilodon from La Brea at the UCMP. The smaller skeleton is that of a domestic cat.}

At least, the UC Museum of Paleontology doesn’t. The great UCMP collection comes almost exclusively from the tar pits at Rancho La Brea, in Los Angeles, and McKittrick just north of Taft in the San Joaquin Valley. Thousands of sabertooth cats were trapped in tar and died in that weird setting over the millennia.

Oakland’s fossils of late Pleistocene megafauna — mastodon, sloth, camel, bison, bear — came to UCMP from excavations in clayey alluvium: the Posey Tube to Alameda (1927), the pond at Montclair Park (1939), the Coliseum site (1964) and up near Oak Knoll (1964), among others. The two higher-elevation sites are dips in the topography related to the Hayward fault, where bones were buried quickly, safe from squirrels and decay.

^{Elbow bones of Arctodus simus, short-faced bear, from the Posey Tube excavation. Photo by Steve Edwards, used with permission. UCMP record}

These animal fossils, classified as a Rancholabrean fauna, represent a community that dominated California and the rest of North America from about a half-million years ago to just the other day, 10,000 BCE or so. A lush savannah of brush and grass supported the large herbivores, and the large carnivores, from sabertooths to condors, fed upon them.

^{Molar of mastodon Mammuthus columbi from the Montclair pond. Photo by Steve Edwards, used with permission.}

We have parts of Oakland made of alluvial sediment like these sites, but much older — the gravelly foothills on the downhill side of Piedmont that I refer to as the Fan. I infer them to be as old as about a million years. This alluvium hasn’t yielded any bones, to my knowledge, but if it ever does I predict they will represent the earlier Irvingtonian fauna. That would be exciting because the Irvingtonian is defined by a spectacular set of fossils dug from a gravel quarry, back in the 1940s, in the Irvington district of Fremont. It would be interesting to study another Irvingtonian locality. (There’s also one near Chowchilla.) In the meantime, you can see genuine Irvington fossils at the Children’s Natural History Museum in Fremont and learn about the city’s effort to build a bigger museum in Sabercat Historical Park.

We should keep our eyes open downtown, too. The Emporium building at Broadway and 20th, now known as Uptown Station, was built in 1928 with two basement levels, and the excavators uncovered ancient tree stumps down there. That forest would have grown at least thirty thousand years ago, perhaps much earlier. As new downtown buildings dig their footings below the Merritt Sand, we may find more.

^{Kneebone of giant ground sloth Glossotherium from the Coliseum site. UCMP record Image reproduced under Creative Commons Attribution 3.0 (CC-BY-3.0) license.}

Keep watching the ground!

Next up, our fossils from the Tertiary rocks.