|See the world (and its fossils) with UCMP's field notes.
|SEARCH | GLOSSARY | SITE MAP|
About halfway through tunnel construction, PaleoResource's contract expired and Caltrans arranged to have Garcia and Associates of San Anselmo, California, continue with the monitoring and collection of fossils. To address preparation, Caltrans approached UCMP Associate Director of Collections and Research Mark Goodwin. As Goodwin explained it, "A master agreement exists between the California Department of Transportation and UC Berkeley. They came to us, discussions proceeded, and I drafted a one-year agreement and budget in collaboration with my colleague at Caltrans. The contract was awarded to UCMP for preparation, curation, data management, and other activities we could provide based on our paleo-expertise."3 At the time, the museum was without a preparator, but in the contract, Caltrans agreed to provide the funds needed to hire a prep lab manager for one year. This individual would manage the prep lab and supervise grad and undergrad students in fossil preparation and curation. Funds were also provided for equipment, tech support, and the development of this web feature.
Foraminifera (foram) tests occur in marine sediments dating back as far as the Cambrian; their high diversity, abundance, and evolution make them useful in determining the relative ages of rocks. They are also important in reconstructing paleoenvironments. For example, the composition of foram assemblages helps determine the paleoenvironment of deposition and the analysis of oxygen isotopes in the tests can be used to establish water temperatures. Forams are sensitive to food availability, as well as salinity and temperature variations, so they are excellent indicators of environmental change.
Ken Finger, UCMP's Senior Museum Scientist in charge of the museum's microfossil collections, identified the three most common foram taxa in the Claremont Formation as Martinottiella communis, Pyramidulina acuminata, and Lenticulina sp. Today this benthic (bottom-dwelling) association occurs on the continental slope, no shallower than 500 meters, suggesting that the waters were quite deep at the time of deposition. The cherts of the Claremont Formation were formed almost entirely from the frustules (cell walls made of silica) of diatoms (a group of single-celled algae), however, they are difficult to process because "… the diatom silica has dissolved and recrystallized, forming a very dense microcrystalline rock."4
Susan's microfossil-separation process did not work so well for the rocks of the Sobrante Formation they proved to be much more resistant to breaking down. Susan experimented with other methods but without success. A few forams were visible on the surface of some rock fragments but identification to genus was not possible.
Although the sediments of the Orinda Formation were of terrestrial origin, Susan applied her process to the Orinda samples hoping to recover some freshwater ostracodes, but the laboratory processing yielded only a few charcoal fragments.5
Aside from some bivalve shell fragments and a single bivalve shell mold from the Claremont Formation, all invertebrate fossils (not counting microfossils) found during the Fourth Bore excavation came from the Sobrante Formation; none, not even freshwater molluscs, were found in the Orinda Formation. Most of the recovered fossils were in a fragmentary condition. These included bivalves, gastropods, scaphopods (Dentalium), and arthropods. Their presence supports the interpretation that sediments from the Sobrante Formation accumulated on a continental shelf.
Although the Orinda leaves are not that well preserved, a number have been assigned to genus; with further study more taxa may come to light. The taxa that could be identified should look familiar to Bay Area residents because they grow here, as well as in other parts of the state, today. Some of the fossil species are very similar to California "native" species such as Umbellularia (California bay laurel), Salix (willow), Quercus (oak), Platanus (sycamore), and Betula (birch). Others like Ulmus (elm) and Persea (evergreens belonging to the laurel family, e.g., avocado), however, are not considered "natives"8 according to our present-day anthropomorphic view, but clearly they were an integral part of the regional flora in Miocene "California." In addition, the Orinda Formation rock samples provide an opportunity for the possible recovery of fossil pollen and spores (palynomorphs), which could reveal much more about East Bay plant diversity in the Middle to Late Miocene, especially for the herbaceous plants. The leaves of herbaceous and ground cover plants are not often preserved as megafossils, therefore, they are not well known or well-represented in fossil floras.
About 16 miles (as the crow flies) east-southeast of the Caldecott Tunnel's east portal is the Black Hawk Ranch Quarry. The fossils found at this historic locality are also Miocene in age, but are perhaps a few hundred thousand years younger (9.0 to 9.7 Myr9) than the Orinda Formation. What is unusual about the rocks from the Orinda Formation and the Black Hawk Ranch Quarry is that both vertebrate and plant fossils are preserved. Normally, a depositional environment will favor preservation of one or the other, but not both. However, having both kinds of fossils is valuable in that you can see what herbivores were present and get a good idea of what they were eating.
Of the 200 fossil leaf specimens collected at the Black Hawk Ranch Quarry in the late 1930s-early 1940s, 195 of them were identified as being from poplar, elm, willow and sycamore.10 The remaining five leaves were from mountain mahogany, sumac and two species of oak. Using modern floral communities for comparison, paleobotanist Daniel Axelrod (B.A., M.A., and Ph.D. from UC Berkeley) reasoned that the dominant assemblage represented a floodplain community receiving a moderate amount of rainfall, including summer rain Axelrod put annual rainfall at about 25 inches. The mountain mahogany, sumac, and oaks were probably from a woodland-chaparral community that existed some distance away.
The leaf fossils from the Orinda Formation also suggest a floodplain community, which is in accord with the geology. And given their somewhat battered condition, the leaves probably underwent some transport before being buried; this too is in keeping with the geology. The presence of Persea, which is not particularly drought tolerant, may indicate that rainfall was a bit higher than at Black Hawk Ranch or perhaps the area benefited from coastal fog; Persea is also not frost tolerant so the area must have had mild winters.
The biggest difference between the East Bay's Miocene climate and today's is the change from year-round rainfall to the current Mediterranean climate: wet winters and dry summers. This change actually began just a little later in the Miocene due to "cooling of deep seas and upwelling of ever-colder water along the Pacific coast."11 The causes of the cooling and upwelling can ultimately be traced to plate tectonics see Minnich 2007 (pp. 43-70) for a complete explanation. It is interesting to note that the annual rainfall for the city of Orinda today is about 27 inches close to Axelrod's estimated annual rainfall for Black Hawk Ranch but between December and March, the city averages 4.8 inches of rain per month, and between June and September, a mere 0.1 inches.12
Plants have often taken a back seat to vertebrates in artists' reconstructions of the East Bay Miocene environment. The first reconstructions, based on the fossils recovered from the Black Hawk Ranch Quarry, were visualized by artist William Gordon Huff in the late 1930s. Looking at Huff's restorations, one might think that the landscape was a barren plain with only the occasional tree cropping up here and there. But one can't really blame Huff he was recreating the scene as envisioned by Charles Camp and Ruben Stirton, two of UCMP's vertebrate paleontologists who were certainly more interested in the animal life. In actuality, the plant fossils from both Black Hawk Ranch and the Orinda Formation support the idea that the landscape was quite heavily wooded. As Diane Erwin, UCMP's Senior Museum Scientist in charge of the paleobotanical collections, put it, "… those browsers and grazers needed to be eating something."13 Compare one of Huff's interpretations (below, top) to a more modern, and accurate, depiction (below, bottom).
In the excavation of the Fourth Bore, an additional 35 vertebrate specimens were recovered from the Orinda Formation, some of them new to the list of known Orinda animals. Only two specimens, a rodent dentary and a turtle plastron, could be identified to genus (Copemys and Hesperotestudo, respectively); still, many could be identified to family. Bones of rhinos, oreodonts, horses, camels, rodents, turtles, tortoises, and even fish (freshwater we assume) have been found.
So far, there is only one entry in the database recording the presence of vertebrates in the marine Claremont Formation: fish scales. However, GSR Ashley Poust reports that the Claremont yielded two bones from marine mammals that have not yet been cataloged.
Over 200 vertebrate specimens from the marine Sobrante Formation primarily fish bones, scales, teeth (shark), and "fish hash" have been cataloged and are in the database.
The Caltrans-UCMP contract was a boon for UCMP; the museum vastly increased its collections of East Bay Hills fossils and received the resources necessary to prepare and catalog them. In providing those resources, Caltrans demonstrated that it appreciates the importance of California's fossils, and, since people are continually fascinated by fossils, it received a good deal of media coverage and good publicity as a result. There is no question that the deal was a win-win for both parties and UCMP looks forward to more such collaborations in the future.
Below is a table of the Fourth Bore fossils that have been cataloged to date (11/25/13). More material remains to be processed and cataloged so the number of specimens and perhaps the kinds of organisms represented are sure to increase.
Fourth Bore fossils in the UCMP database
1 To find out more about these laws and regulations, see the section on Paleontology in the online Standard Environmental Reference (SER). The SER is a "resource to help state and local agency staff plan, prepare, submit, and evaluate documents for transportation projects." [from the SER home page]
2 From this KTVU news story (accessed 10/29/2013).
3 Personal communication, July 12, 2013.
4 Ken Finger, personal communication, Nov. 25, 2013.
5 The presence of charcoal in the Sobrante and Orinda Formations suggests that the coastal area was prone to periodic fires.
6 Evernden, J.F., and G.T. James. 1964. Potassium-argon dates and the Tertiary floras of North America. American Journal of Science 262:945-974.
7 Wolfe, J.A. 1981. A chronologic framework for Cenozoic megafossil floras of northwestern North America and its relation to marine geochronology. Geological Society of America Special Paper 184. Pp. 39-47.
8 Calfora: http://www.calflora.org/
9 Prothero, D.R., and R.H. Tedford. 2000. Magnetic stratigraphy of the type Montediablan Stage (Late Miocene), Black Hawk Ranch, Contra Costa County, California: Implications for regional correlations. PaleoBios 20(3):1-10.
10 Axelrod, D.I. 1944. The Black Hawk Ranch flora. In R.W. Chaney (ed.), Pliocene Floras of California and Oregon. Carnegie Institution of Washington Publications 553:91-101.
11 Minnich, R.A. 2007. Climate, paleoclimate, and paleovegetation. Pp. 43-70 in M.G. Barbour, T. Keeler-Wolf, and A.A. Schoenherr (eds.), Terrestrial Vegetation of California. University of California Press. 712 pp.
12 From http://www.weather.com/weather/wxclimatology/monthly/graph/USCA0813 [accessed Nov. 20, 2013].
13 Personal communication, Nov. 20, 2013.
HOME | SEARCH | GLOSSARY | SITE MAP | FREQUENTLY-ASKED QUESTIONS