Eocene Telephone

Recently, GG concurred in the observation that myths can persist in the scientific community and added his own story of the “ignorant sheepherder” comment supposedly directed by Whitney at Muir.  Some readers might have said so what, these are innocent little pieces of color commentary independent of the march of science.  So for those skeptics, a more significant example.

A lot of recent work has been done on the Auriferous Gravels.  These papers pretty uniformly assign a middle-late Eocene age to these rocks.  For instance, Cassel et al. (2009, Int. Geol Rev.) said “Middle – late Eocene flora from within the upper half of the sequence are the only dateable material in the prevolcanic gravel (MacGinitie 1941).” A later paper gets a bit more precise (Cassel and Graham, 2011, GSA Bull):

The “Chalk Bluffs flora,” from the auriferous gravels at You Bet Diggings (Fig. 1), has been used to estimate the depositional age. Originally described as Capay stage and interpreted as middle Eocene by MacGinitie (1941), the Chalk Bluffs flora is now considered to be early Eocene (48.6–55.8 Ma; Wing and Greenwood, 1993; Wolfe, 1994; Fricke and Wing, 2004), which is consistent with comparable floral assemblages in other recently dated sections (Meyer, 2003; Retallack et al., 2004; Prothero, 2008).

Hren et al. (2010, Geology) similarly date these rocks: “Plant fossils are classified as Chalk Bluffs Flora after their best-preserved occurrence, and are dated at 52–49 Ma by faunal and floral correlation (MacGinitie, 1941; Wing and Greenwood, 1993).” It would seem that these sediments are pretty firmly dated to 49-52 Ma.

Except that in fact there is no firm floral date for these rocks.

Here’s the deal: while there is a nice collection of floral fossils, they in fact have not been used to establish the age of these sediments. The gory details were laid out by Howard Schorn in an unpublished manuscript made shortly before his death. Read the pdf at the link to get the whole story, but here is the essence of the thing:

The original collections from Chalk Bluffs were assigned to the “Neocene” by the first workers in the late 19th and early 20th century. By the 1930s, the plants were suggested to be Eocene. MacGinitie in 1941 took things a bit farther–in fact a step or two too far, it turns out.  Near Oroville (some 45 miles NW of Chalk Bluffs) are marine sediments considered to be Capay stage in the Eocene; these underlie the marine Ione Formation which is generally considered to be correlative at least in part with the Auriferous Gravels. But MacGinitie claimed that the Capay age rocks were correlative with the Ione and thus the gravels, and on this basis assigned the Chalk Bluffs flora to the Capay stage of the Eocene. As the Capay is now thought to be about 49-52 Ma, the source age for the published statements above becomes clear. And yet this was a fallacious correlation that has propagated through the literature

Thus, Schorn goes on to point out, the ages of the Chalk Bluffs flora is in fact poorly known, and he argues that at least some of the collection is much younger (c. 32 Ma).  He suggests that some interpretations of paleoelevation is reflecting an error in age assignment over a time of climatic cooling.

Ironically, Cassel et al. (2012, Am J Sci.) explored the possibility of diachronous ages in the Auriferous Gravels by using some detrital zircon work.  They summarize this thusly:


Thus again does the Chalk Bluffs flora appear to provide a strong age constraint when compared to the parts of the gravels that contain Upper Eocene zircons. And yet it is plausible that the Chalk Bluffs is barely younger than the sediments with the Eocene zircons.

Hopefully this error will get erased in the coming years, as it has served to complicate things greatly.  It seems plausible from the evidence in this paper that the Auriferous Gravels accumulated over a significant time interval, an idea suggested, for instance, by Garside et al. (2005 Geol. Soc. Nevada Symposium paper). The dispute that remains is whether the gravels are diachronous in space (mainly being accumulated in only one place at one time, the depocenter migrating through time as GG thinks Cassel and Graham have suggested) or if they are being rather more uniformly accumulating over time throughout their range of exposure.

Regardless, one can argue that a more careful reading of the primary literature would have turned up the fact that the Capay stage assignment to the Chalk Bluffs was not made because of the characteristics of the flora but was because of a lithostratigraphic correlation that was poorly constructed. From what GG has heard in pestering some paleontologists, the best constraint the fossils themselves provide is merely Eocene. And of course that only applies to the horizons where the fossils were collected; if the Auriferous Gravels extend back into the Paleocene as Garside et al. have suggested, it is possible that there is nearly 30 million years worth of deposition in any one exposure of the gravels.

Unlike the Whitney name calling example, this one carries scientific consequences.  Schorn, for instance, questioned the paleoelevation interpretations of Hren et al as being in error because the assumed ages were wrong, and as climate cooled the cool temperatures inferred from Hren et al.’s proxies would not be the product of height but climate change.  Similarly, inferences of paleoprecipitation from isotopic ratios in clays would be scrambled if in fact the age of the different samples were wildly different, a possibility given the wide range of oxygen isotopes recently reported by Mix et al. (2016 GSA Bull).


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