Observations in the Uinta

Followers Accidental visitors of this blog may have noticed my trademark neglect has been a little higher than usual, but in this case I can justifiably blame it on my new job, which has eaten up more than its fair share of my free time. This new occupation has exposed me to the wonders of the Uinta Formation, a Middle Eocene (46.5 – 40 Ma; Prothero, 1996), predominantly fluvial, accumulation of sediment shed from the Uinta Mountains to the north (Stokes, 1986; Rasmussen et al., 1999; Townsend 2004). Specifically, my observations are based almost exclusively in the middle Wagonhound Member, but that’s probably only of interest to serious stratigraphers…

Marmaduke has died of dysentery.

Part I: Known knowns

The Wagonhound Member (hereafter Uinta B) is composed almost exclusively of thick, trough cross-bedded sandstones, thick overbank fines, and thinner massive sandstones and siltstones, with the thick sandstones forming the most blatant outcrops. These same sandstones are also commonly afflicted with a feature that looks like old war wounds (see left).

This feature can be seen at many scales, from the very small:

…to moderate and localized:

…to occupying an entire cliff face:

This is a geologic feature known as tafoni, a consequence of salt growth inside rocks. Water, either ground or meteoric, contains salts, and in a porous rock such as sandstone, when water in the pore spaces evaporates, it leaves behind salts which crystallize to a larger volume. This displaces grains and increases pore space, and any time additional water is introduced, the process starts over, and a larger void is created. (For a more detailed explanation, as well as appropriate references, I would suggest the excellent Tafoni website.) The occurrence of tafoni in the Uinta B is not at all surprising, as signs of salt precipitate can be seen everywhere:

This feature has erroneously been attributed to fossil termite mounds in the past (J. Strauss, personal communication), which is so wrong on many levels, the least of which being that this feature is most common in channel sandstones, and any termite colony that fancied placing their nests in an active stream channel would be quickly eliminated from the gene pool.

Part II: Known unknowns

Another common feature of the Uinta B is nodules, spherical to subspherical to elongate “balls” of well-cemented sediment which can often be found littering outcrops like an ancient bowling range (see left). Precisely what causes these nodules to form is unknown to me – in part, this is due to my own lack of reading on the subject, but several sources I’ve encountered seem to casually suggest that their formation might be unknown in general. I haven’t seen anything to suggest the nodules are formed by different sediment than their host material – as you can see in example in the lower left, the nodule is eroding at just the same rate as the surrounding rock.

Additionally, in the example below (you may have to click on the picture for full size), you can see in the cross-section of an elongate nodule (L), the sediment is clearly the same cross-bedded sandstone found a few meters away in the same outcrop (R):

Some nodules I have encountered have certainly hinted at the importance of a nucleation site, which, as evidenced by the mammal vertebra (L) and turtle shell fragment (R) below, can often be fossils themselves:

Part III: Unknown unknowns

Tragically, the most intruiging thing I have discovered about the Uinta Formation was not in the field, but in the literature – the sedimentology and stratigraphy of the Uinta is woefully not understood, despite the impressive work of a few individuals (Townsend, 2004, Townsend et al., 2006; Murphey et al. 2011). Part of this is due to the complexity of the Uinta beds and host fossils (Walsh, 1996), but part is probably also due to interest in the Uinta only being recently reignited, thanks to a booming oil industry (in overviews of the Uinta Formation geology, publications between ca. 1930 and 1990 are usually sparse). For someone with time and energy to dedicate to the formation, there’s probably no shortage of geologic information to be uncovered. That won’t be me, ironically, as I will soon be moving out of the area. But that’s a subject for future posts…

REFS

Murphey, P.C., Townsend, K.F.B., Friscia, A.R., and Evanoff, E. 2011. Paleontology and stratigraphy of the middle Eocene rock units in the Bridger and Uinta Basins, Wyoming and Utah, in Lee, J., and Evans, J.P., eds., Geologic Field Trips to the Basin and Range, Rocky Mountains, Snake River Plain, and Terranes of the U.S. Cordillera: Geological Society of America Field Guide 21, p. 125–166, doi:10.1130/2011.0021(06). (pdf here)

Prothero, D.R. 1996. Magnetic stratigraphy and biostratigraphy of the Middle Eocene Uinta Formation, Uinta Basin, Utah, in Prothero, D.R., and Emry, R.J., eds., The Terrestrial Eocene-Oligocene Transition in North America, Cambridge University Press, pp. 75-119.

Rasmussen D.T., Conroy, G.C., Friscia, A.R., Townsend, K.E., and Kinkel, M.D. 1999. Mammals of the Middle Eocene Uinta Formation, in Gillette, D.E., Vertebrate Paleontology in Utah, Utah Geological Survey Miscellaneous Publication, 99-1, pp. 401-410.

Stokes, W.L. 1986. Geology of Utah: Utah Museum of Natural History, University of Utah and Utah Geological and Mineral Survey, Department of Natural Resources.

Townsend, K.E. 2004. Stratigraphy, paleoecology, and habitat change in the Middle Eocene of North America, unpublished dissertation, Washington University, 418 pp.

Townsend, K.E., Friscia, A.R., and Rasmussen, D.T. 2006. Stratigraphic distribution of Upper Middle Eocene fossil vertebrate localities in the eastern Uinta Basin, Utah, with comments on Uintan biostratigraphy: The Mountain Geologist, v. 43, no. 2, p. 115-134.

Walsh S.L., 1996. Middle Eocene mammalian faunas of San Diego County, California, in Prothero, D.R., and Emry, R.J., The Terrestrial Eocene-Oligocene Transition in North America, Cambridge University Press, p. 75-119.

Why Looking for a Hidden River is like Searching for the Right Asylum

Back in my college days, I was lucky enough to intern at the Cleveland Museum of Natural History for one summer. One aspect of my project involved studying the quarrying history of a local building stone. In the course of my research, I noticed that on many 1800s-era geologic maps, asylums were often located very near to quarries. As it turns out, this was not a coincidence – quarries often hired asylums resident as cheap labor, and in turn, the residents got a chance to leave the grounds and get some stimulation. Hence, a somewhat disturbing lesson: if older geologic maps don’t show quarries, look for asylums. Investigating the geologic record often works in the same vein.

As it turns out, the majority of geologic events in the earth’s history were never preserved in the rock record. Even a geologic event that is sufficiently significant, long-lived, and plainly lucky enough to be preserved may never be discovered – it has to be exposed, accessible, and properly observed and studied. Often, geologic events are not interpreted based on their direct effects, but by side effects and related events. Metaphorically speaking, the quarries are gone, but the asylums remain. In a prime example, in looking for evidence of an ancient transcontinental river in the U.S., geologists had to look in….desert deposits.

How to find evidence of a river nobody can see

Many of the archetypical rock formations of the American southwest – the towering red cliffs, psychedelically wavy hillsides, etc (pictures at left from here) – are the deposits of ancient Jurassic ergs* (sand seas). These ergs were particularly enormous – perhaps rivaling in size the Empty Quarter in the Arabian Peninsula (the largest modern erg) (Dickinson and Gehrels, 2009).

With any erg, the obvious question is “where did all the sand come from”? (Granitic crust is only 1/3^rd quartz at most, so a lot of rock has to be eroded to get a sand sea). Two easy answers presented themselves – the (Ancestral**) Rocky Mountains are practically next door, and there were plenty of voluminous sandstones across North America that could have been reworked. But there was also a third possibility, first proposed by Marzolf (1988) – an enormous transcontinental river originating in the Appalachians on the east coast. Unfortunately, Jurassic rocks aren’t exposed anywhere in the U.S. between the Rockies and the Appalachians, so direct evidence of this river was out of the question. But, there are the ergs…

Aspen, Colorado, circa 170,000,000 years ago (from here)

* Deserts and ergs aren’t synonymous, but the two coincided in this instance.

** Not the same as the modern Rocky Mountains, but let’s not get into that right now…

Marzolf’s hypothesis was essentially ignored until University of Arizona geologists William Dickinson (emeritus) and George Gehrels decided to take a look at some of the deposits in the Jurassic ergs in the 1990s. Specifically, they looked at the zircons in the sandstones – unlike the commercials, zircons, not diamonds, are forever in the geologic record, lasting hundreds of millions of years (the oldest known mineral on earth is actually a zircon (Wilde et al., 2001)). Over their long lifespan, zircons can be transported and reworked over thousands of kilometers, but always carry an age signature from their original host rock. When sandstone were sampled from various localities in the ergs, the ages of the zircons fell into three categories: 1/4^th matched ages with the basement rocks of the Ancestral Rockies, 1/4^th matched ages with reworked ancient sandstones, and the remaining half were divided into four ages too young for the previous two categories (Dickinson and Gehrels, 2003). When the younger zircons were investigated further, it was discovered that the four age ranges fit nicely with….granite bodies that compose the Appalachians (Dickinson and Gehrels, 2009). Furthermore, paleowind measurements from the sandstones showed consistent southern winds (Dickinson and Gehrels, 2009). Combined, this suggested there was some major source of transportation that carried sands from the Appalachians to the western U.S., where they were deposited and carried by winds to build ergs to the south (see cartoon below, heavily inspired by Fig. 1 in Dickinson et al., 2010). And a prime candidate would be….a major transcontinental river!

The long history of Canadian immigration

Recently, Dickinson et al. (2010) tested the "transcontinental river" hypothesis from another angle. I mentioned before that Jurassic rocks aren’t exposed between the Rockies and the Appalachians, but they do exist – under a whole bunch of other rocks. In this case, Dickinson et al. (2010) compared sandstones from the Jurassic ergs to the west to subsurface (fluvial) Jurassic sandstones in the Michigan Basin (star in figure above), an area that would have been right in the middle of some of the northern tributaries of the transcontinental river. This time, the zircon ages were more nuanced: the ergs and the fluvial sandstones both contained zircons with ages matching “Grenvillian” source rocks, but the zircons from the Michigan sandstones lacked ages matching “peri-Gondwanan” source rocks (Dickinson et al., 2010). As it turns out, the “Grenvillian” source rocks are found in northeast Canada, and the “peri-Gondwanan” rocks are found in the southern Appalachians. So it would make sense that tributaries from northeast Canada (upper arrow in figure above) would carry zircons that ended up in Michigan and eventually the western ergs, but tributaries from the southeast U.S. (lower arrow in figure above) would carry zircons that ended up in the western ergs only (Dickinson et al., 2010).

 

REFS

Dickinson, W.R., and Gehrels, G.E. 2003. U-Pb ages of detrital zircons from Permian and Jurassic eolian sandstones of the Colorado Plateau, USA: Paleogeographic implications: Sedimentary Geology, v. 163, p. 29–66. (pdf here)

Dickinson, W.R., and Gehrels, G.E., 2009. U-Pb ages of detrital zircons in Jurassic eolian and associated sandstone of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment: Geological Society of America Bulletin, v. 121, p. 408–433.

Dickinson, W.R., Gehrels, G.E., and Marzolf, J.E. 2010. Detrital zircons from fluvial Jurassic strata of the Michigan basin: Implications for the transcontinental Jurassic paleoriver hypothesis: Geology, v. 38, no. 6, p. 499–502.

Marzolf, J.E., 1988. Controls on late Paleozoic and early Mesozoic eolian deposition of the western United States: Sedimentary Geology, v. 56, p. 167–191.

Wilde, S.A., Valley, J.W., Peck, W.H., and Graham C.M. 2001. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago: Nature, v. 409, p. 175-178. (pdf here)