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.
