Introduction
This research is part of an ongoing study of large-diameter structures (LS) in the Salt Wash Member (SWM) of the Upper Jurassic Morrison Formation near Ticaboo and Torrey, Utah. The LS have been described as mammal burrows [3, 4, 5], but a detailed study has never been undertaken. Multiple hypotheses about their origins are being tested. The initial survey noted over 100 structures; the current study has gathered data on over 150 LS and is currently describing their morphology, occurrence, and outcrop distribution.
Methods
Multiple origins are hypothesized for all of the LS: 1) vertebrate burrows—a) mammal, b) therapsid, c) reptile, and/or d) amphibian; 2) rhizoliths; 3) concretions; or 4) fluid escape pipes. To test these hypotheses, we compiled a database of measurements, descriptions, and photographs of the LS from field observations, and have begun comparing their features to an existing database of the architectural and surficial morphologies of structures from known tracemakers and abiotic structures [10].
Study Area
The main study area is near Ticaboo, Utah. The SWM here is composed of interbedded sandstone, siltstone, and mudstone that represent fluvial systems [2, 6, 9, 11, 13]. The depositional environment of the LS-bearing units represents a proximal to distal floodplain [9,13]. The LS are located in the third paleosol from the top of the SWM. Sandstones with pebble conglomerate lenses representing crevasse-splay events are interbedded with the LS-bearing paleosol and non-LS-bearing paleosols [2, 6, 9, 13]. The paleosols contain many slickensides.
A second area was discovered ~80 km northwest near Torrey, Utah. This outcrop was much less laterally extensive. The paleosols here are thinner.
Results
All LS appear to have undergone significant architectural deformation during preservation. The LS have two morphotypes.
Morphotype 1 is more common. The two elements composing this morphotype are a vertical to subvertical helical shaft that leads to one or more horizontal tunnels. In two specimens, the tunnels are also helical. Longitudinal and transverse striations are visible on the top of LS elements, but poorly preserved due to a carbonate coating. The side and bottom surfaces lack striations and are coated with smooth lumps of carbonate. There are more specimens of isolated elements than of complete structures. At the site near Torrey, Utah, the Morphotype 1 LS are smaller. Spacing between the LS is not uniform, but they are rarely tightly clumped.
Morphotype 2 is composed of a single long (~2 m) subhorizontal tunnel. There are fewer longitudinal and transverse striations preserved on the top of these specimens than on Morphotype 1. These striations are also distorted. The major surface texture is again lumpy. There are only three examples of Morphotype 2 at the Ticaboo, Utah, site and none at the Torrey, Utah, site.
Interpretation
With a high level of complexity, the architecture of Morphotype 1 resembles mammal and therapsid burrows. The helical shape is seen in burrows from the Permian and the Miocene that are both in the ichnogenus Daemonelix [8, 12]. The longitudinal and transverse striations are interpreted as scratch marks from the manus and/or pes, or bite marks. The discovery of Fruitafossor, a mammal interpreted as having fossorial adaptions, in the Morrison Formation increases the potential for the LS being constructed by mammals [7].
The simpler architecture of Morphotype 2 most closely resembles the burrows of reptiles. The large size makes a monitor lizard or crocodilian a potential tracemaker. The longitudinal and transverse striations are again interpreted as scratch or bite marks.
References
1) Bown, T.M. and Kraus, M.J., 1983. Ichnofossils of the alluvial Willwood Formation (Lower Eocene), Bighorn Basin, Northwest Wyoming, U.S.A. Palaeogeography, Palaeoclimatology, Palaeoecology, 43: 95-128.
2) Demko, T.M., Currie, B.S., and Nicoll, K.A., 2004. Regional paleoclimatic and stratigraphic implications of paleosols and fluvial/overbank architecture in the Morrison Formation (Upper Jurassic), Western Interior, USA. Sedimentary Geology, 167: 115-135.
3) Hasiotis, S.T., 2004. Reconnaissance of Upper Jurassic Morrison Formation ichnofossils, Rocky Mountain Region, USA: paleoenvironmental, stratigraphic, and paleoclimatic significance of terrestrial and freshwater ichnocoenoses. Sedimentary Geology, 167: 177-268.
4) Hasiotis, S.T., 2008. Reply to the Comments by Bromley et al. of the paper “Reconnaissance of Upper Jurassic Morrison Formation ichnofossils, Rocky Mountain Region, USA: paleoenvironmental, stratigraphic, and paleoclimatic significance of terrestrial and freshwater ichnocoenoses” by Stephen T. Hasiotis. Sedimentary Geology, 208: 61-68.
5) Hasiotis, S.T., Wellner, R.W., Martin, A.J., and Demko, T.M., 2004. Vertebrate burrows from Triassic and Jurassic continental deposits of North America and Antarctica: Their paleoenvironmental and paleoecological significance. Ichnos, 11: 103-124.
6) Kjemperud, A.V., Schomacker, E.R., and Cross, T.A., 2008. Architecture and stratigraphy of alluvial deposits, Morrison Formation (Upper Jurassic), Utah. AAPG Bulletin, 92: 1055-1076.
7) Luo, Z. and Wible, J.R., 2005. A late Jurassic digging mammal and early mammalian diversification. Science, 308: 103-107.
8) Martin, L.D. and Bennett, D.K., 1977. The Burrows of the Miocene beaver Palaeocastor, Western Nebraska, U.S.A. Palaeogeography, Palaeoclimatology, Palaeoecology, 22: 173-193.
9) Peterson, F., 1994. Sand dunes, sabkhas, streams, and shallow seas: Jurassic paleogeography in the southern part of the Western Interior Basin. In: Caputo, M.V., Peterson, J.A., Franczyk, K.J. (Eds.), Mesozoic Systems of the Rocky Mountain Region, USA. Rocky Mountain Section, Society for Sedimentary Geology
(SEPM), Denver, CO, pp. 233-272.
10) Riese, D.J., Hasiotis, S.T., and Odier, G.P., 2011. Synapsid burrows and associated trace fossils in the Lower Jurassic Navajo Sandstone, Southeastern Utah, U.S.A., indicates a diverse community living in a wet desert ecosystem. Journal of Sedimentary Research, 81: 299-325.
11) Robinson, J.W. and McCabe, P.J., 1998. Evolution of a braided river system: the Salt Wash Member of the Morrison Formation (Jurassic) in southern Utah. Relative Role of Eustasy, Climate, and Tectonism in Continental Rocks, SEPM Special Publication No. 59: 93-107.
12) Smith, R.M.H., 1987. Helical burrow casts of Therapsid origin from the Beaufort Group (Permian) of South Africa. Palaeogeography, Palaeoclimatology, Palaeoecology, 60: 155-170.
13) Turner, C.E. and Peterson, F., 2004. Reconstruction of the Upper Jurassic Morrison Formation extinct ecosystem—a synthesis. Sedimentary Geology, 167: 309-355.