PSA Combined Rare Earth Element and Sedimentologic Approach to Taphonomic Interpretations of the Early Cretaceous Crystal Geyser Dinosaur Quarry, Utah*
Celina Suarez1, Marina Suarez1, Dennis O. Terry, Jr. 1, D.E. Grandstaff1, and James I. Kirkland2
Search and Discovery Article #50014 (2005)
Posted August 14, 2005
*Poster presentation at AAPG Annual Convention, with SEPM, Calgary, Alberta, June 19-22, 2005.
2Utah Geological Survey, Salt Lake City, UT
Bone beds are an important source of paleobiological and paleoenvironmental information. However, conclusions drawn from such deposits may be biased by sedimentary processes involved in bone bed formation. Various taphonomic methods have been developed to correct such process-induced biases including measurements of bone sorting, orientation, surface features, and examination of associated sedimentary structures. Recently, Trueman (1999) has suggested that analysis of rare earth elements (REE) in fossil bones may be used to independently assess temporal and spatial averaging associated with modes of bone bed formation.
The Crystal Geyser Dinosaur Quarry (CGDQ), located in lower Cretaceous units southeast of Green River, Utah, is a nearly monospecific bone-bed containing a high density of bones from numerous individuals of a new species of basal Therizinosauroid theropod dinosaur. This unusual group of theropoda is known mostly from Asia (Clark et al., 2004). The only other confirmed occurrence of a therizinosaur in North America is Nothronychus mckinleyi from a site in the Late Cretaceous (Turonian) of New Mexico (Kirkland and Wolfe, 2001). In our study of the Crystal Geyer Dinosaur Quarry, classical methods of sedimentology and taphonomy are coupled with REE analyses of the therizinosaur fossils to constrain the depositional environment and processes by which this fossil assemblage was formed.
The CGDQ is located at the base of the Early Cretaceous (Barremian) Yellow Cat Member of the Cedar Mountain Formation (Figure 1). The Yellow Cat Member is separated from the underlying Jurassic Morrison Formation by an unconformity of approximately 15-25 Ma. Bone-bearing strata include a basal carbonate or pisolitic horizon and an overlying silty to sandy mudstone unit. The carbonate is micritic with clay clasts, pebbles, calcitic feather dendrites, fan-shaped radial calcite, tufa intraclasts, and pisolites (Figure 2a). The mudstone contains silt and sand, pebbles, tufa intraclasts (Figure 3), carbonate nodules, and pisolites. Bones within the mudstone are typically encrusted with micritic carbonate. Approximately 50 cm above the contact is a discontinuous carbonate horizon. Above this horizon, bones tend to be encrusted with thicker carbonate (Figure 1), and carbonate nodules are more abundant. In thin section pisolites are spherulitic (radial needle-like crystals displaying pseudo-uniaxial extinction) (Figure 4a). At the top of the main bone-bearing units (about 80 cm above the Morrison Formation) is a weakly developed (entisol) soil profile (dominated by relict bedding) that is marked by truncated vertical bones and root traces. Above this are interbedded green silty to sandy mudstones and carbonate horizons. The top of the quarry site is marked by a dense limestone caprock of variable thickness consisting of pure micrite to a sandy/peloidal wackestone.
More than 95% of identifiable bones are from a basal therizinosauroid (Kirkland, 2004), with only a few bones from other taxa, including a large ankylosaur, a crocodilian tooth, and a possible chelonian claw. All age groups are represented at the CGDQ, from hatchlings to adults. Bones were deposited in three units (Figure 1) which show an overall fining upward sequence of bone size. Bones from Units 1 and 2 are primarily Voorhies (1969) Group 2 (bones are transported as traction load). Bones in Unit 3 are primarily Voorhies Group 1 (transportation by saltation and floatation) represented by small bones such as small caudals and phalanges. Bones from Units 1 and 2 are generally horizontal; Unit 1 bones from the southwest portion of the quarry have very strong east-west orientation while those in other parts of the quarry have a north-south orientation. This variable orientation is likely the result of braided or radial flow. Bones from Unit 2 are highly fractured, commonly with both articular ends missing. The highest degree of fracturing is in the bones directly on top of the basal carbonate within Unit 2 (Figure 5a). Bones in Unit 3 are generally un-oriented with the exception of a few vertical limb bones truncated by an erosion surface.
Several bones showed unusual puncture marks, especially at the articular ends, which suggest predation or scavenging. Scratch marks were noted on several long bones, possibly due to trampling or transport as traction load. Some bones show weathering, mostly Behrensmeyer (1978) stage 0-1, indicating surface exposure for no more than a few years. An unusual weathering pattern noted by Behrensmeyer (1978) as “mosaic cracking or flaking” was noted on vertebra and two brain cases (Figure 5b). The significance of this particular bone alteration is unknown, but is likely the result of exposure. Abrasion is also noted on the ends of several long bones as well as several rounded bone pebbles near the top of the quarry deposit in Unit 3, and significant abrasion on the top surface of the vertical bones.
REE are incorporated into bone, probably within a few thousand years post-mortem, as the biogenic apatite recrystallizes during fossilization (Trueman, 1999; Patrick et al., 2004). The REE signature in fossils record the REE composition of surface water or groundwater at the time of fossilization (Patrick et al., 2004, Martin et al., 2005) and may be used, in some instances, as paleoenvironmental or redox proxies (Patrick et al., 2004; Metzger et al., 2004; Martin et al., 2005). Trueman (1999) has suggested that variation of REE signatures in fossil bone may be used to identify the degree of time or spatial averaging in a fossil assemblage. In general, REE in bones that were fossilized in-situ, without reworking, should share a common REE signature. If fossils are pre-fossilized in different areas or environments and then mixed (reworked) by re-deposition, they may have varied REE signatures, whose degree of variation reflects the extent of averaging. Signatures of REE in bones from all three units are significantly different from each other (Figures 1 and 6). REE signatures in bones from Units 1 and 2 are variable whereas those from Unit 3 are uniform. The signature data can be plotted on a ternary diagram to show proportions of representative light (Nd), middle (Gd), and heavy (Yb) REE from the three units (Figure 6) (Patrick et al., 2004). Over time (stratigraphically) the signatures become progressively light REE (LREE) enriched. Signatures from the three units do not greatly overlap, indicating three fossilization events, and little time averaging of fossils between the three units. Within Units 1 and 2, data show a trend toward heavy REE enrichment (HREE) (Figure 6). REE in groundwaters may evolve from LREE to HREEenriched along a flowpath (Dia et al., 2000). This trend may be due to adsorption and preferential removal of LREE onto colloids, or to groundwater mixing (Dia et al., 2000; Johanessen et al., 2003). The proximity (within one meter) of such chemically distinct fossils in units 1 and 2 is consistent with spatial averaging (reworking of pre-fossilized bones from different localities formed along a common hydrologic flow path).
The taphonomic history of the Crystal Geyser Dinosaur Quarry is very complex. Based on similarities with calcite morphologies in modern spring deposits (Figures 2b and 4b) and possibly the positive cerium anomalies, the CGDQ was deposited in or near a spring. The basal carbonate and lateral pisolitic horizon were probably deposited first in a spring-fed pool. Input of siliciclastic sediments occurred after the basal carbonate, indicating at least two periods of deposition and bone incorporation. The near monospecific nature of the bone bed itself would normally suggest that one mass mortality event took place. However, the fossils contain three distinct geochemical signatures that differ stratigraphically. These are probably the result of three distinct fossilization events, perhaps separated by thousands of years (Trueman, 1999; Martin et al., 2005). This conclusion could not be made without the results of the geochemical analysis. A discontinuous carbonate horizon about 50 cm above the contact (Figure 1) may represent the base of the third depositional event recorded by the geochemistry of the bones. The geochemical data are consistent with the physical taphonomic data indicating that prefossilized bones have been reworked (spatially averaged) into Units 1 and 2, whereas bones in Unit 3, which are dominated by Voorhies Group 1 (bones rapidly transported) were fossilized after initial fluvial transport and sorting of the un-fossilized bones. It is still unclear how these animals died in such a great abundance at several different times, but it may be related to spring hydrology, poisoning, disease, and/or therizinosaur behavior.
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