Stratigraphic Setting of Fossil Log Sites in the Morrison Formation (Upper Jurassic) near Dinosaur National Monument, Uintah County, Utah, USA

The outcrop belt of the Upper Jurassic Morrison Formation in the northeastern Uinta Basin and southeastern flank of the Uinta Mountains is particularly rich in dinosaurian and non-dinosaurian faunas, as well as in fossil plants. The discovery of several well-preserved, relatively intact, fossil logs at several locations in Rainbow Draw and one location in Miners Draw, both near Dinosaur National Monument (Utah), has provided an opportunity to study the local paleobotany, stratigraphy, and sedimentology of the Morrison Formation in northeastern Utah.


INTRODUCTION
The Upper Jurassic Morrison Formation is a spectacular and easily recognizable formation across the Colorado Plateau, and is distinctive because of its array of rainbow-colored mudstone, siltstone, and sandstone beds that typically form a "badlands" landscape. In northeastern Utah, it is well-exposed along the south flank of the Uinta Mountains (figure 1) and around Split Mountain and the Yampa and Blue Mountain Plateaus in and around Dinosaur National Monument (figure 2). The Morrison gained its fame from the discovery of well-preserved dinosaur bones on the south flank of Split Mountain in August 1909 by Earl Douglass, a paleontologist with the Carnegie Museum of Pittsburgh (Douglass, 2009). Subsequent development of the Carnegie Quarry (figure 2) produced a rich and diverse dinosaur assemblage (Elder, 1999;Engelmann, 1999;Turner and Peterson, 1999;Chure and others, 2006;Carpenter, 2013). In addition to dinosaurs, the Morrison has produced other vertebrate fossils (e.g., pterosaurs, crocodylomorphs, and mammals), invertebrate fossils, and fossil insects (Engelmann, 1999;Evans and Chure, 1999;Hasiotis, 2004;Chure and others, 2006). Fossil wood is also commonly found in the Morrison Formation, but it is mostly scrappy pieces preserved in channel sandstone   Hintze and others (2000). and conglomeratic depositional units in the Salt Wash and Brushy Basin Members (Engelmann, 1999;Gee and others, 2014). In some places, the Morrison contains fossil logs of varying sizes, which occur as solitary logs or in clusters (Gee, unpublished data). Engelmann (1999) surveyed several National Park Service units in Utah and surrounding states and documented 34 sites where log segments a few meters in length were found in the Morrison Formation. He described one site at Dinosaur National Monument as "…a large area covered by silicified logs of considerable diameter, possibly a log jam" (Engelmann, 1993). Escalante Petrified Forest State Park, near the town of Escalante in south-central Utah, is another location with a concentration of fossil logs in the Morrison Formation. These silicified (agatized) logs occur in a conglomerate bed in the upper part of the Brushy Basin Member (Morgan and others, 2012). Many other unpublished sites in Utah had concentrations of fossil logs in the Morrison, but many of these sites have been picked clean over the years by collectors (James Kirkland, Utah Geological Survey, verbal communication, March 2018).
A reconnaissance paleontological survey of the Morrison Formation by Mary Beth Bennis and Dale Gray (Utah Field House of Natural History State Park Museum) on Bureau of Land Management lands east of Vernal, Utah, and outside Dinosaur National Monument, identified two areas that have large, relatively intact, and well-preserved fossil logs. One area is located south of Blue Mountain Plateau near Miners Draw, and the other area is north of Dinosaur National Monument in Rainbow Draw (figure 2). The Rainbow Draw area is of particular importance because it contains a concentration of as many as 11 logs in a relatively small area (240,000 m 2 or about 60 acres).
The Miners Draw site is located in an unnamed tributary of Miners Draw (figure 2) where a single log was found at the base of a Morrison Formation slope (figure 3). The Rainbow Draw area is located northeast of Vernal and north of Dinosaur National Monument (figure 2). There, 10 fossil logs were discovered in beds in the area. The logs are concentrated into two groups within the 60-acre surveyed area. In each group, the fossil logs were somewhat dispersed, except for one site in the western group with a cluster of three large logs (site 3). It is unclear if this cluster of logs represents three individual trees or if it represents only one tree trunk that has broken into segments that have moved downslope. All fossil logs in the Miners Draw and Rainbow Draw areas were found in the Salt Wash Member of the Morrison Formation, but it was not clear at the time of the survey if the logs were at the same stratigraphic horizon or if they were merely preserved in strata of similar depositional environments. Subsequent visits to both areas were made to measure the logs, collect hand samples for paleobotanical analysis, and describe and measure Morrison Formation sections. Geologic mapping was conducted in the field on aerial photographs and imagery to help trace member contacts and key beds. The purpose of this study is to describe the Morrison Formation strata, especially those related to the fossil logs, and to place the logs within a stratigraphic framework for the interpretation of the depositional environment of the log-bearing intervals at both locations.

GEOLOGIC SETTING OF THE FOSSIL LOG SITES
The Miners Draw and Rainbow Draw areas are situated along the flanks of highlands-Blue Mountain Plateau, Yampa Plateau, and Split Mountain-that formed during the Laramide uplift of the Uinta Mountains (figure 2) (Stone, 1993; Gregson and others, 2010). The highlands rose mostly along high-angle reverse faults, Miners Draw fault to the south and Island Park fault to the north. The formations within the highlands were folded into a series of monoclines and asymmetrical anticlines (Hansen and others, 1983;Hansen, 1986;Sprinkel, 2006Sprinkel, , 2007. The bedrock stratigraphy in the Miners Draw and Rainbow Draw areas include (in ascending chronostratigraphic order) the Upper Jurassic Stump and Morrison Formations, the Lower Cretaceous Cedar Mountain and Dakota Formations, and the Upper Cretaceous Mowry and Frontier Formations. In the Miners Draw area, the formations wrap around a gently plunging anticlinal nose that is faulted on the north limb (figure 4). Three partial stratigraphic sections were measured to produce a composite section from the top of the Stump Formation to the base of the Cedar Mountain Formation because of the distance between the exposed base and top of the Morrison Formation and stratigraphic position of the fossil logs. Key marker beds were used to offset between the section segments. In Rainbow Draw, the formations generally dip less than 20° southwest, although the Morrison Formation is very gently folded into a broad, low-amplitude syncline (figure 5).   USA Sprinkel, D.A., Bennis, M., Gray, D.E., and Gee, C.T.

MORRISON FORMATION STRATIGRAPHY
The Morrison Formation in northeastern Utah consists of four members. In ascending chronostratigraphic order, they are the Windy Hill, Tidwell, Salt Wash, and Brushy Basin Members. The Windy Hill Member is composed of siltstone, mudstone, and thin-bedded rippled limestone that ranges from 4 to 20 m thick (figures 6A and 6B), and the invertebrate fossil assemblage and glauconite grains indicate marine deposition (Turner and Peterson, 1999). The Tidwell Member is composed of cross-bedded and rippled siltstone and sandstone, and thin shale beds (figure 6C). Marine      , 1999). The Tidwell also contains abundant pinkish-colored botryoidal chert (figure 6D) (also referred to as "welded chert") near or at the base of the member, which has been recognized throughout the region (King and Merriam, 1969;Peterson, 1980). A marginal marine to coastal plain (supralittoral) depositional environment is indicated for the Tidwell Member in northeastern Utah (Turner and Peterson, 1999), but it may be somewhat fluvial near the top as indicated by the bioturbated and mottled beds (appendix). The Tidwell Member in northeastern Utah ranges from 9 to 17 m thick. The Salt Wash Member is composed of interbedded sandstone, siltstone, and mudstone with some beds of channel-form pebble sandstone, as well as conglomerate that ranges from 75 to 80 m thick ( figure  7A). The Salt Wash was deposited in a fluvial system (Peterson, 1980;Turner andPeterson, 1999, 2004). The capping Brushy Basin Member is composed of a lower banded unit of siltstone, silty sandstone, and clay-rich mudstone ( (Peterson, 1988). However, Turner and Peterson (1999) noted that the J-5 surface is unconformable in some areas and conformable in others. Other workers in northeastern Utah showed the boundary as conformable in the eastern Uinta Mountains and Uinta Basin (e.g., Currie, 1998;Bilbey and others, 2005). Our examination of the contact in the Miners Draw and Rainbow Draw areas indicates that it appears to be gradational and conformable. The Upper Jurassic Morrison Formation is unconformably overlain by the Lower Cretaceous Cedar Mountain Formation. Picking the contact between the two formations can be difficult, despite being an unconformity (K-1), because their similar lithologies are easily weathered to clay flats or clay-covered slopes. Lithostratigraphic, biostratigraphic, and chemostratigraphic criteria are emerging that will help identify the contact (see Cifelli and others, 1997;Kirkland and others, 1999Kirkland and others, , 2003Kirkland and others, , 2011Kirkland and others, , 2016Ludvigson andothers, 2003, 2015;Greenhalgh and Britt, 2007;Kirkland, 2007;Kirkland and Madsen, 2007;Sprinkel and others, 2012). For example, the basal unit of the Cedar Mountain Formation can include the Buckhorn Conglomerate Member, a cobble and boulder conglomerate that varies in thickness and typically forms a resistant cliff above the Morrison Formation (Stokes, 1952;Kirkland and others, 2016), as it does in the Miners Draw area (figure 8A). Where the Buckhorn Conglomerate is missing, the basal bed of the Cedar Mountain Formation is typically mottled, yellowish-orange, chert-pebble-bearing mudstone that underlies the first Cedar Mountain calcrete bed (Sprinkel and others, 2012;Kirkland and others, 2016), as it is in the Rainbow Draw area (figure 8B).

STRATIGRAPHY OF THE FOSSIL LOG SITES
Sections of the Morrison Formation were measured at the Miners Draw and Rainbow Draw areas to describe beds and place the fossil log sites in stratigraphic and sedimentological context. At the Miners Draw area, several sections were measured to capture the entire Morrison Formation (Windy Hill, Tidwell, Salt Wash, and Brushy Basin Members) and the stratigraphic hori-zon of the fossil log site because distance between base and top of the Morrison is large and complicated by being exposed on the nose of a southwest-plunging anticline (figure 4). The contact between the lower banded and upper gray units of the Brushy Basin Member was used to trace around the Miners Draw area to the fossil log site. The section measured at Rainbow Draw included the Windy Hill, Tidwell, and part of the Salt Wash Members; the Salt Wash Member was measured only through the fossil log-bearing unit to a light greenish-to reddish-colored siltstone marker unit, which was overlain by a light-gray sandstone unit that capped the ridge. This distinctive marker unit was mapped and used to correlate between the two groups of fossil logs sites within the Rainbow Draw area.

Salt Wash Member at the Miners Draw Area
The Salt Wash Member at Miners Draw consists of fine-to coarse-grained, cross-bedded sandstone with some conglomeratic sandstone and conglomerate beds, silty sandstone, and siltstone (figure 9). The coarse-grained sandstone, conglomeratic sandstone, and conglomerate beds are mostly shades of brownish gray. The fine-grained and silty sandstone beds are light brownish-gray to greenish gray and medium reddish-brown. The siltstone beds are greenish gray to dark reddish-brown, somewhat clayey, and have a pseudo popcorn-weathering appearance. Black and orange-colored accessory grains are common in the sandstone beds. The coarser grained beds typically form resistant ledges and may include fossil wood and bone fragments, whereas the finer grained beds tend to be slope forming (appendix). The fossil log-bearing interval at Miners Draw is light greenish-gray to brownish-gray, silty to very fine grained sandstone that is slightly clay rich with indistinct bedding and no discernable sedimentary features. The interval is weakly cemented and poorly exposed but the log weathers out in relief. The interval is 4.3 m thick and the fossil log occurs 3.0 m above the base of the unit and 7.8 m below the top of the Salt Wash Member (figure 3). The fossil log is about 6 m long and its long axis is oriented east-west (see Gee and others, 2019, for additional log dimensions).  USA Sprinkel, D.A., Bennis, M., Gray, D.E., and Gee, C.

Salt Wash Member in the Rainbow Draw Area
The Salt Wash Member in Rainbow Draw consists of very fine to fine-grained sandstone, some medium-to coarse-grained sandstone, siltstone, and mudstone (figure 9; appendix). The sandstone beds are very light gray and light to medium greenish-gray and are medium to thin bedded and tabular with small-scale planar  cross-beds and some ripple laminations. The siltstone beds are light to medium greenish-gray and mottled reddish-brown, whereas the mudstone beds are dark greenish-gray, somewhat clayey, slightly micaceous, and fissile. The fossil logs are concentrated in two groups. The measured section of the Salt Wash Member began at the eastern group of logs along a small drainage that contains the top of the Stump Formation to the top of a ridge where the fossil log-bearing interval and the overlying marker bed are exposed ( figure 5A). The fossil log-bearing interval along the measured section is light to medium greenish-gray, very fine to fine-grained, friable sandstone and siltstone with indistinct bedding and no discernable sedimentary structures. The interval is generally slope forming and has fossil wood fragments scattered on the slope. The fossil log along the measured section at site 1 is 11 m in length, moderately well exposed, and fairly intact (figure 10). The log-bearing interval is 10.8 m thick, and the fossil log occurs 1.9 m below the base of the overlying marker bed and 29.7 m above the base of the Salt Wash Member (figure 9). A short fossil log is located a short distance along strike of the measured section. That log occurs about 2 m above the base of the Salt Wash Member. Other fossil log sites of the eastern group are along the ridge within the log-bearing interval. These sites were also in the light to medium greenish-gray, very fine to fine-grained, friable sandstone and siltstone at slightly varying stratigraphic levels within the interval, except the logs at site 9 (see figure 5A). The log at site 9 is about 10 to 12 m lower than the other logs on the ridge. The only other fossil log measured in the eastern group is at site 7 (see figure 5A) where it is partially exposed in the hillside, is slightly more than 0.5 m in length, and has a convoluted wood grain on its surface.
The western group of logs is located along a low ridge about 500 m northwest of the eastern group. No sections were measured through the western group, but detailed geologic mapping and comparison of the stratigraphy indicates that the log-bearing interval in the western group occurs in the same stratigraphic interval as the eastern group (figure 5). As in the eastern group, the log-bearing interval in the western group is light to medium greenish-gray, very fine to fine-grained, fria-ble sandstone and siltstone with indistinct bedding and no discernable sedimentary structures. The measured lengths of the fossil logs range from about 1.3 to 2.3 m, with diameters ranging from 0.5 to 1.1 m.
All sites within each group contain a single log, with the exception of site 1 (eastern group), which has two closely spaced logs, and site 3 (western group), which has multiple closely spaced log segments. The log segments at site 3 may represent multiple logs or a single log that has moved downslope because of erosion and broken apart. The measured azimuths of the long axis of the fossil logs in Rainbow Draw area indicate that they are randomly oriented.

GENERAL DESCRIPTION OF THE FOSSIL WOOD
Only a basic description of the fossil logs is presented here because a detailed description is being made by Gee and others (2019). The fossil logs in both the Miners Draw and Rainbow Draw areas are shades of light to medium brown in color and siliceous, and some have a Figure 10. Fossil log at site 1 in Rainbow Draw (see figure  5A). This fossil log occurs along the measured section in the Salt Wash Member of the Morrison Formation. The fossil log-bearing interval is a light to medium greenish-gray, very fine to fine-grained, friable sandstone and siltstone with indistinct bedding and no discernable sedimentary structures. The log is 11 m in length and is oriented east-west; view is to the east. coaly exterior ( figure 11). The logs are large, mostly intact and have exposed lengths that range from 0.5 to 11 m and diameters up to 1.1 m. The fossil logs have been compressed somewhat due to compaction or have been unevenly abraded so that they have a long and short diameter ( figure 12). The logs are well preserved and can be identified to species level from thin sections made from hand samples collected from both areas. The fossil logs have been identified as conifers that pertain to the same taxon originally described as Araucarioxylon hoodii Tidwell et Medlyn 1993 in the family Araucariaceae from Mt. Ellen in the Henry Mountains of southern Utah (Gee and others, 2019). Concurrent systematic work will prompt a nomenclatural transfer of this species to the genus Agathoxylon (Gee and others, 2019).

The fossil logs in both the Miners Draw and Rainbow Draw areas occur in the Salt Wash Member of the Morrison Formation. The single fossil log at Miners
Draw is found in a silty to very fine grained, slightly clay-rich sandstone, which is 4.3 m thick and located 17 m below the base of the Brushy Basin Member. Unfortunately, the base of the Salt Wash Member is not exposed at the fossil log site, but the Salt Wash is 78 m thick in the section measured to the northeast. Similarly, we have mapped all the fossil log sites in the Rainbow Draw area to a single log-bearing interval, a very fine to fine-grained sandstone and siltstone having indistinct bedding and no discernable sedimentary features (lithologically like the Miners Draw interval) that is 10.8 m thick. Most fossil logs are found in the upper part of the log-bearing interval, but logs are near the base of the interval as well, indicating that the entire interval may contain logs. We did not measure the entire Salt Wash Member at Rainbow Draw, but Turner and Peterson (1999) indicated that this member in Rainbow Draw is about 80 m thick. Our measured thickness to the top of the log-bearing interval is 31.5 m, and we calculate the thickness of the remaining Salt Wash, from the top of the log-bearing interval to the base of the Brushy Basin, to be about 40 m thick. Thus, the measured and calculated thickness of the Salt Wash is about 72 m, which would place the log-bearing interval just below the middle of the Salt Wash Member. Though we may be inclined to correlate the log-bearing intervals at Miners Draw and Rainbow Draw as the same stratigraphic horizon because the strata of all sites are lithologically very similar, the log-bearing interval at Miners Draw seems to be stratigraphically higher in the section and probably does not correlate to the interval at Rainbow Draw.
The Salt Wash Member was deposited in a large flu- vial floodplain system of mostly meandering and braided channels, migrating bar sand, and fine-grained overbank deposits (Peterson and Roylance, 1982;Turner and Peterson, 2004;Kjemperud and others, 2008). Both high-and low-energy deposition is indicated by the range in grain sizes. Much of the fossil wood found in the Salt Wash Member occurs in channel sandstone and conglomeratic sandstone beds, but the fossil logs in the Miners Draw and Rainbow Draw areas are in an interval of very fine to fine-grained sandstone and siltstone beds that lack any discernable sedimentary structures. We interpret the log-bearing intervals as overbank deposits in the floodplain depositional environment. In addition, we speculate that the fossils logs may not have been transported far from their growth locations because of the fine-grained and structureless strata in which they are preserved. We are uncertain if the logs were transported by low-energy currents during a period of flooding or if the trees were toppled by a highwind event. The concentration of fossil logs into two groups in Rainbow Draw suggests that there may have been at least two stands of conifers in this area, if not a larger forest of moderately tall trees.

SUMMARY
Although scraps of fossil wood are commonly found in the Morrison Formation, large fossil logs occur in both the Salt Wash and Brushy Basin Members in northeastern Utah near Dinosaur National Monument. In Rainbow Draw, the fossil logs occur in the middle of the Salt Wash Member based on its measured and calculated thickness. The log-bearing interval is a very fine to fine-grained sandstone and siltstone that lacks sedimentary features, suggesting the logs are preserved in floodplain overbank deposits. The fossil log at Miners Draw is preserved in similar strata in the upper part of the Salt Wash Member and is thought to be deposited in the same depositional environment as the logs in Rainbow Draw. The log-bearing intervals at Miners Draw and Rainbow Draw seem to be at two different stratigraphic horizons and do not correlate with one another. The nature of the sediments in which they are encased suggests that the logs did not travel far from their growth locations, but we are uncertain if the fossil logs were transported by low-energy currents or by some other mechanism. Paleobotanical study of samples collected from both sites indicate the logs pertain to conifers of the family Araucariaceae.