Does Utah Preserve North America’s Oldest Cretaceous Dinosaurs Because of Ancient Salt Deposits?
By James I Kirkland
Utah has the most complete dinosaur record located in any one area in the world.
Currently, the Utah Geological Survey (UGS) Paleontology Section recognizes the presence of 27 sequential, non-overlapping dinosaur faunas spanning 165 million years, from 230 to 65 million years ago. These faunas range from the very first North Ameri-can dinosaur-bearing strata in the Upper Triassic Chinle Formation, through Utah’s real “Jurassic Park” in the Upper Jurassic Morrison Formation, to the uppermost Cretaceous North Horn Formation which has a lone example of Tyran-nosaurus and a record of the extinction of the dinosaurs.
Although the dinosaur record of the Middle Jurassic San Rafael Group is limited to dinosaur tracks, the only real gap in Utah’s extraordinary record is in the transition between the Jurassic and Cretaceous, an interval of up to 25 million years. This interval of non-deposition and erosion extends across the entire interior of North America.
The UGS Paleontology Section has focused on filling this gap by studying the Lower Cretaceous Cedar Mountain Formation (CMF),
which is exposed across east-central Utah (see Survey Notes, v. 37, no. 1). The CMF is complex and we have divided it into a basal lower and upper Yellow Cat Member, a middle Poison Strip Member, and an upper Ruby Ranch Member in the northern Paradox Basin.
As we prepared a guidebook to the CMF as part of co-hosting the 2016 annual meeting of the Society of Vertebrate Paleontology, we combined data generated from our research with that of our extensive network of collaborators. The results from this compilation confirmed our long-held suspicion that Utah’s oldest Cretaceous dinosaurs are restricted to northern Grand County in eastern Utah as a result of local salt tectonics.
When we plotted the distribution of the CMF’s stratigraphic units across the region, we demonstrated that, not only is the basal Yellow Cat Member restricted to the Paradox Basin, but it is thickest in the central part of the basin where salt tectonics had a major influence. Furthermore, the dinosaurs (Gastonia lorriemcwhinneyae and Planacoxa venenica) preserved in the overlying Poison Strip Member appear to be most closely related to dinosaurs in the oldest previously identified Cretaceous fauna in the Lakota Formation of the Black Hills region of South Dakota. Therefore, we have proposed that Utah preserves the two oldest Cretaceous dinosaur faunas in North America, and that both are restricted to the northern Paradox Basin in Grand County.
We also proposed that Utah’s more complete record of the Jurassic-Cretaceous boundary is due to salt tectonics.
During the uplift of the Ancestral Rockies in the late Paleozoic (about 300 million years ago), a deep basin formed on the west side of the Uncompahgre Uplift along the Utah-Colorado border. Shallow seas repeatedly spilled into this basin and evaporated, resulting in the deposition of thousands of feet of salt, which is much more plastic or ductile than other sedimentary rock types. After the salt was buried under thousands of feet of coarse debris shed westward from the Uncompahgre Uplift, it was squeezed and deformed into a series of ridges and depressions that folded and faulted the overlying rock.
The height of salt movement occurred during the early Mesozoic and is well reflected by the Salt Valley anticline at Arches National Park and by the Moab fault. Our research has documented that salt tectonics was also an important control on deposition in this area during the Early Cretaceous, while erosion was occurring across much of the rest of North America. As salt migrated into anticlines causing additional uplift of the terrain, the adjoining areas subsided, leading to local deposition of Lower Cretaceous sediments in these resulting depressions.
The distribution of ancient wetlands in the area during the Early Cretaceous also appears to be controlled by salt tectonics.
In the upper Yellow Cat Member, the presence of an aquatic fauna of diverse fish, freshwater turtles, crocodilians, and aquatic microfossils indicates that an extensive wetland or lake system was present east of the Salt Valley anticline. We believe that salt movement was asymmetric, and more salt entered the Salt Valley anticline from the east than from the west during the deposition of the Yellow Cat Member. This led to greater subsidence east of the Salt Valley anticline and Arches National Park resulting in the development of lakes and wetlands, while the rest of the region records a drying trend.
This pattern is reversed in the overlying Ruby Ranch Member which is much thicker on the west side of Arches National Park, and its upper half preserves a lake system that is dominated by carbonate deposition. Preliminary radiometric dating suggests that these rocks are younger than other Ruby Ranch strata in the area. Pending ages from several volcanic ashes preserved in the lake strata will confidently date these rocks. The dramatic thickening of the Ruby Ranch Member suggests tens of meters of local subsidence along the west side of Arches National Park as salt migrated east into the Salt Valley anticline near the end of the Early Cretaceous.
Perhaps the presence of extensive wetlands in the localized basins in eastern Utah during the Early Cretaceous helps explain the abundance of dinosaur localities in the Lower Cretaceous rocks of Grand County, Utah.
Survey Notes, v. 49 no. 1, January 2017