REVISED LOWER PALEOZOIC STRATIGRAPHY OF THE DINOSAUR NATIONAL MONUMENT REGION: NEWLY RECOGNIZED DEVONIAN STRATA OF THE UPPER "LODORE" FORMATION

One of Bob Dott’s major career contributions was the study of epicratonic lower Paleozoic strata of the mid-continent, particularly basal cover successions of Cambrian age. We present results of a detailed sedimentological, paleontological, and chemostratigraphic study of the lower to middle Paleozoic rocks in Jones Hole, Dinosaur National Monument, Utah, including glauconite-rich, transgressive sandstone deposits of the Cambrian Lodore Formation. A new trilobite fauna recovered from the formation, including undescribed species, shifts the purported age from upper Cambrian to the Ehmaniella Biozone of the Middle Cambrian. All previous work in this region indicate that the Lodore is disconformably overlain by the Mississippian Madison Limestone, although there was past speculation that some Devonian strata may be present. We present lithological, paleontological, and δ¹³C_Carbisotope data to suggest that strata mapped as upper Lodore are in fact the Devonian Parting Formation. The base of the Madison Limestone, which contains a large positive Kinderhookian (Mississippian) carbon isotope excursion, is marked by a previously unrecognized several-meter-thick paleokarst interval that rests on thin bedded, dark dolostone beds. The latter record distinctive negative carbon isotope values that contrast with very positive values in the overlying Madison. These dolostone beds and their isotopic signature correlate well with those of the upper Famennian Parting Formation in central Colorado. Changes in the width of Rusophycus and Cruziana trace fossils between lower and upper “Lodore” also support a Parting Formation interpretation. Finally, calcareous and phosphatic fossil fragments in the putative Devonian strata are tentatively identified as pteraspidomorph (jawless fish) fossils and possible acanthodian spines. In either case, the fossils are consistent with an upper Devonian age, which adds a new formation and rock of a new geologic period to the known strata of Dinosaur National Monument. Detrital zircon data from the definitively Cambrian Parting and Madison strata showthree major age peaks at ~1.1, ~1.4 and ~1.7 Ga. The spectra suggest derivation from local Colorado sources, and remarkably little shift in source rocks from the Cambrian to the Mississippian.