New age constraints on the Late Cretaceous lower Williams Fork Formation, Coal Canyon, Colorado

2021 ◽  
Vol 58 (1) ◽  
pp. 5-26
Author(s):  
Jordan T. Walker ◽  
Andres Aslan ◽  
Rex D. Cole ◽  
Michael T. Heizler

The precise age of terrestrial sediments in the Late Cretaceous Williams Fork Formation of western Colorado is poorly constrained due to a paucity of radiometric data. Sanidine and zircon dating of a volcanic ash encased in coal (i.e., the Coal Canyon ash) within the Cameo-Wheeler coal zone of the lower Williams Fork Formation in Coal Canyon, Colorado provides an important new age constraint for the southwestern Piceance Basin. A 10-30 cm thick, light gray, clayey mudstone encased in coal was sampled for both zircon U-Pb and sanidine 40Ar/39Ar geochronology. The presence of numerous euhedral zircon crystals, a lenticular geometry, and a clayey texture suggest that the mudstone is a minimally reworked and slightly altered volcanic ash. Analysis of the euhedral zircon crystals (n=108) in the ash produced a statistically robust U-Pb date with 93 grains yielding a weighed mean age of 74.52 ±0.11 Ma (1σ analytical uncertainty). 40Ar/39Ar sanidine analyses yielded a younger weighted mean age of 73.10 ±0.12 Ma (1σ analytical uncertainty) based on 6 of the 36 grains analyzed. Our preferred age is given by the weighted mean age of the sanidine as it is based on higher precision analyses that can better discriminate older inherited grains that are likely included in the zircon mean-age calculation. Isotopic data for the Coal Canyon ash overlap in age with a K-Ar date of 72.5 ±5.1 Ma for a widespread Williams Fork Formation tonstein, known as the Yampa Bed, found in coal-bearing outcrops and mine workings throughout the northern Piceance and Sand Wash basins and Axial Basin Uplift. Based on the similarity in isotopic age, sedimentologic context and stratigraphic position, we suggest that the Coal Canyon ash and the regionally extensive Yampa Bed are coeval. Additionally, this correlation corroborates that the Cameo-Wheeler coal zone of the Williams Fork Formation in the southwestern Piceance Basin is correlative with the Middle coal zone of the Danforth Hills and Yampa regions. Lastly, this proposed correlation may suggest that the Coal Canyon ash, like the Yampa Bed, correlates with the Baculites reesidei ammonite zone, which is associated regionally with a bentonite dated to 72.94 ±0.45 Ma. Detrital sanidine geochronology of two lower Williams Fork sandstone units that overly the Coal Canyon ash did not produce grains younger than the ash and thus do not quantitatively improve the chronostratigraphy of these specific units. Lastly, the Coal Canyon ash date serves as a basis for future evaluations of the diachroneity of non-marine strata of the Williams Fork Formation.

2016 ◽  
Vol 155 (1) ◽  
pp. 119-131 ◽  
Author(s):  
V. I. DAVYDOV ◽  
J. L. CROWLEY ◽  
M. D. SCHMITZ ◽  
W. S. SNYDER

AbstractThe discovery and dating of a volcanic ash bed within the upper Phosphoria Formation in SE Idaho, USA, is reported. The ash occurs 11 m below the top of the phosphatic Meade Peak Member and yielded a 206Pb/238U date of 260.57 ± 0.07 / 0.14 / 0.31 Ma, i.e. latest Capitanian, Guadalupian. The stratigraphic position of this ash near the top of the Meade Peak phosphatic Member of Phosphoria Formation indicates plausible completeness of the sedimentation within the Guadalupian–Lopingian and probably at the Permo-Triassic (P-T) transitions. The new radiometric age reveals that the regional biostratigraphy and palaeontology of Phosphoria and Park City formations requires serious reconsideration, particularly in cool water conodonts, bryozoans and brachiopods. The new age proposes that the Guadalupian–Lopingian boundary (GLB) coincides with the Meade Peak – Rex contact and consequently with the end-Guadalupian extinction event. The lack of a major unconformity at the P-T transition suggests that the effects of the Sonoma orogeny were not as extensive as has been assumed.


2016 ◽  
Author(s):  
Benjamin J Burger ◽  
Christopher J Ward

In this paper we describe fossil conifer branches discovered in the Mesaverde Group, Williams Fork Formation in northeastern Utah, along Snake John Reef. Fossil conifers from the Campanian of northeastern Utah have not been previously studied, despite their common occurrence in the formation. The recovered fossils closely resemble Geinitzia known from the late Cretaceous of Europe, with several previous reported occurrences in North America, including New Jersey and Southern Utah. The fossils share morphological characteristics with Geinitzia, exhibiting short spirally arranged thin needles, with appressed scale-like leaves along the shoots. They differ from Araucarites in that the appressed needles are more scale like and smaller, and differ from the members of the modern Araucariaceae in lacking broad bases to the needles, although the fossils resemble the modern species Araucaria heterophylla (Norfolk Island Pine) native to the South Pacific. The observed branching pattern in the fossil reflects similarities found in the Cupressaceae Family, and Geinitzia may be regarded as an early member of this group, or having an affinity to fossil Araucariaceae conifers, which despite having a modern southern hemisphere geographic distribution today were widespread during the late Mesozoic, extending across North America and Europe.


2005 ◽  
Author(s):  
Stephen Lee Wolhart ◽  
Chad Eric Odegard ◽  
Norman Raymond Warpinski ◽  
Charles Kennedy Waltman ◽  
Sean Robert Machovoe

2016 ◽  
Author(s):  
Benjamin J Burger ◽  
Christopher J Ward

In this paper we describe fossil conifer branches discovered in the Mesaverde Group, Williams Fork Formation in northeastern Utah, along Snake John Reef. Fossil conifers from the Campanian of northeastern Utah have not been previously studied, despite their common occurrence in the formation. The recovered fossils closely resemble Geinitzia known from the late Cretaceous of Europe, with several previous reported occurrences in North America, including New Jersey and Southern Utah. The fossils share morphological characteristics with Geinitzia, exhibiting short spirally arranged thin needles, with appressed scale-like leaves along the shoots. They differ from Araucarites in that the appressed needles are more scale like and smaller, and differ from the members of the modern Araucariaceae in lacking broad bases to the needles, although the fossils resemble the modern species Araucaria heterophylla (Norfolk Island Pine) native to the South Pacific. The observed branching pattern in the fossil reflects similarities found in the Cupressaceae Family, and Geinitzia may be regarded as an early member of this group, or having an affinity to fossil Araucariaceae conifers, which despite having a modern southern hemisphere geographic distribution today were widespread during the late Mesozoic, extending across North America and Europe.


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