New palaeoclimatic constraints from paleosols on the Middle-Late Jurassic landscape, Western Colorado, U.S.A

ABSTRACT

The Wanakah Formation and Tidwell Member of the Morrison Formation record intervals of paedogenesis in the Paradox Basin and Central Colorado trough, western Colorado during the Middle-Late Jurassic. Detailed field description of paleosols and paedogenic carbonates at different stratigraphic horizons from three localities document four main pedotypes: vertisols, gleysols, oxisols, and protosols. Generally, gleysols reflect reducing conditions, protosols and oxisols reflect oxidizing conditions, and vertisols reflect fluctuations in oxidizing/reducing conditions (seasonality). Major elemental geochemical ratios in samples from these paleosols suggest variable redox conditions and a sub-humid to humid paleoclimate with seasonal precipitations during paleosol development. Estimated mean annual temperature based on elemental analyses of paleosol B in paleosols from Ribbon Trail and Escalante Canyon in western Colorado suggest mesic – thermic paleoclimate. Mean annual precipitation indicates sub-humid to humid regional palaeoclimatic conditions marked by seasonal precipitation. Clay mineralogy of these paleosols, determined by X-ray diffraction, shows the abundance of illite most likely formed from smectite alteration due to episodic wetting and drying. δ¹³C and δ¹⁸O isotope analyses of carbonate veins in these paleosols suggest that they formed recently from meteoric water and therefore are not considered for palaeoclimatic reconstructions. Mean annual temperature and mean annual precipitation estimates in the Wanakah Formation and Tidwell Member of the Morrison Formation provide new information on the local and regional palaeoclimatic and paleoenvironmental conditions during the Middle Jurassic in western Colorado as well as data for comparison with the Upper Morrison paleoclimate from northern and southern localities in the western United States.

KEYWORDS:

• Jurassic
• Colorado
• Wanakah Formation
• Morrison Formation
• Paradox Basin
• Central Colorado trough
• paleosol geochemistry
• paleoclimate
• paleoenvironment

Acknowledgments

Funding: This work was supported mainly by the American Chemical Society Petroleum Research Fund (ACS-PRF) Grant No. 55161 to Sally L. Potter-McIntyre. The Classen Family Named, American Association of Petroleum Geologists (AAPG) Grants-in-Aid and the National Association of Black Geoscientists (NABG) Scholarship Award to John I. Ejembi (JIE) supported a part of this research as well. Two anonymous reviewers and Editor Bob Stern are gratefully acknowledged for their assistance in improving the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

JIE wishes to thank: Dr. Sue M. Rimmer for providing access to the XRD equipment and reviewing an earlier draft of this manuscript; Dr. Mihai Lefticariu for assisting with the stable isotopes and XRD clay mineralogy experiments; Dr. Liliana Lefticariu for helpful discussions on some aspects of the geochemical data; and Joe Krienert for assistance with compiling the study area map.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2199431.

Additional information

Funding

The work was supported by the American Chemical Society Petroleum Research Fund (ACS-PRF) [55161]; American Association of Petroleum Geologists (AAPG) [Grants-in-Aid]; National Association of Black Geoscientists (NABG) [Scholarship Award].