New Geochronological Age Constraint and Chemostratigraphy for Aptian Lacustrine Strata, Cedar Mountain Formation, Utah

Abstract The Early Cretaceous is an important time of transition in Earth history, marked by a succession of oceanic anoxic events and carbon cycle perturbations that drove changes on land and in the ocean. The need for more precise geochronologic constraints in terrestrial sediments of Early Cretaceous age that record faunal and floral transitions is especially critical. The Cedar Mountain Formation (CMF) is a continental lithostratigraphic unit that hosts a trove of paleoclimate archives and important dinosaurian fossil localities. Determining the timing of deposition of CMF strata has been an ongoing effort for many years. Here, we present new lithostratigraphic and carbon isotope chemostratigraphic data along with high‐precision radiometric ages to further constrain the Ruby Ranch Member of the CMF at a unique locality referred to as “Lake Carpenter,” where a thick section of dominantly lacustrine strata overlies fluvial‐overbank to palustrine strata more typical of other Ruby Ranch Member outcrops. A bentonite bed near the base of the section provides one of the most precise ages yet determined within the Ruby Ranch Member of 115.92 ± 0.14 Ma via CA‐ID‐TIMS U‐Pb analysis of zircons. The age and the trends in the carbon isotope record indicate that the Lake Carpenter sediments were deposited entirely within the late Aptian Stage. These unique new data provide an important step toward improving our understanding of the timing of Early Cretaceous evolutionary and paleoclimate events.