High precision U Pb zircon geochronology for Cenomanian Dakota Formation floras in Utah

Abstract

The Cretaceous Dakota Formation on and near the Colorado Plateau represents the time-transgressive shoreline deposits of the Western Interior Seaway and is a rich repository of fossil plants and animals. However, given the basin architecture and depositional setting, correlations between localities are difficult without geochronology. Here we present new high precision ID-TIMS U Pb zircon dates from five air-fall ash deposits from Westwater (n = 3) and Henrieville (n = 2) that obey stratigraphic superposition and precisely constrain the ages of these two floras. Three air-fall ashes from the Westwater locality record weighted mean Th-corrected 206Pb/238U dates of 97.949 ± 0.037/0.12 Ma, 97.943 ± 0.023/0.12 Ma, and 97.601 ± 0.049/0.13 Ma, providing the first U Pb dates from this locality. Two ash beds from the Henrieville locality record weighted mean Th-corrected 206Pb/238U dates of 95.070 ± 0.036/0.12 Ma and 94.879 ± 0.032/0.11 Ma. The new dates indicate that the two floras are separated by ca 2.5 Ma and are both within the Cenomanian. The assemblage of fossil plants collected from the mudstone and ash-bed localities at Westwater show no compositional overlap, despite the presence of 28 distinct morphotypes. This lack of compositional overlap is repeated between the mudstone facies from the Westwater and Henrieville localities, where two different morphotypes of Liriodendron are the taxa in common. The species incongruence at Westwater is likely due to rapid colonization of early successional plants post deposition of the volcanic ash. For the mudstone floras at Westwater and Henrieville, the different floral composition may be due to rapid species evolution during the rise of angiosperms, the influence of climate in the mid-Cretaceous, the expression of high regional diversity of localities separated by 300 km, or a highly partitioned floodplain vegetation. Estimated sediment accumulation rates from the new radioisotopic dates, combined with existing proxy records for pCO2, suggest that the rise in pCO2 preceding Ocean Anoxic Event 2 (OAE2) began 513 ka (range from 384 to 641 ka) prior to the positive d13C excursion that defines the event. This estimate from terrestrial rocks is within error of estimates for the timing of changes in d34Ssulfate (570 ka; range from 420 to 814 ka) in marine sections at the GSSP for the Cenomanian Turonian boundary. The overlap indicates synchronous perturbation of marine and terrestrial environments related to an increase in pCO2 prior to the onset of OAE2, providing further support for the volcanic initiation hypothesis.