The stratigraphic section located at Toadstool Park, northwestern Nebraska, preserves a detailed sedimentological and fossil record of the Eocene-Oligocene boundary, yet chronologic control is limited to sparse ash dates and magnetostratigraphy. To improve the chronologic control of the Toadstool Park sediments, we examined the fossil collection at the University of Nebraska State Museum and recorded the stratigraphic level of some important biochronologic events. All stratigraphic levels were determined relative to the Upper Purplish White layer (UPW), a distinctive ash that is chemically and mineralogically correlated to an ash dated at 34.6 ± 0.1 Ma. The small artiodactyls Leptomeryx speciosus, Leptomeryx mammifer, and Leptomeryx yoderi co-occur at ‒34 ± 2 m, implying that the early Chadronian–middle Chadronian boundary (35.7 Ma) is located at this level. The first occurrence of L. mammifer (i.e., the middle Chadronian–late Chadronian boundary, 34.8 Ma) is located at ‒18 ± 2 m, and the first occurrence of the camel Poebrotherium wilsoni (34.2 Ma) is located at 0 ± 2 m. The first occurrences of two other artiodactyls (Hypertragulus calcaratus and Leptomeryx evansi), which define the Chadronian-Orellan boundary (33.9 Ma), are located approximately at the UPW, ~5 m lower than in the nearby type sections of Wyoming. This apparent diachroneity may reflect sampling bias in Nebraska, i.e., loose material washed down from higher outcrops. The first occurrences of the dwarfed oreodont Miniochoerus affinis and of the rodent Eumys elegans (i.e., late early Orellan, 33.6 Ma) are located at +8 ± 2 m and +11 m, respectively. Finally, the first occurrence of M. gracilis (i.e., early late Orellan, 33.2 Ma) is located at +13 ± 2 m. These time points, along with magnetostratigraphy and 40 Ar/ 39 Ar dates obtained for this section, were used to refine the age model for a previously published stable isotope record of fossil bone CO 3 . The record is augmented here with 59 bone samples from federal lands near Toadstool Park spanning the critical time period of 33.8–33.4 Ma. The δ 18 O values in the complete data set show a step to higher values (from 23.0‰ ± 0.2‰ to 24.5‰ ± 0.2‰) at ca. 34 Ma, synchronous with the marine record. In combination with unchanged enamel compositions, this increase in bone CO 3 δ 18 O translates into a mean annual temperature drop of 7.1 ± 3.1 °C across the boundary. The time of onset of the climate change coincides with an increase in sedimentation rate from 19 to 37 m/m.y., confirming the strong role of climate on continental erosion and sediment yield.

A zone of intense soft-sediment deformation, with associated spherules and shocked quartz grains, is identified over an area of ∼1000 km 2 in southwestern South Dakota. This Disturbed Zone (DZ) is up to 5 m thick and is preserved within distal deltaic deposits of the Upper Cretaceous Fox Hills Formation. Localized structural development caused thinning and eventual subaerial exposure of several sections within Badlands National Park, whereas sections to the north of the park were unaffected. Although previously interpreted as an intense period of soil formation under tropical conditions, the degree of ancient soil overprinting of these sections is minimal, with the exception of bright coloration of the strata, and appears not to have had any effect on ejecta preservation. Biostratigraphic data suggest a middle to late Maastrichtian age for the DZ. When compared to other Cretaceous impactites, our study sections in southwestern South Dakota are most similar to, and may correlate with, the recently documented 68 Ma impactite within the Vermejo Formation of Berwind Canyon in southeastern Colorado. If this correlation is correct, the size of the ejecta within the Fox Hills and Vermejo Formations suggests that the sections in South Dakota represent distal deposits.

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