James G. Honey

Paleohydrologic and Stratigraphic Significance of Crayfish Burrows in Continental Deposits: Examples from Several Paleocene Laramide Basins in the Rocky Mountains

ABSTRACT Paleocene crayfish burrows are present locally in great abundance in the Greater Green River, Hanna, Wind River, and Piceance basins of Wyoming and Colorado. In the Washakie sub-basin of the Greater Green River Basin, burrows are found in (1) lenticular, cross-bedded sandstones of fluvial-channel origin, (2) massive sandstones and mudrocks of floodplain origin that surround and interfinger with the cross-bedded sandstones, and (3) thin, sandy ironstone beds of overbank and crevasse-splay origin. The burrows, which represent crayfish activity in proximal to distal floodplain settings, are assigned to Camborygma symplokonomos, C. eumekenomos, and C. litonomos on the basis of their architectural morphologies. C. araioklados is not present. Crayfish burrows are part of pedogenically modified channel and overbank deposits. In all the basins studied, burrows are abundant only in non-coal-bearing rocks. The presence of elongate burrows indicates that the landscape was imperfectly drained (on a seasonal basis), and the burrow depths suggest the paleo-water table was 1-4+ m below the paleosurface. Swampy conditions prevailed in areas where carbonaceous shales and coals formed, and thus crayfish did not construct deep burrows. The crayfish burrows and associated lithologies suggest a warm, humid, wet-seasonal climate. In the eastern Washakie Basin, the vertical and lateral distribution of burrows and their abundance reflect changes in the paleohydrologic regime over space and time. Burrow depth and overall abundance decreases towards local basin lowlands, where they are rare. The paleohydrologic regime coupled with a progressively less seasonal climate through time allowed the formation of the Cherokee coal zone in the late Paleocene. In an otherwise lithologically homogeneous, non-variegated deposits, stratigraphically significant surfaces may be indicated only by the tops of burrowed intervals and by other types of paleosols. The tops of burrowed intervals represent paleosurfaces of varying environmental stability on which there are different frequency and magnitude of sedimentation events. Immature cumulative paleosols characterized by less intensely burrowed beds or bedsets record higher sedimentation rates. More intensely burrowed beds or bedsets reflect the activity of many generations of crayfish; hence, the tops of these layers record longer durations of exposure and pedogenic modification with infrequent or no sedimentation. Paleosols represented by burrowed intervals, in conjunction with sandstone and mudrock stacking patterns, can be used to identify significant stratigraphic surfaces, such as parasequences and sequence boundaries, in fully continental environments.

Pedogenic carbonate stable isotope record of environmental change during the Neogene in the southern Great Plains, southwest Kansas, USA: Carbon isotopes and the evolution of C4-dominated grasslands

Fossiliferous strata in the Meade Basin (southwest Kansas) preserve numerous superposed mammalian faunas and calcareous paleosols that range in age from the Clarendonian North American Land Mammal Age (NALMA; 12.0–9.0 Ma, early late Miocene) to the early Irvingtonian NALMA (ca. 2.5–ca. 1.0 Ma, early Pleistocene). Faunas from these sections document the evolution of the small mammal community of the modern grassland ecosystem of the region, and the stable isotope composition of paleosol carbonates provides a means by which the environmental context of the evolution of the modern ecosystem may be documented. We used the stable carbon isotope composition (δ 13 C relative to Vienna Peedee belemnite [VPDB]) of 194 pedogenic carbonates from 19 measured sections to reconstruct the history of C 4 grass abundance in the Meade Basin. Paleosol carbonate δ 13 C values reflect the proportion of C 3 (trees, shrubs, cool-climate grasses) and C 4 (warm-climate grasses) plants that grew in an ancient soil and provide a means with which to reconstruct past mammalian habitats. Paleosol carbonate δ 13 C values record a three-phase increase in the abundance of C 4 biomass during the Neogene in the Meade Basin. Late Miocene sections have mean δ 13 C values of −7.6‰ ± 0.90‰ (Clarendonian) and −6.5‰ ± 0.31‰ (Hemphillian NALMA, 9.0–4.9 Ma), consistent with 17% and 26% C 4 biomass, respectively. Miocene δ 13 C values from Meade are statistically identical to published δ 13 C values for Miocene paleosol carbonates elsewhere in the southern Great Plains, supporting the widespread presence of ∼20% C 4 biomass on average in the region throughout the Miocene. The abundance of C 4 biomass increased between the end of the Hemphillian section and the beginning of the early Blancan NALMA (5.0–3.0 Ma). Early and middle Blancan (3.0–2.5 Ma) carbonates have statistically identical δ 13 C values (−4.9‰ ± 0.90‰ and −5.0‰ ± 1.10‰, respectively), suggesting a stable ecosystem during the early Pliocene, although high δ 13 C variability in densely sampled intervals suggests a high degree of landscape-scale variation in C 4 abundance. The final phase, geochronologically controlled by two well-characterized ashes (Huckleberry Ridge, 2.10 Ma; Cerro Toledo B, 1.47–1.23 Ma) and magnetostratigraphy, is a trend to higher δ 13 C values from the late Blancan to early Irvingtonian (ca. 2.5–ca 1.0 Ma) from −4‰ at the base of the section to ∼1‰ at the top, corresponding to an increase from almost 50% to 65% C 4 biomass. The abundance of C 4 biomass first reaches modern levels for the region (78% ± 10.9%) around the level of the Cerro Toledo B ash, indicating that a modern-like grassland ecosystem first appeared in the region ca. 1.3 Ma, although δ 13 C values do not remain consistently high through the rest of the section.

Pedogenic carbonate stable isotope record of environmental change during the Neogene in the southern Great Plains, southwest Kansas, USA: Oxygen isotopes and paleoclimate during the evolution of C4-dominated grasslands

Neogene strata in the Meade Basin (southwest Kansas) preserve numerous superposed calcareous paleosols in sections that range in age from the Clarendonian North American Land Mammal Age (NALMA; 12.0–9.0 Ma, early late Miocene) to the early Irvingtonian NALMA (ca. 2.5–ca. 1 Ma, early Pleistocene). The carbon isotope compositions (δ 13 C relative to Vienna Peedee belemnite [VPDB]) of pedogenic carbonates from these sections record the protracted regional increase in the abundance of grasses using the C 4 photosynthetic pathway over this interval, with the first appearance of a modern-like grassland ecosystem having >70% C 4 biomass around 1.3 Ma. We use the stable oxygen isotope composition (δ 18 O relative to Vienna standard mean ocean water [VSMOW]) of 194 paleosol carbonates from 19 measured sections to reconstruct the climatic conditions in the Meade Basin during the rise of C 4 grasses to ecological dominance in the region. Pedogenic carbonate δ 18 O values are sensitive to soil temperature and the δ 18 O of soil water, and hence they are a paleoclimate proxy. Carbonate δ 13 C values do not exhibit consistent trends in relation to δ 18 O values, indicating no consistent relationship between short-term climatic conditions in terms of temperature or aridity and the abundance of C 4 biomass. Mean carbonate δ 18 O values within biostratigraphic intervals decrease from the Clarendonian (25.3‰ ± 0.72‰) to the early and middle Blancan (21.8‰ ± 0.87‰ and 22.1‰ ± 0.69‰, respectively), and they also exhibit a decreasing trend in the late Blancan–early Irvingtonian, from ∼25‰ to ∼21‰. The increase in δ 18 O values between the end of the middle Blancan and the beginning of the late Blancan–early Irvingtonian sections could reflect the onset of Northern Hemisphere glaciation and a change in the isotope composition of the hydrosphere due to increased ice volume. The long-term trend in δ 18 O values suggests that C 4 biomass increased coincident with some combination of decreasing temperature, increasing proportion of winter precipitation recharge of soil water, and/or increasing soil moisture. Thus, the ecological dominance of C 4 grasses in the region today does not seem to be linked to warmer temperatures or increased aridity.

LATE PLIOCENE AND EARLY PLEISTOCENE RODENTS FROM THE NORTHERN BORCHERS BADLANDS (MEADE COUNTY, KANSAS), WITH COMMENTS ON THE BLANCAN-IRVINGTONIAN BOUNDARY IN THE MEADE BASIN

Abstract New fossiliferous localities in the Borchers Badlands of southwestern Kansas add to our knowledge of rodent community change across the Plio-Pleistocene and Blancan-Irvingtonian boundaries. We report fossil rodents from ten local faunas in the Badlands that range stratigraphically from beneath the Huckleberry Ridge ash (2.10 Ma [million years ago]) to a level just beneath the Cerro Toledo B ash (1.23–1.47 Ma). The late Blancan Borchers local fauna (l.f.), includes the Meade Basin highest stratigraphic datum (HSD) for the following taxa that characterize or are found in earlier Blancan faunas: Alilepus, Geomys quinni, Sigmodon minor, and Geochelone. Borchers currently also has the HSDs for Reithrodontomys pratincola and Ondatra zibethicus /idahoensis, but the prior temporal distribution of these taxa in the Meade Basin is unknown. The stratigraphically lowest Pleistocene (Irvingtonian) l.f. in the Badlands, Nash 72, includes the lowest stratigraphic datum (LSD) for Cynomys, Reithrodontomys moorei, Microtus, and Mictomys kansasensis, all of which are found above Nash 72 and beneath the Cerro Toledo B ash. Prodipodomys is last seen at Nash 72, and Sigmodon curtisi is first encountered at Short Haul, a locality slightly younger than Nash 72. A preliminary hypothesis of age based on stratigraphic position places the Nash 72 l.f. at about 1.80 Ma, suggesting that this fauna and the genus Microtus may characterize both the Plio-Pleistocene and Blancan-Irvingtonian boundaries in the Borchers Badlands.

BLANCAN LAGOMORPHS AND RODENTS OF THE DEER PARK ASSEMBLAGES, MEADE COUNTY, KANSAS

Abstract A new collection of lagomorphs and rodents from the Deer Park B local fauna (l.f.) of Meade County, Kansas is described and compared with other small mammal assemblages of the Meade Basin, including the underlying Deer Park A l.f. Deer Park A was correctly assigned by Hibbard to the Blancan, bridging the gap between earlier Blancan faunas such as Fox Canyon and the late Blancan Sanders l.f. Recent fieldwork indicates that the Deer Park quarries may lie in the Rexroad Formation, rather than in the Ballard Formation as previously assumed. The geology and extinct mammalian contingent at Deer Park suggest that the lower horizon of Deer Park A was an active spring that gradually turned into a marshy environment during Deer Park B time. The rodents of Deer Park B are indicative of an open prairie ecosystem that might have been somewhat more arid than that of southwestern Kansas today.

Five million years of pocket gopher history in the Meade Basin of southwestern Kansas and northwestern Oklahoma

ABSTRACT The Meade Basin record of pocket gophers extends from the latest Miocene (Buis Ranch local fauna) to modern time. A primitive species with hypsodont but rooted cheek teeth, Pliogeomys buisi, characterizes the late Hemphillian. Another species of Pliogeomys, P. louderbachi, is described here as a new species from the early Blancan. It is intermediate in dental and mandibular morphology between Pliogeomys and Geomys. The Geomys minor (= G. smithi) lineage displays a stepped dwarfing trend prior to its extinction, whereas the G. jacobi lineage demonstrates an overall pattern of stasis in size. G. jacobi is replaced by G. quinni at the end of the Pliocene, within which there is a significant directional size increase. A late Pliocene immigrant, the new species G. floralindae, appears in the Sanders assemblage. It is briefly replaced by an indeterminate small species in the Nash 72 local fauna. G. tobinensis is found in the Cudahy local fauna. Morphologically modern G. bursarius appears in the Meade Basin during the early Rancholabrean. A small, primitive species, Geomys adamsi, appears only during the early Pliocene at Fox Canyon, and transient Thomomys appear at various times during the Pleistocene, both apparently during cold intervals. A phylogenetic analysis suggests two clades, one uniting Pliogeomys russelli and Geomys adamsi and another including the remaining Meade Basin geomyines. Enhanced species turnover and the last push for modern mandibular morphology is most pronounced in sediments younger than about 2.6 million years ago, corresponding to the first global cooling heralding the Pleistocene.

Plesiadapidae (Mammalia, Primates) from the Late Paleocene FORT Union Formation of the Piceance Creek BASIN, Colorado

Abstract Utilizing the occurrence of plesiadapid mammals, we recognize both Tiffanian and Clarkforkian ages in the Fort Union Formation of the Piceance Creek Basin of Colorado. Five species of plesiadapids are described from eighteen fossil localities. The presence of Nannodectes gazini, Plesiadapis fodinatus and a new species Chiromyoides gigas indicate that at most eleven of the reported fossil localities are Tiffanian in age. The majority of the fossil specimens come from UCM locality 92177 (USGSD-2001) located 35.8 meters above the base of the Fort Union Formation, which we place in the Ti5 lineage zone. The occurrence of Plesiadapis dubius and Chiromyoides caesor in the Plateau Valley area suggests a younger Clarkforkian age for these stratigraphically higher localities. We are the first to recognize Tiffanian aged fossils in the Piceance Creek Basin, extending the mammalian record back to more than 58 million years ago.