Volume 38 Number 1

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Jason F. Hicks¹,Kirk R. Johnson^1*, John D. Obradovich², Daniel P. Miggins²,and Lisa Tauxe³

¹Departmentof Earth Sciences, Denver Museum of Nature & Science, 2001 ColoradoBoulevard, Denver, CO 80205-5798, U.S.A.

²U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, CO 80225,U.S.A.

³ScrippsInstitution of Oceanography, Geological Research Division, 9500 Gilman Drive,La Jolla, CA 92093-0220, U.S.A.

*Author to whom correspondence should be addressed: kjohnson@dmns.org

The Denver Basin is a Laramide foreland basin that filled with synorogenic sediment shed from the rising Rocky Mountains from the end of the Cretaceous through the Eocene. This sedimentary sequence contains a rich and diverse biota that is difficult to correlate because of the low relief and poor exposures characteristic of this region. This study has correlated the stratigraphic sequence and fossil localities using a combination of three techniques. Magnetostratigraphy has proven to be an effective way to date these rocks as they contain a measurable and interpretable reversal sequence that can be correlated to the latest Cretaceous through Tertiary geomagnetic polarity time scale (GPTS). The palynostratigraphy of the region is well known and the rocks contain multiple levels that yield palynomorphs. Volcanic ashes found in both the Cretaceous and Tertiary units can be dated using the ⁴⁰Ar/³⁹ Ar isotopic dating method. Each technique has its advantages and disadvantages, but in combination, these three chrono- and biostratigraphic methods have the potential to date with a high level of precision virtually every fossil locality sampled in the Denver Basin. To effectively date the exposures from across the entire basin, a reference or benchmark section had to be established against which the biostratigraphic zonation could be directly correlated to the chronology. Due to the paucity of long surface outcrops, drilled wells were the only way to obtain a continuous rock sequence from which a reference section could be constructed. Two cores, one drilled at Castle Pines on the western margin of the basin, and another at Kiowa in the central part of the basin, provided a continuous rock sequence from the top of the Maastrichtian Pierre Shale to the Eocene rocks of the D2 synorogenic sequence.

The magnetostratigraphic study of these cores established a reversal sequence that could be correlated to the GPTS ranging from polarity chron 31 through to chron 24. The palynostratigraphy yielded a zonation ranging from the Aquilapollenites striatus Interval Zone through to the early Eocene, and accurately placed the Cretaceous-Tertiary (K-T) boundary in each core. Isotopic ages were obtained from the Maastrichtian, early Paleocene, and early Eocene parts of the section, and allow us to independently confirm the calibration of the units to the time scale. With this chronostratigraphic framework in place, the individual fossil-bearing localities and surface sections from across the entire basin can be correlated and dated to a precision that is comparable to the calibrating isotopic ages.

The measured sedimentation rates vary across this asymmetric basin, with higher rates in the western, proximal part of the basin, and a pronounced increase in sedimentation rate across the K-T boundary. Two separate packages of strata, separated by a regional unconformity between the D1 and D2 synorogenic sequences, were dated. The Maastrichtian through Paleocene sequence that encompasses the Pierre Shale, Fox Hills Sandstone, Laramie Formation, and D1 strata dates from about 69 to 64 Ma. The overlying D2 synorogenic strata are poorly constrained and date from about 54 Ma. The reversal pattern of the D2 sequence varies across the basin, which indicates that sedimentary hiatuses, probably caused by tectonically quiet intervals along the mountain front, were followed by differential subsidence across the basin as sedimentation resumed at different times across the 100 km (60 mi) breadth of the basin.

Key Words: Late Cretaceous • Tertiary • Maastrichtian • Paleocene • Denver Basin • K-T boundary • paleomagnetism • magnetostratigraphy • isotopic age

Elisabeth A.Wheeler¹* and Thomas C. Michalski²

¹Department of Wood and Paper Science, North Carolina State University, Box 8005, Raleigh,NC 27695-8005, U.S.A.

²CoreResearch Center, U.S. Geological Survey, Denver, CO 80225, U.S.A.

*Author to whom correspondence should be addressed: xylem@unity.ncsu.edu

Silicified woods are common in the upper D1 (Paleocene – Puercan and Torrejonian) and D2 (Eocene – Wasatchian) sequences of the Denver Basin. Almost all derive from angiosperms. Woods from the upper D1 sequence are the second set of angiosperm woods described from Paleocene strata of the Rocky Mountain region. Wood assemblages from the upper D1 sequence differ from Paleocene wood assemblages of the San Juan Basin, New Mexico and the Big Bend region of Texas, indicating variation within the Western Interior. Lauraceous woods with oil cells are common in the Denver Basin assemblages while they are not known from the San Juan Basin or Big Bend. This initial survey suggests that the early Paleocene D1 (7 wood types) and early Eocene D2 (5 wood types) wood assemblages differ. Lauraceous woods with oil cells apparently are not common in the D2 sequence. The early Eocene Denver Basin wood assemblages differ from the early Eocene Yellowstone Fossil Forest wood assemblages in which conifers are common and phyllanthoid woods are rare. Growth rings are present but not well defined in the D1 and D2 sequence woods. The Denver Basin angiosperm woods are neither semi-ring porous nor ring porous, features that are common in present-day northern temperate forests and in angiosperm woods from the late Eocene Florissant Fossil Beds.

Key Words: fossil wood • paleobotany • Paleocene • Eocene • Lauraceae • Paraphyllanthoxylon • Platanaceae

Richards. Barclay¹*, Kirk R. Johnson¹, William J. Betterton²,and David L. Dilcher³

¹Denver Museum of Nature & Science, 2001 Colorado Blvd., Denver, CO 80205, U.S.A.

²U.S.Geological Survey, MS 973, Box 25046, Denver Federal Center, Denver, CO 80225,U.S.A.

³Universityof Florida, Florida Museum of Natural History, Box 117800, Gainesville, FL32611-7800, U.S.A.

*Author to whom correspondence should be addressed: sihetun@hotmail.com

A 50-m-thick section of the Denver Formation (D1 sequence of Raynolds) exposed at the West Bijou Site of the Plains Conservation Center, Arapahoe County, Colorado, contains the first complete Cretaceous-Tertiary (K-T) boundary section in the Denver Basin. The 3-cm-thick boundary claystone coincides with a 21 percent palynological extinction, contains iridium and shocked-mineral anomalies, and is immediately overlain by a 7-cm-thick, fern-spore anomaly interval. The entire 50-m section is of reversed magnetic polarity and is interpreted to be subchron C29r because of paleontological data, the presence of a tuff with an ⁴⁰ Ar/³⁹ Ar date of 65.73 ± 0.13 Ma, and by correlation to the Kiowa cored well. A diagnostic basal Puercan (Pu1) mammal jaw was found 12 m above the boundry clay, and ceratopsian and hadrosaurian dinosaur fragments occur 4 m below the boundary clay. Estimates of the basin sedimentation rate derived from the duration of C29r based on marine cyclostratigraphy suggest that the 28 m of basal Paleocene strata represent approximately 200,000 years. Abundant fossil leaves found within the Paleocene sedimentary rocks at the West Bijou Site K-T boundary section allow assessment of early Paleocene patterns of plant diversity. Nine leaf localities at eight stratigraphic levels in the basal 22 m of the Paleocene section were sampled and analyzed to better understand the flora that survived the global K-T catastrophe. The Paleocene flora of this site is taxonomically dominated by dicotyledonous angiosperms (74%), with lesser numbers of monocotyledons (10%), ferns and allies (11%), and conifers (5%). By number of specimens, angiosperms comprise greater than 95 percent of the flora. Within the sampled section, there were no recognizable directional trends in diversity or abundance, suggesting that earliest Paleocene vegetation was stable, although not particularly diverse. The West Bijou Megaflora is strikingly similar in composition and relative abundance to basal Paleocene floras from the northern Great Plains. This K-T boundary disaster-recovery flora, also known as the FUI flora, is shown to have been widespread, ranging from central Colorado to southern Canada, a distance of at least 1,100 km. The West Bijou Site megaflora is markedly different from coeval floras along the western margin of the Denver Basin, supporting the hypothesis that orography and elevation provided greater influence on basal Paleocene floral composition and diversity than did latitude.

Key Words: Paleocene • Puercan • paleobotany • Cretaceous-Tertiary boundary • shocked quartz • iridium • biostratigraphy • Denver Basin • Denver Formation • D1 sequence

Beth Ellis*,Kirk R. Johnson, and Regan E. Dunn

Department of Earth Sciences, Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, CO 80205-5798, U.S.A.

*Author to whom correspondence should be addressed: bellis@dmns.org

A very diverse, early Paleocene (63.8 ± 0.3 Ma) fossil leaf site located in Castle Rock, Colorado represents nearly autochthonous burial of a rainforest floor. This is an unusual fossil flora preserved in an unusual manner. The site, on the western margin of the Denver Basin in synorogenic sediments associated with the rise of the Laramide Front Range, is dated using multiple methods. Leaves are preserved in three distinct units overlying a poorly developed paleosol that contains in situ tree trunks. Fossil-bearing units are continuous along 150 m of outcrop. The leaves were apparently preserved as a result of rapid deposition of sand and mud onto the floor of a mature rainforest via overbank flooding. Five quarries were excavated and the leaves from these quarries were segregated by morphotype and scored for leaf area and margin type. From 1030 specimens, we document 93 unique dicotyledonous angiosperm leaf types, three cycads, three ferns, two conifers, and seven seed types. There is little taxonomic variation among leaf-bearing units of a single quarry, but the taxonomic composition varies significantly among laterally spaced quarries, suggesting that the fossil leaf litter reflets the original growth positions of the source trees. We compare the fossil leaf litter to leaf litter of modern forests and show that the Castle Rock flora has numerous features in common with extant equatorial rainforests, including dominance by angiosperms, high species richness, large leaves that often have smooth margins and drip tips, and high spatial heterogeneity from quarry to quarry.

Key Words: Paleobotany • Paleocene • fossil rainforest • Denver Basin • paleoclimate • Denver Formation • D1 sequence • Castle Rock rainforest

Kirk R. Johnson¹*, Michele L. Reynolds¹,Kevin W. Werth¹, and Joseph R. Thomasson²

¹Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, CO 80205,U.S.A.

²Department of Biological Sciences, Sternberg Museum of Natural History, Fort Hays State University, Hays, KS 67601, U.S.A.

*Author to whom correspondence should be addressed: kjohnson@dmns.org

Late Cretaceous and Paleogene plant fossils collected at 149 localities in the Denver Basin, Colorado, are placed into a stratigraphic framework based on palynostratigraphy, magnetostratigraphy, vertebrate paleontology, geochronology, sequence stratigraphy, electric well logs, and two cored wells. Between 69 and 54 Ma, the Denver Basin accumulated sedimentary rocks that recorded the withdrawal of a seaway, the uplift of a mountain range, and evidence of the Cretaceous-Tertiary and Paleocene-Eocene boundary events. Fossil floras deposited in the Denver Basin record these events as variations of floral composition, species diversity, and leaf margin and size (used to estimate mean annual temperature and precipitation, respectively). Attention to these details and to the position of the floras relative to the basin margins and sedimentary facies allows for the recognition of six megafloral associations (K-L, K-D1, P-D1-West, P-D1-Central, P-D1-East, and E-D2). Preliminary comparison of these assemblages documents: floral change at the K-T boundary; a strong paleoenvironmental gradient probably associated with increased topographic relief along the basin margin in the early Paleocene; and a warmer, drier Eocene vegetation.

Key Words: Paleobotany • paleoclimate • Cretaceous • Paleocene • Eocene • Denver Basin • Laramie Formation • D1 sequence • D2 sequence

J. Howard Hutchison and Patricia A. Holroyd*

Museum of Paleontology, 1101 Valley Life Sciences Building, University of California,Berkeley, CA 94720-4780, U.S.A.

*Author to whom correspondence should be addressed: pholroyd@uclink4.berkeley.edu

The record of turtles in the Denver Basin spans four formations (Fox Hills, Laramie, Arapahoe, and Denver) that range from Late Cretaceous (Lancian) to early Paleocene (Puercan) in age. We recognize at least fifteen distinct, although fragmentary, species-level taxa, including Pleurosternidae (Compsemys), Baenidae (Neurankylus, Plesiobaena, Stygiochelys, and Palatobaena), Kinosternia (Hoplochelys), Adocidae (Adocus), Nanhsiungchelyidae (Basilemys), Trionychidae (Axestemys, Aspideretoides, Helopanoplia, and another plastomenine), Macrobaenidae, and Chelydridae. Notable among these are the best-preserved skull of Compsemys, a new chelydrid genus, the most northerly confirmed record of Hoplochelys, and the most southerly records of Helopanoplia, Stygiochelys, and Macrobaenidae in the Rocky Mountain region. We also present evidence for synonymy of Paleotrionyx and Conchochelys with Axestemys, and the first cranial material assignable to Axestemys. The early Paleocene (Puercan) part of the Denver Formation yielded the most diverse assemblage, followed by the Cretaceous (Lancian) part of the Denver Formation and Laramie Formation. The Cretaceous samples are not demonstrably different from more northerly comparable faunas, but the more diverse Paleocene part of the Denver Formation exhibits a unique combination of taxa compared to contemporaneous faunas to the north and south.

Key Words: Chelonii (= Chelonia) • Paleocene • Cretaceous • biogeography • Denver Basin • Arapahoe Formation • Fox Hills Sandstone • Denver Formation • Laramie Formation

Jaelyn J.Eberle

University of Colorado Museum, 265 UCB, Boulder, CO 80309, U.S.A.

As a result of the Denver Basin Project, several more fossils of Puercan mammals are reported here from five areas in the Denver Formation: South Table Mountain, Greater Denver, the West Bijou Site, Big Gulch, and Corral Bluffs. Systematic description and discussion are provided for one multituberculate and 11 ungulate taxa, including a new species of Baioconodon. Some taxa represent extensions of previously recognized temporal and geographic ranges. Notably, the ungulate Protungulatum donnae from strata of early Puercan (Pu1) age in the Denver Formation represents the southernmost occurrence of the species, while Oxyclaenus simplex, in probable early Puercan strata of the Denver Formation, appears to represent both a temporal and geographic range extension from middle Puercan (Pu2) strata of the San Juan Basin, New Mexico. Additionally, I report the first occurrence in the Denver Basin of the arctocyonid Loxolophus faulkneri. Refined biostratigraphic interpretations, resulting from new discoveries and incorporating paleomagnetic, palynological, and radioisotopic analyses presented elsewhere, suggest that Puercan interval zones Pu1 and Pu2 are both represented by mammalian faunas in the Denver Formation. Localities at South Table Mountain, as well as the Denver Oxyclaenodon Site (DMNH loc. 299) and Nicole's Mammal Jaw locality (DMNH loc. 2557), are Pu1 correlatives. Discoveries reported here support previous interpretations that the Alexander and South Table Mountain localities are probably similar in age (i.e., Pu1) and are included here in the Littleton fauna. Based upon comparison to other Puercan faunas, the Littleton fauna may be temporally intermediate between typical Pu1 assemblages known north of the Denver Basin and earliest Pu2 assemblages from the Hanna Basin, Wyoming. Alternatively, faunal differences between the Littleton fauna and other Pu1 faunas may reflect ecological and biogeographic differences. The unusually high diversity of ungulates in the Littleton fauna suggests some of the diversification that elsewhere characterizes the onset of Pu2 may already have begun by Pu1 in the Denver Basin. The mammalian assemblage at Corral Bluffs is interpreted here as a probable Pu2 correlative, based upon occurrence of Loxolophus faulkneri, Conacodon entoconus and C. delphae, and absence of Pu3 index taxa. Pu2/Pu3 correlatives (i.e., faunal assemblages that are either Pu2 or Pu3) also are reported here from the Big Gulch area, although more fossils are needed to refine their ages. The present study and others in this issue demonstrate that the Denver Basin is among the few places wherein correlation between Puercan mammalian biostratigraphy and paleomagnetic, palynological, and radioisotopic analyses is an attainable goal.

Key Words: biostratigraphy • Paleocene • Puercan • Denver Formation • Denver Basin • Colorado • Eutheria • Multituberculata • Ungulata

Robert G. Raynolds* and Kirk R. Johnson

Department of Earth Sciences, Denver Museum of Nature & Science, 2001 Colorado Boulevard,Denver, CO 80205, U.S.A.

*Author to whom correspondence should be addressed: denverbasin@dmns.org

In a multidisciplinary effort, uppermost Cretaceous and lower Tertiary Laramide synorogenic strata in the Denver Basin have been dated using biostratigraphy, magnetostratigraphy, and radioactive isotopes. The resulting calibrated framework permits biotic and stratigraphic events to be examined in a spatial and temporal context. Synorogenic sediments accumulated in two distinct pulses separated by about eight million years. Faunal changes are evident across time and, to a lesser extent, space. In addition to evolutionary changes through time, floras show distinctive distributional patterns that reflect the ancient landscape and indicate the presence of Paleocene monsoons on eastern flanks of the Rocky Mountains. Observations by the Denver Basin Project team have been used to create a series of rigorously documented paintings reconstructing ancient Denver Basin landscapes. As a result of our stratigraphic work, bedrock aquifer patterns in the Denver Basin are better quantified. The Arapahoe aquifer is comprised of a series of buried alluvial fans and does not form a uniform layer across the Denver Basin.

 Key Words: Denver Basin • biostratigraphy • paleontology • palynology • chronostratigraphy • hydrogeology