Magnetostratigraphyof
Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver
Basin,Colorado
Jason F. Hicks1,Kirk
R. Johnson1*, 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.
*Authorto
whom correspondence should be addressed: kjohnson@dmns.org
ABSTRACT
The Denver
Basinis a Laramide foreland basin that filled with synorogenic sediment shed
fromthe rising Rocky Mountains from the end of the Cretaceous through the
Eocene.This sedimentary sequence contains a rich and diverse biota that is
difficultto correlate because of the low relief and poor exposures
characteristic ofthis region. This study has correlated the stratigraphic sequence
and fossillocalities using a combination of three techniques.
Magnetostratigraphy hasproven to be an effective way to date these rocks as
they contain a measurableand interpretable reversal sequence that can be
correlated to the latestCretaceous through Tertiary geomagnetic polarity time
scale (GPTS). Thepalynostratigraphy of the region is well known and the rocks
contain multiplelevels that yield palynomorphs. Volcanic ashes found in both
the Cretaceous andTertiary units can be dated using the 40Ar/39Ar
isotopicdating method. Each technique has its advantages and disadvantages, but
incombination, these three chrono- and biostratigraphic methods have
thepotential to date with a high level of precision virtually every
fossillocality sampled in the Denver Basin. To effectively date the exposures
fromacross the entire basin, a reference or benchmark section had to be
establishedagainst which the biostratigraphic zonation could be directly
correlated to thechronology. Due to the paucity of long surface outcrops,
drilled wells were theonly way to obtain a continuous rock sequence from which
a reference sectioncould be constructed. Two cores, one drilled at Castle Pines
on the westernmargin 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
PierreShale to the Eocene rocks of the D2 synorogenic sequence.
Themagnetostratigraphic
study of these cores established a reversal sequence thatcould be correlated to
the GPTS ranging from polarity chron 31 through to chron24. The
palynostratigraphy yielded a zonation ranging from the Aquilapollenitesstriatus
Interval Zonethrough to the early Eocene, and accurately placed the
Cretaceous-Tertiary(K-T) boundary in each core. Isotopic ages were obtained
from theMaastrichtian, early Paleocene, and early Eocene parts of the section,
andallow us to independently confirm the calibration of the units to the
timescale. With this chronostratigraphic framework in place, the
individualfossil-bearing localities and surface sections from across the entire
basin canbe correlated and dated to a precision that is comparable to the
calibratingisotopic ages.
The
measuredsedimentation rates vary across this asymmetric basin, with higher
rates in thewestern, proximal part of the basin, and a pronounced increase in
sedimentationrate across the K-T boundary. Two separate packages of strata,
separated by aregional 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 64Ma. The overlying D2 synorogenic strata are
poorly constrained and date fromabout 54 Ma. The reversal patternof the D2 sequence varies across the
basin, which indicates that sedimentaryhiatuses, probably caused by
tectonically quiet intervals along the mountainfront, were followed by
differential subsidence across the basin assedimentation resumed at different
times across the 100 km (60 mi) breadth ofthe basin.
KEY WORDS: LateCretaceous,
Tertiary, Maastrichtian, Paleocene, Denver Basin, K-T boundary,paleomagnetism,
magnetostratigraphy, isotopic age.
Paleocene
andearly Eocene woods of the Denver Basin, Colorado
Elisabeth
A.Wheeler1* and Thomas C. Michalski2
1Departmentof 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.
*Authorto
whom correspondence should be addressed: xylem@unity.ncsu.edu.
ABSTRACT
Silicified
woodsare common in the upper D1 (Paleocene Puercan and Torrejonian) and D2
(Eocene Wasatchian) sequences of
the Denver Basin. Almost all derivefrom angiosperms. Woods from the upper D1
sequence are the second set ofangiosperm woods described from Paleocene strata
of the Rocky Mountainregion. Wood
assemblages from theupper D1 sequence differ from Paleocene wood assemblages of
the San Juan Basin,New Mexico and the Big Bend region of Texas, indicating
variation within theWestern Interior. Lauraceous woods with oil cells are
common in the DenverBasin 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) andearly Eocene D2 (5 wood types) wood assemblages differ.
Lauraceous woods withoil cells apparently are not common in the D2 sequence.
The early Eocene DenverBasin wood assemblages differ from the early Eocene
Yellowstone Fossil Forestwood 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 inangiosperm woods from the late Eocene
Florissant Fossil Beds.
KEY WORDS: fossilwood,
paleobotany, Paleocene, Eocene, Lauraceae, Paraphyllanthoxylon,
Platanaceae.
Stratigraphyand
megaflora of a K-T boundary section in the eastern Denver Basin, Colorado
RichardS.
Barclay1*, Kirk R. Johnson1, William J. Betterton2,and
David L. Dilcher3
1DenverMuseum
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.
*Authorto
whom correspondence should be addressed: sihetun@hotmail.com.
ABSTRACT
A
50-m-thicksection of the Denver Formation (D1 sequence of Raynolds) exposed at
the WestBijou Site of the Plains Conservation Center, Arapahoe County,
Colorado,contains the first complete Cretaceous-Tertiary (K-T) boundary section
in theDenver Basin. The 3-cm-thick boundary claystone coincides with a 21
percentpalynological extinction, contains iridium and shocked-mineral
anomalies, andis immediately overlain by a 7-cm-thick, fern-spore anomaly
interval. The entire50-m section is of reversed magnetic polarity and is
interpreted to be subchronC29r because of paleontological data, the presence of
a tuff with an 40Ar/39Ar date of 65.73 ± 0.13 Ma, and by
correlation to the Kiowa coredwell. A diagnostic basal Puercan (Pu1) mammal jaw
was found 12 m above theboundary clay, and ceratopsian and hadrosaurian
dinosaur fragments occur 4 mbelow the boundary clay. Estimates of the basin
sedimentation rate derived fromthe duration of C29r based on marine
cyclostratigraphy suggest that the 28 m ofbasal Paleocene strata represent
approximately 200,000 years. Abundant fossilleaves found within the Paleocene
sedimentary rocks at the West Bijou Site K-Tboundary section allow assessment
of early Paleocene patterns of plant diversity.Nine leaf localities at eight
stratigraphic levels in the basal 22 m of thePaleocene section were sampled and
analyzed to better understand the flora thatsurvived the global K-T
catastrophe. The Paleocene flora of this site istaxonomically dominated by
dicotyledonous angiosperms (74%), with lessernumbers of monocotyledons (10%),
ferns and allies (11%), and conifers (5%). Bynumber of specimens, angiosperms
comprise greater than 95 percent of the flora.Within the sampled section, there
were no recognizable directional trends indiversity or abundance, suggesting
that earliest Paleocene vegetation wasstable, although not particularly
diverse. The West Bijou megaflora isstrikingly similar in composition and
relative abundance to basal Paleocenefloras from the northern Great Plains.
This K-T boundary disaster-recoveryflora, also known as the FUI flora, is shown
to have been widespread, rangingfrom central Colorado to southern Canada, a
distance of at least 1,100 km. TheWest Bijou Site megaflora is markedly
different from coeval floras along thewestern margin of the Denver Basin,
supporting the hypothesis that orographyand elevation provided greater
influence on basal Paleocene floral compositionand diversity than did latitude.
KEYWORDS: Paleocene, Puercan,paleobotany,
Cretaceous-Tertiary boundary, shocked quartz, iridium,biostratigraphy, Denver
Basin, Denver Formation, D1 sequence.
Evidence for anin
situ early Paleocene rainforest from Castle Rock, Colorado
Beth
Ellis*,Kirk R. Johnson, and Regan E. Dunn
Departmentof
Earth Sciences, Denver Museum of Nature & Science, 2001 ColoradoBoulevard,
Denver, CO 80205-5798, U.S.A.
*Authorto
whom correspondence should be addressed: bellis@dmns.org.
ABSTRACT
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 isan unusual fossil flora preserved in an unusual manner. The site, on
thewestern margin of the Denver Basin in synorogenic sediments associated with
therise of the Laramide Front Range, is dated using multiple methods. Leaves
arepreserved in three distinct units overlying a poorly developed paleosol
thatcontains in situ tree trunks. Fossil-bearing units are continuous along 150
mof outcrop. The leaves were apparently preserved as a result of
rapiddeposition of sand and mud onto the floor of a mature rainforest via
overbankflooding. Five quarries were excavated and the leaves from these
quarries weresegregated by morphotype and scored for leaf area and margin type.
From 1030specimens, we document 93 unique dicotyledonous angiosperm leaf types,
threecycads, three ferns, two conifers, and seven seed types. There is
littletaxonomic variation among leaf-bearing units of a single quarry, but
thetaxonomic composition varies significantly among laterally spaced
quarries,suggesting that the fossil leaf litter reflects the original growth
positionsof the source trees. We compare the fossil leaf litter to leaf litter
of modernforests and show that the Castle Rock flora has numerous features in
commonwith extant equatorial rainforests, including dominance by angiosperms,
highspecies 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, DenverFormation, D1
sequence, Castle Rock rainforest.
Overview of
theLate Cretaceous, early Paleocene, and early Eocene megafloras of the
DenverBasin, Colorado
Kirk R. Johnson1*, Michele L. Reynolds1,Kevin
W. Werth1, and Joseph R.
Thomasson2
1DenverMuseum of Nature & Science,
2001 Colorado Boulevard, Denver, CO 80205,U.S.A.
2Departmentof Biological Sciences,
Sternberg Museum of Natural History, Fort Hays StateUniversity, Hays, KS 67601,
U.S.A.
*Authorto
whom correspondence should be addressed: kjohnson@dmns.org.
ABSTRACT
Late
Cretaceousand Paleogene plant fossils collected at 149 localities in the Denver
Basin,Colorado, are placed into a stratigraphic framework based
onpalynostratigraphy, 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 recordedthe
withdrawal of a seaway, the uplift of a mountain range, and evidence of
theCretaceous-Tertiary and PaleoceneEocene boundary events. Fossil
florasdeposited in the Denver Basin record these events as variations of
floralcomposition, species diversity, and leaf margin and size (used to
estimate meanannual temperature and precipitation, respectively). Attention to
these detailsand to the position of the floras relative to the basin margins
and sedimentaryfacies 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 theseassemblages documents: floral change at the K-T boundary; a
strongpaleoenvironmental gradient probably associated with increased
topographicrelief along the basin margin in the early Paleocene; and a warmer,
drierEocene vegetation.
KEY WORDS:Paleobotany,
paleoclimate, Cretaceous, Paleocene, Eocene, Denver Basin, LaramieFormation, D1
sequence, D2 sequence.
Late
Cretaceousand early Paleocene turtles of the Denver Basin, Colorado
J.Howard
Hutchison and Patricia A. Holroyd*
Museumof
Paleontology, 1101 Valley Life Sciences Building, University of
California,Berkeley, CA 94720-4780, U.S.A.
*Authorto
whom correspondence should be addressed: pholroyd@uclink4.berkeley.edu.
ABSTRACT
Therecord
of turtles in the Denver Basin spans four formations (Fox Hills,Laramie,
Arapahoe, and Denver) that range from Late Cretaceous (Lancian) toearly
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-preservedskull of Compsemys, anew chelydrid genus, the most
northerly confirmed record of Hoplochelys, and the most southerly
records of Helopanoplia, Stygiochelys, and Macrobaenidae in the
Rocky Mountainregion. We also present evidence for synonymy of Paleotrionyx
and Conchochelys with Axestemys, and the first cranial
materialassignable to Axestemys.The early Paleocene (Puercan) part of
the Denver Formation yielded the mostdiverse assemblage, followed by the
Cretaceous (Lancian) part of the DenverFormation and Laramie Formation. The
Cretaceous samples are not demonstrablydifferent from more northerly comparable
faunas, but the more diverse Paleocenepart of the Denver Formation exhibits a
unique combination of taxa compared tocontemporaneous faunas to the north and
south.
KEY WORDS: Chelonii(=
Chelonia), Paleocene, Cretaceous, biogeography, Denver Basin, Arapahoe
Formation,Fox Hills Sandstone, Denver Formation, Laramie Formation.
Puercan
mammaliansystematics and biostratigraphy in the Denver Formation, Denver Basin,
Colorado
Jaelyn J.Eberle
Universityof
Colorado Museum, 265 UCB, Boulder, CO 80309, U.S.A.
ABSTRACT
As a result ofthe
Denver Basin Project, several more fossils of Puercan mammals are reportedhere
from five areas in the Denver Formation: South Table Mountain, GreaterDenver,
the West Bijou Site, Big Gulch, and Corral Bluffs. Systematicdescription and
discussion are provided for one multituberculate and 11ungulate taxa, including
a new species of Baioconodon. Some taxa represent extensions ofpreviously
recognized temporal and geographic ranges. Notably, the ungulate Protungulatumdonnae
from strata ofearly Puercan (Pu1) age in the Denver Formation represents the
southernmostoccurrence of the species, while Oxyclaenus simplex, in
probable early Puercan strata of theDenver Formation, appears to represent both
a temporal and geographic rangeextension from middle Puercan (Pu2) strata of
the San Juan Basin, New Mexico.Additionally, I report the first occurrence in
the Denver Basin of thearctocyonid Loxolophus faulkneri.Refined
biostratigraphic interpretations, resulting from new discoveries
andincorporating paleomagnetic, palynological, and radioisotopic
analysespresented elsewhere, suggest that Puercan interval zones Pu1 and Pu2
are bothrepresented by mammalian faunas in the Denver Formation. Localities at
SouthTable Mountain, as well as the Denver Oxyclaenodon Site (DMNH loc. 299)
andNicole s Mammal Jaw locality (DMNH loc. 2557), are Pu1
correlatives.Discoveries reported here support previous interpretations that
the Alexanderand South Table Mountain localities are probably similar in age
(i.e., Pu1) andare included here in the Littleton fauna. Based upon comparison
to otherPuercan faunas, the Littleton fauna may be temporally intermediate
betweentypical Pu1 assemblages known north of the Denver Basin and earliest
Pu2assemblages from the Hanna Basin, Wyoming. Alternatively, faunal
differencesbetween the Littleton fauna and other Pu1 faunas may reflect
ecological andbiogeographic differences. The unusually high diversity of
ungulates in theLittleton fauna suggests some of the diversification that
elsewherecharacterizes the onset of Pu2 may already have begun by Pu1 in the
DenverBasin. The mammalian assemblage at Corral Bluffs is interpreted here as
aprobable Pu2 correlative, based upon occurrence of Loxolophus faulkneri,
Conacodon entoconus, and C. delphae, and absence of Pu3 index
taxa. Pu2/Pu3correlatives (i.e., faunal assemblages that are either Pu2 or Pu3)
also arereported here from the Big Gulch area, although more fossils are needed
torefine their ages. The present study and others in this issue demonstrate
thatthe Denver Basin is among the few places wherein correlation between
Puercanmammalian biostratigraphy and paleomagnetic, palynological, and radioisotopicanalyses
is an attainable goal.
KEY WORDS:biostratigraphy,
Paleocene, Puercan, Denver Formation, Denver Basin, Colorado,Eutheria,
Multituberculata, Ungulata.
Synopsis of
thestratigraphy and paleontology of the uppermost Cretaceous and lower Tertiarystrata
in the Denver Basin, Colorado
Robert
G.Raynolds* and Kirk R. Johnson
Department
ofEarth Sciences, Denver Museum of Nature & Science, 2001 Colorado
Boulevard,Denver, CO 80205, U.S.A.
*Author to
whomcorrespondence should be addressed: denverbasin@dmns.org.
ABSTRACT
In
amultidisciplinary effort, uppermost Cretaceous and lower Tertiary
Laramidesynorogenic strata in the Denver Basin have been dated using
biostratigraphy,magnetostratigraphy, and radioactive isotopes. The resulting
calibratedframework permits biotic and stratigraphic events to be examined in a
spatialand temporal context. Synorogenic sediments accumulated in two distinct
pulsesseparated by about eight million years. Faunal changes are evident across
timeand, to a lesser extent, space. In addition to evolutionary changes
throughtime, floras show distinctive distributional patterns that reflect the
ancientlandscape and indicate the presence of Paleocene monsoons on eastern
flanks ofthe Rocky Mountains. Observations by the Denver Basin Project team
have beenused to create a series of rigorously documented paintings
reconstructingancient Denver Basin landscapes. As a result of our stratigraphic
work, bedrockaquifer patterns in the Denver Basin are better quantified. The
Arapahoeaquifer is comprised of a series of buried alluvial fans and does not
form auniform layer across the Denver Basin.
KEYWORDS: Denver Basin, biostratigraphy, paleontology, palynology, chronostratigraphy,hydrogeology.