High Precision U/Pb geochronology on the Dakota Formation, Utah: implications for paleobotany and the transgression of the Western Interior Seaway preceding Oceanic Anoxic Event 2

Laura Meyer

Submitted to the Department of Earth, Atmospheric and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology

May 7, 2010 ?Jeg \ Copyright 2010 Laura Meyer. All rights reserved.

The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created.

Author Signature redacted Department of Ftdh, Atmospheric and Planetary Sciences - 1102May 7, 2010 Certified by Signature redacted ay 7,2010 Samuel Bowring Thesis Supervisor bSignature redacted Samuel Bowring Chair, Committee on Undergraduate Program

MASSACHSEITS INSTITUTE OF TECHNOWGY

SEP 28 Z017 LIBRARIES ARCHIVES

1 Acknowledgements: I first like to thank Sam Bowring, my advisor, for his patience, humor, and encouragement not only on this project but throughout my undergraduate career. His guidance has been indispensable in my education. Special thanks also go to Kirk Johnson and Ian Miller, who opened their collections as well as their ideas to me without hesitation. I would also like to thank Matt Rioux who gave so much of his time, patience, optimism, and I am sure at times his sanity, in training me in at the MIT geochronology lab.

2 Table of Contents

Abstract:...... 4

Introduction to the Dakota Formation Regional paleogeography and depositional environment...... 4 Previous studies of the Dakota Formation...... 6 Radiometric dating introduction...... 8 Site descriptions and geology of Henrieville and Westwater, Utah...... 11

Methodology of high precision U/Pb dating techniques...... 14

Results

Paleobotany...... 17

Uranium-Lead ...... 21

Discussion/ Interpretation...... 25

C onclusions...... 30

R eferences...... 31

Appendix

A: Site photographs...... 36

B: Paleobotany data...... 37

C: Geochronology raw data...... 39

D. Accepted geological time scale with fossil correlations..... 40

3 Abstract The Dakota Formation was deposited during the Cenomanian, a time when the Western Interior Seaway spanned much of North America. The Dakota Formation contains a rich record of plant and fossils. Key to understanding their importance, it is imperative to precisely place the Dakota Formation within the geological time scale. Volcanic ash samples interlayed with fossil bearing sandstones of the Dakota were collected from two localities in Westwater, and Henrieville, Utah. Samples were dated using isotope dilution thermal ionization mass spectrometry (ID-TIMS). The samples collected from the Westwater, Utah localities have a weighted mean 206Pb/2 "U date of 97.656+0.082 Ma, while the samples taken from Henrieville, Utah have weighted mean 2 38 206Pb/ U dates of 95.170 0.056 Ma and 94. 941 0.032 Ma. Based on an analysis of stratigraphic, paleontological, and geochronological data, the Dakota Formation was then correlated across much of the Western Interior. These dates provide a base of the initiation of the transgression across Utah, and set a context for the CT boundary and OAE2 that follow.

Introduction to the Dakota Formation During the early (145.5 +/- 4.0 to 99.6+/- 0.9

Ma; Gradstein et. al., 2004), a broad sea flooded continental

North America from the north and the. By the end of the early

Cretaceous, the Western Interior of North America was covered by a large, shallow marine seaway, the Western Interior Seaway

(WIS) (Obradovich and Cobban, 1975). At its peak, the WIS extended through parts of Canada and the United States (Figure Figure 1: Cenomanian Paleogepgraphy of North America showcasing the Western Interior 1). To the west of the seaway lay the Sevier orogen, and the to the Seaway (adapted from Cobban and McKinney, USGS) east the cratonic platform (Kaufmann, 1984). The Dakota

Formation, a series of alternating shales and mudstones bounded stratigraphically above and below by massive sandstone deposits, were deposited along the seaway, from the

4 Era Period Epoch Stage Age (Ma) -65-50' Maastrichtian 70.60 Late Albian to ages (Figure 2). While the

Campanian Dakota Formation was deposited during a time of Late 85.53 Santonian 8585 Coniacian 8927 major marine transgression, Haq et.al (1987) found Turonian 0 93.55 - Cenamanian 99.6 these to be superimposed by regressive phases N Albian (marked by shoreline deposits such as sandstones) 112.0 that were periodically deposited within the more Aptian Early 125.0 phase of the seaway (marked Barremnlan expansive transgressive 130.0 Hauterivian in deep water areas by marine shales). The 136.4 Valanginian 1401 Berriasian 14r r. I deposition of the Dakota Formation, as considered in Figure 2: Geological time scale for the Cretaceous (adapted from Gradstein et. this paper, began in the early Late Cretaceous near the al, 2004) base of the Cenomanian. At this time, deposition was

occurring in coastal environments as the base level of rivers rose related to the

transgression of the inland Western Interior Seaway.

The Dakota Formation, often overlain by the Tropic Shale Formation, marks a

rise in sea level and the beginning of a minor mass extinction event around the

Cenomanian-Turonian boundary. This extinction is coincided with a negative carbon

isotopic excursion and sea level rise (Sageman, 2006; Barclay 2010). This interval of

time and associated perturbation of the carbon cycle is also known as an "Oceanic

Anoxic Event" (OAE2), and is characterized by an accumulation of organic rich

sediments (Schlanger and Jenkyns, 1976). Cretaceous OAEs, and specifically OAE2

occurred on a warm planet with high concentrations of atmospheric CO 2. OAE2 (-93.5

Myr ago (Gradstein et al, 2004)) peaks in the uppermost Cenomanian, and is the most

widespread of the three recognized oceanic anoxic events in the mid-Cretaceous. The

5 increased deposition and burial of organic material can be explained through enhanced primary productivity and/or different preservation possibilities, but the causal trigger of the ocean anoxic event has not been clearly identified. Despite the controversy, it is undeniable that OAE2 marks a major change in climate including a 40-80% reduction in

CO2 levels (Kuypers et. al., 1999) although an exact sequence of events has never been established. This disturbance stressed the ecosystem, forcing a major biotic crisis (Hallam and Wignall, 1997). The onset in the Western Interior was abrupt, and in central

Colorado resulted in 6"Ccarb and 8"Corg records which display positive excursions of up to 2%o-4%o (Sageman 2006; see Figure 14). The two main hypotheses for the cause of the carbon isotope excursion are a period of enhanced seafloor spreading and orbitally forced climate change (Schlanger and Jenkyns, 1976; Kerr, 1998; Kuroda, 2007). This paper focuses on the age and paleontology of the Dakota Formation, in order to provide an environmental context for the boundary and OAE2, as well as place an age on the initiation of the transgression of the Western Interior Seaway.

The Dakota Formation has historically been subject to controversy and misunderstandings surrounding its age and flora. Many of these arguments stem from the

incorrect usage of the term "Dakota flora", which makes assigning an exact age difficult

at best. The type section near the Nebraska-Iowa border was first studied by Meek and

Hayden (1858). This section has since been referred to as the "Dakota Formation", the

"Dakota Sandstone", the "Dakota Group", the "Dakota Conglomerate", and simply as the

"Dakota". When referencing the Dakota fauna, this diversity of names can lead to

confusion and misinterpretations. Often, the Dakota is used to mean any Cretaceous flora

with dicot leaves older than the Benton Formation (Berry, 1920). But Witzke and

6 Ludvigson (1994) suggested a simplification and unification of terms, stating that "The

Dakota Formation should define as a body of rock of eastern provenance, primarily

nonmarine fluvial to marginal marine and deltaic deposits, that were deposited during

transgressive phases of the Greenhorn marine cyclothem." Taking this into account,

Figure 3 spans the western United States and correlates the Dakota Formation as it is

defined in this paper to other formations and localities.

S"t j K " Utah % *"Il-w (Ameueai Kkoh& CI&Ka", (AIL111ma 1 '4

Pa-t 364w-6Mak. Mfl nGram-f Granm Graner

-- _____ I~~aL..t D

Sh"ah. -~ SuJ.

gnred tratgrpi cunS:a t.i ale CooaPla (Young, Nrhae S%) C Shoa(le%)

Da~w

Figure 3: Correlation of geologic rock formations across the western interior of the US. Adapted from Lane (1963) with generalized stratigraphic columns: Utah (this paper), Colorado Plateau (Young, 1960), Northwest Colorado (Lane 1963), North Front Range Foothills (Waage, 1955), South-Central Colorado (Waage, 1953), Kansas and Oklahoma (Merriam, 1950), and Northwest Nebraska (Tester 1929)

In order to synthesize, this paper not only describes the Dakota Formation from

two sites in Utah, it also integrates the implications of the transgressive nature of deposit

in the description of the fauna as well as depositional environment. Originally described

by Hayden and Meek in 1858 as Cretaceous, the Dakota Formation's age has been

controversial ever since. Hayden and Meek's assertion was disputed until the late 1860s

7 when Marcou and Capellini agreed with a Cretaceous age assignment. Marcou, in 1864, also posited a freshwater depositional environment for the Dakota, until Hayden found marine invertebrates with fossil plants as well (Rushforth, 1971). Lesquereux interpreted the main depositional environment of the Dakota to be low-relief beach with muddy shorelines. In 1883, Lesquereux first assigned a Cenomanian age to the Dakota

Formation, but it wasn't until the 1960s that most other workers agreed. Today, it is a widely held belief that indeed the Dakota Formation is within the Cenomanian, but with the advent of high precision radiometric dating techniques a new wave of controversy has arisen. New high-precision geochronology is the subject of this paper, along with a discussion of stratigraphy, paleobotany, and depositional environment.

In order to study a formation such as the Dakota it is imperative to place it within a larger framework of Cretaceous earth history. While this sounds like a simple enough task, many of the problems and issues surrounding the Dakota come from such attempts.

The history of studies of the Dakota Formation is one of specialists, each describing one or more aspects of the formation in single localities, making it difficult to develop an integrated picture. Integration and interpretation of both vertebrate and invertebrate paleontology, paleobotanical data, and sedimentology is crucial for understanding the significance of the Dakota Formation. Paleobotanical data, for example, can be utilized to constrain paleoecologies and paleoclimates and play an important role in the understanding of the depositional environments and climatic changes during Dakota time.

Paleobotanical and invertebrate paleontology can be used to correlate the formation over a wide area.

8 Radioisotopic dating techniques are transforming the way in which we think about geological time and the stratigraphic record. These techniques allow for an

"absolute" date to be attached to the geological time scale built on hundreds of years of paleontologic and stratigraphic studies to establish the major subdivisions of Phanerozoic time. Two techniques in particular, U-Pb zircon geochronology and 40Ar/ 39Ar sanidine and biotite geochronology have been applied to dating volcanic ashes. Volcanic ashes are ideal as they are often easily traceable for tens to hundreds of kilometers, and represent isochronous layers deposited in different depositional environments.

In 40Ar/39Ar geochronology, the age of the sample is given by the basic equation:

t = 1/X In (J x R + 1) (Eq. 1) where X- 5.5 x 1010 yr-1 (radioactive decay constant of 40K); J: J-factor associated with irradiation; and R: 40Ar/39Ar ratio.

The 40Ar/ 3 9 Ar method is an outgrowth of the K-Ar method in which parent and daughter isotopes are both measured, using different techniques to solve the basic equation. In the 40 39 Ar/ Ar method, K-bearing minerals are irradiated in a reactor and the neutron flux produces 39Ar from K and thus a proxy for K. That means that only one kind of mass spectrometer is necessary to make the measurements. The technique does not, however, provide an absolute date as it is based on an unknown/standard comparison where the age of the unknown or monitor must be assumed. Thus, it is a very accurate relative dating technique. In order to calculate an age, the J-parameter (Eq. 1) must be determined by irradiating the unknown sample along with a known, standard sample that has been dating using another technique (typically the K/Ar technique is used). This and relatively

9 large uncertainties in the decay constant for K causes a discrepancy with U/Pb geochronology. Kuiper (2008) recently suggested that this discrepancy is easily estimated and indicates Ar-Ar dates are 0.65% lower than U-Pb dates.

Uranium-Lead geochronology is not a relative dating method. It instead relies on two separate decay chains, 2 38U to 2 06Pb and 2 3 5U to 2 07Pb. The equations for these decay series are:

6 238 20 *Pb / U = e 238t- I (Eq. 2) 7 235 20 *Pb / U = e X235t- 1 (Eq. 3)

2 06 204 206 2 2 3 8 2 04 t206= 1/ X 1 In [(( Pb/ Pb)- ( Pb/ 04Pb); / ( U/ Pb)) +1] (Eq. 4) where X: half life (4.47 billion years for 238U and 704 million years for 23 5U), and t: time, and * is used to indicate a radiogenic component.

U/Pb dating can in principle be applied to any U-bearing system. Zircon (ZrSio 4) is desirable, however, as it preferentially incorporates uranium into its crystal structure while rejecting lead. Thus, any lead found in the zircon can be assumed to be produced by in-situ decay of U and is termed radiogenic lead. Zircons crystallize from magmas, and when erupted in a volcanic event, we assume that the crystallization age and the depositional age are approximately the same, and inferentially the age of any fossils buried in the ash layer or bracketed between multiple layers. Zircons are remarkably durable and not affected by weathering, diagenesis, or metamorphism, making them excellent for radiometric dating. This resilience, while invaluable in for dating, can occasionally cause complications. In magmas that are, at least in part, produced from melting and/or assimilation of older rocks, new zircons may crystallize around "cores" of older zircons that are "inherited". These cores can range in age from being identical to just slightly older than the eruption to hundreds of millions of years older. Thus, if not

10 detected, the inclusion can yield a date that is a mixture and thus older than the true eruption age of the volcanic rock. In order to minimize this complexity, we polished mounts of grains using cathoduluminescence (CL-imaging), which allows detection of growth zones and cores (see Methodology for further details on U/Pb dating techniques) .

Being able to pinpoint volcanic eruptions in the geologic record provides a setting for tectonic events, climatic changes, and in some cases, insight into mass extinction events.

It is therefore together that paleontology and geochronology can create a more complete picture of environmental, tectonic, and climatic events within the context of the geologic time scale.

A brief review of the geology of Henrieville and Westwater, Utah sample localities

This paper is focused on two localities: Henrieville and

Westwater, Utah (see Figure 4). Plant fossils discussed were collected in 1991 and 1994 by Kirk Johnson and crew for the Denver Figure 4: Map of Utah with general locations of Westwater Museum of Nature and Science (with exception of some of the and Henrieville localities. original Westwater samples, which were donated to the museum). The Westwater ash samples were collected in 2008 on a DEAPS field trip led by Kirk Johnson, Ian Miller, and Sam Bowring, and attended by the author. Notes on the stratigraphic relationships of the ashes and plant fossils taken from Kirk Johnson's field notebooks were utilized in the paper. Pictures of the Westwater site are included in Appendix A, along with arrows

11 indicating exact location of samples KJ08105 and KJ08108. These two samples dated

were dated using the high-precision U/Pb dating techniques developed in the Bowring lab

at MIT (see methods). Along with the Westwater, two samples from Henrieville collected

by Kirk Johnson were also analyzed: KJ08142 and KJ08143. Appendix A has

photographs of the Henrieville outcrop. Henrieville plateau CanKJ0S142 The Henrieville site is located on the western margin of the Kaiparowits in southern Utah, where widespread coal deposits of the Cenomanian Dakota Formation

were deposited (Kirschbaum and McCabe; 1992). The measured section containing the

sampled volcanic ashes is -34 m thick (Figure 5). Others who have studied the area

(Kirschbaum and McCabe, 1992) studied a slightly thicker section, -50 m thick. Here,

the Dakota Formation unconformably overlies strata of the Entrada (locally

referred to as the Henrieville Formation). Peterson (1969) and Gustason (1989) together

defined three "members" within this formation in southwest Utah. These members are

informal, and are descriptions of lithological variations which correspond to different

depositional environments. In this scheme, the lower member is made up of

conglomerates and a thick layer of sandstone, interpreted as braided stream deposits

C3 KJO8143 deposited on an erosional surface defining the unconformity (Peterson, 1969 and

Gustason, 1989). The middle "member" is comprised of a series of mudstones and coals, 2m indicating lacustrine and/or flood plain deposits. The upper member

Figure 5: Stratigraphic column of consists of sandstones, mudstone, and coal layers that are laterally Henrie'ville locality, based on notes of Kirk Johnson; lowest sandstc e extends -20m; continuous, and interpreted as shoreline, lagoonal, and mire deposits stratigraphic relationship of ash sample s KJ08142 and KJ08143 are not ed (Kirschbaum, 1992; Peterson, 1969; Gustason, 1989). The site as described

by Kirk Johnson indicates a similar breakdown of "members" with a lower member of

12 white sandstone, overlain by a series of ashes, mudstones and coals, and topped by another member of fine, white sandstone (Figure 5). Note that in the middle of the study area, there is a large section of covered slope in which no outcrop was exposed.

Overlying the Dakota Formation in Henrieville, the Tropic Shale of the Upper Cretaceous

is a deeper water marine deposit. Ammonites in this shale immediately above the Dakota

Formation in the study area have been assigned a Late Cenomanian age (Kaufmann et. al., 1987). Volcanic ashes in this area have been previously dated using 40Ar/39Ar radiometric dating techniques, placing the middle member of the Dakota Formation between 92.9 +0.2 Ma at the base and 90.5+0.1 Ma near the top (Bohor, 1991). However, previous dates of this same area and stratigraphic horizon place the middle Dakota between 94.7 0.4 Ma and 94.5 0.2 Ma (Kowallis, 1989). These ranges were assuming a calibrated age of 28.02 Ma for Fish Canyon sanidine. Not only is Kowallis presenting a significantly older Ar/Ar date for the middle Dakota than Bohor, but his data also considerably compresses the range of the middle member. Thus the incompatibility of these two sets of data has made it difficult to place an isotopic age range on the deposition of the middle member of the Dakota. This is but one example of the problems surrounding the dates of the deposition of the Dakota.

Sample KJ08143 (Latitude: 370 35' 17.8" N; Longitude: 1110 58' 44.4" W) is derived from a small 3 cm thick ash near the base of the study area (see Figure 5). The ash is light in color, contrasting sharply with the dark lignitic coal on either side of the deposit. It has been noted that the same ash layer exposed in other areas is directly

overlain by a layer containing small chert pebbles (Kirschbaum, personal correspondence). Sample KJ08142 (Latitude: 370 35' 17.7" N; Longitude: I11 ' 58' 43.9"

13 W) is stratigraphically above KJ08143. This sample is a white/ light grey ash -23 cm

thick, again bracketed by dark lignitic coals.

The study sites at Westwater represent a collection of four ash samples by leaves

were also collected at this locality by MIT field trip participants. Though less well

studied than Henrieville, this sample site has several fossil localities in the collection at

Westwater the Denver Museum of Nature and Science. Ash samples include KJ08105, KJ08106,

KJ08107, and KJ08108 (see Figure 6). Note that these samples were collected in

2008 in the same locality as the fossil plants collected in 1991 by Kirk Johnson.

KJ08105 Sample KJ08105 (Latitude: 39.10604 N; Longitude: 109.15157 W) is

stratigraphically the highest, and is located within a section of alternating sandstone

and mudstone. Several of the sandstone layers have dinosaur footprints preserved in

them, the most prominent of which is near the top of the outcrop. Separated from

KJ08105 by 5.93 m of covered slope, KJ08106-108 are also ash samples. These,

however, are not surrounded by sandstone and mudstones, but rather are interbedded

with coal seams. Sample KJ08108 is located at latitude 39.10580* N and longitude

109.151620 W. K108108 Figure 6: Stratigraphic column of KJ08107 Westwater locality, based on notes of Kirk Johnson (distance between upper and lower sections -5.93m) KJ08106

Methodology

Samples of ash were processed and analyzed in the MIT geochronology lab using

isotope dilution thermal ionization mass spectrometry (ID-TIMS). This technique is

14 typically an order of magnitude more precise than the competing microbeam zircon dating technique. (Bowring et. al., 2006)

Samples were first crushed using a jaw crusher and disk mill to <420 micrometers, and then processed with a Wilfley table. This separates the minerals with high specific gravity (e.g. zircon and pyrite) from the rest of the material (clays, quartz, feldspar). The high-specific gravity fraction or "heavies" were then further purified using high density liquids, or "heavy liquids". Zircons are almost completely non-magnetic, so in order to remove magnetic minerals in the samples, a Frantz magnetic separator was used before and after the heavy liquids step. The sample was then handpicked for zircons to be dated. All samples had abundant zircons, and I focused on handpicking the longest, most needle-like zircons which are thought to form during the final stages of crystallization immediately prior to eruption. A set of 30-40 zircons were picked from each sample, and annealed at high temperatures (~90 0 *C) for around 60hours. Then the best four to six grains were chosen to date, and imaged under the microscope to better understand the growth structures and history of the grains. Grains were then transported into clean Teflon hex-beakers, and loaded into pre-cleaned Teflon microcapsules. Once samples were loaded into microcapsules, they were subjected to a partial dissolution in hydrofluoric acid following the chemical abrasion procedure of Mattinson (2005). This process allows the parts of the zircons which have been damaged by radiation or Pb-loss to preferentially dissolve more quickly. This leaves crystal domains with lower uranium concentrations, which is most likely to have remained a closed system since formation, and thus yield an accurate age for the zircon crystallization. Thus, this leaching process results in an elimination or "mining out" of the damaged zones within the grain. Results

15 of this leaching were typically easily visible under the microscope as small holes in the zircon. Following the leach step, grains were rinsed and dissolved in HF acid. Included with the IF step is an isotopic tracer (ET535-C) which contains 205Pb, 2 33 U, and 235U.

Once fully dissolved, samples were converted to HCl, and Pb and U were separated from all other components. The final sample was captured in an acid-cleaned Teflon beaker and a drop of phosphoric acid to make the sample visible to the naked eye was added.

Uranium and lead were then dissolved in a drop of silica gel to enhance ionization in the mass spectrometer, and mounted on outgassed Rhenium (Re) filaments. Samples were then loaded in a turret and analyzed using a thermal ionization mass spectrometer, equipped with seven faraday collectors used for U analysis and an ion-counting Daly detector for small ion beams (here used for Pb analysis). Data were analyzed using

Tripoli 4.6, a graphical data processor for mass spectrometry, and synthesized in U-Pb

Redux, a program designed to reduce raw mass spectrometer data, as well as blank and trace data to collect U-Pb dates.

Precision on the U-Pb dates is controlled by counting statistics and correction for non-radiogenic Pb or blank Pb. Much stress was placed on the minimization of non- radiogenic Pb included in the samples, so all work was done in a clean lab. Still, the addition of some Pb during the sample processing is inevitable. For example, from the

Teflon containers, silica gel and or phosphoric acid, and random air contamination all add some Pb into the sample. Total common Pb was monitored measuring 204Pb during analysis and corrected for using the isotopic composition of measured total procedural blanks. This can then be corrected for numerically when analyzing the date acquired.

16 Results

Paleobotany:

An in-depth study of the Westwater flora was done by Rushforth in 1971, which resulted in a similar stratigraphy to that discussed above in the site/geology section. He generally describes his stratigraphy as a massive sandstone layer (~30 feet thick) overlain by a series of shales-mudstone deposits (where fossil plant flora was found within an ash layer) about 30-40 feet thick, which were in turn overlain by another thick sandstone deposit of ~30 feet . Rushforth (1971), focused solely on the plant fossils found within the ash layers in Westwater with no mention of a mudstone fossil layer at all, though such a layer was found and is discussed in this study. His leaves were found in an ash layer about 40-45 feet below the Gryphaea newberryi zone from the overlying Mancos Shale.

Rushforth described a mainly fern-dominated flora, though he proposes a fern- angiosperm-gymnosperm coexistence not seen previously. His describes a flora in which angiosperms are plentiful, but in which ferns ultimately dominates in abundance.

This contrasts sharply with the flora described by Lesquereux of the Dakota Formation in

1892. He describes 460 species, of which 6 are ferns, 12 are cycadean species, 15 conifer species, and 437 are angiosperm species (making up -92.6% of the flora). This discrepancy may be related to sampling bias, for as this study has shown, the facies in which the plants are found bears on the abundance of species ferns versus angiosperms. It could also, however, be due to the transgressive nature of the Western Interior Seaway.

Lesquereux described a type section of the flora in Nebraska, whereas the same Dakota

17 Formation in Utah may have a different environment closer to the seaway at a later or earlier time. Likewise, Brown (1950) studied a flora in the Burro Canyon Formation which is similar to a flora described in part of the Cedar Mountain Formation (Young,

1960), and is equivalent to a flora from the Dakota Formation (Naturita, CO). Like the flora described at Westwater by Rushforth (1971), these sites show a high percentage of ferns in comparison to the number of angiosperms (Tidwell and Thayn, 1976).

In Utah, the Henrieville site has one fossil plant locality, collected by Kirk

Johnson for the DMNS in 1991 (DMNH site 404). The fossils were found in a layer of lithified mudstone, indicative of a wet floodplain. Fossils were primarily angiosperm based, with few ferns present at the locality.

The Westwater site area has four fossil localities: Coal Draw, Angiosperm, Slide

Ash, and Westwater II.

Table 1: Fossil Plant localities

DMNH No. Locality name Latitude (N) Longitude W 404 Henrieville 37.58805556 111.9786111 947 Coal Draw 39.11111111 109.1377778 1191 Angiosperm 39.10916667 109.1419444 1192 Slide Ash 39.10888889 109.1422222 1193 Westwater 1 1 39.11055556 109.1380556

Coal Draw was collected by Howard and Darlene Emry, and now resides in the collections at DMNS. The stratigraphy for this roadcut locality is generalized, as notes for the site do not specify thickness of layers, but merely the order (from top to bottom): a tan sandstone layer, a lignitic coal layer, a thin (~7cm thick) white ash layer, a second coal layer, and then a second white ash layer (~15 cm thick). The fossil plants were found in the middle of the lower ash layer, and consisted primarily of fossil ferns with few angiosperms. This layer extends for several hundred feet along the roadcut, though leaves

18 1'

are less plentiful in some areas than others. The remaining three localities were all collected by Kirk Johnson in 1994 for the DMNS collections. Westwater Angiosperm fossils were embedded in a lithified mudstone, and like the Henrieville site, is interpreted to be a wet floodplain depositional environment. Slide Ash was, as the name implies, found in a thin ash layer in Westwater. And finally, Westwater II is an ash layer which produced only 8 fossil leaf specimens in an ash layer.

When analyzed together, the prominent feature of the leaf data is the prevalence of fossil fern specimens versus angiosperms at the different localities (Numerical summary for abundance and diversity of the five localities can be found in Appendix B)

The localities found in ash layers consist almost entirely of fern specimens, whereas those found in the mudstone are dominated by angiosperm specimens.

Plant Types for Five Fossil Localities

1000

800

C. 600 400

E 200

0 " Henrieille WW Coal Draw WW Angiosper WW Slide Ash WW 2 Locality name Fern M Dicot Other

Figure 7: Graph of five plant fossil localities at Henrieville and Westwater (WW Coal Draw, WW Angiosperm, WW Slide Ash, and WW2) showing the prevalence of ferns vs dicot leaves.

This bias could be explained by a predominance in different paleoecologies of ferns versus angiosperms, but palynological evidence does not show the same magnitude

19 just north of Henrieville. Their flora was preserved in a carbonaceous shale unit, and

found of the 500 pollen grains counted that ~65% were ferns and 33% were angiosperms

(the opposite majority from Henrieville). Therefore, it is more like a preferential preservation pattern based on depositional environments. Fine grained mudstones such

as those found in these localities, are typically formed in any moderate to low energy environment including lakes, lagoons, and shallow marine environments. One likely possibility, therefore, is that these facies were formed in part of a shallow offshore lake close to the margin of the WIS. If this was the case, then plant leaves would be expected to accumulate over time at the bottom of that lake as they are transported by wind and

stream flow. It stands to reason, therefore, that these facies primarily preserve angiosperm dicot leaves as these are the most likely to end up in lakes. Ashes, on the other hand, typically preserve leaves and plants in situ as it buries vegetation as it accumulates across the landscape. It would, therefore, preferentially fossilize the groundcover plants (the ferns) rather than the higher angiosperm leaves. The only angiosperm leaves to be fossilized would be those that had fallen or were knocked down as the ash fell. The density of leaves on the ground would thus be dependent on the season in which the ashfall occurred. Together, these observations show marked evidence for freshwater, marine and/or brackish depositional environments. Such a variety indicates a flood plain, estuary or swamp affected by the large scale marine advance of the interior seaway.

20 Geochronology

From the upper ash in Henrieville, sample KJ08142 yielded a large number of zircons of varying size and shape. An example of a grain using Backscatter Electron (BSE) and Cathodluminensece (CL) imaging is shown in Figure 8. Of the nine analyzed zircon fractions, six cluster together with a weighted mean 2 06Pb/ 238U date of

94.914 0.032 Ma (95% confidence intervals). This age is interpreted to reflect the eruption/depositional age. Three other analyses were Figure 8: Sample grain from KJO8142 Top: CL imaging on electron microprobe shows growth lines and inclusions distinctly older with 206Pb/ 238U dates of 95.436 0.056 Ma, Bottom: BSE imaging shows high contrast

95.034+0.094 Ma, and 95.046+ 0.086 Ma; these are interpreted to be slightly older grains from an older eruption or contain a small amount of older inherited core that escaped detection.

The stratigraphically lower ash, sample KJ08143 also yielded a large number of zircons, many of them with long needle-like shapes and melt inclusions. Of the nine fractions analyzed a cluster of three yields a weighted mean 206Pb / 238U age of

95.170+0.056 Ma. This age is slightly older than KJ 08142, consistent with its startigraphic position. This sample yielded three discordant zircon analyses (zl,z3,z4), with 206Pb/238U dates of 99.352 0.071 Ma, 95.455 0.075 Ma, and 95.170 0,056

Ma, respectively indicating the presence of an older inherited component. (Table CI in

Appendix C)

Seven zircon fractions were analyzed for sample KJ08105, of which four cluster

2 2 38 with a 0Pb/ U weighted mean date of 97.656t0.082 Ma and interpreted as

2 238 depositional age. Two fractions (z5 and z7) produced 0Pb/ U ages slightly older than

21 the weighted mean date for the sample. Fraction z5 yielded an age of 97.832 0.078 Ma, while z7 is statistically identical with an age of 97.826 0.099 Ma. These are slightly older than the weighted mean 20Pb/ 238U date from the cluster of three zircon analyzes and are interpreted to reflect grains from an earlier eruption and/or inherited cores. The

2 238 final fraction is shown to be slightly younger than the other grains, with a 0Pb/ U age of 96.75 0.14 Ma. Two interpretations are possible, the first is that the rock is this age or younger and the second is that the weighted mean of 97.656 0.082 Ma is the eruption age and this represents residual Pb-loss not removed by CA-TIMS.

The other sample from Westwater, KJ08108, proved to be more of an enigma than the other three samples. The 206Pb/ 238U ages for the fractions were highly variable and a single weighted mean date for this sample was not determined.

*Raw data for all samples can be found in Appendix C

22 I.! KJa142

F1

20@bM3 ffta-eneutwOs

sta e 'dW u-n -sa a.,. - a 0.3132 0.0.0.0372 2 0.09= 0.66" ...

Figure 9: Sample KJ08142 from Henrieville. Nine zircon grains were analyzed, six clustered around a weighted mean 2PW 'U age of 94. 914 0.032 Ma. *Raw Data in Appendix C

a o p e~a16. 231 1.410. aso 'p

E= I :c~os~e

1708143 -F 36.4 4.P

37.4

36.4 s

vimWD i 2.3. n = 1

3.10 7. 9 06 .10 29 of 0 M

~7. __ / 0.1016 0.D46 0.0172 0.0M 0.1024 0.10"0 WM(GS13G Figure 10: Sample KJ08143 from Henrieville. Eight zircon grains were analyzed, I I W m 241 S three clustered around a weighted mean 2PW "U age of 95.170t0.056 Ma *Raw Data in Appendix C

23 -__j

97. A IJO105 7.

* 3 97. 7.

97.2 an Cq 37.1 S

77.0/ -j~es dmmWam14 * 4 87 10.&M as a91 MI 0US 0 : 2 ,2.A 0 0 96.7 i0 sic 2071b'23 6h EBD.L"4 OM342 0.2.90N .IM 0.:IM 0. IMs

Figure 11: Sample KJ08105 from Westwater, Utah. Seven zircon grains were analyzed, four clustered around a weighted mean 2Pb/ 'U age of 97.656+0.082 uenamawoeo Ma *Raw Data in Anvendix C

I.,

s3

a 0.1016 0.1030 0.1644 6.1*50 Figure 12: Sample KJO818 from Westwater, Utah. Four zircon grains were 2 6 analyzed. 0 MP2/U ages for the fractions were highly variable and a single weighted mean date for this sample was not determined *Raw Data in Anoendix C

24 Interpretation/ Discussion A The stratigraphic relationships with and A between the Henrieville and Westwater sites have A *- A important implications for the transgression of the A

Western Interior Seaway. The thickness of the Dakota AhmW A Formation thickens to the west s (2m to 350 m according to Kaufmann et al, 1987) in southern Utah. Figure 13: Generlized depositional environment of the Dakota Formation near the Western Interior This trend can be seen between Henrieville and eway. (Adapted from Roberts and Kirschbaum, 1995) Westwater sites. The Dakota Formation at these sites, as well as many localities across the western US, represents a time of episodic sediment accumulation. During this time,

peat accumulation was intermingled with shorter intervals of clastic sedimentation

(Kirschbaum, 1992). In order to examine the Dakota Formation is a broader sense, a

series of generalized stratigraphic sections are shown in Figure 14. These stratigraphic

columns showcase the different depositional environments represented in Figure 13.

Fossil data has been historically used to bracket the age of the Dakota Formation

across the Western Interior, as noted in the discussion of the stratigraphy above. In

Arizona, for example, the ammonites Calycocerasobrieni and Metoicocerassp. were

found near the base of a sandstone in the upper Dakota Formation (Fursich, 1986). These

are indicative of the Late Cenomanian Dwrneganocerasalbertense Zone of Kirkland

(1983) (see Appendix D). Similarly, at Grand Staircase-Escalante National Monument,

the oldest fossils that have a marine origin are located in the upper section of the Dakota

Formaiton. The highest sandstones within this formation contain assemblages of

Vascoceras diatrianum,while the lowermost marine shales yielded fossils indicative of

25 WA

Calcocerascanitaurinum. These bracket the stratigraphy for the late Cenomanian in the

Dakota Formation in Utah (Dyman et al. 1994). Figure 14 traces the fossil zones of

Sciponocerasgracile, Neocardiocerasjuddii,Metoicoceras mosbyense, C. Canitaurinum,

and W devonsense from southwest Utah through Colorado and into Kansas (for accepted

fossil ages and zones see Appendix D).

P1,k

.W--, tu- VW. i niii

11.m OMb~ I. 6L 1* I I = 1* I m

SM Sm

SOM *1' 21 1

Figure 14: Correlation across the Upper Cenomanian of fossil zones, with ages (Ma) from the USGS Open File Report in Appendix D3. This diagram places the Henrieville localities within a larger context in the Western Interior. It also demonstrates the implications for the onset of OAE2 following the deposition of the Dakota Formation. (Larger chart available in Appendix D)

Sources: Henrieville, Utah (this paper). Kaiparowits Plateau, Utah; Southwestern Utah; Greenhorn Anticline, Colorado; Four Corners Section, New Mexico; Bunker Hill Section, Kansas (Elder 1994). Southern Utah and Western Colorado (Barclay, 2010). Portland, Colorado (Sageman, 1998); OAE2 carbon excursion adapted from Pratt (1985)

26 In addition to the macrofossil evidence, several pollen taxa have been historically used to "date" non-marine layers of the Dakota Formation. Nichols (1994) devised a revised palynostratigraphic zoning scheme for the western United States' Upper

Cretaceous deposits. This scheme is based on the correlation of pollen taxa to marine ammonite zones. According to this new scheme, the first appearance of psilate tricolporate pollen (e.g. Nyssapollenites microfoveolata) and obligate tetrads (e.g.

Artiopollis indivisus Agasie 1969), indicate a middle Cenomanian- Coniacian age (Hu,

2008; Nichols and Sweet, 1993; Nichols,1994). Similarly, palynomorphs from sites in eastern Utah indicate that the Dakota Formation is Late Albian-Early Cenomanian in age

(Pierson, 2006). The occurrence of Fraxinoipollenitesinaequalis, Liliacidites peroreticulatus,L. inaequalis, and Rugubivesiculites rugosus leads to an interpretation of the Dakota Foramtion being no older than Late Albian. The age is further constrained by the occurrence of middle Cenomanian palynomorphs from the basal Mancos Shale overlying the Dakota Formation, including include Cicatricosisporitescrassiterminatus,

Cribroperidiniumedwardsi, Palaeohystrichophorainfusorioides, and Subtilisphaera terrula (Pierson 2006). In Kansas and Nebraska, however, the age of the underlying

Kiowa Formation foraminifera and palynology to be Albian in age (Loeblich and Tappan

1950; Ward 1983). The Graneros Shale overlying the Dakota Formation is dated by the arenaceous foraminifera to be Cenomanian, and age further constrained by marine invertebrates of the Uppermost Dakota Formation which were middle Cenomanian

(Eicher, 1965; Hattin 1965, 1967). It is the absence of Normapolles group pollen, however, that implies that the Dakota Formation can be no older than late Cenomanian in age as this was the first appearance of that group in North America (Farley, 1986).

27 It is evident, based on fossil evidence, that in Utah and western Colorado the

Dakota Formation is contained entirely within the Cenomanian, but extends to the

Turonian boundary. This is important in understanding the relationship between the

Dakota Formation and the Cretaceous Ocean Anoxic Event 2 which lead up to the C-T boundary (base of the Sciponocerasgracile through the Watinoceras devonense zones)

(Barclay 2010). The OAEs are manifested in the rock record by fine-grained, laminated strata with elevated organic carbon contents (Schlanger and Jenkyns, 1976). These events reflect major disruptions to the carbon cycle and it has been suggested that OAE2 may have been large enough to shift the ocean to a net carbon sink state (Barclay, 2010). One proxy for measuring pCO 2 change is stomatal indexing of fossil leaves (Woodward,

1987). This index shows the inverse relationship between pCO 2 and the frequency of leaf stomata. Thus, studying the paleobotanical evidence in the Dakota provides an environmental context for the onset of the transgression as well as the beginning of

OAE2.

The localities at the Henrieville site (KJ08142 and KJ08143) are immediately overlain by the Tropic Shale, indicated by a bed of Sciponoceras gracile ammonites

(Ulicny, 1999). Thus, the study site is late Cenomanian in age (Kauffinan, 1987).

40Ar/39Ar isotopic ages from an ash stratigraphically above KJ08143, and a bentonite bed in the lower Tropic shale right above KJ08142 suggests that the interval between was deposited in 2-5 million years (Bohor, 1991). The age of KJ08142 and KJ08143 indicate a shorter time scale for the deposition of the Dakota Formation at least in this area of

Utah. The dates of 94. 934+/-0.036 Ma (for KJ08142) and 95.170+/- 0.056 Ma (for

2 06 2 8 KJ08143) precisely constrain the time scale of deposition. Moreover, the Pb/ 1 U dates

28 of KJ08142 and KJ08143 mark the beginning of the transgression of the Western Interior

Seaway across the Kaiparowits Plateau and correlate to deposits across much of Utah and

Colorado (Figure 14). The combination of this paleobotanical analysis and environmental studies with high precision U/Pb dating sets the stage for an in-depth study of the timing and environmental changes across this oceanic event and the CT boundary.

These dates also have implications for the proposed 40Ar/39Ar dates from for the

Cenomanian-Turonian boundary. Wing et. al. (1991) proposes a date of 93.3 Ma (relative to the calibrated age of 28.02 Ma for Fish Canyon sanidine) from analysis on sanidine crystals from bentonites. He also places the Albian-Cenomanian boundary at 98.6 Ma. In

2001, Prokoph et al suggested a new 40Ar/39Ar calibration for the Cenomanian-Turonian boundary, placing it at 96.4 +/- 1 Ma. More recently, in 2009 at the Geological Society of

America annual meeting, the original Obradovich ages were modified slightly by Singer et al. The new ages are from both older samples (Obradovich 1993) as well as collected material from a variety of ammonite zones across the C-T boundary (including E. septemseiatum, N. juddii, W devonse, and V birchbyi). These combined with an ash sample from the juddii zone (94. 11+/- 0.11 Ma) yielded an overall age of 93.97 +/- 0.20

Ma relative to the astrochronologically-calibrated age of 28.201 Ma for Fish Canyon sanidine (new calibration from Kuiper et al., 2008; new ages from Singer et al, 2009).

This is significantly younger than the placement of the CT boundary based on sample dates provided in this study from Henrieville. This can have serious implications as to the duration, timing, and causes of the OAE2. Previous hypotheses (eg Mitchell, 2008) propose that the ocean anoxic events coincide with a harmony in eccentricity, obliquity and precession of the earth. This may not be the case, however, as this proposed nodal

29 agreement is thought to have begun some 590 kyr after the ages suggested by Sageman

(2006). Thus, enhanced volcanic activity near the CT boundary would seem to be a more likely cause of the OAE2 event rather than the cyclical association with the earth's eccentricity (Turgeon and Creaser, 2008, Kudora, 2007). Evidence of large volcanic events, in the Madagascar and Carribbean flood basalts, associated with large igneous provinces and sea floor spreading coincide with the timing of OAE2 (Kudora, 2007).

Another possibility, however, is that given the offset of the model ages is a little more than the 400 kyr cycle, the chance exists that the calibration is slightly off and needs to adjusted. Either way, examining the events preceding the OAE2 allows for a better understanding of the future work that must be done.

Conclusions

The 2 06Pb/ 238U dates determined in this study provide new constraints on middle

Cretaceous earth history. By providing a minimal basal age for the Tropic Shale in the

Kaiparowits Plateau and eastern Utah, this work offers a context for the Cenomanian-

Turonian boundary as well as the coinciding Ocean Anoxic Event. The stratigraphic relationships discussed provide a relationship across the Western Interior for the beginning of the WIS transgression, and the paleobotanical floras contain important depositional environment information. Further work on volcanic ashes across this transgressional plane in Utah and Colorado would provide a more complete picture of the

Western Interior Seaway's initiation and rapidity of transgression.

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35 Appendix A: Site Photographs B *Courtesy of Kirk Johnson

A

4 1

4 A4

C

A: Henrieville locality looking east; Dakota Formation unconforamably %3"8 15 overlies the Entrada Formation

B: Westwater locality featuring Ian Miller: Samples KJ08106-KJO8108 collected; KJ08108 analyzed

C: Westwater locality from a distance, featuring MIT DEAPS students. - - 4 KJ08105 collected near the top of the hill, KJ08106-KJO8108 near students -- (distance between KJ08105 and KJ08108 is 5.93m) 4.

36 Appendix B: Paleobotany data

Table B 1: Numerical summary of abundance of fossil plants in five localities Denver Museum of Nature and Science

Henrieville WW CD WW Ang WW SA WW 2 UD001 260 34 UD002 479 119 6 UD003 142 UD004 8 UD005 35 5 2 UD006 7 3 UD007 9 UD008 1 UD009 4 UD01 0 5 UD01 1 5 UD012 41 1 UD013 3 UD014 3 2 UD015 2 UD016 1 UD017 13 UDOI8 1 UD019 5 UD020 1 UD021 7 7 UD022 3 UD023 1 UD024 1 UD025 1 UD026 22 UD027 2 UD028 1 UD029 15 UDO30 9 UDO31 3 UD032 1 unidentified 61 41 12 1 0

37 Table B2: Numerical Summary of diversity of fossil plants in five localities

(Denver Museum of Nature and Science)

WW Coal WW WW Slide Henrieville Draw Angiosperm Ash WW 2 Fern 4 916 2 163 8 Dicot 106 10 39 Other 15 7 Total 1093 941 173 170 8

%Fern 0.04 0.97343252 0.04878049 0.95882353 1 %Angio 0.96363636 0.01062699 0.95121951 0 0

38 Appendix C: Raw Geochronology Data

Table Cl: KJ08142 Dates (1a) Co.position Isotopic Ratios 20rb 2a 2075*, &2 2O7PbI k2 Corr. Thi Pbw Pbc Pb*/OP/2~ Fraction 2w/ abs 235Ua 26 abs cf. %dcH - pg)d (pg)* Ph 204M 23 *2a % 2 2w % P 2w % Fraction A1rmo Z1 95.362 0.056 96.55 0.39 126.6 9.1 0.587 24.70 0.88 22.0 0.66 33 1819 0.0149030 0.059 0.09978 0.42 0.04856 0.39 zi z3 94.559 0.094 95.23 0.78 105 19 0.511 9.29 0.97 9.20 0.50 18 96 0.014824 0.10 0.09633 0.86 0.04811 0.81 z3 z4 94.844 0.066 95.71 0.64 117 16 0.515 19.17 0.80 23.3 1.27 18 1037 0.014821 0.070 0.0965 0.70 0.04837 0.67 z4 Z5 94.960 0.065 95.41 0.57 107 14 0.632 10.92 0.89 25.3 1.20 21 1163 0.014840 0.069 0.09652 0.62 0.04815 0.58 zS z6 94.873 0.073 95.75 0.75 118 18 0.669 19.34 1.04 11.7 0.67 18 936 0.014826 0.077 0.0989 0.82 0.04837 0.77 z6 7 94.871 0.064 95.37 0.571 108 14 0.586 12.08 0.93 11.5 0.54 21 1170 0.014826 0.068 0.09645 0.63 0.04812, 0.59 27 zx 94.768 0.081 95.04 0.75 102 18 0.623 6.98 0.84 14.4 0.76 19 1060 0.014810 0.0"6 0.09612 0.82 0.04805 0.77 z8 Z9 95.046 0.086 95.17 0.57 98 14 0.522 3.35 0.63 13.7 0.37' 37 2115 0.014853 0.091 0.0926 0.63: 0.04796 0.58 29 210 94.76 0.13 94.8 1.6 97 41 0.603 2.17 0.99 5.85 0.65 9 496 0.014809 0.14 0.0979 1.8 0.04795 1.7 zi1

Table C2: KJO8143 Dates (0h) Composition Isotopic Ratios 20F1ct2w 207Pbf 2 207Pb/ 2w Corr. Thi Pb' Pbc Pb' j 206Pb/ Fraction 2385$ abs 23Sti abs 206Fb' abs coat Meisc UO (pg)d (pg). Pbc 04b2381P 2w % 3h *2w % 2061*( *2a % Fraction Zircon 21 99.352 0.063 100.37 0.41 124.7 9.2 0.610 20.34 0.53 14.9 0.46 33 1964 0.015531 0.064 0.10390 0.43 0.04852 0.39 21 z2 95.034 0.077 94.71 0.71 87 18 0.552 -9.66 0.41 7.65 0.45 17 1061 0.014851 0.062 0.09777 0.79 0.04775 0.75 z2 23 96.309 0.071 97.11 0.63 117 IS 0.602 17.65 0.34 21.2 1.30, 16 1036 0.015052 0.074 0.10037 0.68 0.04836 0.64 z3 24 95.455 0.075 96.67 0.34 126.7 8.1 0.431 24.69' 0.42 21.51 0.51 43 2633 0.014918, 0.080! 0.099M5 0.37 0.0456 0.34 24 Z6 95.52 0.41 96.4 4.7 118 110 0.659 19.30 0.48 2.04 0.77 3 177 0.014927 0.43 0.0996 5.1 0.0484 4.8 ME Z7 94.676 0.093 92.6 1.0 40 27 0.634 -136.60 0.44 5.47 0.42 13 510 0.014795 0.10 0.0955 1.2 0.04682 1.1 z7 xl 95.094 0.096 94.9 1.2 91 29 0.678 -4.56 0.46 6.11 0.66 9 575 0.01461 0.10 0.0980 1.3 0.04783 1.2 z8 Z9 95.23 0.16 95.6 2.1 106 52 0.727 9.79 0.52 2.30, 0.37 6 385 0.014M51 0.17 0.09M5 2.3 0.0481 2.2 23 12 95.13 0.10 94.36 0.56 75 21 0.724 -26.80 0.45 13.1 0.78 17 1034 0.014866 0.11 0.09739 0.95 0.04751 0.38 z12

Table C3: KJ08105 Dates (Ph) Composition Isotopic Ratios 206P*? 2w 2075*/ 2f207Pb/ Cor. Th/ P *Pbc Pb2 206Pb/ 2061,/ 207ft/ 207 2 c/ Fraction 238UA abs 23SUA abs 2O6Pb = coef. %disc' Wj (PO) d (pg), Pbcf 2041Pb23sua 2w % Z3S1r 2w % 206FV 2u % Fraction zircon z3 97.73 0.17 99.5 2.0 143 46 0.714 31.61 1.30 4.37 0.62 7 368 0.015275 0.18 0.1030 2.1 0.04890 2.0 z3 z4 97.53 0.14 97.1 1.2 "6 27 0.742 -12.80 0.53, 9.14 0.70 13 794 0.015244 0.15 0.1003 1.3 0.04774 1.2 z4 Z5 97.832 0.075 98.28 0.73 109 17 0.600 10.29 0.42 21.2 1.42 15 928 0.015292 0.081 0.10163 0.75 0.04820 0.73 25 z6 97.57 0.16 99.3 2.2 142 51 0.772' 31.28 1.23 3.37 0.40 8 438 0.015251 0.17 0.1028 2.3 0.0489 2.2 x6 7 97.826 0.099 98.36 0.62 111 14 0.778 12.04 0.57 12.4 0.55 23 1345 0.015291 0.10 0.10171 0.67 0.04824' 0.59 z7 29 97.54 0.19 98.6 1.8 126 42 0.677 22.32 1.15 5.27 0.64 8 435 0.015246 0.19 0.1020 1.9 0.04854 1.8 29 210 96.75 0.14 97.3 1.6 110 39 0.703 12.28 0.55 7.52 1.00 8 462 0.015122 0.14 0.1005 1.7 0.04822 1.6 z10

Table C4: KJ08108

Dates (Pk) Composition Isotopic Ratios 206Pb/ 2w 207P*1 a2& 207P*1 *2w Corr. Th/ Pb' Pbc Pb*( 206P206Pb/ Fraction 2381$ abs 235UM ab 206W abs coef. % dise UO (Wgd (")a PbC 3P 2w % Zot 2w S 2w % Fraction zircon 21 97.637 0.095 98.93 0.85 130 20 0.625 25.06 0.61 22.0 1.69 13 775 0.015261 0.098 0. 10234 0.90 0.04864 0.84 z1 z2 99.572 0.058 102.14 0.93 163 21 0.647 38.77 0.33 8.82 0.76 12 748 0.015586 0.089 0.1058 0.95 0.04931 0.90 z2 z3 97.954 0.072 96.22 0.56 105 13 0.651 6.48 0.45 11.0 0.50 22 1366 0.015311 0.074 0.10157 0.60 0.04811 0.55 z3 7 95.56 0.12 100.2 1.4 140 32 0.696 29.44 1.14 4.42 0.36 12 652 0.015407 0.12 0.1037 1.4 0.04883 1.3 z7

39 Appendix D: Fossil and Stratigraphic Tables/Figures

Table ID

USGS Open-File Report 2006-1250 Figure 1 A USGS Zonal Table for the Upper Cretaceous Middle Cenomanian - Maastrichtian of the Western Interior of the United States Based on Ammonites, Inoceramids, and Radiometric Ages

Wiliarn A. Cobban, John D. Obradovich, Ireneusz Walaszcyk, and Kevin C. McKinney

2 Stages and Stage Western Interior Ammonite Age Western Interior Substages Boundaries Taxon Range Zones Ma Inoceramid Interval Zones Ma - 6.*0.3 -8%.51* 0.10 C Upper 6amssesmee

EINGUNIS cloobalus 86.69*20.36 7nocmunus b5 70.00 *0.45 Tfncomrmus raclosms Lower Swcum$s gacfs 70.6*0.6 71.98 * 0.31 7noceamusredhensts Bemes Poo" 7noc -us-eins Eureper l172.94 0.45

Bacun" sUMM 633*0.39 74.67*0.15 Upper 7n00 0.11 sp -asotmuperfenus is 75.19 0.28 1075.56i 0.11 Bmo9swem866mer 756* 0.26 7noce.lnmadn " I. Upper 9 emmacmreus BaaibswUnhn Middle C---

scbboosehmep * 066 7nowmnuVazer6e~~zeeab

Lower Ba- ansprrte m 0.3. Cabwnbefibu Lower scpnsdepr111 63.6 0.7 - C 430* 0.34 spmnoeraminharm*na Upper 6ecsqmaeeawaf A Bcupamsp. norme) Middle .5 Lower iC 666 0.7 - -anoin Upper 87.14 0.39 Aftp eren-ciue. Middle -0 amd crennoermmus aucrassustsa Lower

9.31 1.0 - -

Upper 7N-owfeWilmmml - - -ga---am- subq--- CecmeAmwhewenab 7cgocessua on S-4 -__p i - 9021 0.54 hCW08061158masuum e -6w 6e asb Middle enScpess diprn bloogmu nep, SScocess Mi a

DeernoseaeWM boessie 93A9*0.42 P'Srftsnctns obee -AFYIaNDdes helffm 63.5*0.3- - -c -- -- 93.32 *026

Eeermihakimmassep6emb&mrn 93.66 * 0.50 Vaeooem~M nM Upper 93.99*0.72 '-..,.s, . Psedwnw tomm~a

--- ama 94.71*0.46 4 tdcmeloosenske onocensompffla amepommahm 94.0650 Middle Va9somm bkdkb me warvan thooerawo enou 95.73*0.61

Lower 12.331 0.37 99.* 0.9 1 -WDJO-d dWWiMiS 2 Grddsin 7 S~oobbanmeens of Cooper 199412 roused radorm Wfr lri meswokU of Canpiern end On 2004 6 Choscas6iVlof Cooper 194 from HcMe J r1 t al.. 2W4 genralfly eo-pbd in Europe 4 ==217= 1904 3 Tateca*o s of cooper 1998 9 Hicn et al., 2002. K-T Boundary 8 lwInzone NObbadomi nd gmas ga of 2034 Me for rxi nf

o hispu ) an op of 28.32 Mft for calbrati Figure D2

Planktonic Calcareous Western Interior Stage Foraminifera Nannofossil Ammonite Zone Zone Zone

Praeglobotruncana Eprolithus helvetica floralis M. nodosoides Zone Zone Turonian

W. coloradoense Parhabdolithus asper Whiteinella Zone archaeocretacea N. juddii Zone

& gracile Cenomanian

Axopodorhabdus V. mosbyense Rotalipora albianus cushmani Zone C. canitaurinum Zone

P. wyomingense

C. amphibolum

Simplification of fossil zonations, including a correlation between Ammonite zones, Foraminifera zones and Nanofossil zones. (Bralower, 1988)

41 4

E

Figure D3

a vt

Ir"I

hL 11 I I

Rui I

flf

aoi I1Ik1;qrQ 1DI1~4Jy

/ I.-- -e I

/ ~}T-, / /

.I &1w.,sL

.10