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9-1-2015

Sediment Analysis of the Late Cretaceous Hell Creek Formation Sediment Analysis of the Late Cretaceous Hell Creek Formation

of Western North Dakota of Western North Dakota

Henry Cole Winona State University

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Methodology

Sediment and fossil samples were collected at Triceratops excavation site in the Hell Creek For-

mation near Marmarth, North Dakota (Figure 2). Sediment samples were collected from strata in

the butte containing the Triceratops fossils. Samples were obtained by digging into the butte at

least 30 cm deep to ensure collection of originally horizontal sediments rather than slumped ma-

terial from above (Figure 3). Unique strata were identified using textural and color sediment analy-

sis practices. Samples from each strata were collected using Ziploc® bags and individually coded

according to their stratigraphic position, and stored for later examination in the fossil Preparation

lab at Winona State University in Winona, MN. Sediments were catalogued using the Shepard clas-

sification (Shepard, 1954). Additional sediment samples were set aside for possible future pollen

analysis.

Abstract

This study focuses on the lithology of a Triceratops excavation site near Marmarth, North Dakota.

The excavation site, Hell Creek Formation, consists of sedimentary rocks, predominantly clays,

sandstones, and mudstones dating to the Late Cretaceous period approximately 66 million years

ago. Fossilized dinosaur skeletons and preserved concentrations of microfossils, or fossil micro-

sites, can be found in exposures of the Hell Creek Formation throughout Montana and North and

South Dakota. Sediment samples were collected directly from and near the Triceratops excavation

site. The majority of sediments present at the site are sand and silt-sized, representing floodplains

and channel sands, with occasional plant fossils and thin coal layers. The types of sediment found

at the site, along with evidence supporting the presence of terrestrial species, supports the inter-

pretation that during the Cretaceous Period, the depositional environment of western North Da-

kota was warm and wet, likely a paludal or transitional coastal environment similar to North

America’s Gulf Coast today.

Results

Sediment sizes ranged from about 3.9 μm clays to 200 μm fine sands and vary in composition. Nine

separate lithologies were identified, and are presented in Figure 6. The Predominant lithologies are

mud, clay, and siltstones. Two distinct sandstone beds are present in the top section of the sample

butte, both displaying cross bedding and evidence of root fossils. All sampled sediments were rich in

organic material, identified as small dark spots in the rock. Siltstone SRP_C16-03 contained a 6cm lay-

er of lignite, with preserved plant fossils seen in figure 4. The Triceratops skeleton was found in clay-

stone SRP_C16-08 (figure 7) approximately 3 meters away from the sample column.

Discussion

The sediments collected from the study site coincide with the currently accepted interpretation of deposi-

tional environments, samples collected and previous studies of the Hell Creek formation (Murphy et al.).

The very fine clays (figure 7) with high organic matter suggests an environment of either low energy stand-

ing water, or a floodplain. The latter theory is also supported by the cross-bedding present in the sand-

stones just above and below the upper clays, interpreted as channel sands. These observations suggest dep-

ositional environment to be interpreted as streams cutting across a floodplain. The silts, clays and muds

would have been deposited during high water events when the stream channels overflow. During times

when water exceeds channel capacity and overflows to the flood plain the sediment choked water rapidly

spreads over the floodplain, loses energy and the suspended sediment drops out. This drastic loss of energy

causes a build-up of sediment creating an embankment close to the channel, while also depositing finer ma-

terial across the plain. As an additional line of evidence, the Triceratops excavation indicates a terrestrial

depositional environment, as Triceratops were terrestrial animals.

Introduction

The clays, sandstones and mudstones of the Hell Creek formation are considered to be terrestrial

river channel and flood plain deposits. They were deposited at the edge of an inland sea that

stretched across North America 66 million years ago, during the Cretaceous Period (Murphy,

2002). Many of these deposits contain fossilized skeletons, such as Triceratops, along with micro-

fossil beds. Interpreting sediment data paired with fossil sites, geologists can ascertain the deposi-

tional environment present during the cretaceous.

Sediment Analysis of the Late Cretaceous Hell Creek formation of

Western North Dakota Henry Cole

Figure 1: Triceratops Excavation prior to study

Figure 3: (Left) Strat column for sampling strata adjacent to Triceratops excavation

Figure 6: Stratigraphic column with labeled strata

Figure 4: (Right)

Lignite with pre-

served plant fossils

Figure 2: (Below)

Marmarth North

Figure 7: Shepard Classification ternary plots for all nine separate units.

References

Arens, N.C., and Allen, S.E., 2014, A florule from the base of the Hell Creek Formation in the type area of

eastern Montana: Implications for vegetation and climate: Geological Society of America Special Papers

Through the End of the Cretaceous in the Type Locality of the Hell Creek Formation in Montana and Adjacent

Areas, p. 173–207.

Johnson, K.R., Nichols, D.J., and Hartman, J.H., 2002, Hell Creek Formation: A 2001 synthesis: Special Paper

361: The Hell Creek Formation and the Cretaceous-Tertiary boundary in the northern Great Plains: An Integrat-

ed continental record of the end of the Cretaceous, p. 503–510.

Moore, W.L., 1976, Stratigraphy and environments of deposition of the Cretaceous Hell Creek Formation

(reconnaissance) and the Paleocene Ludlow Formation (detailed), southwestern North Dakota. Report of in-

vestigations No. 56.

Murphy, E.C., Hoganson, J.W., and Johnson, K.R., 2002, Lithostratigraphy of the Hell Creek Formation in

North Dakota: Special Paper 361: The Hell Creek Formation and the Cretaceous-Tertiary boundary in the

northern Great Plains: An Integrated continental record of the end of the Cretaceous, p. 9–34.

Scannella, J.B., and Fowler, D.W., 2014, A stratigraphic survey of Triceratops localities in the Hell Creek For-

mation, northeastern Montana (2006–2010): Geological Society of America Special Papers Through the End of

the Cretaceous in the Type Locality of the Hell Creek Formation in Montana and Adjacent Areas, p. 313–332

Shepard, F.P., 1954, Nomenclature Based on Sand-silt-clay Ratios: SEPM Journal of Sedimentary Research,

v. Vol. 24,