The Geology Department at Macalester College is committed to collaborative
faculty-student research. To this end, many of our majors complete senior
honors theses. A typical thesis-related research experience includes one
or more summers of data collection and analysis (often in far-off places such
as Crete, Zimbabwe, Mexico, Montana, Madagascar, and even southwestern Minnesota),
followed by an oral defense and submission of a thesis to the library. In recent
years, our students have conducted projects that focus on structural geology,
geochemistry, paleontology, and sedimentology/stratigraphy.
The general requirements to participate in the Honors program may be found by clicking
here.
Mara Brady
Advisor: Ray Rogers
Senior Honors Thesis: 2005
An Experimental and Field-Based Approach to the Taphonomy of
Microvertebrate Assemblages: A Case Study in the Judith River Formation of North-Central Montana
Microvertebrate assemblages, or “microsites,” are fossilized concentrations of small physico-chemically
resistant vertebrate elements, including small bones and teeth from mammals, crocodiles, turtles, fish,
and dinosaurs. These assemblages are generally preserved in two distinct depositional settings: 1)
low-energy pond/lake environments and 2) high-energy channel facies. Many previous works have focused
on the origins of channel-hosted concentrations. Several of these studies examined the behavior of bone
in an active flow, and most concluded that bone material has great dispersal potential in water. Other
studies focused on the characterization of fossil microvertebrate assemblages and generally discount
the possibility of using these sites for paleobiological reconstruction, claiming that the bones in
these assemblages have been transported great distances from their original habitats. However, previous
studies generally fail to provide a mechanism of bone concentration in a fluvial environment. How does
bone material concentrate in an active channel if its predicted behavior in water is dispersal?
This study takes a two-fold approach to understanding the origins of microvertebrate assemblages by 1)
comparing sites found in both lacustrine and channel deposits and 2) conducting flume experiments in
which an active flow is directed through a bed of sediment with a known concentration of dispersed bone
material. Lacustrine and fluvial assemblages can be readily distinguished based on sedimentary context
and the overall richness, or abundance, of bone material. Moreover, taphonomic features, such as the
degree of surface alteration and trends in size sorting, differentiate the two site types. Bone material
from the low-energy lacustrine assemblages (Type I) is dispersed as a background concentration (<1% bone
by volume) throughout the fine-grained mudstone deposits, while bone material in channel-hosted assemblages
(Type II) is concentrated in discrete lags (up to 3% bone by volume) localized at the base of fine-grained
fluvial deposits. Microvertebrate elements from both types of sites exhibit a range of rounding, fragmentation,
alteration, size, and shape. However, bone material from Type II assemblages shows evidence of greater surface
alteration (discoloration, weathering) and is better sorted compared to the bone material from Type I assemblages.
Flume experiments were conducted to better constrain potential origins of microvertebrate assemblages.
An active flow was directed through beds of fine-grained sand and dispersed bone material with three
different initial background bone concentrations (0.1%, 1%, and 10% bone by volume). In all three
experiments, bone material became less evenly dispersed through time as local accumulations developed.
Fine bone material (0.5-1 mm) was preferentially winnowed to the distal portions of the flume, while
the coarsest fraction of bone material (2-4 mm) was transported minimally and generally remained in
the proximal portion of the flume as lag deposits.
The potential for natural channels to interact with Type I (low-energy) assemblages and the characteristics
of Type I and Type II (high-energy) assemblages indicate that Type II assemblages could readily be generated
by reworking preexisting Type I deposits. Experimental results further demonstrate that erosional flow
processes can generate localized concentrations of bone material from a background of widely dispersed
material. Taken together, these findings indicate that microvertebrate assemblages found in ancient
channel scours could plausibly be sourced locally from preexisting concentrations. In turn, this
study has important paleoecological implications, specifically in relation to the spatial fidelity and
temporal resolution of microvertebrate assemblages.
Chris Dwyer
Advisor: Ray Rogers
Senior Honors Thesis: 2005
A Comparative Investigation of Diagenesis in Fossil Teeth and Fish Scales from the Upper
Cretaceous Two Medicine and Judith River Formations of Montana
The Campanian Two Medicine (TMF) and Judith River (JRF) Formations of Montana preserve
coeval deposits of the semi-arid alluvial uplands (TMF) and the coastal plain lowlands
(JRF). Previous work has documented the abundance and quality of vertebrate fossils in
both formations. In this study I explore the diagenesis of dinosaur teeth, crocodile
teeth, and gar scales using REE content and authigenic cements. The fossils under
investigation were collected from several well-documented sites of fluvial and lacustrine origin.
Authigenic cements filling cracks and dentine tubules were examined in 23 theropod teeth,
19 hadrosaur teeth, 6 crocodile teeth, 4 ceratopsian, ankylosaur and myledaphus teeth,
and 8 gar scales using polarized light microscopy and SEM-EDS. Fills include calcite,
sulfates, sulfides, bornite, authigenic and detrital clays, iron oxides, and clastic
particles. Patterns of authigenic cementation serve to describe the general process
of permineralization, and also characterize pore water chemistry in the burial environment.
Rare earth element (REE) concentrations were determined for the same sample of teeth and
scales using LA-ICP-MS. TMF teeth have higher REE concentrations than JRF teeth and are
enriched in middle REE. JRF teeth are enriched in HREE. TMF teeth also tend to show more
variability in REE concentrations. These REE patterns suggest that (1) TMF teeth likely
spent the early stages of diagenesis in or upon alluvial soils, (2) TMF teeth experienced
greater small-scale variability in the factors controlling REE uptake, and (3) JRF
teeth likely spent early diagenesis in stagnant aqueous environments. These
findings are consistent with independent sedimentologic and taphonomic data,
and are also in line with previous studies of other workers that focused on REE
patterns in TMF and JRF bone.
Finally, the overall REE patterns in enamel and dentine are similar within a given
element, but normalized total REE concentrations vary in these tissues from
relatively low (enamel) to relatively high (dentine). This pattern is consistent
with porosity and crystallite density contrasts in enamel and dentine, and
illustrates the importance of these factors in diagenetic processes.
Brett Dennis-Duke
Advisor: Ray Rogers
Senior Honors Thesis: 2005
Revisiting the Magnetostratigraphy of the Upper Cretaceous Berivotra and
Maevarano Formations, Northwestern Madagascar
The Maevarano and Berivotra Formations of the Mahajunga Basin in northwestern
Madagascar yield a diverse assemblage of paleobiologically and paleobiogeographically
significant vertebrate fossils. In an effort to correlate these units to the
geomagnetic polarity time scale, oriented rock samples were collected from 28
sites. Magnetic remanence was detected with a Superconducting Rock Magnetometer
(SRM) at the University of Minnesota’s Institute for Rock Magnetism, employing
both alternating field (AF) and thermal demagnetization techniques. Although
signals were weak, analysis thus far corroborates work done by Michelle
Casey in 2003. Both studies correlated the Berivotra Formation and the
uppermost Maevarano Formation (more tentatively) to Chrons 29R and 30N
(late Maastrichtian). Additionally, trends were observed that might offer
insight into future study: 1) Fidelity of magnetic signal seems to be
distinctly tied to Lithologic character, and 2) There may be a reversed
polarity or excursion recorded in Berivotra sediments. Based on this
study, it is recommended that further work be conducted on specific
units that may have more interpretable signals (specifically the
Berivotra Formation and the Miadana Member of the Maevarano Formation).
It is also suggested that extensive sampling be done in the interval
15-20 meters below the Berivotra Formation – Betsiboka limestone contact
to test the hypothesis that a reversal or excursion is present.
Josephine Williams
Advisor: Ray Rogers
Senior Honors Thesis: 2005
Authigenic cements and Rare Earth Element Concentration in Fossil Bones from
the Upper Cretaceous Two Medicine Formation, Northwestern Montana
The Upper Cretaceous (Campanian) Two Medicine Formation of northwestern
Montana preserves abundant vertebrate fossils. An understanding of the
changes that take place within a given bone as it becomes a fossil is
essential to an informed interpretation of that fossil material and its
associated depositional and diagenetic system. I investigated authigenic
cements and rare earth element (REE) concentrations in fossil bones from a
variety of terrestrial depositional environments, including high-energy fluvial
deposits (sandstones) and low-energy floodplain and coastal deposits (mudstones).
I explored preservational patterns in a paleoenvironmental context by comparing
trends in authigenic cementation and REE concentration across differing
lithologies, between similar depositional environments, and between individual
fossil bones within a site. Authigenic cements were investigated using polarized
light microscopy and scanning electron microscopy (SEM). REE concentrations
in fossil bone tissue were examined using laser ablation inductively
coupled plasma mass spectrometry (LA-ICP-MS).
The nature and relative abundance of authigenic cements vary among sites.
This suggests that authigenic cements track environments, and that there
exists a correlation between depositional environment and authigenic mineralization.
REE concentrations tended to display distinctly different fractionation patterns
by site, and thus tracked depositional settings. In general, fossil bones from
sandstones show the most variation in both authigenic cements and REE
concentration, and are typified by HREE enrichment and abundant calcite
and iron oxide void fill. Fossil bones from a coastal mudstone locality
exhibit the least variability in preservational state, and are characterized
by presence of pyrite, replacement of bone tissue by calcite, and very low
levels of REE fractionation. Fossil bones from upland low-energy sites
display intermediate variability and are characterized by MREE enrichment,
and the presence of chlorite and detrital clay. Additionally, certain authigenic
minerals and REE fractionation patterns are indicative of specific Eh/pH conditions.
Overall, the data collected during the course of this multifaceted study allow
for characterization of local conditions during early diagenesis, and reveal
novel insights into the regional microtaphonomy of the Two Medicine system.
Kirsten Fristad
Advisor: John Craddock
Senior Honors Thesis: 2005
The Valley of Ten Thousand Smokes in Katmai National Park, Alaska as a Mars Analogue
Recent NASA missions to Mars have returned increasingly detailed surface imagery, motivating the effort to find and study analogue features on Earth to understand processes on Mars. The 1912 eruption of Novarupta in Katmai National Park created a barren volcanic landscape, the Valley of Ten Thousand Smokes (VTTS), which remains devoid of vegetation today. The precisely dated VTTS, created by pyroclastic volcanism, contains many geomorphic features similar to Mars and is an excellent test case for studying life in an inhospitable environment. The landscape would be beneficial for planetary science field studies, remote sensing and rover testing.
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