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.
Anna Jerve
Advisor: Ray Rogers
Senior Honors Thesis: 2004
Geochemical analysis and characterization of paleosols from the Upper Cretaceous Masorobe
Member of the Maevarano Formation, Mahajunga Basin, northwestern Madagascar
Paleosols, or ancient soils, are excellent indicators of climate because they tend
to maintain much of their formative chemical and biological signatures. They have
been identified and used for paleoenvironmental reconstruction in localities
worldwide and in rocks dating as far back as the Precambrian. Upper Cretaceous
terrestrial strata of the Maevarano Formation of the Mahajunga Basin in northwestern
Madagascar preserve numerous exquisite paleosol profiles. Thirty-six well-preserved
paleosol units were recently identified in the Masorobe Member, which comprises the
lower 80 m of the Maevarano Formation, and six of the paleosols were described in detail
and sampled at 10-cm increments. The Masorobe paleosols are alluvial in origin, compound,
and generally red in color. They tend to consist of clay-rich silty sandstone. They are
also characterized by pervasive pedogenic features such as color banding, strong vertical
root traces, drab root halos, and slickensides. Pedogenic carbonate, in the form of
carbonate nodules and carbonate-encrusted rhizoliths, is also common. The geochemistry
of the six well-sampled paleosol units was analyzed using X-ray fluorescence
spectrometry to assist in identifying diagnostic horizons indicative of organismal
nutrient intake (A-horizons) and the illuviation of minerals and elements (B-horizons).
Horizons were then used to classify the paleosols units from the Masorobe Member
using a paleosol classification scheme designed specifically for fossil soils.
Lithologic attributes suggest that these paleosols formed under alkaline conditions
in a semiarid climate. Molecular weathering ratios, which represent the degree of
hydrolysis and leaching, support a general interpretation of minimal to moderate
weathering and leaching. Subtle differences developed among he Masorobe paleosols may
possibly represent climatic variations that transpired during deposition of the 80-m-thick
unit. Alternatively, the minor variations documented among the Masorobe paleosols may be
due to (1) local variations in formative environment (topography and/or position on
the floodplain), or (2) complications involving paleosol classification. Overall,
the Masorobe Member paleosols provide key insights into paleoclimate of Madagascar.
Brady Z. Foreman
Advisor: Ray Rogers
Senior Honors Thesis: 2004
Geochemical Charaterization and Discrimination of Bentonites in the Upper Cretaceous
Two Medicine Formation, Northwestern Montana
Volcaniclastic terrestrial sediments of the Upper Cretaceous Two Medicine Formation of
northwestern Montana preserve several discrete bentonite beds. These beds hold
significant potential for geochemical fingerprinting and high-resolution correlation.
This study focuses on three bentonites within the formation, and employs geochemistry
to distinguish the "Seven Mile Hill bentonite" from the "Bed 2 bentonite" both found
near Choteau, Montana, and the "Hadro-Hill bentonite" on the Two Medicine River.
Major and trace element concentration were determined using X-ray fluorescence
spectroscopy. The bentonite units successfully discriminated using bivariate plots
and ternary diagrams utilized by previous researchers. In addition, discrimination
was refined by plotting source-related and petrogenically-significant rations of
elements after a rigorous element mobility analysis. Th/Zr, V/Ti. U/Nb, and V/Y
proved to be particularly useful for discriminating between the two bentonite
begs. Analysis of these ratios and others not only provides geochemical
signatures, but can be used to describe general volcanic processes and
possible magmatic source composition changes overtime. This study indicates
that geochemical discrimination of Two medicine bentonites can be used to
generate a high-resolution stratigraphic framework, and this in turn can be
employed to frame a variety of paleobiological questions.
Macalester College · 1600 Grand Avenue, St. Paul, MN 55105 USA ·
651-696-6000
Comments and questions to mac.geology@gmail.com
