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

Anna Jerve 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

Brady Z. ForemanVolcaniclastic 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.