Student Research

Steve Welch

M.S. ‹swelch1@utk.edu›

Project title: Deciphering eustatic and tectonic influences during parasequence development in the Mesoproterozoic Helena/Wallace Formations, Belt Supergroup

Project description: Parasequences in the Helena/Wallace formations are composed of a lower, argillaceous interval and an upper, carbonate-rich interval that contains variable amounts of silt and quartz sand. Argillaceous intervals are variable in thickness and thin and fine to the east, suggesting a western, tectonically-driven input. In contrast, coarse-grained sands are more abundant to the east and occur largely in the upper portions of shoaling cycles, suggesting derivation from the east during relative falls in sea-level. Steve is testing the hypothesis that eustatic parasequences were variously overprinted by tectonically-derived sediment by combining stratigraphic modeling (STRATA) with field work (to identify parasequence stacking patterns) and XRF analyses (to fingerprint sediments and identify changes in sediment type that might reflect tectonic activity).

A quote from Steve about his research: "Petroleum? That's right between dairy and fruits & vegetables on the food pyramid, right? Gotta have it then!"

Curt Crawford

M.S. ‹curtcrawford@comcast.net›

Project title: Investigating the origin of Precambrian molar-tooth carbonate

Project description: Molar-tooth (MT) structures consist of a complex array of variously shaped voids filled with an unusual calcitic microspar cement. MT cement consists of equant, non-interlocking crystals whose rounded shape and uniform, small crystal size (7-11 microns D) suggests original precipitation as vaterite. Vaterite is a highly unstable polymorph of CaCO3 that precipitates as homogeneously nucleated sub-spherical crystals under supersaturated laboratory conditions and converts rapidly to calcite. Initial cathodoluminesence (CL) of MT cements reveals dully-luminescent, sub-spherical cores surrounded by a more luminescent, polygonal cement rim, which appears to have preserved this unusual crystal morphology. Curt’s current work involves characterizing the nature of MT cement via standard petrographic, CL, and SEM analysis; constructing crystal size distributions (CSD’s) to yield qualitative and quantitative information about crystal nucleation, nucleation density, crystal growth rate, and growth time; detailed trace-element imaging via SIMS; and, finally, high-resolution laser-Raman microprobe spectroscopy to define the current mineralogy and lattice structure of MT cement. [Raman spectra are characteristic of lattice structure and should reveal any lattice disorder resulting from remnant cation content or structural defects in the crystal lattice associated with vaterite-to-calcite transformation.] Together, this multi-faceted approach should help constrain the origin of this unusual carbonate cement. If originally vaterite, then MT cement may provide critical link between our understanding of changing carbonate saturation state in the Proterozoic and the precipitation of specific carbonate fabrics.

A quote from Curt about his research: "Apatite is for pansies ... I have calcite in my molar-teeth!"

Quintin Overocker

M.S. ‹qoverock@utk.edu›

Project title: Origin of breccias in the Wallace Formation, Mesoproterozoic Belt Supergroup, Montana and Idaho

Project description: An enigmatic series of breccias occur in the middle Wallace Formation of the Mesoproterozoic Belt Supergroup in the northwestern U.S. The origin of these breccias has been disputed for some time with varying hypotheses offered, including syndepositional downslope slumping, evaporite-solution collapse, and post-depositional tectonic brecciation. Unfortunately, the origin of many breccias in the geologic record remain poorly constrained, because compatible features occur within breccias of several different origins. The goal Quintin’s work is to attempt to constrain the mode of origin of the middle Wallace breccias by a combination of field mapping and petrographic and geochemical analysis on breccia clasts, matrix and surrounding strata. If a distinct mode of origin can be determined, this work will help place constraints on depositional and/or postdepositional features of the Wallace Formation. Downslope slump breccias imply basin geometries that may help explain facies distributions in the Belt; evaporite collapse implies significant evaporite deposition at 1.45 Ga – prior to the first known massive sulfates in the geologic record – and thus helps to place constraints on Precambrian ocean chemistry; and tectonic breccias can help to clarify possible fluid migration pathways related to regional trends in ore mineralization.

A quote from Quintin about his research: "I need to have a beer and think about it."

Emily Goodman

B.S. ‹egoodman@utk.edu›

Project title: Trace sulfate concentrations as an indicator of depositional environment: examination of possible calcitized evaporites from the Proterozoic Atar Group, Mauritania

Project description: The scarcity of bedded gypsum in Proterozoic successions has been attributed to both the high solubility/low preservation potential of gypsum/anhydrite, and the difficulty of precipitating these evaporite minerals under the extremely low marine sulfate conditions that potentially characterized Proterozoic marine systems. Recognition of ancient evaporite deposits is therefore a critical step both in constraining depositional environments and in interpreting the long-term geochemical evolution of the Proterozoic biosphere. Emily has been examining a series of unusual limestones (characterized by a bright white color, nodular to irregular to massive bedding, breccia horizons, microkarstic surfaces, and interlamination with terrigenous deposits) in the Atar Group, Mauritania. Standard petrography showed no characteristic evaporite replacement textures, yet cathodoluminescence (CL) revealed clusters of unzoned crystals with 90° crystal faces, suggesting a possible halite precursor. Concentrations of carbonate-associated sulfate (CAS; trace sulfate that substitutes for the carbonate ion and acts as a proxy for fluid sulfate concentration at the time of deposition or diagenesis), however, average 242 ppm, which is significantly higher than that preserved in similarly aged marine rocks, even those that are interbedded with gypsum or contain diagnostic pseudomorphs after gypsum. High CAS concentrations suggest either deposition under highly evaporative conditions or recrystallization in the presence of sulfate-rich fluids, such as those resulting from evaporite dissolution.

A quote from Emily about her research: "Why are Nature papers so short? They leave out all the explanation!"

David Teal

B.S. ‹dteal@utk.edu›

Project title: Using C-isotopes to constrain intrabasinal stratigraphic correlations: Mesoproterozoic Atar Group, Mauritania

Project description: Parasequences in the Helena/Wallace formations are composed of a lower, argillaceous interval and an upper, carbonate-rich interval that contains variable amounts of silt and quartz sand. Argillaceous intervals are variable in thickness and thin and fine to the east, suggesting a western, tectonically-driven input. In contrast, coarse-grained sands are more abundant to the east and occur largely in the upper portions of shoaling cycles, suggesting derivation from the east during relative falls in sea-level. Steve is testing the hypothesis that eustatic parasequences were variously overprinted by tectonically-derived sediment by combining stratigraphic modeling (STRATA) with field work (to identify parasequence stacking patterns) and XRF analyses (to fingerprint sediments and identify changes in sediment type that might reflect tectonic activity).

A quote from David about his research: "Are you sure this is the same basin?"

Adam Kirthlink

B.S. ‹akirthli@utk.edu›

Project title: Determining environmental conditions controlling formation of unusual morphologies of calcium carbonate: experimental precipitation of vaterite

Project description: In a continuing examination of the enigmatic Precambrian carbonate fabric known as “Molar-tooth” (MT), Adam is taking a different look at the unusual carbonate microspar that fills MT structures. Previous studies have suggested that this unusual microspar may have originally precipitated as vaterite – a highly unstable polymorph of CaCO3 has precipitates in a variety of different morphologies and converts rapidly to calcite. If this is the case, determining environmental parameters (pH, temperature, carbonate saturation, presence of dissolved organics, etc.) that control the precipitation of spheroidal vaterite morphologies may provide an invaluable constraint on the chemistry and evolution of marine waters in the Precambrian. To this end, Adam is currently developing laboratory protocols for a series of vaterite precipitation experiments aimed at precipitating spheroidal carbonate crystal morphologies and determining the range of conditions under which these morphologies are reproducible.

A quote from David about his research: "Meet my best friend ... the Fisher Scientific catalog"

Past Students

Mark Pollock

M.S. (2003) ‹mpollock@utk.edu›

Project title: Morphology of molar-tooth structures in Precambrian carbonates: influence of sustrate rheology and implications for genesis

Project description: An unusual Precambrian carbonate fabric, termed molar-tooth structure (MT), is thought to form by expansion of gas produced within shallow sedimentary substrates by decomposition of organic material. Mark constructed a model for how gasses would be expected to interact with sediments of differing rheology and tested his model with MT samples from the Belt Supergroup, Montana. He examined MT in the field to determine if there was any facies preference for MT formation, then petrographically examined samples to detail the behavior of MT as structures crossed laminae of different lithology and, presumably, initial rheology. Mark ultimately was able to construct a scenario of MT formation that can account for much of the observed morphological variation.

A quote from Mark about his research: "You mean I can't use the term 'mushy' in my petrographic descriptions?"

Currently: Mark is currently a Geologist with the international enginerring services firm AMEC (www.amer.com) in Nashville, TN, which provides project design, delivery, & support for oil and gas, transport, & industry.

Curt Crawford

B.S. ‹curtcrawford@comcast.net›

Project title: Chemostratigraphy as a tool for constraining the age of subsurface Proterozoic strata, southern Urals, Russia

Project description: The age of subsurface strata from the southern Urals, Russia has variously been interpreted as Vendian (~650 Ma), based on the presence of ornamented acritarchs, and Middle Riphean (~1300 Ma), based on lithologic similarity with surface strata. The marine C-isotope curve for the Proterozoic suggests that these two time intervals should be easily distinguished by their C-isotope signatures. Curt has been using light microscopy and cathodoluminescence petrography to distinguish between carbonate phases, microdrilling phases, and running elemental (Ca, Mg, Sr, Mn, Fe) and isotopic (C, O) analyses to construct a C-isotope profile for two cores that penetrate the strata in question.

A quote from Curt about his research: "I am the gas!!!"

Future Projects

• Origin of molar-tooth microspar and implications for marine carbonate saturation
• Determining marine compositions from fluid inclusions in evaporite minerals
• Experimental precipitation of unusual carbonate crystal forms
• Biogeochemistry of Precambrian organic matter
• Diagenesis and fabric development in Precambrian carbonate rocks
• Sedimentary fabric recognition on hand-lens scale & application to planetary geology
• A project of your own design!