Date of Award

2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

J. Hartman

Abstract

The uppermost Cretaceous Hell Creek Formation and Paleocene Ludlow Member of the Fort Union Formation in easternmost Montana and western North Dakota produce ironstone preserved freshwater molluscan fossils. Ironstone preserved mollusks are associated with ironstone nodules, which have been described as composed of iron carbonate, iron oxide, or manganese oxide by various researchers. To date, the exact composition of the ironstone preservation has not been satisfactorily determined to allow agreement between researchers. Freshwater mollusk fossils, preserved as ironstone external casts, molds, and steinkerns, tend to be highly weathered. The poor preservation of the fossils has resulted in little professional interest, with a limited understanding of the geochemical conditions that produced this preservational phenomenon. The intent of this project was to determine the composition of the ironstone, and attempt to constrain the geochemical conditions necessary to produce the ironstone preservation.

Ironstone preserved fossils and nodules were collected from five localities in Bowman and Slope Counties in southwestern North Dakota. In order to determine the mineralogical composition, samples of mollusks and nodules were analyzed using x-ray diffraction (XRD). Four fossil and four nodule samples were sent to the University of Arizona for 18O and 13C isotope analyses. The elemental composition of three mollusks and two nodules were analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS). Composition information collected from the XRD analyses were entered into the computer program Geochemist’s Workbench®, in which Eh-pH stability diagrams were created to constrain the geochemical conditions necessary to produce ironstone preservation.

XRD analyses have identified the current mineralogical composition of the ironstone nodules and fossils as siderite (FeCO3), quartz (SiO2), and goethite (FeOOH). The original nodule-forming iron mineral was identified as siderite. Analysis of thermodynamic relationships and stability diagrams indicates that siderite formation occurs within a fairly restricted range of ion activities and Eh-pH conditions. Because sulfate will preferentially combine with ferrous iron to form pyrite, the system must have little to no sulfate activity. Similarly, the fugacity of carbon dioxide must be relatively high in order to encourage the precipitation of siderite. The activity of iron must be above 10-6 mol/kg for siderite precipitation; however, increased iron activity beyond 10-6 mol/kg does not appear to increase the overall stability of siderite. From Eh-pH diagrams, it can be determined that siderite is only stable in a neutral to basic and a moderately to severely reducing environment.

Ferrous iron ions may have directly replaced the calcium ions in aragonitic shells without the dissolution of aragonite and precipitation of siderite. A replacement scenario allows for the preservation of shell ornamentation observed on many of the ironstone preserved mollusks. SEM and isotope analyses indicate that the siderite was formed in a completely continental environment. The restrictions for siderite precipitation and stability provide a guide for the geochemical pore water conditions that may have existed during early diagenesis of the Hell Creek Formation and Ludlow Member.

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