PRELIMINARY RESULTS OF A PROTEROZOIC ENVIRONMENTAL MAGNETISM STUDY: QUANTIFICATION AND ANALYSIS OF THE COPPER HARBOR CONGLOMERATE MAGNETIC MINERALOGY FROM THE NORTH AMERICAN, MID-CONTINENT RIFT

The arrangement, composition, abundance, and grain size of magnetic minerals present in a sedimentary section is often reflective of past environmental or climatic conditions. This study investigates the sandstone section of the 1.1 Ga Copper Harbor Conglomerate in Michigan’s upper peninsula. The purpose of this study is to quantify the magnetic mineralogy and evaluate its suitability for use in an environmental magnetism study. The goal is to differentiate between depositional processes and to constrain the interval of formation. Thereby, providing insight into the environmental setting and influence that climate maintained over the development of this formation which contains some of the oldest evidence for the colonization of microbial life on land.

Previous work has demonstrated that the unit contains detrital magnetite but is dominated by authigenic hematite. To infer paleoclimatic or paleoenvironmental conditions linked to deposition, it is necessary to isolate the detrital signal. This study uses susceptibility and isothermal remanent magnetization (IRM) measurements to assess the relative contributions of the two components. We also use anisotropy of magnetic susceptibility (AMS) to evaluate paleocurrent direction, and will present preliminary field susceptibility data that aims to assess timescales of variations through cyclostratigraphy.

A lineation in AMS data identified a paleocurrent towards the S-SE or N-NW. The latter is consistent with paleocurrent estimates based on field observations at the site. Consistent with prior studies, we find two major magnetic components. High-coercivity, authigenic hematite dominates the remanence, contributing about 65%, while low-coercivity, fine-grained detrital magnetite contributes the remainder. The magnetite component can be isolated through unmixing of detailed IRM acquisition curves or by simple application of a single back field 0.3 T IRM following saturation. Further, the magnetite component strongly dominates low-field susceptibility measurements, meaning that cyclostratigraphy based on outcrop susceptibility measurements should reflect only the detrital signal. These results suggest a more detailed environmental magnetism study should provide real insight into variations in environmental and depositional conditions.