T68 Evolution of the Midcontinent Rift: A Window into Proterozoic Environments, a Repository of Minerals, and a Lesson in Rifting II

Sunday, 4 November 2018
1:30 PM - 5:30 PM
Room 242

THE NONESUCH FORMATION: AN ENIGMATIC WINDOW INTO TERRESTRIAL ENVIRONMENTS DURING THE MESOPROTEROZOIC

Paper number: T68-13
Program: Technical Sessions
Time: 4:30 PM – 4:50 PM

Authors

Abstract

The Nonesuch Formation is a relatively thin unit when compared to the total thickness of sediments deposited within the Midcontinent Rift System. Yet it continues to garner much attention due to its association with important economic deposits and growing interest in life and environments on the Early Earth. The Nonesuch Formation is generally thought to have been deposited in a freshwater lacustrine setting and has complex assemblages of acritarchs throughout, making it a rare archive of early terrestrial life. Its value as a paleoenvironmental archive is further enhanced by its relatively moderate post-depositional history. Despite its age, the Nonesuch Formation was never deeply buried and experienced only moderate burial temperatures (< 150° C) during a relatively brief interval.

In this study, we present inorganic (major and trace element, Fe speciation) and organic (TOC, bulk carbon isotopes) geochemistry data from seven different drill cores dispersed across the Upper Peninsula of Michigan (White Pine and Iron River synclines) and Wisconsin (Ashland syncline) to provide further constraints on the Nonesuch depositional environment. These data indicate that environmental conditions varied significantly throughout deposition of the Nonesuch Formation, with evidence supporting oxic, anoxic, and possibly sulfidic conditions during different intervals. For example, molybdenum concentrations range from 0.1 to 40 ppm, FeHR/FeTotalratios vary between 0.1 and 0.4, and Fepyrite/FeHRratios range from 0.02 to 0.92. These results are broadly consistent with previously observed spatial and temporal trends in depositional environments, which vary range from proximal fluvial/lacustrine to distal lacustrine environments. The fidelity of these environmental signals will also be discussed in light of possible complications, such as changes in source material, intermittent marine influences, and shifts in lake hydrology.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-321724

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