T134 Clear as Mud: Stratigraphic, Diagenetic, Sedimentologic, Geomechanical Analyses, and Modern Analogs of Ancient Mudrock Systems

Monday, 23 October 2017
1:30 PM - 5:30 PM
Ballroom 6C

DETERMINING MOLYBDENUM SPECIATION TO UNDERSTAND THE DEPOSITIONAL HISTORY OF BLACK SHALES

Paper number: T134-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

Unaltered black shales are used to interpret past depositional redox conditions by examining rock geochemistry. Measuring redox sensitive trace element concentrations and isotopic signatures, can successfully identify ancient oxygen depleted episodes. For example, molybdenum (Mo) isotopic measurements have allowed researchers to make breakthroughs reconstructing the timing of early Earth’s oxygenation. Additionally, higher Mo concentration are usually associated with severe euxinia (e.g., free sulphide in the water column). However, the interpretation of Mo systemics is too qualitative which limits the full potential of the Mo proxy. Our research uses X-ray Absorption Fine Structure (XAFS) as an innovative approach to determine the molecular geochemistry (speciation) of Mo in rocks.

The early Triassic (~250-240 Ma) Montney formation is in the Alberta Sedimentary Basin along the Canadian Rockies. It is composed of black shales and interbedded siltstone/sandstone packages in transgressive-regressive cycles. Field and well observations the Lower Montney have interpreted these cycles to indicate distal offshore provenance with an anoxic or possibly euxinic water column due to the amount of organic matter preserved. Yet, there is some debate on the degree of reducing conditions during deposition of the Lower Montney. Recent research characterizing organic matter (O.M.) fractions found that O.M. migrated into the formation post-depositionally, complicating the distal offshore interpretation. To untangle the depositional history of black shales within this formation, we measured the Mo concentration and speciation using inductively coupled plasma mass spectrometry (ICP-MS) and XAFS, respectively. Our preliminary results reveal that low temperature diagenesis altered Mo speciation indicating depositional euxinia is not the main mechanism concentrating Mo.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-306845

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.