RECOGNIZING THE INFLUENCE OF BURROWING MEIOFAUNA IN MARINE MUDROCKS AND WHAT IT MEANS FOR PALEOREDOX: A CASE STUDY OF MICROSCOPIC TRACE FOSSILS IN THE ORGANIC RICH HORN RIVER GROUP DEVONIAN MUDROCKS, NORTHWEST TERRITORIES

Although the fossilized tracks and trails (i.e. trace fossils) of macrofauna are well understood, the burrowing habits and resultant trace fossils of meiofauna (<0.5 mm) have, for the most part, been overlooked. This lack of discussion focused on meiofaunal contributions to paleo-sediments stems from the difficulty associated with identifying both body fossils and trace fossils; due to their microscopic size and low preservation potential. This deficit is amplified for organic rich mudstones, as their fine grain size, dark colour and significant post depositional compaction makes the identification of any sub-millimeter features difficult.

The Hare Indian and Canol Formations, comprising part of the Horn River Group (HRG) in the Northwest Territories, primarily consists of organic rich mudstones deposited during the Middle to Late Devonian. The mudstones are considered to represent marine basin fill accumulated in an oxygen starved (anoxic to euxinic) distal shelf setting, which is evidenced by their organic-rich character, pyrite content, and lack of macro-scale bioturbation. Detailed petrographic sedimentological and ichnological analyses were carried out on 248 thin sections taken from several cored HRG intervals, to assess for evidence of microscopic trace fossils. This analysis revealed several morphologically distinct microscopic biogenic-sedimentary structures, which were grouped into a classification scheme based on characteristics such as orientation, burrow-fill and -infill organization, and the presence or absence of burrow linings. Identification of such microburrows indicates that the HRG sediments were at least episodically partially oxygenated during deposition.

By matching trace fossil morphologies with the prospective burrowing strategies used to create them, a suite of potential paleo-bioturbators may be defined. Based off the known oxygen tolerances of various meiofaunal species one may thus constrain a minimum depositional dissolved oxygen (DO2) concentration. This may lead to an ability to reconstruct larger scale oxygenation trends during the deposition of organic rich mudstones. Applying the proposed criteria for the identification and evaluation of microbioturbation to other “anoxic” organic rich mudstones may spark re-evaluations of DO2 interpretations.