--> ABSTRACT: Bioturbation Patterns and Near-Anoxic Events in Frobisher-Alida Beds of Western North Dakota, by Jeffrey R. Valvik; #91033 (2010)

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Bioturbation Patterns and Near-Anoxic Events in Frobisher-Alida Beds of Western North Dakota

Jeffrey R. Valvik

A study of the upper part of the informal Frobisher-Alida interval, which includes portions of the Mississippian Charles and Mission Canyon Formations, was conducted over an eight township area of western North Dakota. Fourteen cores totaling 1,157 ft in length were slabbed and polished in order to (1) interpret the cored rocks for environments of deposition, and (2) examine the bioturbated sequence and determine its significance.

The sublittoral rocks within the study area could be grouped into beds of two main lithotypes: (1) bioclastic wackestone-packstones and (2) bioturbated mudstone-wackestones. The bioclastic wackestone-packstone lithotype contains a stenohaline fauna such as echinoderms, corals, bryozoans, trilobites, and mollusks. The bioturbated mudstone-wackestone lithotype contains a variable ichnofauna as well as a euryhaline fauna including ostracods and green algae. Thin-shelled brachiopods indicative of stressed environmental conditions are also present.

The ichnofossils examined within the mudstone-wackestone lithotype can be used to characterize relative changes in oxygenation levels at or near the sediment-water interface. As levels decrease from well oxygenated to near anoxia, the ichnofaunas' diversity decreases, culminating in the deposition of a thinly laminated, highly organic layer. The ichnogenus Chondrites is the first to reappear in the sequence of ichnofauna as currents replenish oxygen levels.

These brief periods of anoxia were localized and confined within the shallow depressions of a hummocky sediment surface.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988