Facies patterns and depositional environments of the Peltura scarabaeoides trilobite biozone sediments, Upper Cambrian Alum Shale Formation, southern Sweden
Date
2012
Authors
Newby, Warren, author
Egenhoff, Sven O., advisor
Hannah, Judith L., committee member
von Fischer, Joseph C., committee member
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Abstract
The Upper Cambrian-age Peltura Scarabaeoides Biozone represents 2.2m - 5.94m of the Alum Shale in Västergötland, southern Sweden. The Alum Shale succession has historically been characterized as an accumulation of dark, organic-rich "monotonous" mudstones deposited in an anoxic and tranquil setting. However, the results of this study indicate the contrary and will place the Alum Shale alongside many other high-TOC mudstones that document abundance of benthic life and advective sediment transport. Description of continuous thin sections at the microscopic-scale enabled the subdivision of the Peltura scarabaeoides Biozone into four carbonate and three siliciclastic facies. The entire biozone represents an overall shallowing trend that can be sub-divided into eight medium-scale shallowing-upward cycles consisting of intercalated carbonates and siliciclastic mudstones. These medium-scale cycles can further be sub-divided into numerous small-scale shallowing- and deepening-upward siliciclastic mudstone cycles. Time estimates suggest that long eccentricity explains the eight medium-scale cycles whereas the driver of the small-scale cycles remains unclear. Along a shelf transect, the proximal sections contain high energy shell debris and pack- to grainstones that grade into carbonate wackestone and mudstones; the distal sections are composed of dark organic-rich shales. All siliciclastic mudstone facies are characterized by abundant bioturbation, with more proximal types being horizontal and larger, and distal ones multidirectional and smaller. All facies also show signs of bed load transport in the form of irregular laminae or ripple structures. Several millimeter-thick slump units are present in the succession, indicating active tectonic movements in the area. The Peltura scarabaeoides Biozone therefore exemplarily shows that early in the Phanerozoic the deep shelf environment was already colonized by a variety of organisms burrowing millimeters into the soft substrate. Silt transported from the shoreline by currents reached even the most distal parts of this shelf. Therefore, the Cambrian deep shelves must have already presented an environment similar to later Paleozoic examples with probably dysoxic to oxic, not anoxic conditions controlling the accumulation of high-TOC sediments.
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Subject
alum shale
Peltura scarabaeoides