Investigating the carbonate-shale facies transition and deposition on the Scandinavian Ordovician Shelf – the Arnestad Formation, southern Norway
Date
2018
Authors
Schuller, Kathryn M., author
Egenhoff, Sven O., advisor
Harazim, Dario, committee member
Schutt, Derek, committee member
von Fischer, Joe, committee member
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Journal ISSN
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Abstract
The Upper Ordovician Arnestad Formation was located on the northwestern edge of Baltica in the region near modern day Oslo, Norway. This formation was found to be 60 m thick and consists of intercalated siliciclastic mudstones and thin, nodular carbonate beds. Six outcrops and 38 thin sections were used to describe the Arnestad Formation in detail and divide the formation into six key facies. Dark grey silicilastic mudstones dominate the succession and contain lenses of fossil fragments. Interbedded nodular carbonates are mud- to wackestones, contain lenses of fossil fragments, and often form continuous beds. All facies show varying amounts of bioturbation from Chondrites, and Phycosiphon-like fecal strings can be found in the siliciclastic mudstone facies. The Arnestad Formation can be divided stratigraphically into a lower portion with siliciclastic mudstones and continuous carbonate beds, a central part dominated by siliciclastic mudstones with local ash and carbonate beds, and an upper portion containing both thick siliciclastic mudstone beds and intercalated stacks of siliciclastic and carbonate mudstones. The Arnestad Formation is interpreted as representing sedimentation on a ramp-like shelf with carbonate facies deposited proximally to siliciclastic mudstone facies, below normal wave base with sediment and bioclasts being transported basinward due to bed load processes. Fair-weather and storm generated deposits are found in all facies, with storms eroding the sediment and producing shell lenses throughout the formation, but increasing in frequency upwards independent of sea-level. In the lower part of the succession, the Arnestad Formation records a relatively low sea-level stand, shifting to an overall high sea-level position during the middle part, and back to another low sea-level position in the upper portion. Ash beds are found almost exclusively in the middle portion and seem to have higher preservation potential during high versus low sea-level positions. There were likely more ash beds deposited than were found that were later biogenitically homogenized, rendering the beds mostly unrecognizable from the surrounding sediment. The frequent intercalation of the siliciclastic mudstones with carbonate beds most likely shows the influence of climate cycles on deposition. Based on dividing the length of deposition (~3.5 my) by the estimated cycle count (215 – 220), these small-scale cycles were found to have periods between 15,900 and 16,300 years and are interpreted as precessional Milankovitch cycles.