Depositional environment and facies architecture of the Lower to Middle Ordovician carbonate ramp succession, Öland, southern Sweden
Date
2014
Authors
Cigri, Ali Riza, author
Egenhoff, Sven, advisor
Ridley, John, committee member
Çavdar, Gamze, committee member
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Journal ISSN
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Abstract
The Lower to Middle Ordovician carbonates of Öland, southern Sweden, exhibit outcrops of up to 12 m thickness and rest conformably on Cambrian black shales of the Alum Formation. Based on lithological and sedimentological characteristics, nine carbonate facies were identified within the successions that are grouped into four facies associations (FAs). FA 1 is composed of glauconite- bearing mud- to wackestone facies. FA 2 consists of three glauconite- and one glauconite- and Fe-ooid bearing mud- to packestone carbonate facies. Deposits of FA 3 are carbonate mud- to wackestone facies. FA 4 is characterized by one Fe- ooid bearing and four other mud- to packstone carbonate facies. The studied carbonate succession is subdivided into three stratigraphic units referred to as Intervals 1 to 3. Interval 1 consists of the Köpingsklint and Bruddesta Formations located at the base of the succession. Interval 2 is composed of the Horns Udde formation in the middle level of the succession, and Interval 3 is characterized by the Gillberga Formation situated at the top of the succession. FA 1 rocks were deposited in an offshore proximal setting, whereas FA 2 records an offshore distal setting during "starved" times. FA 3 and FA 4 rocks were deposited during the "normal" times reflecting offshore proximal and distal settings, respectively. Heterozoan assemblages, allochthonous Fe- ooids, and abundance of carbonate mud within the facies suggest that the studied carbonate succession experienced temperate to sub-tropical sea-water conditions during deposition despite its paleo-latitudes equivalence to what would be temperate to cool-water environments of modern examples. Three 3rd order sequences having regressive systems tracts (RSTs) and transgressive systems tracts (TSTs) bounded by maximum regressive surfaces (MRSs) were determined within the studied succession based on the transgressive versus regressive sequence stratigraphic model. The MRSs are situated at the top of the RSTs reflecting maximum sea-level drops. Interval 1 is interpreted to represent both TST and RST characterized by glauconite-bearing facies, and have a sequence boundary (MRS) at the base. Interval 2 contains two sequence boundaries (MRSs) and is comprised of two TST and one RST characterized by carbonate facies. Interval 3 has a sequence boundary (MRS) at the top and is composed of one RST discriminated by glauconite-bearing facies. The Lower to Middle Ordovician carbonates and their subsurface equivalence have high potential in terms of conventional and unconventional petroleum exploration based on their lithologic character and positions in the Paleozoic petroleum system in the Baltic Sea of Scandinavia and adjacent areas.
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Zip file contains appendix 1.
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Subject
carbonate rocks
Southern Sweden
petroleum
Öland
Lower to Middle Ordovician