Pyritization history in the late Cambrian Alum Shale, Scania, Sweden: Evidence for ongoing diagenetic processes

Research output: Chapter in Book/Report/Conference proceedingBook chapter

Abstract

Detailed diagenetic studies of the late Cambrian Alum Shale in southern Sweden were undertaken across an interval that includes the peak Steptoean Positive Carbon Isotope Excursion (SPICE) event to evaluate the pyrite mineralization history in the formation. Samples were collected from the Andrarum-3 core (Scania, Sweden); here the Alum was deposited in the distal, siliciclastic mudstone-rich end of a shelf system. Abundant cryptobioturbation is observed in the Alum, which points to oxic–dysoxic conditions prevailing during deposition. Petrographic examination of polished thin sections (n = 65) reveals the presence of numerous texturally distinct types of pyrite, including matrix framboids, two different types of framboid concretions (those with rims of iron-dolomite and those lacking rims), disseminated euhedral pyrite crystals, concretions of euhedral pyrite crystals, overgrowths of pyrite on these different pyrite generations, anhedral pyrite intergrown with bedding parallel mineralized fractures (i.e., “beef”), and massive vertical/subvertical accumulations of pyrite.
Paragenetic relationships outline the relative timing of formation of the texturally distinct pyrite. Framboids and framboid concretions formed prior to precipitation of any euhedral pyrite crystals, and these pyrite generations precipitated prior to the pyrite overgrowths on them. As Alum Shale sediments are all distorted by these texturally different pyrite generations, they are likely to have formed early in the postdepositional history of the formation. In contrast, pyrite associated with “beef” is likely temporally related to the onset of hydrocarbon generation, which in this part of Sweden is thought to have been many tens of millions of years after deposition. Because vertical/subvertical massive pyrite features distort “beef,” they clearly postdate it. Of all these pyrite textures, only framboid concretions appear to be restricted to the SPICE interval.
The texturally distinct nature of the pyrite generations, along with evidence of their formation at different times in the postdepositional history of the Alum Shale, is the key outcome of this petrographic study. Because the petrographic data presented herein point to a postdeposition origin for all generations of pyrite, diagenetic processes—not those processes associated with deposition—were responsible for the complex pyritization history observed in the Alum, in the Andrarum-3 core.

Details

Authors
  • Neil S. Fishman
  • Sven O. Egenhoff
  • Heather A. Lowers
  • Adam R. Boehlke
  • Per Ahlberg
Organisations
External organisations
  • Colorado State University
  • US Geological Survey, Denver
  • PetroLogic Solutions, LLC
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Geology
Original languageEnglish
Title of host publicationMudstone diagenesis: Research perspectives for shale hydrocarbon reservoirs, seals, and source rocks
EditorsW. Camp, K. Milliken, K. Taylor, N. Fishman, P. Hackley, J. Macquaker
PublisherThe American Association of Petroleum Geologists
Chapter6
Pages83-102
VolumeMemoir 120
ISBN (Print)9780891814252
Publication statusPublished - 2020 Feb 3
Publication categoryResearch
Peer-reviewedYes

Publication series

Nameaapg memoir
PublisherThe American Association of Petroleum Geologists
Volume120