Strata-bound vein array in the basal Pierre Shale, Lake Francis Case, South Dakota, U.S.A.

Harmon Maher, Sarah Ferguson, Ryan Korth, Justine Marshall, Laura Pickett

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

A distinctive strata-bound vein array occurs in the basal Pierre Shale exposed along the shores of Lake Francis Case, a reservoir on the Missouri River in south-central South Dakota. Typically 2-4 meters in thickness, the array consistently outcrops over a >50-km distance, a significant areal footprint. Ash layers define the upper and lower bounds of the vein array. Two, suborthogonal, preferred directions of vertical veins (northeast and southeast strike) define a regional pattern. By volume, vertical veins comprise 1-2% of the rock. Thinner, more discontinuous, and irregular horizontal veins also occur. Comparisons between array orientations and the joint/vein pattern in the immediately underlying marls of the top of the Niobrara Chalk identify distinct differences. Traverse data suggest that the vein arrays are characterized by uniform horizontal extension. Vertical veins in the array are typically 1-2 centimeters thick and contain massive jarosite, selenite, and fibrous gypsum. The abundance of jarosite and fibrous gypsum distinctly correlates with position in the weathering profile, and these phases are interpreted as due to replacement of original selenite during modern weathering. However, for initial vein array formation, the following suggests that they are not related to modern weathering and formed at depth: (1) a lack of correlation of vein width/frequency with position in the weathering profile; (2) the regional extent; (3) the consistent preferred orientations; (4) the uniform horizontal extension; and (5) the coarsegrained character of the selenite. The consistent strike pattern suggests influence of a regional stress field. The mechanism/timing of vein array formation is unclear. Formation due to diagenetic processes, which are especially significant in mud rocks, would explain the strata-bound character and isotropic horizontal strain and is considered most likely. Formation during glacial loading is one intriguing possibility. Localization of the vein array may be due to the organic-rich character of the host Burning Brule Member of the Sharon Springs Formation.

Original languageEnglish (US)
Pages (from-to)153-165
Number of pages13
JournalRocky Mountain Geology
Volume50
Issue number2
DOIs
StatePublished - Oct 1 2015

Fingerprint

shale
lake
selenite
weathering profile
jarosite
gypsum
weathering
regional pattern
preferred orientation
chalk
footprint
rock
stress field
outcrop
mud
ash
replacement
river

Keywords

  • Gypsum
  • Jarosite
  • Joints
  • Lake Francis Case
  • Niobrara Chalk
  • Pierre Shale
  • Stresses
  • Structural diagenesis
  • Veins

ASJC Scopus subject areas

  • Geology
  • Stratigraphy
  • Palaeontology

Cite this

Strata-bound vein array in the basal Pierre Shale, Lake Francis Case, South Dakota, U.S.A. / Maher, Harmon; Ferguson, Sarah; Korth, Ryan; Marshall, Justine; Pickett, Laura.

In: Rocky Mountain Geology, Vol. 50, No. 2, 01.10.2015, p. 153-165.

Research output: Contribution to journalArticle

Maher, Harmon ; Ferguson, Sarah ; Korth, Ryan ; Marshall, Justine ; Pickett, Laura. / Strata-bound vein array in the basal Pierre Shale, Lake Francis Case, South Dakota, U.S.A. In: Rocky Mountain Geology. 2015 ; Vol. 50, No. 2. pp. 153-165.
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AB - A distinctive strata-bound vein array occurs in the basal Pierre Shale exposed along the shores of Lake Francis Case, a reservoir on the Missouri River in south-central South Dakota. Typically 2-4 meters in thickness, the array consistently outcrops over a >50-km distance, a significant areal footprint. Ash layers define the upper and lower bounds of the vein array. Two, suborthogonal, preferred directions of vertical veins (northeast and southeast strike) define a regional pattern. By volume, vertical veins comprise 1-2% of the rock. Thinner, more discontinuous, and irregular horizontal veins also occur. Comparisons between array orientations and the joint/vein pattern in the immediately underlying marls of the top of the Niobrara Chalk identify distinct differences. Traverse data suggest that the vein arrays are characterized by uniform horizontal extension. Vertical veins in the array are typically 1-2 centimeters thick and contain massive jarosite, selenite, and fibrous gypsum. The abundance of jarosite and fibrous gypsum distinctly correlates with position in the weathering profile, and these phases are interpreted as due to replacement of original selenite during modern weathering. However, for initial vein array formation, the following suggests that they are not related to modern weathering and formed at depth: (1) a lack of correlation of vein width/frequency with position in the weathering profile; (2) the regional extent; (3) the consistent preferred orientations; (4) the uniform horizontal extension; and (5) the coarsegrained character of the selenite. The consistent strike pattern suggests influence of a regional stress field. The mechanism/timing of vein array formation is unclear. Formation due to diagenetic processes, which are especially significant in mud rocks, would explain the strata-bound character and isotropic horizontal strain and is considered most likely. Formation during glacial loading is one intriguing possibility. Localization of the vein array may be due to the organic-rich character of the host Burning Brule Member of the Sharon Springs Formation.

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