The nature and significance of rift-related, near-surface fissure fill networks in fractured carbonates below regional unconformities

Hardman, Kit; Holdsworth, Robert E; Dempsey, Edward; McCaffrey, Ken

Abstract

Hosted in carbonate and crystalline basement rocks below regional unconformities, fissure-fill networks are a widely recognized, but relatively little described near-surface phenomenon (< 1-2km). Faults and fractures in otherwise tight Devonian carbonate basement rocks of the Tor Bay region, Devon, are associated with the development of millimetre- to decametre-wide fissures containing red early Permian sedimentary material, vuggy calcite mineralisation and wall-rock collapse breccias. These features preserve evidence concerning the style and history of fault deformation and reactivation in near surface settings, and on fluid-related processes such as elutriation and/or mineralization. Field observations, palaeostress analysis, and fracture topology analyses show that the rift-related faults and fractures created a network of long-lived open cavities during the early Permian development of the Portland-Wight Basin. Once formed, they were subjected to episodic, likely seismically-induced fluid fluxing events and local karstification. The large, well-connected networks of naturally propped fractures were - and possibly still are - important fluid migration pathways within otherwise low permeability host rocks. These structures are likely equivalent to those observed in many other rift-related, near-surface tectonic settings, and suggest that the Tor Bay outcrops can be used as a global analogue for sub-unconformity open fissure systems hosted in low permeability basement rocks.

Citation

Hardman, K., Holdsworth, R. E., Dempsey, E., & McCaffrey, K. (2020). The nature and significance of rift-related, near-surface fissure fill networks in fractured carbonates below regional unconformities. Journal of the Geological Society, https://doi.org/10.1144/jgs2020-074

• Journal Article Type: Article
• Acceptance Date: Jun 15, 2020
• Online Publication Date: Jun 18, 2020
• Publication Date: Jul 22, 2020
• Deposit Date: Jun 24, 2020
• Publicly Available Date: Jun 25, 2020
• Journal: Journal of the Geological Society
• Print ISSN: 0016-7649
• Electronic ISSN: 2041-479X
• Publisher: Geological Society
• Peer Reviewed: Peer Reviewed
• DOI: https://doi.org/10.1144/jgs2020-074
• Keywords: Geology
• Public URL: https://hull-repository.worktribe.com/output/3525658
• Publisher URL: https://pubs.geoscienceworld.org/jgs/article-lookup?doi=10.1144/jgs2020-074