The transition from planar to en echelon morphology in a single vein in shale; insights from X-ray computed tomography scanning

En echelon fractures and veins are among the most common and distinctive geological structures, yet their three-dimensional forms and relationships to surrounding structures are commonly unclear. X-ray computed tomography (CT) offers an unrivaled ability to examine structures within rocks in three d...

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Bibliographic Details
Published in:Geosphere (Boulder, Colo.) Colo.), 2020-04, Vol.16 (2), p.646-659
Main Authors: Andrews, Graham D. M, Brown, Sarah R, Moore, Johnathan, Crandall, Dustin, Mackey, Paige
Format: Article
Language:English
Subjects:
boreholes
clastic rocks
computed tomography data
cores
Devonian
en echelon faults
faults
fractures
imagery
Marcellus Shale
Middle Devonian
morphometry
Paleozoic
Pennsylvania
sedimentary rocks
shale
southwestern Pennsylvania
Structural geology
United States
veins
visualization
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Summary:En echelon fractures and veins are among the most common and distinctive geological structures, yet their three-dimensional forms and relationships to surrounding structures are commonly unclear. X-ray computed tomography (CT) offers an unrivaled ability to examine structures within rocks in three dimensions, and it is applied here to a sample of drill core from the Marcellus Shale of southwestern Pennsylvania (USA). CT images yield qualitative and quantitative data on the transition from a pyrite-rich planar vein to an en echelon veinlet array, and on the heterogeneity of veinlets within the array. Using a combination of three- and two-dimensional images, geometric data, and traditional petrography, we identify a range of veinlet shapes consistent with deformation during formation of an antitaxial graphite-calcite-pyrite vein system. Each of the veinlets is rooted in the underlying planar vein where it is narrowest. The transition from planar vein to en echelon array coincides with a change in bedding, suggesting that competency contrasts between adjacent beds controlled the fracture morphology. Veinlets initiated as short, lenticular fractures at ∼45° to the planar vein before lengthening, dilating, and rotating. None of the veinlets are strongly sigmoidal, nor is there measurable offset across the margins of the planar vein; therefore, finite non-shear strain was very limited, and fluid overpressure-induced fracturing during burial and diagenesis is probably the most likely process for fracturing and vein formation.
ISSN:1553-040X
1553-040X
DOI:10.1130/GES02191.1