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Sedimentary dikes, mostly of the neptunian type, are common if not universal features of submarine carbonate buildups, particularly reefs, and also have been reported from submarine volcanic mounds and pelagic marine sediments. Most of the neptunian dikes occupy widened joints that exhibit clear evidence of tensional origin. Large neptunian dikes reported from Gotland, Sweden, from central Europe, and from the upper Wabash Valley region of the United States also exhibit multiple (polyphase) banding, suggesting repeated joint formation.
The largest dikes appear to occur only in the margins of reefs or other sediment mounds, suggesting an origin related to local sediment accumulation rather than to regional tectonic forces. Dike joints that occur in reef flank rocks of the upper Wabash Valley region exhibit both radial and concentric orientation with respect to reef centers, and most of the dikes are essentially vertical. The vertical dike crevices appear to be simple extension fractures, but the orientation of a few outward-dipping dikes and sills in some reefs is somewhat anomalous. The joint crevices which they occupy have the orientation of shear fractures, but probably represent extension fractures that were diverted from their initial vertical orientation by the inclined bedding of the reef flank rock.
The fact that most neptunian dikes are oriented vertically strongly suggests that principal compressive stress axes were oriented vertically throughout the development of reefs and other submarine mounds, which in turn suggests that the dike crevices probably were produced in response to gravitational load stresses developed within the mounds. The existence of both radial and concentric dike crevices in reefs suggests that the intermediate and least principal stress axes were oriented both radially and concentrically during reef development. This in turn suggests that the confining pressures of surrounding interreef sediment, and the internal cohesion within the reefs, were great enough to prevent reef expansion by concentric joint formation at some times, but not at other times. Reli f of stress by reef expansion along concentric vertical fractures probably increased the likelihood of later expansion along the radial fractures.
Dike crevice formation probably is closely related to differential compaction or other differential volumetric changes that occurred within the reef parts, or that occurred between the reefs and the interreef sediments, but crevice formation also may have been affected to some extent by sagging of the sea floor beneath the reefs and other sediment piles.
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