INVESTIGATING THE EFFECT OF MECHANICAL DISCONTINUITIES ON JOINT SPACING

In rocks without systematic mechanical discontinuities (e.g., granite) , joint spacing follows an approximately log-normal frequency distribu tion (i.e., the distribution has a kurtosis near zero). Joint spacing in rocks with systematic mechanical boundaries differs from the spacin g in isotropic rocks, exhibiting consistently positive values of kurto sis (i.e., the distribution is more clustered around the mode than a p erfectly log-normal distribution). We attribute this difference in joi nt-spacing distribution to mechanical boundaries such as bed partings in sedimentary rocks that constrain joint height and control joint spa cing. Existing systematic joints can also act as mechanical boundaries during the development of later 'cross' joints. Two parallel mechanic al boundaries determine the mechanical-layer thickness that influences joint spacing. In this paper, we investigate the effect of sampling g eometry and mechanical discontinuities on joint-spacing statistics. In many situations, neither pavement surfaces nor properly oriented bore holes are available for measuring the spacing of cross joints that dev elop between existing systematic joints. When joint-spacing data come from scanlines that are oblique to a systematic joint set (e.g., cross ing many systematic joints on a sub-vertical outcrop face), we conside r whether the median spacing of the systematic joint set is statistica lly equivalent to the mechanical-layer thickness thought to control cr oss-joint spacing between individual pairs of systematic joints. The b asis of our analysis is the fracture spacing index (FSI), which is the slope of a line fitted to a plot of mechanical-layer thicknesses vs, median joint spacing. We collected joint-spacing data along oblique sc anlines from a large outcrop of the Devonian Brallier Formation, a dis tal turbidite sequence, near Huntingdon, PA. Our analysis indicates th at the spacing of cross joints correlates better with a mechanical-lay er thickness defined by the median systematic ('strike') joint spacing (r(2) = 0.78, FSI = 1.02) than with a mechanical-layer thickness defi ned by the stratigraphic bed thickness (r(2) = 0.69, FSI = 0.97). This is consistent with the conclusions of previous workers. We also note that the spacing data from the cross joints exhibit a higher (i.e., mo re positive) value of kurtosis than the data from the earlier strike j oints (1.66 vs. 0.98). This is consistent with the idea that mechanica l discontinuities alter joint spacing in a systematic manner. In this case, the cross joints may have been influenced by two sets of mechani cal boundaries (bedding and existing joints), whereas the earlier stri ke joints were constrained by only one set (bedding). (C) 1998 Elsevie r Science B.V. All rights reserved.