A comparison of the strain of crinoid columnals with that of their enclosing silty and shaly matrix on the Appalachian Plateau, New York
A pole-figure goniometer was used to measure the preferred orientation of the basal planes of chlorite in 87 samples from the Upper Devonian clastic rocks of the Appalachian Plateau, New York. Interpretation of the preferred orientation according to the theory of March yields an estimate of how much an element of the sediment has changed shape since deposition; this deformation can be interpreted according to three distinct models for the strain history. All begin with compaction, followed by one of three types of tectonic strain: (1) a plane strain, that conserves bedding-plane area by compensating for horizontal shortening with horizontal elongation at right angles; (2) one that uniaxially shortens a horizontal line normal to the fold axes without compensating elongation: and (3) a plane strain again, which conserves the area in the vertical plane containing a shortened horizontal direction by stretching it vertically. For lack of suitable names, we will simply call the three total strains type (b), (u) and (v) from the initials of the distinguishing features of their tectonic increment. The horizontal strain components estimated from preferred orientation are compared at a number of localities with those implicit in the principal horizontal diameters of the elliptical outlines of deformed crinoid columnals found on bedding planes. Expressed for constant bedding-plane area, or type (b) total strain, the strain in the direction of greatest elongation measured by preferred orientation was found to range from no recorded strain to 0.19, with a median of 0.04. As a rule, however, the more intensive strains were recorded in shales with a fine-grained phyllosilicate content exceeding 85% by volume, in which the mean was 0.07. Deformed crinoid columnals in the same set of samples have elongations parallel to their long axes that range from 0.04 to 0.10, with a median and average of 0.07. The discrepancy between the two sets of strain measures, at least in the coarser and more permeable rock types, may either be real and caused by easier flushing of dissolved ions or may be caused by imperfect strain recording in coarse matrix materials.
Journal of Structural Geology
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