JOINT STUDIES IN THE HILLSBORO 7 1/2' QUADRANGLE, NEW HAMPSHIRE: A SAMPLING PROBLEM

In the course of mapping bedrock geology in the Hillsboro 7 ½' quadrangle, NH, an effort was made to characterize the rocks' water-bearing properties. Characteristics of selected joints were recorded as follows: orientation, spacing, mineralization and geometry (through-going, abutting, crossing, blind, or en echelon). Length and aperture were not noted. In addition, orientation and spacing of joints at five large exposures (two pavements, three road cuts) were sampled using inventory and tag-line methods. A rose diagram for all 639 measured joints shows no strongly preferred orientations, but the 376 joints measured in smaller outcrops indicate a NNW set and more scattered NE set. The scatter becomes less pronounced when the data are plotted by geographic domain, and other preferred orientations appear in some areas: an E-W set near a brittle fault in West Deering and a N-S set near Bennington village.

Data were also plotted by dip angle, geometry, rock unit and rock type. Many steep, through-going joints lie parallel to a regional NNW ductile fabric. Most joints dipping less than 60˚ strike NNE. Rocks in the quadrangle are Silurian and Devonian metamorphic rocks intruded by Kinsman Granite and granodiorite of the Antrim pluton. Plutonic units are well jointed, with wide spacing; granofels and quartzite are more densely jointed; schist outcrops are least well jointed. Two tag-lines at different orientations on a Kinsman Granite pavement gave very different diagrams, but the combined diagrams are similar to the joint pattern at a Kinsman road cut five miles away. Diagrams from the tag-line and inventory methods at one Antrim Granodiorite road cut differ significantly from each other.

Discrepancies between the various rose diagrams provoke questions about the best approach to characterizing joint sets in a quadrangle. The selective method at numerous outcrops in a quadrangle may provide a better idea of the total range of joint orientations, whereas individual outcrop studies are needed to analyze the pattern at a specific site. Studying joint sets by several methods can be a very useful teaching tool in structural geology or hydrogeology courses because students see clearly that how one samples the data can affect the results, even before one attempts kinematic analysis or application to groundwater movement.