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Millar, S.W.S., 1995

Clast fabric in periglacial mass-movement deposits

Bibliographic Reference

Millar, S.W.S., 1995, Clast fabric in periglacial mass-movement deposits: New Brunswick, New Jersey, Rutgers, The State University of New Jersey, Ph.D. dissertation, 158 p.


The orientation of stones in slope materials has been used frequently as a criterion for identifying mechanisms of downslope movement. Colluvium in parts of the eastern United States south of the glacial margin has been interpreted as having a periglacial origin, possibly from solifluction. This is based on a general statement that the downslope component of solifluction causes elongated clasts to become oriented in the direction of the slope. A detailed, systematic evaluation of this statement has not previously been attempted. The initial research presented here addresses the lack and incompatibility of data from fabric studies on periglacial slopes. Second, these data allowed for a more rigorous evaluation of probable relict periglacial deposits. Fabric data were collected in areas of active solifluction at Eagle Summit, Alaska, and Niwot Ridge in the Colorado Rockies. Further data were collected from deposits interpreted as resulting from solifluction in Fairbanks, Alaska, and from periglacial colluvium in western New York State. Eigenvalue analysis was used to assess the characteristics of the fabrics. The results from Eagle Summit indicated that, contrary to previously reported solifluction fabrics, a high degree of variability was evident across a meso-scale slope. Three factors were studied to illuminate the reasons for the disparity between this and previous studies. (1) Following theoretical predictions, more elongated clasts generated a stronger preferred orientation. (2) The orientation of the exposure from which the data were collected, whether from the horizontal floor of a soil pit or from a vertical face, led to a biasing of fabric. Clasts protruding out of a vertically cut face are easier to see and measure, thus result in stronger preferred orientations in the downslope direction. (3) A systematic sampling strategy was critical for highlighting variations across a slope. Selecting optimum sites on well-formed lobe features biases the data, leading to overly strong fabrics. Implications for assessing relict deposits are further complicated in light of results from Fairbanks and New York. Post-depositional modification can weaken fabric signatures created by former slope processes. In Fairbanks ice lensing has led to smeared fabrics. In New York, stone upfreezing has increased clast dip angles.

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