Abstract

THE EFFECT OF PURE AND SIMPLE SHEAR DEFORMATION ON CLUSTERED, RANDOM, AND UNIFORM TRACE FOSSIL DISTRIBUTIONS: UNCERTAINTY IN THE NEAREST NEIGHBOR TECHNIQUE RESULTS

SIMPSON, E. L.; SUMMERS, S. A.; JAROSYNSKI, W. M., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530.

The two-dimensional distribution of trace fossils can yield important paleoecological information with regards to feeding habit, such as competitive exclusion. The Clark and Evans nearest neighbor technique (1954), which uses the mean distance from each point to the nearest neighbor point, has been applied often to Cambrian and Silurian Skolithos trace fossil distributions. In this technique, the R-value is the ration of observed mean tube spacing to the mean tube spacing for a random distribution. Measured R-values of less than, equal to, or greater than one indicate a clustered, random, or uniform distribution respectively. This study examines the effect of increasing increments of pure and simple shear on known grids and calculates the resultant R-values.

Grids of clustered, random, and uniform point distributions were entered into Earth'nWare Inc.'s CarDec 2.0 software and subject to pure and simple shear. output was digitized and R-values were calculated. Within the program, grids were deformed at 5 degree increments of angular shear with a maximum angle of 55 degrees. 55 degrees corresponds to a shear strain of 1.43; shear values of less than 10 are considered to be low strain. In the clustered, random and uniform distributions subjected to increasing angular shear, R- values progressively changed from 0.70 to 0.44 (clustered to clustered), 1.05 to 0.70 (random to clustered), and 2.02 to 0.85 (uniform to clustered), respectively. Grids were deformed by pure shear with increasing strain ratios (Rs) of approximately 0.1 increments for values of less than one and 0.5 for values of greater than 1; 1 is the undeformed state. Rs values varied from 0.5 to 6.5. With increasing and decreasing strain ratios, R-values for clustered, random, and uniform did not vary greatly.

As a result of these experiments, R-values gleaned from slightly deformed strata must be viewed with caution. Reported trace fossil distributions can be the result of paleoecological parameters. If grids are subjected to very low angular shear then the original R-value may be slightly to significantly modified.

Kutztown University of Pennsylvania

A member of the State System of Higher Education

D.J. Moore
Oct. 2002
Last modified Sept. 2004
moore@kutztown.edu