NEAREST NEIGHBOR ANALYSIS OF LOWER CAMBRIAN SKOLITHOS AND MONOCRATERION ICHNOFOSSILS: IMPLICATIONS FOR PRESERVATION AND SEQUENCE BOUNDARY IDENTIFICATION

HARRISON, M. J., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530; VENN, C., Department of Geosciences, Indiana University of Pennsylvania, Indiana, PA 15705; SKOOG, S.Y., Department of Biology, Indiana University of Pennsylvania, Indiana, PA 15705; GLEDHILL, H. M.; NOTO, R. C.; and SIMPSON, E. L., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530.

 

The ichnofossil Skolithos is ubiquitous in Lower Cambrian strandline deposits. Monocraterion, also cylindrical in shape but with a funnel-shaped top, is less abundant and commonly associated with Skolithos. Paleoenvironmental interpretation of deposits is often hindered by destruction of primary sedimentary structures due to intense bioturbation. Near Natural Bridge, VA, tidal-flat deposits can be identified based on primary sedimentary structures, permitting delineation of a sequence boundary between Skolithos-bearing shoreface deposits and higher tidal-flat deposits containing both Skolithos and Monocraterion. Tidal-flat strata are overlain by Skolithos-bearing shoreface deposits. Bedding plane exposures across the sequence boundary permit analysis of the trace fossil distribution using Clark and Evan's (1954) nearest neighbor technique. In this technique, R-values (observed mean tube spacing divided by the mean tube spacing predicted for a random distribution) less than, equal to, and greater than 1 suggest clustered, random, and uniform distribution, respectively.

Mean density of Skolithos is highest in the shoreface deposits (2770/m2 below the sequence boundary and 4270/m2 above). Mean R-value of 1.04 and 1.14, respectively, for the shoreface deposits below and above the sequence boundary. R-values for the shoreface deposits above the boundary range from 1.09 to 1.24. Skolithos densities within the lowest tidal-flat deposits range from 690 to 2750/ m2 (mean density = 1460/ m2); R-values vary from 0.99 to 1.34. Monocraterion densities in the tidal-flat deposits immediately below the shoreface deposits vary from 320 to 1240/ m2 (mean = 705/ m2); associated R-values range from 1.03 to 1.35. In those deposits, R-values calculated for Monocraterion to the nearest Skolithos vary from 0.85 to 1.56.

R-values within shoreface deposits show less variability than those for tidal-flat deposits. In strata without primary sedimentary structures identification of beds with highly variable R-values contrasted with less variable R-values may indicate the presence of a sequence boundary. The even greater variability in R-values calculated for Monocraterion to nearest Skolithos record the combination of two possible sequences of trace formation. Clustered to random distribution (R< or = 1) results from Monocraterion then subsequent Skolithos formation, and random to uniform distribution (R > or = 1) results from Skolithos then subsequent Monocraterion formation.

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D.J. Moore
Oct. 2002
Last modified Sept. 2004
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