QUADRANT VARIANCE ANALYSIS OF LOWER CAMBRIAN ICHNOFOSSIL DISTRIBUTION ACROSS A SEQUENCE BOUNDARY, CENTRAL VIRGINIA

SIMPSON, E.L., Dept. of Phy. Sci., Kutztown Uni., Kutztown, PA 19530; BENNINGTON, J.B., Dept. of Geology, Hofstra Uni., Hempstead, NY 11550; HARRISON, M.J.; GLEDHILL, H.M.; NOTO, R.C.; DILLIARD, K.A., Dept. of Phy. Sci., Kutztown Uni. Kutztown, PA 19530.

Can variation in trace fossil spatial pattern be used to identify sequence stratigraphic boundaries when primary sedimentary structures are missing? The cylindrical ichnofossil Skolithos is ubiquitous in Lower Cambrian strandline deposits, often existing in high densities that obliterate primary structures and make it difficult to identify sequence boundaries. Near Natural Bridge, VA, tidal-flat deposits were identified based on primary sedimentary structures, permitting delineations of a sequence boundary between Skolithos -bearing shoreface deposits and higher tidal-flat deposits containing both Skolithos and Monocraterion (similar to Skolithos but with a funnel-shaped to). Tidal-flat strata are overlain by additional Skolithos -bearing shoreface deposits. Bedding plane exposures permit analysis of the trace fossil spatial pattern (random, uniform, or clumped) across the sequence boundary using quadrant variance methods. Two methods were applied to test for differences in trace fossil pattern across the sequence boundary - the Two Term Local Quadrant Variance method (TTLQV) developed by Hill (1973) and the Paired Quadrant Variance method (PQV) developed by Ludwig and Goodall (1978). For a uniform distribution variance remains low and constant as quadrant size or distance increases along a transect. Random distributions show random variances. Clumped distributions display a marked peak in variance corresponding to the size or distance between clumps. The TTLQV and PQV methods give no strong indication of pattern in Skolithos and Monocraterion tube distribution along any transect above and below the sequence boundary. Plots of block size/distance versus variance for all transects are consistent with a random distribution across the transect. Average variances range from 1.8 to 3.7 for the TTLQV method and from 1.4 to 3.0 for the PQV method. The two transects on the sequence boundary are similar to shoreface deposits above the sequence boundary, but with a slightly higher than average variance. Mean density of Skolithos is highest in shoreface deposits (27700/m2 below the sequence boundary and 4270/m2 above). Skolithos densities within the lowest tidal-flat deposits range from 690 to 2750/m2 (mean density = 1460/ m2). Monocraterion in the tidal-flat deposits immediately below shoreface deposits vary from 320 to 1240/m2 (mean density = 705/ m2).

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