MICROSTRATIGRAPHY OF AN AMALAGAMATED STORM BED: SPATIO-TEMPORAL RESOLUTION IN THE FOSSIL RECORD

Studies of modern storm processes indicate that post-mortem transport of benthic marine organisms is often negligible, suggesting that subfossil death assemblages may commonly preserve spatial community structure with good fidelity, albeit time-averaged. To test the implication that the same fidelity is to be expected in ancient assemblages, the Upper Ordovician strata of the type Cincinnatian provide highly fossiliferous, laterally extensive outcrops that are ideally suited for assessing fine-scale spatial and temporal patterns in a storm-dominated setting.

In this investigation, we analyzed spatial and microstratigraphic patterns of fossil distribution preserved in an amalgamated, multi-event Cincinnatian horizon that is traceable for a lateral distance of some 60 km. A previous study of this horizon, contained in the lower Fairview formation, suggested that biologically meaningful spatial variation was preserved down to the 10 m scale, despite extensive storm reworking of the unit. However, in the earlier study, no effort was made to assess variation among the microhorizons that comprise the bed. By contrast, we conducted a microstratigraphic dissection of samples collected at 1- and 10-m lateral scales along a 150-m transect at the primary, “anchor” locality. Collections were also made of the same horizon at four additional localities arrayed throughout the study area.

Analysis of the 72 bulk samples (5,150 specimens) indicated that a subtle gradient is present at the anchor locality. Statistical analyses, including randomization tests, demonstrated that greater heterogeneity was preserved among the samples than can be expected by chance. Moreover, in several instances, microhorizons within a given bulk sample of the entire interval maintained a consistent biotic composition that was distinguishable from that of adjacent samples.

Thus, non-random spatial variability appears to be preserved in these ancient fossil assemblages, despite the myriad of storms and other post-mortem processes that may affect faunal assemblages between death and final preservation. Furthermore, the discovery of a consistent stratigraphic signal through several microhorizons suggests that biological patchiness on the Ordovician sea floor sometimes survived the effects of several storms.