FAUNAL ZONATION IN A MIDDLE PALEOZOIC CARBONATE BUILDUP: IMPLICATIONS FOR A DEPOSITIONAL MODEL

Carbonate buildups, common in Mid-Paleozoic strata worldwide, supported a moderately diverse assemblage of benthic suspension and deposit feeders. The communities present in most buildups were significantly different from those present in the equivalent level bottom strata. This difference in community makeup between the two adjacent facies has been invoked to assert that the buildups were indeed biogenically produced, and offered a different environment from the surrounding seafloor. However, few studies have addressed the internal biotic structure of carbonate buildups, the analysis of which could provide a much needed paleontological perspective on the still uncertain origin of buildups worldwide.

This study examined all of the macrofauna in and around a well-exposed buildup in the Fort Payne Formation (Lower Mississippian) of Kentucky. Through the use of multivariate statistical techniques, distinct faunal zonations were identified not only between the buildup and surrounding strata, but also within the mound itself. Two main faunal zones are present in the buildup, and are characterized by two dominant taxonomic groups, pelmatozoan echinoderms and fenestrate bryozoans. These organisms seem to have been mutually exclusive during mound growth, likely relating to their different substrate preferences. Thus, the distribution of the faunal zones throughout the buildup is decidedly non-random, with echinoderms preferring soft substrates and bryozoans associated with a firmer substrate.

This finding contributes to the understanding of early Carboniferous carbonate buildups by providing paleobiological evidence supporting a buildup-origin hypothesis which postulates a progression of surface sediment consistency from soupy, to gel-like, to lithified. This progression, mediated by the presence of a surficial microbial mat, is thought to have occurred continuously, but in small patches, so that at any given time all sediment consistencies were present at the buildup surface. The faunal zonation recognized in this study is an expression of this phenomenon, in that the two distinct faunal zones recognized represent end-members of sediment consistency on the buildup surface.