GENESIS AND PRESERVATION OF MOUNDED HARDGROUNDS IN THE LATE ORDOVICIAN (KATIAN) OF KENTUCKY AND ONTARIO: THE PERSISTENCE AND MIGRATION OF THE "KIRKFIELD FAUNA"

Two complex, mounded hardgrounds that share remarkably similar encrusting communities occur in transgressive systems tracts of adjacent 4th order Upper Ordovician sequences (Katian M5A & M5B) from central Kentucky and southern Ontario, respectively. These assemblages record the persistence and migration of the “Kirkfield fauna." During the interval separating these two occurrences, several important events took place, including a global sea level highstand, a major positive isotopic carbon excursion (GICE-Logana), and a turnover in global ocean microflora. Yet, these assemblages display remarkable similarities. Here, we examine how this type of community evolves over time scales of a few hundred thousand years and demonstrate the processes of formation of the mounded topography characteristic of many Ordovician hardgrounds.

Mid-Late Ordovician carbonates record the first major diversification of hard-substrate communities. These hummocky hardground occurrences display a unique topographic heterogeneity with a variety of microhabitats and accommodated a large diversity of nearly one hundred species. The encrusting assemblages, dominated by bryozoans, sponges, and eight classes of echinoderms, provide "snapshots" of hard-substrate sclerobiont communities at the culmination of the Great Ordovician Biodiversification Event when echinoderms reached maximum disparities, occupying nearly every benthic niche.

The hardgrounds are analyzed using microstratigraphic analysis and structure-from-motion photogrammetry to understand the similarities between depositional/erosional and diagenetic processes. This allows us to examine the formation, development, colonization, and preservation of these unique mounded hardgrounds and study ecological processes in ancient communities on a fine scale. The benthic paleocommunities are examined at the species level to determine the extent to which the fauna, preserved in situ, evolved over a hundred thousand-year timescale. Although the echinoderm faunas display strong stasis at the generic level, there are Important community changes from the older (Kentucky) to the younger (Ontario) sample, including the disappearance of hexactinellids, proliferation of blastozoans, and a general increase in biodiversity.