Paleoenvironmental reconstruction based on horn corals, with an example from the late Ordovician of North America

Rugosan horn corals are a common constituent in many Ordovician and younger Paleozoic marine units. Analyses of coral curvature, orientation, condition of the coral surface, associated borings and epizoans, and features in the surrounding matrix can provide information on the nature and intensity of environmental energy, mode and rate of sedimentation, and changes in sediment consistency. This permits paleoenvironmental reconstruction on a local and regional scale. A thin unit of dolomitic limestone and dolostone at the top of the Elgin Member in the Maquoketa Formation (Maysvillian-Richmondian) of southeastern Minnesota is unique among documented North American Late Ordovician strata bearing horn corals. A substantial proportion of individuals in these beds are preserved in situ and some of them are in life position. Deposition evidently occurred in relatively deep water above but near storm wave base, where the competence of currents generated during high-energy events was only moderate. These are considered to be the deepest-water solitary Rugosa known from Maysvillian-Richmondian epicontinental deposits of eastern and central North America. Within this region, horn corals in units that are lateral equivalents of the Maquoketa were transported and generally reworked repeatedly in shallower-water environments where the competence of storm-generated currents was high. The results of this study support the hypothesis that water depth was an important factor controlling the distribution of Late Ordovician solitary rugosan species.