The development of an Early Ordovician hard ground community in response to rapid sea‐floor calcite precipitation

The Kanosh Shale (Upper Arenig) of west-central Utah, contains abundant carbonate hardgrounds and one of the earliest diverse hardground communities. The hardgrounds were formed through a combination of processes including the development of early diagenetic nodules in clay sediments which were exhumed and concentrated as lags by storms. These cobble deposits, together with plentiful biogenic material, were cemented by inorganically precipitated calcite on the sea floor, forming intraformational conglomerate hardgrounds. Echinoderms may have played a critical role in the development of hardground faunas since their disarticulated calcite ossicles were rapidly cemented by syntaxial overgrowths, forming additional cobbles and hardgrounds. A significant portion of the hardground cements may have been derived from the early dissolution of aragonitic mollusk shells. The Kanosh community marks the transition from the Cambrian Fauna to the Paleozoic Fauna in the hardground ecosystem.