MIDDLE ORDOVICIAN SPONGE-MICROBIAL MOUNDS FROM WESTERN UTAH: TRANSITIONAL PRECURSORS OF LATER METAZOAN-DOMINATED REEFS

During the Great Ordovician Biodiversification Event (G.O.B.E.), diversity increased dramatically within the major metazoan invertebrate phyla, and reef communities underwent a transition from microbial- to metazoan-dominated assemblages. Reefs that developed during the G.O.B.E. interval varied in complexity from mud mounds, which were bound together chiefly by microbial communities (e.g., Johns, 1997), to more complex reef structures, which contained both microbial textures and abundant metazoan constituents, including sponges, stalked echinoderms, bryozoans, and, later, stromatoporoids (e.g., Rigby, 1971; Church, 1974) . The Early Ordovician portion of this critical interval in reef history has been the focus of a number of recent studies (e.g., Adachi et al., 2009, 2011; Cao et al., 2009), but limited work has been devoted to Middle Ordovician sponge-microbial buildups found in western Utah (e.g., Rigby, 1965; Wyatt, 1979; Johns, 1994). As part of an ongoing investigation into connections between changes in benthic ecology and metazoan radiation during the Ordovician, field studies were conducted at well-exposed Middle Ordovician (Whiterockian) sections near Ibex, western Utah. Sponge-microbial mounds, varying in size from less than one meter to five meters in diameter, and large individual sponges, ranging from 15-31cm in diameter at the base, are common in the upper portions of the Middle Ordovician Juab and Wah Wah Limestones (Rigby, 1965; Johns, 1994). Mounds have not been observed in the overlying Kanosh Formation (Rigby, 1965; Ross et al., 1989), although rare individual sponges are present. Most Middle Ordovician sponge-algal mounds likely formed on stabilized substrates (Li et al., 1993) and in turn served as hard substrates to which other organisms, such as echinoderms, could attach (Johns, 1995, 1997). In the Wah Wah and Juab Limestones, contacts between mounds and the surrounding strata are commonly sharp, with little deformation of the underlying beds apparent. The primary components of these mounds, which vary in their proportions from one mound to another, include lithistid sponges, putative microbially-bound micrite, and massive skeletal-rich wackestones to grainstones. Sponge morphologies vary considerably and include conical, branching, bowl-shaped, and laminar forms.