SCLEROBIONT PALEOECOLOGY OF THE CARMEL FORMATION (MIDDLE JURASSIC, BAJOCIAN) OF SOUTHWESTERN UTAH, USA

The Middle Jurassic was a critical interval for the diversification of sclerobiont (hard substrate dwelling) faunas. The Carmel Formation (Bajocian) was deposited in an unusual set of shallow carbonate environments at the distal southern end of the long Carmel-Twin Creek Seaway. Occupied hard substrates here include extensive hardgrounds (some undercut and hosting cryptic faunas), calcitic bivalve shells, and ostreoliths (mobile circumrotatory accumulations of oysters). The sclerobionts in the Co-op Creek Limestone Member of the Carmel Formation exposed in the southwestern corner of Utah represent a unique community that was a product of hard substrate availability and restricted marine conditions. The encrusters are dominated by the common oyster Liostrea strigilecula, followed by plicatulid bivalves and cyclostome bryozoans. Minor encrusters are sabellid and serpulid tubeworms, thecideide and disciniscid brachiopods, anomiid bivalves, and possible foraminiferans. The most common borings are Gastrochaenolites (some with mytilid bivalves inside) in the hardgrounds and the likely phoronid boring Talpina in some shells. There are cryptic/exposed differences with the faunas on both undercut hardgrounds and concave shell surfaces. Storms appear to have been a major influence on sclerobiont community development through frequent burial and scouring of the hard substrates. Most surfaces have only thin encrusting layers, with the bryozoans being predominantly juvenile colonies buried early in their development. An exception is the fauna represented by the circumrotatory ostreoliths which apparently survived most storm events because of their mobility, developing multi-generational layers of oysters, plicatulids, bryozoans and borings. The paleoenvironments of these sclerobionts were mostly shallow normal marine near fair weather wavebase, as shown by periodically abundant crinoids. Conditions were restricted and sometimes hypersaline as indicated by a lack of cephalopods and occasional layers of mudcracks and calcite-replaced gypsum nodules. The Carmel Formation sclerobionts thus represent a unique Middle Jurassic community most comparable to equivalents in the Middle East but considerably less diverse.