A carbonate hardground in the carmel formation (middle jurassic, sw utah, usa) and its associated encrusters, borers and nestlers

Abstract

A rare example of a North American Jurassic hardground is found in the Carmel Formation of southwestern Utah. The Carmel hard‐ground was formed across a carbonate lagoon from an oolitic shoal seaward to a subtidal shelly facies landward. It has an abundant bivalve fauna consisting of thick layers of encrusters (the oyster Liostrea and the plicatulid Plicatula), borers (the ichnofossil Gastro‐chaenolites with the mytilid Lithophaga often preserved inside), and nestlers (the mytilid Modiolus). A rare soft‐bodied bryozoan (Arach‐nidium) is preserved by bioimmuration in the attachment scars of Liostrea; this is the first bioimmuration recorded from the Jurassic of North America, and the first bioimmuration recorded from a hard‐ground. The phoronid boring Talpina is present in some Liostrea shells; it was apparently excavated after the death of these oysters. The Carmel hardground community does not contain other fossils, such as serpulids, brachiopods, foraminiferans, and skeletal bryo‐zoans, typical of Jurassic hardgrounds elsewhere. It represents a low diversity molluscan community developed in a restricted marine environment.

Key words:

• Jurassic
• Carmel Formation
• Utah
• hardgrounds
• bivalves
• bivalve borings
• bioimmuration