Abstract

Abundant and diverse trace fossil suites are characteristic of fully marine, deeper subtidal, carbonate environments, yet relatively few studies have focused on the ichnology of carbonate depositional environments. Integration of ichnology, paleontology, and sedimentology can provide a powerful approach to paleoenvironmental reconstruction.

The Midale carbonates in the Radville area consist of two facies associations: 1) a muddy facies association and 2) a bioclastic facies association. Vertical distribution and relative thicknesses of these facies associations and diversity of associated ichnofossil assemblages indicate deposition in a relatively deep subtidal, quiescent outer-to-inner shelf marine environment which was episodically influenced by storm activity. Thick bioturbated dolomitic mudstones are dominant components of the muddy facies association. Interbedded thinner coarse bioclastic units represent episodic higher-energy storm events which interrupted fairweather mudstone deposition.

Trace fossils in this succession are typical of the Cruziana ichnofacies. Two assemblages were recognized: 1) a distal Cruziana ichnocoenose, and 2) a proximal Cruziana ichnocoenose. The distal Cruziana assemblage consists of Planolites, Chondrites, Rhizocorallium, Teichichnus, Thalassinoides, Asterosoma, Zoophycos, Helminthopsis, Subphyllochorda, Palaeophycus and locally, Siphonichnus. The proximal Cruziana assemblage includes predominant Planolites and Chondrites, few Rhizocorallium, Asterosoma, Teichichnus and Palaeophycus, as well as rare Anconichnus, Zoophycos and Phycodes. Also, rare escape traces (fugichnia) are associated with the bioclastic units.

The vertical distribution of bioclastic facies associations show overall coarsening-upward and thickening-upward trends. At least six major coarsening-up units are recognized on the basis of depositional textures and trace fossil assemblages. These subtidal units show an overall shallowing-upward trend which culminate in restricted intertidal deposits near the top of the Midale carbonates.