New Discoveries of Diplocraterion and Tidal Rhythmites in the Upper Devonian Rocks of Grab-all Bay, Cork Harbour: Palaeoenvironmental Implications

Abstract

Geological mapping of the Upper Devonian succession on the northern side of Grab-all Bay in Cork Harbour has led to a new interpretation of the local lithostratigraphy. Rocks previously mapped as the Ballyknock Member of the Gyleen Formation are now reassigned to the Toe Head Sandstone Formation which is here divided into two new members, the Rams Head and Grab-all Bay Members and to the Old Head Sandstone Formation. The axis of the Grab-all Bay Syncline is re-positioned 160m further to the north in Grab-all Bay, revealing a younger and previously undescribed interval of strata here assigned to the Old Head Sandstone Formation. Miospore assemblages recorded from the Toe Head Sandstone and the lower part of the Old Head Sandstone Formation are assigned to the Retispora lepidophyta—Knoxisporites literatus (LL) Miospore Biozone, indicating a late Devonian (Uppermost Famennian) age for the succession. New discoveries of U-shaped burrows, assignable to the trace fossil Diplocraterion, are recorded from several intervals in the Toe Head Sandstone Formation and the lower part of the Old Head Sandstone Formation. These monospecific ichnofaunas are believed to have colonised finely laminated sediments of the intertidal flat environment. In addition, rhythmically laminated sandstone and siltstone beds interpreted as tidal rhythmites are recorded for the first time in the Toe Head Sandstone Formation. The paleoenvironmental significance of the tidal rhythmites and Diplocraterion beds in the Toe Head Sandstone is important in understanding the nature of the fluvial–tidal transition zone in the Cork Harbour region in late Devonian times. It is proposed that in late Devonian (LL Biozone) times the upper Toe Head Sandstone Formation sediments in the Cork Harbour area were deposited in the upper reaches of an estuarine embayment within a low gradient fluvial coastal plain.