Lateral variability in shallow marine ichnofabrics: implications for the ichnofabric analysis method

Abstract

Ichnological analysis, commonly undertaken both in the field and in core studies, is generally carried out in vertical logged successions. This work documents the results of a detailed ichnofabric study of two highly bioturbated sandstone beds separated by a mudstone, within the thick, Lower Jurassic succession of the Yorkshire coast, UK. The two sandstone beds contain relicts of hummocky cross-stratification and rare bioclasts of bivalves and belemnites. Bioturbation of the beds is intense and the diversity of trace fossils is considerable. Trace fossils present include Teichichnus rectus , Teichichnus isp. A, Thalassinoides isp., Palaeophycus heberti , Diplocraterion parallelum , Rhizocorallium , Rosselia socialis , Schaubcylindrichnus isp., Chondrites isp., Planolites isp. and Phycosiphon incertum . Ichnofabrics were studied at 5 m intervals parallel to the modern cliff-line, which is parallel to the inferred Jurassic palaeo-coastline. The results demonstrate that there is considerable patchiness to ichnofabric even in a comparatively simple depositional setting such as a storm-influenced shelf. This observation is in line with studies of the modern sea floor. The implications of patchiness in ichnofabric are broad-ranging, but most importantly suggest that caution is needed in the use of ichnofabric analysis for intra-regional correlations.