Distinguishing borings and burrows in intraclasts: Evidence from the Cambrian (Furongian) of North China

Abstract

Hardgrounds represent synsedimentary cemented stratigraphic beds that form at or near the seafloor. Borings represent a key line of evidence for investigations of hardground development and record the evolution of bioerosion and boring organisms. The unequivocal identification of borings is done through identification of the crosscutting relationship between the proposed boring and a hard substrate, such as lithoclasts and/or shells, with morphological criteria able to be used when dealing with a homogeneous substrate, such as micritic hardgrounds. Bioeroded hardgrounds and burrows with a micrite halo/lining are subject to fracturing and reworking, resulting in accumulations of intraclasts in flat-pebble conglomerates (FPC). The recognition of borings and broken burrows with a halo can be challenging in FPC. Using trace fossils preserved in situ and in FPC from late Cambrian carbonates of North China, we establish a set of criteria for distinguishing borings from burrows with a halo in FPC. Features such as the relative volume of burrows and borings versus the host pebble and the number of traces per pebble, the cross-cutting relationship with different colored laminae, and the presence of pyrite or glauconite encrustations can all be invoked to aid recognition of borings. Examination of the cross-cutting relationship and encrustation of trace fossils are not sufficient on their own. Our results suggest caution is necessary in defining borings in FPC, particularly as synsedimentary deformation of burrows with a halo in late Cambrian FPC can create structures that resemble borings.