THREE-DIMENSIONAL RECONSTRUCTION OF "PHYCOSIPHONIFORM" BURROWS: IMPLICATIONS FOR IDENTIFICATION OF TRACE FOSSILS IN CORE

Phycosiphon-like trace fossils are some of the most common and important ichnofabric forming trace fossils in marine facies. This study was conducted to reconstruct the three-dimensional (3D) morphology of a Phycosiphon-like trace fossil from Cretaceous turbidites in Mexico in order to test the validity of criteria used to recognize such fossils in vertical cross sections similar to those seen in cores through hydrocarbon reservoir intervals. The geometry of the trace fossil was computer-modeled using a series of consecutive images obtained by serial grinding. The recognition of Phycosiphon in cross section is usually based on comparison with hypothetical cross sections of bedding-parallel specimens. The authors critically reassess Phycosiphon-like burrows in the light of existing conceptual and deterministic models, for comparison with three-dimensional reconstruction of Phycosiphon-like trace fossils from the Cretaceous Rosario Formation of Baja California, Mexico. Observed morphological differences between our material and typical Phycosiphon suggest that the characteristic “frogspawn” ichnofabric that is usually attributed to Phycosiphon (sensu stricto) can be produced by other similar taxa. Our palaeobiological model for the formation of the studied Phycosiphon-like trace fossil is fundamentally different to that proposed for Phycosiphon, but produces remarkably similar vertical cross sections. We consider that identification of Phycosiphon incertum in core is not possible without detailed 3D examination of burrow geometry. We propose the term “phycosiphoniform” for this group of ichnofabric-forming trace fossils.