Conclusion

Mud-rich siltstones from Rosario Formation are characterized by dense monospecific assemblages of phycosiphoniform burrows and are analogous to many shale-gas reservoir facies. Local concentrations of burrowing may reflect patches of labile organic matter. The phycosiphoniform burrow-makers are thought to be selective deposit feeders that ingested clay-grade material and left a clean mud-poor feeding halo of processed sediment.

Our image analysis of two-dimensional slices allows reconstructing the three-dimensional geometry of the Phycosiphoniform trace fossil. The reconstructed burrow is unlike Phycosiphon (sensu lato), but produces very similar "frogspawn texture" ichnofabrics. The cross sections of our burrow system are distinguished from those of Phycosiphon s.l. in that the halo is generally present only beneath the level of clay-rich burrow cores.

The examined phycosiphoniform burrow geometry presents the following characteristics that allow differentiation from Phycosiphon incertum: 1) Arms of the single lobe are parallel in the vertical plane (Figure 8.1-8.2), and the lobe is seen to bend into a half ellipse when viewed in the plane of the lobe (Figure 8.3); 2) In side view, the lobe arms extend parallel to bedding and are steeply bent downward at the termination of the loop (Figure 8.1); 3) in axial view, the lobe is steeply inclined relative to the bedding plane (Figure 8.2); 4) The halo of the burrow is present only below the level of the burrow core and completely fills the space between the lobe arms (Figure 1 and Figure 9); 5) The halo can be several times thicker than the burrow core (Figure 8.4); and 6) No spreiten have been observed.

Our palaeobiological model for the formation of the studied Phycosiphoniform 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 three-dimensional examination of burrow geometry. We propose the term "phycosiphoniform" to describe this group of ichnofabric-forming trace fossils. We consider that, at present, our material should be left in open nomenclature pending thorough three-dimensional analysis of the type material of other phycosiphoniform burrows including Anconichus horizontalis. We note that there are many possible burrow geometries that can produce phycosiphoniform cross sections, but that much work needs to be done before many taxa can be convincingly recognized in vertical cross section.