PETROGRAPHIC ANALYSIS OF CROWDED ROSSELIA ICHNOFABRICS FROM THE TREMADOCIAN OF NORTHWESTERN ARGENTINA. ETHOLOGIC MEANING AND DIAGENESIS

Rosselia socialis were studied in the Ordovician Áspero Formation, in order to explore theirsedimentary and diagenetic fingerprint in the substrate. These trace fossils are found formingcrowded Rosselia ichnofabrics, described for the first time in pre-Quaternary strata ofArgentina. We identified three microstructures corresponding to the central shaft complex,the burrow lining and the host rock of a trace fossil assigned to a terebellid polychaete. Theinfill of the central shaft complex represents downwards advection of surficial depositslocated close to the burrow opening: fine-grained fecal mounds, and sandy mounds and lagdeposits of manipulated, non-ingested material. Abundant phylosillicates in the central shaftcomplex and burrow lining evidence mechanical selection of particles with high specificsurface area by the tracemaker. The fine-grained composition and multilayered organicallybound structure of the burrow lining generate an impermeable and reinforced burrow, whichcombined with crowding grants physical and chemical stability to its inhabitants. This isespecially advantageous in the high energy environments with shifting substrates wherecrowded Rosselia ichnofabrics are typically found. The central shaft complex and burrowlining are enriched in secondary iron minerals with respect to the host rock. Mineralizedbacterial structures in the burrow lining evidence biologically induced precipitation of ironoxides and possibly sulphides. This coupled with the distribution of iron minerals in theburrow lining and central shaft complex suggests the occurrence of early diageneticprocesses of organic matter decomposition and precipitation of authigenic iron minerals inRosselia burrows, as observed in modern terebellid polychaetes.Keywords: