A NEW OCCURRENCE OF HERRINGBONE CARBONATE WITHIN CAMBRIAN ARCHAEOCYATH MOUNDS, SARDINIA (ITALY)

Analysis of unusual marine carbonate phases with distinctive distributions through time and space aid in interpreting and constraining ancient marine conditions. We investigated the spatial and temporal distribution of one such phase, herringbone (HB) carbonate. Unique in its crystallography, HB appears as mm-scale light-dark crenulated banding with sweeping extinction along the length of each crystal. This phenomenon is attributed to the rotation of the c-axis relative to crystal elongation. Initial identification of HB in Archean rocks by Sumner & Grotzinger (1990) suggested that both enhanced alkalinity concentrations and an anoxic water body may be crucial during HB precipitation. Since its discovery, HB has been observed in a variety of younger successions as well. In the Mesoproterozoic, HB is typically associated with marine flooding surfaces (cf. Kah et al., 2006; 2012), implying that the interaction between chemically distinct marine water bodies may facilitate HB precipitation. Occurrences of HB in younger deposits, although poorly studied, appear to be concentrated at the interface between shallower and deeper-water environments that enable potential interactions between chemically distinct water bodies.

Here we report a new discovery of HB within the Cambrian strata of southwestern Sardinia. Sardinian HB was first identified in a sample at the Natural History Museum of Paris. Following this observation, a field expedition was led to confirm its presence and understand occurrences across the carbonate platform. Sardinian HB generally occurs in void spaces and shelter cavities within archaeocyath/calcimicrobial mounds. Field work confirmed the presence of HB within the Matoppa Formation (Nebida Group) predominately inside archaeocyath-rich bioherms, and potentially within the Santa Barbara Formation (Gonnesa Group) calcimicrobial boundstones. The stratigraphic transition from the Matoppa Formation to the Santa Barbara Formation records a broad shallowing upwards sequence associated with a decrease in terrigenous input. Thus, the identification of HB within both formations allows for improved constraints on the behavior of distinct water bodies across carbonate platforms and for enhanced interpretations of HB formation.