INVESTIGATING THE ROLE OF AMORPHOUS FABRICS ASSOCIATED WITH CALATHIUM IN THE LOWER ORDOVICIAN FILLMORE FORMATION, UTAH, USA, USING PETROGRAPHY AND GEOCHEMISTRY

Calathium is an open-cup shaped genus of receptaculitids, an enigmatic fossil group that acted as reef builders during the Paleozoic. Their calcitic skeletons are mostly preserved as a three-layered conical wall: the middle layer has a dark micritic matrix and recrystallized meroms, while the outer two layers are composed of transparent coarse spar and are intersected by dark, micritic pore casts. In reef settings, pervasive amorphous fabrics often co-occur with calathid fossils (Li et al., 2015), both within the inner cavity and outside the wall structure (e.g., Liu et al., 2005). The amorphous fabrics, which we refer to as clotted networks, feature well-defined dark micritic channels surrounding transparent calcite spar partially replaced by peloidal fabrics. Previous studies on Calathium morphology have interpreted clotted networks either as skeletal outgrowths (e.g., Li et al., 2015) or as encrusting organisms which used the calathid skeleton as a substrate (e.g., Church, 2009).

We examined calathid samples collected from the Lower Ordovician Fillmore Formation to investigate the possible role of the clotted networks. Elemental abundance and cathodoluminescence analyses reveal that the fossil specimens are composed of low magnesium calcite and exhibit relatively low Mn content in both the walls and clotted networks. While both results indicate little diagenetic alteration, Calathium walls and clotted networks bear noticeable morphological and geochemical differences. Calcite crystal size is significantly larger within the outer two walls than in the clotted network. In addition, the walls contain higher Fe and Mn abundances than the clotted network, suggesting different degrees of alteration. Therefore, our results tend to support the hypothesis that the clotted network fabrics are not part of the Calathium skeleton, but instead are encrusting organisms associated with the Calathium skeletal structure. Although the clotted network appears to be biologically distinct from Calathium, the close spatial association between the two organisms suggests that the encrusting network likely helped anchor and support the calathid skeleton within the surrounding reef mounds.