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Journal of Paleontology 91(1):73–85


Tremadocian), ~20m above the Cambrian-Ordovician bound- ary. These levels probably correlate in time with those of the St. George Group in Newfoundland, but could be slightly older than the thrombolites-Lichenaria mud-mounds of Pratt and James (1982).


Reef-mound interval.—Within the La Silla Formation, benthic biota seems both sparse and low diversity in composition, especially in lacking pelmatozoan ossicles, as Pratt et al. (2012) have pointed out. However, complex microbial consortiums forming thrombolites, the new coralomorph described herein, and calcareous algae such as Nuia sp. begin to occur in these limestones at the lower interval of the unit (uppermost Cambrian–lowermost Ordovician). The coralomorphs are restricted to a 2m thick massive,


matrix-rich interval (Fig. 3), where they seem to be in life position. The outcrop is partially covered by debris and vegetation, making it difficult to precisely track the coralomorph-bearing boundstone interval laterally. However, this interval completely disappears laterally at a scale of tens of meters, suggesting low-relief mound geometry. The individual coralomorphs may be grouped in low-relief


(up to 10 cm) radial clusters or alternatively may be isolated crusts and slightly broken fragments embedded in peloidal micrograinstones that are intimately associated with various microbial communities. In polished slabs (Fig. 4), radial clusters seem to be passively covered by laminated peloidal muds or abruptly truncated by rugged surfaces capped by peloidal grainstones (Fig. 5). Within the grainstones, the calcareous alga Nuia sp. is present (Fig. 5.3). Radial clusters seem to be nucleated on the peloidal intraclastic grainstones (hardgrounds) or directly stacked on small-diameter domical microbial heads.


Intercolumnar spaces between coral clusters are alternatively filled by peloidal muds, peloidal grainstones, or complex calcified microbial consortiums (Fig. 5). This allows for the interpretation that colonial growth occurred synchronous with energy fluctuations and somehow colonies were interacting with microbial communities and periodically interrupted by higher- energy events, developing erosion surfaces, and grainy facies. Microscopic observations in thin sections seldomshow terminal module walls protruding out of these irregular surfaces, indicating that not all of these surfaces are strictly erosive, but depositional. Irregular micritic to micropeloidal laminae, occasionally with filamentous textures, point to a stromatolite- like habit and trapping and binding processes. Microscopically clotted-peloidal dense micrite with diverse shapes and growth patterns suggest pervasive bacterially induced precipitation within microbial biofilms, as well as cryptic microbialites


(Chafetz, 1986; Reitner, 1993; Riding, 2002; Adachi et al., 2004; Flügel, 2004; Chen and Lee, 2014). As pointed out in other examples (e.g., Sun andWright, 1989; Riding and Tomas, 2006), microbial textures occur both as irregularly laminated crusts on framework elements, as complex open-cavity fills within the boundstone framework, and as internal fills in cavities (intraskeletal and boring). Among distinct cavity-filling microbial consortiums, we recognized Renalcis-like chamber arrays (Fig. 5.1) (e.g., Riding, 1991; Chafetz and Guidry, 1999; Riding and Fan, 2001; Stephens and Sumner, 2002), as well as dendritic forms (Fig. 5.5) (e.g., Pratt and James, 1982; Pratt, 1984; Riding, 1991; Shen et al., 1997) and clustered globous forms (Fig. 5.4). Internal fills have a patchy intraskeletal clotted homogeneous pattern (Figs. 5, 6) and may correspond to later eodiagenetic non-photosynthetic cryptic bacteria that grew within available pore spaces, largely represented by decaying of coralomorph colonies. Most of the microfacies, including the microbial peloidal-


clotted matrix, the grainstones, and the coralomorph clusters themselves, are sparsely bioturbated. Field, slab-, and thin- section analysis is consistent with a combination of processes typical of microbial boundstones and microbial-coralomorph boundstones. Due to their relative abundance, the complex microbial consortium, together with the coralomorphs, seems to represent the major framework builders in our case. During their growth and metabolic activities, microbes and cyanobacteria


may have induced calcite precipitation in cyanobacterial filaments, algal sheaths, and extra-polymeric substances (EPS) within the boundstone and/or periodically helped trap and bind lime peloidal muds that in some cases seem to drape coralomorph colonies (Fig. 4). Together with early cementation, microbial activity may have contributed to substrate stabiliza- tion, encrustation, and development of the metazoan (coralo- morph) skeletal frameworks. It is therefore evident that microbes may have played roles as binders and substrate stabilizers, further allowing encrustation by coralomorphs. It is also clear that erosion and fragmentation periodically affected these low-relief reef-mounds, as suggested by the presence of irregular erosive surfaces (Fig. 5.3) and interbedded peloidal intraclastic grainstones with fragmented and disrupted colonies.


Repository and institutional abbreviation.—The specimens are housed under the prefix CEGH-UNC in the paleontological collection of Centro de Investigaciones Paleobiológicas (CIPAL), at the building of Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), Universidad Nacional de Córdoba, Argentina.


Figure 5. Thin section photomicrographs and boundstone petrography showing textures and distinct microbial components, including Amsassia argentina n. sp. colonies; scale bars = 1mm, except in Fig. 5.3: (1) Renalcis-like chambers (white arrows) isolated and forming arrays within the microbial consortium growing towards center of cavity (filled with sparite); (2) view of the colony in sharp contact with a peloidal grainstone through a stylolite (white arrow); (3) peloidal grainstone overlying a cemented erosive surface (hardground). Note oblique section of Nuia sp. (black arrow) within grainstone, scale bar = 0.5mm; (4) detail of termination of a colony showing a complex microbial consortium with chambered, dentritic, filamentous, and globous forms growing toward a framework reef cavity; (5) similar to 4. Note the contrast between cryptic microbial communities (represented by homogeneous clotted peloidal automicrite) growing within modules (black arrow) and those more oriented forms colonizing the surface (white arrow); (6) finely laminated peloidal micrograinstone (stromatolite crust, black arrow) draping the termination of a colony. Note trilobite section (white arrow) and other bioclasts within upper part of photograph and some infiltration within the upper modules of the underlying colony. 1, 2, 4, 5 and 6 show partial views of longitudinal to transverse sections of Amsassia colonies. Note that in most cases, individual modules are partly occluded by homogeneous clotted automicrite of bacterial origin.


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