Carrera et al.—Coralomorph in lowermost Ordovician reef mounds


75


have been recognized (Cañas, 1999; Keller, 1999; Pratt et al., 2012) within the three major subdivisions. Stromatolites, and particularly thrombolites, are ubiquitous


and very common components of the higher-frequency cycles (Armella, 1994; Cañas and Carrera, 2003). Thrombolites vary in diameter from meter scale to several meters, and can be isolated, coalescent, distributed in clusters or patches, and forming laterally persistent biostromes remarkably linked to shallow subtidal high-energy grainy facies (intraclastic to peloidal grainstones). Calcarenites are mainly composed of peloids, well-rounded intraclasts, aggregate lime grains, micritized ooids, and a few bioclasts. Bioclasts are very scarce and include rare calcareous algae such as Nuia sp. and scattered fragments of gastropods, trilobites, and nautiloids within the cross-bedded subtidal grainstone-packstone intervals. Abundant in situ gas- tropods seem intimately related with domical thrombolites also are common components in associated storm layers and within intercolumnar spaces between domical thrombolites.


Stratigraphic position and age.—The tabulate-like form is included in patchy, mud-rich microbial mounds that grew immediately above the major flooding surface that initiated the second accommodation-related shallowing-upward cycle, ~95m above the base of the La Silla Formation (Fig. 2). The fossiliferous level is within the Cordylodus angulatus Conodont Zone (lower Tremadocian), slightly above the Cambrian- Ordovician boundary, indicated by the presence of the Clavo- hamulus hintzei and Cordylodus lindstromi zones (Buggisch et al., 2003). The age of the La Silla Formation has been established by


combining information from conodonts and sparse trilobites (Lehnert and Keller, 1993; Keller et al., 1994; Lehnert, 1995; Lehnert et al., 1997; Albanesi and Ortega, 2002). The trilobite Plethopeltis obtusus (Rasetti, 1945), which in North America ranges from the Saukia to the Missisquoia trilobite zones, has been found near the base of the unit (Vaccari, 1994), indicating a late Sunwaptan (late Furongian) age for this part of the section. Four conodont associations were described from intervals B through E within the La Silla Formation (Lehnert, 1995; Buggish et al., 2003). The oldest recorded conodont fauna of the unit occurs


~55m from its base (point B in the section, Fig. 2), containing the species Clavohamulus hintzei Miller, 1969, which is diagnostic of the Clavohamulus hintzei Conodont Subzone of the Cordylodus intermedius Zone. The next conodont fauna (point C in the section, Fig. 2) was correlated with the Rossodus manitouensis Conodont Zone of the Skullrockian Stage (early Tremadocian) (Ibexian Series) of North America. Point D conodonts were correlated with the ‘Low Diversity Interval’ and with the lower Macerodus dianae Conodont Zone in North America, whereas point E near the top of the section is correlated with the Paltodus deltifer Conodont Zone, indicating a late Tremadocian age for the top of the La Silla Formation. According to this scheme, the base of the Tremadocian Stage


Figure 2. General stratigraphy and conodont biostratigraphy (from Buggish et al., 2003) of the La Silla Formation. CRM: stratigraphic position of the coralomorph reef-mounds; SJF: San Juan Formation; LFF: La Flecha Formation; A-E: main section points for conodont biozones.


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