Carbonates in Skeleton-Poor Seas: New Insights From Cambrian and Ordovician Strata of Laurentia

Geobiology of a lower Cambrian carbonate platform, Pedroche Formation, Ossa Morena Zone, Spain

“…Skeletons, especially archaeocyaths, are locally conspicuous and abundant, but because facies other than archaeocyath rudstone contain such a low skeletal component, the total measurable contribution of carbonate skeletons to the platform accumulation is low, perhaps 5 -6% by volume. This is not too different from Cambrian samples measured in China, Newfoundland, Labrador and the North American Cordillera, and well below skeletal abundances in most Middle Ordovician and younger deposits (Hicks and Rowland, 2009;Pruss et al, 2010Pruss et al, , 2012. That is, despite the fact that Pedroche Formation and coeval carbonates accumulated during the acme of Cambrian body plan diversification (Knoll and Carroll, 1999;Erwin et al, 2011), and despite the observation that most of the skeletal designs evolved by animals appeared during this interval (Thomas et al, 2000), skeletons remained a subsidiary component of carbonate deposits.…”

“…Point-counts of petrographic thin-sections enabled us to quantify the proportional contributions of constituent components to carbonate volume (e.g., Flügel, 2004;Payne et al, 2006;Pruss et al, 2010;Pruss and Clemente, 2011). Of the 74 thin-sections examined for lithofacies description, we point counted 30 thin sections for quantitative analysis of constituent components, with 5 representing the calcimicrobial thrombolite facies (facies A), 9 representing the ooid/oncoid/peloid grainstone facies (facies C), and 16 representing the interbedded carbonate mudstone, bioclastic wackestone, archaeocyathan floatstone-packstone-rudstone and siliciclastic siltstone facies assemblage (facies D; see text for lithofacies description; Table 2).…”

“…13d). Thus, if archaeocyaths were not actively displacing other taxa from ecological niches, the evolution and expansion of the Archaeocyatha during Cambrian Series 2 likely buoyed skeletal carbonate production beyond the low diversity, low abundance observed in other places and at other times in the early Cambrian (Pruss et al, 2010). Yet, even in lower Cambrian lithologies bearing archaeocyaths, skeletons do not play the depositional role that they do later in the Paleozoic Era (Rowland and Gangloff, 1988;Hicks and Rowland, 2009;Pruss et al, 2012).…”

“…Might the Cambrian Period represent a time when organisms had evolved biomineralized skeletons but not yet transformed ecosystem ecology? Analysis of upper Cambrian and lower Ordovician carbonates from Laurentia provides support for this view; in successions from Newfoundland and the North American Cordillera, skeletal material rarely exceeds 25% of bulk carbonate volume and is commonly much less (Pruss et al, 2010). Upper Cambrian rocks, however, might understate the case for skeletal input to Cambrian carbonates, in large part because archaeocyaths, the most conspicuous Cambrian reef-constructing animals, had nearly disappeared by 510 Ma (Debrenne, 2007).…”

“…Next, we analyze the architecture of thrombolite and associated carbonate facies from this mixed carbonate-siliciclastic platform developed on a peri-Gondwana terrane during the early Cambrian. To do so, we follow the methodology of Pruss et al (2010) and use nested scale observations, from outcrop stratigraphy to petrographic analysis of carbonate facies, to quantify the contribution of skeletal material to carbonate…”