Elena Moreno-Eiris

Sedimentary patterns across the Lower–Middle Cambrian transition in the Esla nappe (Cantabrian Mountains, northern Spain)

In the carbonate platforms of the western Gondwana margin, the extinction recorded at the Lower–Middle Cambrian boundary is accompanied by a profound change in the style of carbonate deposition. The Lancara Formation of the Esla nappe (Cantabrian Mountains, northern Spain) contains a distinct sedimentary turnover due to a combination of tectonism, eustatic fluctuations, and immigration and colonization of new benthic communities, such as the youngest archaeocyathan assemblage of the entire Iberian Peninsula. During latest Early Cambrian times, a regressive trend is recorded in the Lancara Formation. This regression was recorded on a peritidal-dominant, homoclinal ramp that is topped by a tectonically induced discontinuity (D1). The latter surface marks the beginning of a last prograding, regressive tendency recorded on an intra-shelf ramp with ooidal/bioclastic shoals protecting archaeocyathan-microbial patch reefs. The overlying discontinuity (D2) corresponds to a major erosive unconformity, which coincides with the Lower–Middle Cambrian boundary in the Cantabrian Mountains. The subsequent, long-term, earliest Middle Cambrian rise in relative sea-level allowed deposition of low-relief, bioclastic shoals bearing a diverse and cosmopolitan assemblage of benthic fauna. Finally, the previous evolution is bounded by a third discontinuity (D3), which marks the beginning of a rhythmic sedimentation indicative of a major phase of tectonic breakdown and drowning of platforms recognised throughout southwestern Europe. Two associations of calcimicrobes occur in the latest Early Cambrian regressive trend of the Lancara Formation: (i) Proaulopora and Subtiflora are identified in peritidal, high-energy settings, lacking self-supported structures, whereas (ii) intergrowths of Epiphyton, Renalcis and Girvanella encrusted branching colonies and solitary archaeocyaths in protected (back-shoal) patch reefs. The latest Early Cambrian regression is correlated in southwestern Europe in both siliciclastic (Iberian Chains and Ossa–Morena) and carbonate-dominant platforms (Cantabrian Mountains, Montagne Noire and Sardinia). Its tops are recognised as diachronous unconformities ranging in age from early Bilbilian to the Bilbilian–Leonian or Lower–Middle Cambrian boundary.

Early Cambrian coelobiontic communities in tectonically unstable crevices developed in Neoproterozoic andesites, Ossa-Morena, southern Spain

A well-preserved biota of Lower Cambrian cavity-dwelling organisms is recorded within fissures in Neoproterozoic andesites in Ossa-Morena (southern Spain). The cavities are unique among described Lower Cambrian coelobiontic communities due to the igneous character of the host rock. Coelobiontic habitat was episodically enlarged by synsedimentary tectonic fracturing reflecting polyphase infill of recurrent facies. The pioneer coelobiontic biota was diverse, and consisted of encrusting stromatolites and thromboids (dominated by Epiphyton and Renalcis), attached to walls and ceilings of the cavities, associated with archaeocyaths. Sponge spicules and chancelloriid sclerites occur as dense clusters indicating in situ growth, death and decay of spiculate sponges and coeloscleritophorans. Other organisms, such as echinoderms, trilobites and brachiopods, are also found within the cavities as reworked skeletons, indicating that they were washed in from the overlying, open seafloor. The main feature of the coelobiontic biota is the dominance of a sessile, chemosynthetic and filter-feeding epibenthos, composed of microbial communities, archaeocyaths, spiculate sponges (demosponges and rarer hexactinellides) and coeloscleritophorans.