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 r ecorded at the Lower-Middle Cambrian boundary is accompanied by a profound change in the style of carbonate deposition. The Lancara Formation of the E sla nappe (Cantabrian Mountains, northern Spain) contains a distinct sedime ntary turnover due to a combination of tectonism, eustatic fluctuations, an d immigration and colonization of new benthic communities, such as the youn gest archaeocyathan assemblage of the entire Iberian Peninsula. During late st Early Cambrian times, a regressive trend is recorded in the Lancara Form ation. This regression was recorded on a peritidal-dominant, homoclinal ram p that is topped by a tectonically induced discontinuity (D1). The latter s urface marks the beginning of a last prograding, regressive tendency record ed on an intra-shelf ramp with ooidal/bioclastic shoals protecting archaeoc yathan-microbial patch reefs. The overlying discontinuity (D2) corresponds to a major erosive unconformity, which coincides with the Lower-Middle Camb rian boundary in the Cantabrian Mountains. The subsequent, long-term, earli est Middle Cambrian rise in relative sea-level allowed deposition of low-re lief bioclastic shoals bearing a diverse and cosmopolitan assemblage of ben thic fauna. Finally, the previous evolution is bounded by a third discontin uity (D3), which marks the beginning of a rhythmic sedimentation indicative of a major phase of tectonic breakdown and drowning of platforms recognise d throughout southwestern Europe, Two associations of calcimicrobes occur i n the latest Early Cambrian regressive trend of the Lancara Formation: (i) Proaulopora and Subtiflora are identified in peritidal, high-energy setting s, lacking self-supported structures, whereas (ii) intergrowths of Epiphyto n, Renalcis and Girvanella encrusted branching colonies and solitary archae ocyaths in protected (back-shoal) parch reefs. The latest Early Cambrian re gression is correlated in southwestern Europe in both siliciclastic (Iberia n Chains and Ossa-Morena) and carbonate-dominant platforms (Cantabrian Moun tains, Montagne Noire and Sardinia). its tops are recognised as diachronous unconformities ranging in age from early Bilbilian to the Bilbilian-Leonia n or Lower-Middle Cambrian boundary. (C) 2000 Elsevier Science B.V. All rig hts reserved.