Geology without borders: a review of the carbonate record in the Iberian Massif hinterland

Abstract

Carbonate sedimentation has evolved over nearly 3.5 billion years, shifting from early inorganic/microbial precipitates from seawater to massive biogenic systems throughout the Phanerozoic. This is related to the appearance of carbonate exoskeletons and the first reef-dwelling organisms: archaeocyathans, followed by sponges and corals, and other carbonate-skeleton-bearing reef-associated faunas such as pelmatozoans (e.g. crinoids) and bryozoans. The Iberian Massif allows to trace this connection between life, paleogeography, paleoenvironments, and geodynamic evolution, presenting a relatively continuous sedimentary record from the lowermost Cambrian to the Lower Devonian, including carbonate units deposited during this time span. The Iberian Massif has been subdivided into several paleogeographic zones based on their sedimentary record, tectono-metamorphic and magmatic features. Although the hinterland paleogeographic zones, namely the Central Iberian Zone (CIZ) and the Ossa-Morena Zone (OMZ), are characterized by markedly different Paleozoic lithostratigraphic successions, a comprehensive analysis shows that the carbonate sedimentation episodes record display similar, or at least partially overlapping ages. This is likely related to a common evolution during the rift and drift stages along the northern Gondwana margin throughout the Variscan Cycle. Indeed, four carbonate depositional events are represented in both geological zones: Cambrian (Series 2), Upper Ordovician, upper Silurian, and Lower-Middle Devonian. These episodes have been identified either in the OMZ or the CIZ, some of them also occurring in the imbricated successions of the Lower Allochthonous units of the Galicia-Trás-os-Montes Zone. Furthermore, carbonates occasionally occur associated with Mississippian-aged basins. However, the spatial distribution of these carbonate units is not ubiquitous throughout these paleogeographic zones. The Cambrian episode (Ovetian to Marianian) featuring an extensive archaeocyathan-bearing carbonate platform in the OMZ, being also represented by the Navalucillos Limestone in the CIZ. The stratigraphic record of the Upper Ordovician “Pelmatozoan Limestone” and upper Silurian “Scyphocrinites Limestone” is local. The Ordovician episode is only preserved in few localities (Valle [OMZ], Buçaco and Almadén [CIZ] Synclines). However, it remains unclear whether its absence elsewhere reflects non-deposition or erosion related to instability associated with the end-Ordovician glaciation, as stratigraphic gaps affecting the upper Katian deposits are common. The Silurian carbonate event is lacking from most Pridoli successions, cropping out in the Moncorvo [CIZ] and Valle Synclines. The Devonian carbonate episode shows discrepancies in age between the two paleogeographic domains. In the Obejo-Valsequillo Domain (southernmost CIZ, along the contact with the OMZ), the Devonian episode is represented by Pragian to Emsian coral-rich limestones in the Amêndoa-Carvoeiro Syncline and the Zújar region. Conversely, in the OMZ, the oldest carbonate sequences are Emsian in age and extend up to the Givetian, being restricted to the SW border of this zone. This partially overlapping age range of the Devonian episode in both paleogeographic zones may indicate a diachronous depositional process. Nonetheless, most carbonate lithostratigraphic units in Portugal (e.g. Bateiras, Estremoz, Ficalho, Portel, or Trigaches carbonate units) remain chrono- and biostratigraphic poorly constrained, with their ages assigned mainly through lithostratigraphic correlations with neighboring Spanish sections, where the ages of the carbonate successions are better constrained. Furthermore, the fossil content differs markedly between the Portuguese and Spanish successions, mostly due to post-depositional processes, namely Variscan tectono-metamorphism and late dolomitization.