Dissolutions and Fe-crusts related to the OAE 1a in La Florida carbonate platform (NW Cantabria)

The early Aptian succession of La Florida platform in northwestern Cantabria exhibits two stages of platform carbonates separated by a dissolution surface preceding the drowning of the platform. The dissolution surface was draped by a thin Fe-crust and the cavities subsequently filled with calcarenites of the second platform stage before being sealed by open marine marls formed as a consequence of the platform drowning. The marls correlate in age with the Early Aptian Oceanic Anoxic Event (OAE 1a). The aim of this work is to provide new data that may help to know the causal link between the karst surface, the Fe-crust and the drowning of the platform during the OA

Diagenetic study of brachiopod shells from the Guadiato basin (Mississippian, SW Iberian Massif)

Detailed petrographic and geochemical analyses of brachiopod shells from the Guadiato Carboniferous basin (Córdoba, SW Iberian Massif) has been carried out in order to evaluate their textural and geochemical preservation degree and their potential to be used as indicators of paleoenvironmental changes during the Carboniferous. The preservation degree of the shell´s microstructure has been evaluated by petrographic microscope, cathodoluminescence (CL) and scanning electron microscope (SEM). Under CL, ventral valves are mainly nonluminescent (NL) or nonluminescent with small slightly luminescent areas (NL-SL). In contrast, most of the dorsal valves are luminescent with moderate luminescent areas (ML-L). Under SEM, the samples show very well preserved lamellar and fibrous microstructures. The elemental contents (Ca, Mg, Sr, Mn and Fe) for the NL portions agree with elemental contents of modern brachiopods in equilibrium with ambient seawater and that have not undergone significant diagenetic modification. Therefore, our findings reveal a better microstructural preservation of the ventral valves of Gigantoproductids than the dorsal valves. The NL and NL-SL shell portions present the better microstructural and geochemical preservation and are interpreted to reflect the original isotopic and chemical composition of the marine water from which they precipitate

Microcodium proliferation during Cenomanian-Turonian transition in southern Iberia: a consequence of OAE-2 in emerged areas?

En el sur de Iberia (Cordillera Ibérica meridional y Prebético) el tránsito Cenomaniense-Turoniense está definido por una sucesión de carbonatos marinos muy someros que presentan abundantes evidencias de exposición subaérea y paleosuelos (Fm.Alarcón). En esta sucesión se registra la primera aparición conocida en Iberia de Microcodium. Esta aparición es abrupta y podría estar relacionada con los cambios ambientales asociados a la perturbación global del ciclo del carbono que indujo el evento de anoxia oceánica OAE2In southern Iberia (southern Iberian chain and Prebetic Zone) the Cenomanian to Turonian transition is characterized by a sedimentary succession of very shallow marine carbonates with abundant evidences of subaerial exposure and paleosols (Alarcón Fm.). That succession records the oldest known Microcodium in Iberia. Its first appearance is abrupt and probably related to the environmental changes associated to the global perturbation in the C cycle that trigger the oceanic anoxic event OAE

The effects of astronomically forced climate change on hemipelagic carbonate sedimentation in a tectonically active setting: the Albian Mioño Formation in Castro Urdiales (Cantabria, N Spain)

Our understanding of the stratigraphic expression of astronomically driven climate-change cycles in the Milankovitch frequency band has improved significantly in recent decades. However, several aspects have been little studied to date, such as the nature of the climatically regulated environmental processes that ultimately control cyclic sedimentation. Similarly, relatively little is known about the expression of Milankovitch cycles in successions accumulated in tectonically active basins. In order to fill this knowledge gap, the Albian hemipelagic deposits of the Miono Formation exposed in Castro Urdiales (Basque-Cantabrian Basin) are studied herein. These deposits were accumulated during a rifting phase with strong tectonic activity. The sedimentological, petrographic and cyclostratigraphic analysis demonstrates that, despite the synsedimentary tectonic instabilities and some diagenetic overprinting, the hemipelagic carbonate alternation was astronomically forced 110.68-110.47 Ma. Seasonality fluctuations driven by precession cycles caused periodic (20 ky) variations in the rate of carbonate productivity (abundance of pelagic calcareous plankton and micrite exported from adjacent shallow-water areas) and/or siliceous dilution (terrestrially derived siliciclastic sediment supply and siliceous particle production by sponges). These variations resulted in the formation of marly limestone beds when annual seasonality was low (i.e., boreal summer at aphelion, winter at perihelion) and the accumulation of marlstones when seasonality increased (i.e., boreal summer at perihelion, winter at aphelion). The incidence of these processes increased and decreased in line with seasonality modulation by short-eccentricity cycles of 100 ky. In conclusion, this study shows that Milankovitch cycles can be reliably recorded in hemipelagic successions accumulated in tectonically active settings if sediment gravity flows or other disturbances do not affect autochthonous sedimentation.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. Research funded by the MCIN/AEI project PID2019-105670GB-I00/AEI/10.13039/501100011033 of the Spanish Government and by the Consolidated Research Group IT160222 of the Basque Government

Structural and facies architecture of a diapir-related carbonate minibasin (lower and middle Jurassic, High Atlas, Morocco)

We report the structural geometry and facies architecture of a small diapir-related carbonate-dominated basin from the Jurassic rift of the Moroccan High Atlas. The Azag minibasin is a lozenge-shaped depocenter completely enclosed by tectonic boundaries that we interpret as welds after former salt anticlines or salt walls. The exposed ca. 3000 m-thick infill of the Azag minibasin is asymmetric; layers are tilted to the W defining a rollover geometry. Areally-restricted sedimentary discontinuities and wedges of growth strata near the basin margins indicate sedimentation contemporaneous with diapiric rise of a Triassic ductile layer. Facies evolution through the basin reflects local accommodation by salt withdrawal and regional events in the High Atlas rift. The early basin infill in the Sinemurian and Pliensbachian shows thickness variations indicative of low-amplitude halokinetic movements, with reduced exposed thicknesses compared to surrounding areas. The exposed Toarcian and Aalenian deposits are also reduced in thickness compared to areas outside the basin. Subsidence increased dramatically in the Bajocian-early Bathonian (?), the main phase of downbuilding, when over 2600 m of carbonates and shales accumulated at a rate > 0.5 mm/a in the depocentral area of the minibasin governed by W-directed salt expulsion. The stratigraphic units distinguished often show maximum thicknesses and deeper facies in the depocentral area, and rapidly change to shallower facies at the basin margins. The Bajocian carbonate facies assemblage of the minibasin include: reservoir facies as microbialite-coral reefs in the basin margins (formed during periods of strong diapir inflation and bathymetric relief), basin-expansive oolite bars (formed during episodes of subdued relief), and organic-rich, dark lime mudstones and shales that show source-rock characteristics. The Azag basin is a good analog for the exploration of salt-related carbonate plays in rifts and continental margins where source-rock and reservoir can form in a same minibasin

High-frequency subtital-peritidal carbonate cycles of the Lower Aptian (San Esteban Fm., Basque-Cantabrian basin): a record of climatic change

El análisis detallado de facies de las calizas de la Formación San Esteban del Aptiense inferior de Cantabria ha permitido caracterizar 15 tipos de litofacies depositadas en ambientes submareales a supramareales. Las facies se organizan en ciclos somerizantes que culminan con superficies de exposición subaérea, evidenciado variaciones relativas del nivel del mar de alta frecuencia/baja amplitud. Se distinguen dos tipos de ciclos en función de las facies y naturaleza de las superficies que los culminan. Los ciclos Tipo-1 están dominados por facies submareales. La secuencia ideal comienza con rudstone bioclástico-intraclástico, seguido de wackestone de Chondrodonta, floatstone de rudistas, boundstone de rudistas y mudstone de milólidos. El techo es una superficie neta con bioturbación y marcas de raíces estilo pseudomicrokarst. Los ciclos Tipo-2 comienzan con floatstone oncolítico-litoclástico, seguido de floatstone de rudistas, packstone-grainstone de foraminíferos, boundstone de rudistas y finalmente rudstone oncolítico, mudstone fenestral y/o estromatolitos laminares a hemiesféricos. Culmina con rasgos subaéreos de desecación y conglomerados calcáreos de cantos negros con microestructura alveolar y rizolitos calcificados. Los ciclos Tipo-1 caracterizaron condiciones ambientales húmedas, mientras que los de Tipo-2 caracterizaron condiciones semi-áridas. La evolución vertical de los ciclos refleja una transición climática de tipo greenhouse a otra coolhouse tras el OAE-1a.High-resolution facies analysis of Lower Aptian platform carbonates from Cantabria has allowed to characterize 15 lithofacies types, which reflect environmental conditions ranging from subtidal to supratidal. The facies are organized in shallowing-upward cycles capped by subaerial exposures, attesting for high-frequency and lowamplitude relative sea-level fluctuations. According to the facies types and nature of the subaerial capping surfaces, two types of cycles have been differentiated, characterizing respectively the lower and upper part of the succession. Type-1 cycles are dominated by subtidal facies. They ideally start with transgressive bioclastic-intraclastic rudstone, followed by Chondrodonta wackestone, rudist floatstone, rudist boundstone and finally miliolid mudstone. They culminate with burrowed and rooted surfaces that resembles to pseudomicrokarst. Type-2 cycles start with transgressive oncoidal-pebbly floatstone, followed by rudist floatstone, foraminiferal packstone-grainstone, rudist boundstone and ending eventually with either oncoidal rudstone, fenestral mudstone and/or laminoid to hemispherical stromatolites. These cycles are capped by subaerial desiccation and limestone conglomerates rich in black-pebbles with alveolar septal microstructure and calcified rhizoliths. Type-1 cycles developed under humid environmental conditions, whereas Type-2 cycles could have formed under semiarid conditions. The cycle stacking pattern may reflect the sedimentary evolution of the platform during the transition from greenhouse to coolhouse conditions after the OAE-1a.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Economía y Competitividad (MINECO)pu

Stage of regional dolomitization in La Florida-El Soplao area, Cantabria

Dolomitization played a very important role in producing porosity within the Late Aptian shallow marine carbonates hosting the El Soplao Cave and La Forida mine deposit. A detailed petrographic study of samples from a stratigraphic section across these limestones and dolostones has revealed three diagenetic stages from shallow to late burial and finally uplift. During these stages, at least four phases of calcite cementation (C1 to C4) and five phases of dolomite formation and recrystallization (D1 to D5) took place. The timing of dolomitization is still uncertain, but the first phase (D1) occurred shortly after limestone deposition (shallow burial diagenesis) as cement filling primary and early secondary (mouldic) porosity. Calcite cements C1 and C2 occurred also in this diagenetic stage. Later, pervasive replacement dolomitization (D2, idiotopic) and dolomite recrystallization (D3, xenotopic) took place in the burial realm. Saddle dolomite (D4) occurred in continuity with xenotopic dolomite D3 as pore-lining cement in vuggy porosity. Late-stage coarse saddle dolomites (D5) were precipitated as cement in fractures, hydrofractures and hydrothermal breccias cross-cutting the previous dolomite stages. Coarse blocky calcite cement (C3) fills the remaining pore space during latest burial diagenesis. Finally, dedolomitization, iron oxides and calcite C4, are observed in discrete zones within dolomite crystals, and are related to uplift meteoric diagenesis and karst-related dissolutio

Early Toarcian (Jurassic) brachiopods from the Balearic Islands (Spain) and their paleobiogeographic context.

The record of brachiopods in the Lower Toarcian of the Balearic Islands is described after a reassessment of previous works and new samplings in the Tramuntana Range of Mallorca. The recognized species have been assimilated to the assemblages described in the Iberian Range in the Tenuicostatum and Serpentinum biozones. Moreover, a detailed comparison with other Western Tethys basins and the study of the dispersion of the brachiopod faunas in relation with the Early Toarcian Oceanic Anoxic Event, allow to refine the paleobiogeographic relationship of the Balearic brachiopods. They belong to the Euro-Boreal Province of brachiopods in the Tenuicostatum Biochron and correspond to the”Spanish Fauna” that emerged after the deep paleobiogeographic disruption that took place at the base of Serpentinum Biochron. The occurrence of Prionorhynchia msougari Rousselle in the Serpentinum Zone of Mallorca indicates that, within this last assemblage, there would be a closer connection between the Balearic region and the southern margin of the Tethys (Betic and North African basins) than with the northern margin (Eastern Iberian Platform System and Eastern Pyrenees). This paleobiogeographic conclusion is consistent with the supposed position of the Balearic area in the Early Toarcian, on the southeastern margin of Iberia and near the Alboran and Kabylian margins