Óscar Merino-Tomé

Mélanges and other types of block-in-matrix formations in the Cantabrian Zone (Variscan Orogen, northwest Spain): origin and significance

Block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath or ahead of submarine thrust systems. These mélanges may reach up to kilometric thickness and are mostly composed of broken formations (boudinaged sequences) of late Carboniferous age and scattered ‘exotic’ blocks derived from older Palaeozoic formations. Moreover, the mélanges in the Cantabrian Zone also include subordinate debris flow deposits with a chaotic block-in-matrix fabric (olistostromes). The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated through submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones, whereas the unlithified muddy matrix underwent continuous deformation. The scattered ‘exotic’ blocks ranging in age from early Cambrian to early Carboniferous were incorporated into the mélanges as individual blocks from competent well-lithified formations, originally located in the lower part of the nappe stacks. Although the Cantabrian Zone mélanges include olistostromic intervals, most of the olistostromes of this zone occur in a different geological setting. They are usually intercalated in the normal marine deposits of the Variscan foreland basin and, in contrast to the mélanges, they are mostly related to the margins of carbonate platforms, ahead of moving nappes. Finally, other instances of olistostromes are related to slopes generated by limb rotation of growth folds, which developed on submarine wedge-top successions.

LA-ICP-MS U-Pb dating of Carboniferous ash layers in the Cantabrian Zone (N Spain): stratigraphic implications

Seven centimetre-thick volcanic ash-fall layers interbedded within the thick Carboniferous successions of the Cantabrian Zone in northern Spain were dated by U–Pb zircon laser ablation inductively coupled plasma mass spectrometry across an interval ranging from Visean to Kasimovian, thus covering most of the Carboniferous period. All these ash layers occur in fossiliferous successions, allowing us to insert the radiometric data within a well-constrained biostratigraphic framework. Considering the analytical uncertainty, the obtained ages match the ages inferred from the conodont biostratigraphy established in the Mississippian succession (which hosts the oldest two ash layers, Visean in age), and the fusuline and mega- and microflora data from the strata hosting the Moscovian and Kasimovian (Westphalian–Stephanian) tonsteins. The age of a Langsettian tonstein along with data provided by several papers stating that in the Cantabrian Zone Langsettian floras were contemporaneous with lowermost Moscovian fusulines suggest that Langsettian floras could have been younger in Spain than in other areas. Our absolute ages provide new constraints not only for the correlation of the Carboniferous successions of the Cantabrian Zone with time-equivalent reference successions in other parts of the world but also for calibrating the Carboniferous global chronostratigraphic units based on marine fossils with the West European regional units. Supplementary material: Stratigraphic position and biostratigraphic information on the studied ash-fall layers, analytical methods and geochronological results are available at https://doi.org/10.6084/m9.figshare.c.3768701

Intact seismic-scale platforms and ramps in the Lower to Middle Jurassic of Morocco: Implications for stratal anatomy and lithofacies partitioning

The Jurassic carbonate platforms of the central High Atlas in Morocco are well known for several high-quality outcrops. In the central High Atlas, there are two complementary locations that offer critical lessons for our understanding of Jurassic carbonate system evolution in extensional basins: a Lower Jurassic high-relief, carbonate platform with steep slopes that developed on the footwall of a rotating fault block in an active half-graben (Djebel Bou Dahar [DBD]) and an upper Lower to Middle Jurassic low-angle prograding carbonate ramp rich in ooids (Amellago ramp [AR]). The DBD and AR outcrops provide superbly exposed, structurally intact, and fully accessible platform to basin transects. They provide valuable analogs for depositional geometries at reservoir and seismic scales that are highly relevant for hydrocarbon exploration and production. The DBD serves as an analog for isolated carbonate platforms developed in rift basins (particularly synrift carbonate platforms with a coral calcareous sponge and microbial boundstone facies belt in the upper slope and margin). The AR provides one of the rare examples whereby a large-scale oolitic ramp can be examined in great detail, providing an analog for a range of oolitic reservoirs, mostly Mesozoic.

Constraining the Timing and Amplitude of Early Serpukhovian Glacioeustasy With a Continuous Carbonate Record in Northern Spain

During the Late Paleozoic Ice Age (LPIA, 345–260 Ma), an expansion of ice house conditions at 330 Ma caused a nearly synchronous, global unconformity. Subaerially exposed paleotropical carbonates were dissolved by meteoric waters, mixed with the light terrestrial carbon, and recrystallized with overprinted, diagenetic dC values. In Northern Spain, development of a rapidly subsiding foreland basin kept local sea level relatively high, allowing continuous carbonate deposition to record dC without meteoric overprint. The Spanish sections show a 2& increase in dC that can be modeled as the ocean’s response to the creation of a significant light carbon sink through widespread meteoric diagenesis of marine carbonates during the near-global hiatus. About 15–35 m of sea level fall would have exposed a large enough volume of carbonate to account for the positive excursion in dC of oceanic DIC. Combining the dC data with high resolution biostratigraphy and new ID-TIMS U-Pb zircon ages from interbedded tuffs, we calculate that the depositional hiatus and glacioeustatic fall caused by the early Serpukhovian phase of ice growth lasted for approximately 3.5 My.