KEYWORDS:STEEPCARBONATI~;SLOPI; !VIICROBIALP, OLiNI)STONI-. blI(71{()I".,\CIIzS-f.it{()MIblTI~Y ,c, PAIN LII>Ptr.R('AllIT, ONIH{I4()t.;S SumnlaryThe Carboniferous, particularly during the Serpukhovian and Bashkirian time. was a period of scarce shallow-water calcimicrobial-microbialite rccf growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsulface, Kazakhstan, Russia, Japan and Spain and rcpresenl uncommon occurrences within the general trend of low accumulation rates and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate platform of Lale Carboniferous (Bashkirian-Mosc(~vian) agc with a microbial boundstone-dominated slope dipping from 2(F up to 45 ~ Kilometer-scale conlJrlunus exposures allow the detailed documentation of slope eeomctry and lithefacies spatial distribution. This study aims to clcvclop a depositional model of steep-margined l.ate Palc(~zoic platforms built by microbial carbonates and lo contribulc to the understanding of the controlling factors on lithefacies characteristics, stacking patterns, accumulation rates and evohltion of the depositional architecture of systems, which differ from light-dependent coralgal platform margins.From the platform break to depths of nearly 300 m. the slope is dominated by massive cement-rich boundstone, which accumulated through the biologically induced precipitation of micrite. Boundslonc facies (type A) with peloidalcarbonate thud, fenestellid and fisluliporid bryozoans, sponge-like molds and primary cavities I'illed by radiaxial fibrous cement occurs all over the slot)e but dominates the deeper settings. Typc B b(~undstone consists ofglobose centimeter-scale laminated accretionary structures, which commonly host bolryddal cement in growth cavities. The laminae nucleate around fenestcl lid bryozoans, sponges, Retialcis and (;trade,ella-like filaments. Type B boundstone typically occurs at depths between 20-150 m to locally more than 300 rn and forms the bulk of the Bashkirian prograding slope. The uppermost slope boundstone (type C: bctwcei~ 0 and 20-100 in depth) includcs peMdal micritc, radiaxial fibrous ccmem. brx.ozoans, sp~mgc mohis. Dr,~eg.ella, Renalui.s. G i r t ' c e l i a . ()rtc~tlello. calcareous algae and calcitorncllid foramini Icrs. From depths of 80-200 m to 450 m. 1-30 m thick Ictlses ofcrinddal packstcme, spiculitic wackesionc, and bryozoan biocmnentstone with rcd-siaincd micrite matrix arc episodically inlcrcalaled with boundsttme and bieccias. These layers incrca.,,c in number Irom the uppermost Bashkirian to the Moscovian in parallel with thcchangc [ rolll {;| rapidly prograding t~ an aggrading architecture. The red-stained strata share comparable Iealures with Lower (2arbonilErous decper-watcr mud-mound Facies and were deposited durin.~ rclative rises of sea Icx.cl and pauses in boundstone production. Rapid relative sea-level rises might have been a.,,sociatcd v,.it h changes in oceanograpllic conditions not ...

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The fi nal stages of the Variscan orogeny (mostly Carboniferous) in the Western European Variscan belt involved the development of the Iberian-Armorican arc, which is cored by the Cantabrian zone (NW Iberia). TheCantabrian zone is the foreland of the Western European Variscan belt, and it is interpreted to record the waning stages of the closure of the Rheic Ocean. The distalmost tectonic units within the Cantabrian zone (the Cuera Unit and the Picos de Europa Province) were the last tectonic units emplaced at the core of the Western European Variscan belt orocline. Together, they form an imbricate system and associated wedge-top basins that are key to understanding the development of the orocline. The emplacement of the Cuera Unit and the Picos de Europa Province occurred in the latest Pennsylvanian, between Moscovian and Gzhelian times. New detailed mapping together with stratigraphic, sedimentologic, and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system. Our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the Cantabrian Variscan foreland fold-and-thrust belt, most probably during the oroclinal bending of the SW European Variscan belt that formed the Iberian-Armorican arc. The N-S-directed imbricate system was characterized by a shallow dip of the topographic surface (α < 1°), allowing for the development of wedge-top basins. The large amount of shortening (>150 ± 15 km) and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal décollement during bending of the underlying Gondwana lithosphere and may refl ect the closure of the Iberian-Armorican arc.

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The 14 km wide Valdorria outcrop (Pennsylvanian, Northern Spain) is one of the few examples of entirely exposed flat-topped and high relief carbonate platforms reported in the fossil rock record. Laterally and vertically traceable stratal patterns expose three phases of growth. Phase I is a 430 m thick platform to slope succession that prograded over 6 km, and is dated as Early Bashkirian (Akavasian–Askynbashian). Phase II aggraded and prograded, exhibiting 180 m thickness of cyclical platform top deposits, dated as Late Bashkirian (Asatauian). Phase III is a mound-shape structure that developed over the platform top of Phase II as a new phase of platform nucleation. It is 535 m thick and 2 km wide, and dated as Late Bashkirian (Asatauian–Transition interval). The observed changes of growth styles during platform evolution, from a prograding to an aggrading–prograding system, and a rapid aggradational phase, are inferred to be controlled by flexural subsidence in the active Cantabrian foreland basin, at the Variscan orogenic front. The metre-scale shallowing-upward cycles of the platform top are most probably due to glacioeustasy, as evidenced by well-recorded subaerial exposure surfaces superimposed on subtidal deposits, and by a stratal pattern recurrent in a short interval of about 160 Kyr. Observations of outcropping Bashkirian cyclothems in an isolated carbonate system, devoid of siliciclastic input, are relevant for a better understanding of the impact of high-frequency sea-level fluctuations on the carbonate factory. Moreover, progradation of the platform margin during Phase I reaches a rate of 2500 m/Myr, and 1810 m/Myr during Phase II; rates that are high when compared to other Pennsylvanian examples. The aggradation rate of 447 m/Myr calculated for Late Bashkirian–Transition interval (Phases I and II; uncorrected for compaction, missing beats and erosion) is uncommonly high in comparison to coeval Pennsylvanian examples. The platform exhibits a self-nourishing prograding microbial boundstone-dominated slope. Thus, the slope shedding model applies well to Valdorria. However, Phase II recorded eustatic variations able to inhibit the slope microbial boundstone factory during low sea-level stands; this is marked by common slope red-stained breccias synchronous to platform top subaerial exposure phases. Contrarily, periods of relative high sea-level and rapid subsidence in Phase III registered a greater development of cemented microbial boundstone. These observed, partly opposing relationships of sea-level stands, shedding modes and slope architecture provide an improvement of the currently used slope shedding model. The overall architecture of the Valdorria outcrop compares well with that of other contemporaneous platforms, such as Sierra del Cuera and Bolshoi Karatau. Valdorria shares the high-relief and flat-topped, steep slopes, cyclothemic patterns and occurrence of karst features with the Pricaspian Basin platforms (Tengiz, Karachaganak and Kashagan), with minor variations in facies distribution of the...