Positive shifts in global seawater δ13CDIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ18O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow‐marine settings. The model involves geochemically altered platform‐top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ13C and δ18O relative to typical oceanic values. During high‐amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and 13C‐depleted platform‐top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate 13C and 18O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.

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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 ...

Seismic-scale continuous exposures of an Upper Carboniferous (Bashkirian-Moscovian) carbonate platform (N Spain) provide detailed information about the lithofacies and stratal geometries (quantified with differential global positioning system measurements) of microbial boundstone-dominated, steep prograding and aggrading platform margins. Progradational and aggradational platform-to-slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable, high-relief depositional systems. The Bashkirian is characterized by rapid progradation at rates of 415-970 m My )1 . Characteristic outerplatform facies are high-energy grainstones with coated intraclasts, ooids and pisoids, moderate-energy algal-skeletal grainstones to packstones and lower energy algal packstone and boundstone units. The Moscovian aggradational phase is characterized by aggradation rates of 108 m My )1 . Coated-grain shoals are less common, whereas crinoidal bars nucleated in well-circulated settings below wave-base. Boundstones form a belt (30-300 m wide) at the platform break and interfinger inwards with massive algal-skeletal wackestones (mud-rich banks). The progradational phase has divergent outer-platform strata with basinward dips of 12°to 2°. Steep clinoforms with dips of 20-28°are 650-750 m in relief and possibly sigmoidal to concave in the lower part. The basinward-dipping outer-platform strata might be depositional for less than 6°, consistent with lithofacies deepening seaward. The basinward dip is attributed to the downward shift of upperslope boundstone, forced by late highstand and relative sea-level fall, and to compaction-induced differential subsidence during progradation. The aggradational phase is characterized by horizontally layered platform strata. Clinoforms steepen to 30-45°reaching heights of 850 m and are planar to concave. The evolution from progradation to aggradation, at the Bashkirian-Moscovian boundary, is attributed to increased foreland-basin subsidence and decreased boundstone accumulation rates. Progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top. Within the major phases, aggradational-progradational increments are produced by third-to fourth-order relative sea-level fluctuations.

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