Pennsylvanian carbonate platforms in distal shelf areas of a marine foreland basin (Escalada Fm., NW Spain)
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Data di Pubblicazione:
2011
Citazione:
Pennsylvanian carbonate platforms in distal shelf areas of a marine foreland basin (Escalada Fm., NW Spain) / J.R. Bahamonde, G. Della Porta, O.A. Merino Tomé. ((Intervento presentato al 28. convegno IAS Meeting of Sedimentology tenutosi a Zaragoza, Spain nel 2011.
Abstract:
The Carboniferous foreland basin located in the western European Variscan belt (Cantabrian Mountains, N Spain) was filled by progradational clastic wedges showing a regional lateral transition from proximal alluvial conglomerates and coal-bearing deltaic sequences to distal fine-grained marine deposits including carbonates. Carbonate beds alternating with sandstones, siltstones and mudstones distally converged forming flat-topped carbonate platforms. The two most extensive and thick (hundred of metres) carbonate units, developed in the Ponga Nappe Province, have been addressed jointly as Escalada Fm.
During a first stage (Kashirian-Podolskian), the Escalada Fm. platform covered nearly 5000 km2 and reached a mean thickness of 300 m, interfingering landward with deltaic cyclothems (Sama Group). Distally, the carbonate platform developed a steep, 500 m-high slope, fronting a starved basinal area (Beleño Fm.). The depositional slope consisted of thick steeply dipping strata (in situ microbial boundstones), which pinched out into calciturbidites, argillaceous spiculites and breccias at the toe of slope. Platform margin mostly aggraded, but minor progradational and retrogradational phases occurred. Platform-top carbonate successions comprise: 1) a lower part consisting of coated-grain and ooidal-grainstone packages (accumulated in high-energy areas close to the platform margin); and 2) a 200-300 m-thick upper part, mainly composed of tabular and mound-shaped beds of skeletal pack- to wackestones including algal colonies (beresellids and red phylloids), in situ precipitated microbial micrite and locally early marine cementation.
In a second stage (lower Myachkovian) a new Escalada Fm. platform developed beyond the previous platform margin, once the starved basinal areas (Beleño Fm.) were filled-up by a renewed clastic input. The 200 m-thick succession is arranged in tens of metre-thick, 4th-5th order shallowing upward cycles, characteristic of icehouse periods: absence of peritidal carbonates and presence of palaeokarsts at the cycle boundaries. Shoreward, limestone strata intercalated with sandstones, siltstones and mudstones (Fito Fm.) mainly consisting of two lithofacies: a) nodular, skeletal wackestones including siliceous sponge spicules, Chaetetes, calcispheres, foraminifers and echinoderms; and, b) up to 70 m-thick, massive algal boundstones with microbial micrite and early marine cements. Landwards, carbonate facies are marls, nodular wackestones and red and Anthracoporella algal bafflestones.
The Escalada and Fito Formations record the interaction between carbonate production and siliciclastic input into an active foreland basin, when not only tectonics but also glacioeustasy affected relative sea-level changes. The carbonate platforms are interpreted as highstand and transgressive deposits of 3rd order sequences, covering vast distal realms of a siliciclastic shelf, while basin infill phases mostly coincided with lowstands.
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Keywords:
carbonate platform ; Carboniferous ; Spain ; microbialite
Elenco autori:
J.R. Bahamonde, G. Della Porta, O.A. Merino Tomé
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