Pennsylvanian ice-house carbonate platform cycles, dominated by algal-microbial reef mounds, accumulated during highstands in a marine foreland basin located in the eastern coast of Pangea and facing, in the W, the advancing Variscan orogen (Cantabrian Mountains, NW Spain). The upper Kashirian-Myackkovian nearly 2000 m-thick succession in the Ponga Nappe area comprises numerous metre- to decametre-thick cyclothems composed of: 1) mostly carbonates (Escalada Fm); 2) siliciclastics and carbonates (Fito Fm); and 3) mostly siliciclastics (Fito Fm). Stacked carbonate-dominated cyclothems accumulated in two stages forming two distinctive and mappable carbonate units. Unit 1 (late Kashirian-early Podolskian in age) covered a wide area of ~200 km2 (Caso tectonic unit) developing a flat-topped geometry with a steep and high-relief distal margin (Tiatordos Mt.). Once the Unit 1 was buried and the deep-water basin in front of the distal margin was filled-up by clay wedges, Unit 2 (early Myachkovian in age) developed in the more external Beleño and Sebarga tectonic units. In both cases, the carbonate platform pinched out toward the proximal sector of the foreland basin (in the west), where the carbonate succession was laterally replaced by siliciclastics that alternated and interfingered with limestone beds. Recent biostratigraphic data indicate that Unit 2 can be correlated westward with siliciclastic-carbonate cyclothems (Fito Formation) overlying? Unit 1. Both Unit 1 and 2 show an internal arrangement into metre-scale, shallowing-upward platform cyclothems consisting of 5-25 m thick, largely subtidal deposits and absence of peritidal carbonates. These features are common in glacio-eustatic cycles. Subtidal deposits mostly consist of mound-shaped accumulations with a vertically stacked disposition, and including algal colonization (beresellids and red phylloids), in situ precipitated micrite of possible microbial origin, and locally early marine cementation. Dark skeletal packstone and wackestone intervals are common in the lower part of the cycles; and skeletal and minor ooidal, grain-supported deposits form their upper part. Often, a thin, compacted and bioturbated interval of calcisphere-rich skeletal wackestones (including biomoulds of siliceous sponge spicules and colonization of Chaetetes) caps the cycles. In the upper Kashirian-lower Podolskian cycles, subaerial exposure surfaces are poorly developed or absent; whereas in the lower-Myachkovian cyclothems, prominent subaerial exposure surfaces are associated to palaeokarsts. In these cases, above the discontinuity, argillaceous palaeosols and coal seams show evidence of alteration by sea water, such as abundant pyritisation attributed to marine flooding of the subaerially exposed platform top during the initial transgression of a new cycle. An estimate of the cycle duration in Unit 1 and 2 yielded approximate values of 100ky. This is based on the presence of 11 cycles deposited during the late-Kashirian to early-Podolskian interval and considering a total duration of 2.5 and 1.5 My for the complete Kashirian and Podolskian time intervals, respectively (according to Menning et al., 2006). Similarly, Unit 2 includes 9 cycles deposited during the early Myachkovian, being 1.5My the total duration of the Myachkovian period (according to Menning et al., 2006). That is in agreement with the general-accepted idea that ice-house high frequency cyclicity reflects 100,000 year eccentricity rhythm-generated climatic changes (4th-order sequences). Additional studies of the complete Pennsylvanian succession, including revised biostratigraphic analysis, are necessary to confirm this calculation. Considering cycle duration of 100,000 year, calculated subsidence rate should range between 0.33 to 0.50 m/ky in Unit 1 and ~0.2 m/ky in Unit 2. These differences in flexural subsidence within the active foreland basin might be responsible for the development or absence of palaeokarst in the platform cyclothems.
Ice-house carbonate cycles in a Pennsylvanian carbonate platform of the Cantabrian Mountains (the Escalada Fm., Northern Spain) / J.R. Bahamonde, G. Della Porta, O.A. Merino-Tomé. ((Intervento presentato al 27. convegno IAS Meeting of Sedimentology tenutosi a Alghero nel 2009.
|Titolo:||Ice-house carbonate cycles in a Pennsylvanian carbonate platform of the Cantabrian Mountains (the Escalada Fm., Northern Spain)|
DELLA PORTA, GIOVANNA (Secondo)
|Data di pubblicazione:||21-set-2009|
|Parole Chiave:||carbonate platform cyclicity ; Carboniferous ; glacioeustasy ; Spain|
|Settore Scientifico Disciplinare:||Settore GEO/02 - Geologia Stratigrafica e Sedimentologica|
|Citazione:||Ice-house carbonate cycles in a Pennsylvanian carbonate platform of the Cantabrian Mountains (the Escalada Fm., Northern Spain) / J.R. Bahamonde, G. Della Porta, O.A. Merino-Tomé. ((Intervento presentato al 27. convegno IAS Meeting of Sedimentology tenutosi a Alghero nel 2009.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|