Growth and demise of Late Carboniferous carbonate platforms in the eastern Cantabrian Zone, Asturias, northwestern Spain

Abstract

Geometric relationships of seismic-scale stratal patterns observed on aerial photographs of Late Carboniferous carbonate platforms in the Cuera Region in Asturias, northwestern Spain, allow the identification of two discrete stages of platform development with different styles of deposition. The first phase, represented by the Bashkirian Valdeteja Formation, is characterized by progradation of more than 10 km with steep clinoforms of up to 35°, and slope heights up to 650 m. During the second phase, corresponding to the Moscovian Picos de Europa Formation, the platform aggraded mainly vertically, nearly 850 m. Both phases of platform growth were terminated by major relative sea-level lowstands in conjunction with local filling of the adjacent basins by shales and mudstones that onlap the platform flanks. Calibration with outcrops suggests a subdivision into the following lithofacies, directly related to the depositional profile: (1) platform interior, parallel, well- to thick-bedded skeletal pack- to grainstones; (2) margin, algal- and sponge-dominated bioherms with little internal structure; (3) slope, clinothems of medium- to thick-bedded margin-derived breccia beds; (4) toe-of-slope, alternating grain- and packstones with spiculites; and (5) basin, marly shales, shales, sandstones, and calcareous mudstones. Relative sea-level changes caused by regional tectonic movements are suggested to be responsible for the generation of the two platform phases; sea-level highstands triggered the nucleation and growth of the platforms, whereas major sea-level lowstands in combination with terrigenous infill of the basins terminated the platform growth. In addition, it is proposed that the termination of Late Carboniferous carbonate platforms is caused by the combined effect of a sea-level fall, subaerial exposure and erosion, and, finally, a rapid transgression that drowned and polluted the platform with shales and mudstones. Local variations in the infilling of the basins controlled the variations in the style of progradation. The subsidence rates for the vertically aggrading phase (Picos de Europa Formation) were approximately twice that for the progradational phase (Valdeteja Formation) and may explain the aggradational versus progradational character.