Abstract: 

Carbonate mounds are widespread in many places around the world and are especially well exposed in the Cantabrian Mountains, northern Spain. This study focuses on the composition and growth dynamics of two large mound complexes well exposed near the village of Puebla de Lillo, 70 km northeast of León, and evaluates the impact of sea level, accommodation, and siliciclastic input on mound growth.

The mound substrate consists of bedded dark skeletal bindstone. The core facies exhibits patchy growth forms of alternating massive microbial–algal boundstone and skeletal and nonskeletal wackestone, reflecting the overall mound internal composition of smaller coalescent bodies. Microbial activity, as inferred from the volumetrically important clotted peloids, appears to be the main factor leading to the high accumulation rate in the cores. The steeply dipping flanks presumably resulted from early cementation and subsequent lithification and stabilization induced mainly by microbial activity. In the intermound strata, carbonate beds and small mounds alternate with fine-grained sandstone, siltstone, and shales. The intermound carbonates consist of skeletal and intraclastic mudstone to wackestone, whereas algal–microbial boundstone is a minor component. Overall, facies transition from the mound core towards the flanks or the mound-capping beds is gradual. However, the volume of bioclasts and intraclasts increases towards the flank and capping facies, inversely to amount of clotted peloids or micrite.

The substrate and the cores of the two mound complexes likely formed in the photic zone, as inferred from the common calcareous algae. Carbonate precipitation favored by microbial activity and algal growth along with gain in accommodation space led to mound-core accretion. The growth of the small mound complexes was most likely stopped by recurrent sea-level fall and siliciclastic input. The large, high-relief mound complex actually kept growing until reaching shallow-water and high-energy conditions. Unlike many late Paleozoic counterparts, these mounds were not subaerially exposed during growth. The great thickness and the aggradational mound growth are indicative of a high subsidence rate. At a later stage, however, siliciclastic input also buried the large mound complex. Rapid subsidence, sea-level fluctuations, and siliciclastic input were thus decisive factors controlling carbonate production.

This study reveals that mound complexes nucleated and grew on preexisting paleohighs, maintaining consistent growth in these specific settings. Paleohighs, recurrent sea-level fluctuations (presumably linked to glacio-eustasy), and episodic siliciclastic input are the principal factors that controlled the shape, size, and growth evolution. Furthermore, the internal architecture as a mosaic of juxtaposed small bodies composing the complexes represents an important finding for better understanding of mound growth and, thus, for better constraining reservoir heterogeneity, particularly for upper Paleozoic deposits.

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