| Summary: | Well-exposed mounds are common in limestone of the Late Carboniferous San Emiliano Formation, Cantabrian Mountains (Northern Spain). They occur as obvious primary topographic features. Careful study of the mound intervals and surrounding strata revealed the internal structures of mounds and the factors controlling their growth. The substrate (2–3 m) of the mounds consists of greyish to reddish, bedded oolitic and oncolithic packstone and grainstone. Crinoids, fragments of the alga Epimastopora, and, rarely, bryozoans are present. Ooids and oncoids indicate a wave-dominated high-energy environment. Presence of quartz indicates the influence of terrigenous siliciclastic input. Mound intervals (6–12 m thick) are characterized by skeletal–microbial boundstone. Donezellid algae, agglutinated worm tubes, and calcisponges are the dominant fossils. Smaller foraminifers, gastropods, and brachiopods are also present. A peloidal-clotted matrix is characteristic and accounts for more than 30% of the mound volume. Intraframe pores are mainly filled by peloidal sediment and early marine cement. Intermound strata are approximately one-third as thick as time equivalent mounds. Mound fossils (algae, agglutinated worm tubes, and sponges) are uncommon. However, intermound strata are generally more diverse than the mounds, containing fusulinids, smaller foraminifers, bryozoans, gastropods, crinoids, and bioclasts. Some of these fossils have micritic envelopes. Bedded packstone and grainstone, 3–6 m thick, with siliciclastic debris, rugose corals, and chaetetid sponges characterize the capping facies. Coated grains and small ooids are uncommon. This facies indicates shallowing to a higher energy environment and/or a higher input of siliciclastics, inhibiting mound growth. Mounds are interpreted to have accreted in a quiet environment below wave base. This position is comparable to the depositional environment inferred for many Late Paleozoic mounds described elsewhere, e.g., from Texas and New Mexico, Canadian Archipelago, and ...
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