Elsevier

Sedimentary Geology

Volume 145, Issues 3–4, 15 December 2001, Pages 235-252
Sedimentary Geology

Internal structure and depositional environment of Late Carboniferous mounds from the San Emiliano Formation, Cármenes Syncline, Cantabrian Mountains, Northern Spain

https://doi.org/10.1016/S0037-0738(01)00150-6Get rights and content

Abstract

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 Carnic Alps in Austria. Mound relief is explained by (1) accumulation of peloidal-clotted sediments limited to boundstone and probably related to microbial activities, (2) widespread marine cementation within this area, and (3) low export of mound fossils to intermound areas.

The position of the mounds within the sequence, and their initiation, size and termination, seem to be mainly controlled by sea-level fluctuations and siliciclastic input.

Introduction

Most studies of Upper Carboniferous buildups focus on phylloid algal mounds, especially those from the American Midcontinent. Although phylloid algae are the major mound builders, mounds built by other algae have been reported, e.g., Anthracoporella dasycladalean algal mounds (Samankassou, 1998). The growth fabric of algal mounds is an important aspect, which has not been investigated in detail: whether they consist of detrital accumulations of algal thalli (Ball et al., 1977) or of in situ thalli, implying a framework (examples in Samankassou, 1998, Samankassou and West, 2000) needs further investigation. Another important topic is the role of microbial activity in the accumulation, consolidation, and lithification of the mound structure Pratt, 1982, Pratt, 1995, Webb, 1996, Kirkland et al., 1998.

Algal mounds in the Cantabrian Mountains have been described by Bowman (1979) and Riding (1979). Diagenetic features of the mounds described herein have been studied by Hensen et al. (1995). The results presented here are based on detailed sampled sections focusing on the microfacies, paleontology, and paleoecology of the mounds. Additional sampling of the facies surrounding the mounds (base, top, and intermound rocks) provided data on the depositional environment of the mounds and on the dynamics of mound growth.

Section snippets

Location, stratigraphy

The Cantabrian arc is the northernmost zone of the Iberian Massif Lotze, 1945, Julivert, 1971. It is subdivided in the Cantabrian, West-Asturian-Leonese, Galacian-Castillan, Ossa-Morena, and South Portuguese zones (see a recent overview in Dallmeyer and Martinez Garcia, 1990). The Cantabrian Zone is subdivided into the allochthonous Asturian-Leonese and the autochthonous Palentine domains. The Asturian-Leonese domain comprises the Somiedo-Correcilla, Sobia-Bodón, Aramo, Ponga, Central Coal

Facies analysis

The studied succession is characterized by a cyclic alternation of fine-grained marine sandstone, siltstone, grey shale, and limestone (see Dingle et al., 1993 for a recent review). Bedded limestone overlies siliciclastic rocks (fine-grained sandstone and shale), and passes upward into mounded limestone (Fig. 3). Bedded limestone, with siliciclastics, overlies the mounds.

Mound paleontology and paleoecology

Mound-forming fossils are briefly described in this section, and their relationships discussed. To better characterize mound biota, some fossils that are rare within the mound but common in rocks above, below, and in intermound areas are also considered.

Growth and demise of mounds, controlling factors

Ooids occur below the mounds, and as they are not continuous laterally, suggest the presence of ooids shoals/bars on which the mounds grew. Ooid shoals (unstable seabed; Flügel, 1982) were first stabilized, as indicated by the occurrence of coated grains (stable seabed), before mound nucleation. Ooid bars and the oncolitic facies above these formed the mound substrate facies. In this siliciclastic–carbonate system, shoals built local highs, which seem to be the sites appropriated for growth

Comparison

Detailed reports of Donezella-dominated mounds are scarce. Data presented in this paper are compared only to that contained in a recent report of Donezella-dominated mounds by Choh and Kirkland (2000) from Oklahoma, USA.

⋅ The mound substrates are similar in both areas, characterized by high-energy components.

⋅ The boundstone facies of the mound intervals are similar in that both contain pores, encrustations, micritic cements, and Archaeolithophyllum. They differ in the abundance of siliceous

Conclusions

The 6–12-m-thick mounds in the Cármenes area are characterized by skeletal–microbial boundstone. Donezellid algae, agglutinated worm tubes, and calcisponges are the dominant fossils. Accessory fossils are smaller foraminifers, gastropods, and brachiopods. A peloidal-clotted matrix is characteristic and accounts for more than 30% of mound volume. Most intraframe pores are filled with peloidal sediment and early marine cement. Mounds grew on stabilized ooid shoals as indicated by the occurrence

Acknowledgements

I address my thanks to Beate Fohrer (Erlangen, Germany) for fieldwork assistance and to Priska Schäfer (Kiel, Germany) and her working group for their cooperation. Greg Wahlman (Texas, USA) gave helpful advice on agglutinated tubes and discussed some aspects of Upper Paleozoic buildups; Elisa Villa (Oviedo, Spain) kindly made her knowledge on stratigraphy of the San Emiliano Formation within the frame of the Cantabrian Mountains available to me. Thorough reviews of an earlier version of the

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