Development of albian microbialites and microbialite reefs at marginal platform areas of the Vasco-Cantabrian Basin (Soba reef area,Cantabria, N. Spain)

The Middle Albian sequence from the western marginal area of the Vasco-Cantabrian Basin contains calcified microbialites in different marine depositional environments, individually well defined by microstructure, lamina characteristics and mode of formation. Microbialites may form the primary framework of reefs, which occur as composite stacks in mid to lower slope environments or as isolated bodies in small intraplatform basins. In most areas microbialite reef growth was initiated below the photic zone. Stratiform intercalations of microbialites and composite microbialite/foraminifer oncoids are restricted to well bedded carbonate platform deposits (Urgonian). Three basis types of microbialites are recognized:

(i)	Dense micritic/fenestral microbialites corresponding to laterally linked, stacked stromatolitic hemispheroids. The development and preservation of stromatolitic structure is a function of sediment supply and secondary obliteration by succesive boring activities. They were calcified in situ at the surface with irregularly curved linings of microcrystalline carbonate. Dense micritic/ fenestral microbialites, variously developed and preserved, are the main contributors to microbialite reefs. Microbialites form hard substrates bored by lithophagous pelecypods and boring sponges (Aka sp.). The main associated faunal elements include lithistid and coralline demosponges, hexactinellid sponges, encrusting foraminifera, brachiopods, polychaetes, and bryozoans.
(ii)	Dense micritic/peloidal microbialites with subplanar, arhythmic lamination (binding habit). They were calcified in situ below the surface in conjunction with decaying organic matter. At large scale, they occur in shallow water, i.e. within the photic zone. They cover earlier microbialite reefs or occur on and in episodic deposits of coarse biodebris. At small scale they occur in protected microenvironments (e.g. intraparticle space, boring cavities).
(iii)	Peloidal/in situ ooid microbialites with subplanar/ wavy lamination occur as small-scale stratiform intercalations in carbonate platform deposits, episodically revealing physical reworking. Other features are very similar to dense micritic/peloidal microbialites.

The results of geochemical analyses indicate a rock-buffered diagenetic system during early diagenetic and burial history of microbialite reefs. Independent of microbialite type residual MgCO₃-contents are in the range of 1.20 to 3.57 mole %, agreeing well with those from isopachous rim cements and indicating a high Mg-calcite precursor of microbialite micrites. Stable isotope values (δ¹³C) are in the range of 3.13 to 3.80 (permil, vs PDB), close to the internal standard, the coralline spongeAcanthochaetetes (Albian species=2.93; Recent species=3.27) and comparable with inorganically precipitated Mg-calcite.