Lithofacies and depositional processes on a high, steep-margined Carboniferous (Bashkirian-Moscovian) carbonate platform slope, Sierra del Cuera, NW Spain

摘要

The depositional slope of the Sierra del Cuera, a Carboniferous (Bashkirian-Moscovian), high, steep-margined carbonate platform, provides excellent cross-sections of lithofacies zonations and associated stratal geometries. The steep (30-40°) and nearly planar upper slope is dominated by massive sheet-like layers of microbial, cement-rich boundstone, which alternate with intercalations of red-stained bryozoan cementstone with crinoids and brachiopods. The slightly gentler (20-26°) and concave-upward lower slope is characterized by clast-supported resedimented deposits. The upper slope extended from platform break to ca. 300 m water depth, whereas lower slope sediments were deposited in water depths up to 600-700 m, at which level the slope beds flatten to a few degrees (toe-of-slope) and interfinger with spiculitic and argillaceous basinal sediments. Between 250 and 450 m water depth, boundstones and breccias alternated in a transitional zone. The lower slope sediments include clast-supported breccias with radiaxial fibrous cement in interparticle space, mud- to clast-supported breccias with red-stained carbonate mud matrix and packstone to grainstone and rudstone beds. Most of the clasts comprise boundstone reworked from an upper slope setting and smaller grains are platform and slope derived. A pervasive submarine cementation occurs along the upper two-thirds of the flank and this stabilized the slope. Slope deposition is interpreted as follows. During active boundstone accretion, microbial boundstone layers slid off and formed breccia tongues extending from the lower upper slope down to the toe-of-slope. Rock falls and avalanches were generated whenever the shear strength of the substrate of loose (or partly lithified) sediment was exceeded. Upper-slope boundstone accretion and shedding, independent of the depth of light penetration, controlled most of the depositional processes on the slope. Cement-dominated intervals are considered to be related to early highstand (and/or flooding) phases. Relative sea-level fluctuations and/or associated changes in the water conditions are believed to be responsible for intervals of low boundstone production or cement precipitation. Whether the in situ boundstone and breccia are preferentially related to lowstand or highstand periods is, as yet, unclear.