Spatial and temporal distribution of microbial carbonates, skeletal and non-skeletal grains in a Pennsylvanian carbonate platform (Valdorria, Northern Spain)
Abstract The Valdorria carbonate platform, in northern Spain, features a well-preserved and continuous outcrop of a Bashkirian platform to basin transect. During the Asatauian (late Bashkirian), the platform growth changed gradually from a progradational to an aggradational mode, recording 10 most likely short-eccentricity-controlled cyclothems. The facies of the platform-top and slope have been mapped, and the architecture of each cyclothem schematically reconstructed. The distribution of microbial carbonates (microbially mediated precipitates), skeletal (grazers/burrowers, corals/filter feeders, algae, foraminifera, Osagia-like oncoids and Thartarella-Terebella worm tubes) and non-skeletal grains (faecal pellets) has been quantified for the transgressive–regressive periods of deposition (transgressive, maximum flooding, early and late regressive intervals) of each cyclothem. This study shows that the distribution of microbial carbonates, skeletal and non-skeletal grains along a carbonate platform transect is variable through time and mainly governed by a set of measurable interconnected factors regulating the local palaeoenvironments: the water depth and the wave energy (facies) along the platform profile (inner, outer, break, slope) during periods of sea-level fluctuations (transgressive to regressive intervals of deposition). Auloporid corals, siliceous sponges, phylloid algae and Osagia-like oncoids are characteristic of a low-energy environment situated from estimated palaeo-water depths of 25 to 80 metres below sea level (mbsl onward) that formed in the outer platform during the maximum-flooding intervals. Anthracoporella–Archaeolithoporella boundstone is characteristic of the moderate-energy environments created in the horizontal inner-platform during the transgressive intervals. Microbially mediated precipitates are reliable indicators of the slope and Masloviporidium(?) indicator of low-energy environments of the upper slope. On the platform-top, Ungdarella and Donezella are more abundant during the early regressive intervals than during any other interval. Stacheoids are mainly present in the water depth range of 10 to 60 mbsl, tournayellids from 15 to 35 mbsl and archaediscids from 15 to 75 mbsl, all in low- to moderate-energy conditions. Rugose corals are common in the water depth range of 5 to 35 mbsl, either during the transition from the early to the late regressive intervals, as solitary forms or forming boundstone, or during periods of higher sea-level stand, as solitary forms in floatstone. Fenestellids, fistuliporids and trilobites occur mainly in the slope, but are also common in the platform top during the maximum flooding and/or the early regressive intervals. Archaeolithoporella is a reliable indicator of the platform top deposits, but its abundance is closely linked to the presence of adequate substrates. A high abundance in the assemblage of gastropods, endothyrids, palaeotextulariids, bradyinids and fusulinids characterizes the platform top shallow-water coated-grain grainstones formed under high-energy currents during the late regressive intervals. Tetrataxids act as encrusters on microbial precipitates of the slope or on phylloid algae and Archaeolithoporella of the platform top, but are also observed as nuclei of coated grains. Brachiopods are more abundant in low- to moderate-energy and 35 to 55 mbsl environments that prevailed during the transgressive intervals. Faecal pellets were commonly observed in low-energy environments, from 50 to 325 mbsl, at the platform break and in the slope, and the same applies for Thartarella-Terebella worm tubes that are virtually absent in high-energy deposits. Finally, crinoids, lasiodiscids and tuberitinids are randomly distributed and are not indicative of any specific environment. The here well-constrained distribution patterns of microbial carbonates, skeletal and non-skeletal grains in Valdorria outcrop hold the potential to serve as a reference for interpretation of depositional environments of other platforms using biota associations and depositional texture, particularly in regions where the platform geometry is not preserved or is not fully exposed.