Laminación y bioturbación en carbonatos lagunares: interpretación genética (Cuenca del Guadiana, Badajoz)

Abstract

The sedimentological study of both polished and thin sections of samples taken from the 1-2 m thick unit of laminated carbonates overlying the Miocene succession infilling the Neogene Guadiana Basin (western Spain) reveals the existence of prominent features that are discussed in the paper.
The lamination visible in hand specimens is due to the alternance of massive and laminated layers. In thin sections, the massive layers are made or intraclastic, ooidal pelletoidal particles, whereas the laminated ones show gently undulating, alternating light and dark-coloured layers. Inside the laminated and, to a lesser extent, the massive leyers it is possible to recognize; (a) abundant vestiges of algal filaments, (b) abundant remains of charophites, (c) small, usually grouped-up, cavities (mean diameter about 250 microns) and bigger, pellet-filled cavities, both types related to burrowing; (d) sparse prisms of calcite resembling those of Microcodium, and (e) cclusters of trapezohedral crystals of calcite with rhomboidal and. lenticular sections,these being more abundant in the massive layers.
The sedimentological interpretation assumes that sedimentation took place in swamps with periodic oscillations of the water level. The laminated layers, el stromatoIithic nature, correspond to quiet realms under constant water level during dry stages. The massive layers record higher environmental energy during flood stages, that is able to remobilize the cracked sediments of the former dry stages.
The vital activity of Chironomid insects lead to the two described types of burrowing: the small grape-like cavities correspond to eclosing eggs whereas the bigger pellet-filled cavities resulted from mining by larvae