Mesozoic Stratigraphic Breaks and Pelagic Stromatolites in the Betic Cordillera, Southern Spain

Abstract

In the Jurassic and the Cretaceous of the Betic Cordillera (Southern Spain) numerous examples of pelagic stromatolites have been found. In the present study a close relationship between the situation of pelagic stromatolites and stratigraphic discontinuity surfaces is shown to exist. These surfaces can be encrusted by ferruginous, manganiferous or phosphatic stromatolites, or covered by a bed very rich in oncoids with a core made up of ammonites and/or pelagic calcareous sediment. The textural and morphological features of these structures show that they are microbial (possibly bacterial and/or fungal) in origin. They form irregular and laterally discontinuous, decimetric to millimetric horizons which grow in areas of pelagic swell with a very slow sedimentary rate. The discontinuity surfaces are generally related to falls in sea-level that provoked interruptions of sedimentation reflected in omission surfaces, hardgrounds and, more locally, paleokarstic surfaces. The growth of stromatolites and oncoids indicate the base of transgressive cycles, with very slow sedimentation. The discontinuity surfaces with pelagic stromatolites and oncoids are situated in the following: a) Middle Liassic, b) Base of Middle Jurassic, c) Top of Bathonian, d) Dogger-Malm boundary, e) Kimmeridgian, f) Upper Valanginian-Hauterivian, g) Upper Albian and h) Cretaceous-Tertiary boundary. Stromatolite growth accounted preferently on swells, in submarine and moderatelly deep to deep environments, during sea-level rises after lowstands. They mark the beginning of transgressive cycles and, usually, are finally buried under pelagic sediments when sea-level rise progresses. Their growth always accounts in extremely starved environments, and was favoured by submarine hydrothermalism and bottom currents. Under these hard environmental conditions, microbial communities and, in a lesser extent, encrusting foraminifera and serpulids, were the only agents able to survive and accrete pelagic biosedimentary deposits.