Mesozoic Pelagic Phosphate Stromatolites from the Penibetic (Betic Cordillera, Southern Spain)

Abstract

In the Penibetic (Southern Spain), a Mesozoic pelagic swell of the Western Betic Cordillera, several examples of pelagic phosphate stromatolites have been detected. They lie on three regionally important stratigraphic discontinuity surfaces of different ages respectively: the Dogger-Malm boundary, the base of the Upper Valanginian and the Lower Albian. The stromatolites are usually several centimetres in size and their morphology varies between ovoidal, subspheroidal or flattened nodules (oncoids) and laminated crusts of planar, cupola-like or irregular shape. Microscopic examination reveals that stromatolite building is essentially the result of the textural interaction of four types of elements: 1) microbial filaments of probable bacterial origin; 2) phosphates and minor amounts of other minerals (goethite, pyrite, etc.); 3) pelagic sediment rich in planktonic foraminifera and coccoliths; and, 4) encrusting foraminifera. These elements appear interrelated in various ways giving rise to several mesoscopic morphologies and microscopic stromatolite fabrics, sometimes arranged in cycles. Stromatolite accretion occurred as a result of the piling-up of microbial, especially bacterial filament bundles and, due to their life processes, these bacteria precipitated pyrite and floculated jelly-phosphate substances around the filaments. Small pelagic particles, especially coccoliths, were trapped and bound during stromatolite growth. Encrusting foraminifera consolidated the structure. The growth rhythms or cycles reveal that there have been periodic variations of different duration in the speed and mode of stromatolite growth (variations which are especially obvious in Albian-Vraconian examples). These periodic changes that affected a pelagic realm could have been caused in the last instance by climatic factors which have modified environmental parameters (currents, temperature, planktonic productivity, etc.) and/or determined the existence of complex microbial ecological successions. The formation of Penibetic phosphatic stromatolites occurs in not very deep (several hundred meters at most) zones of very low sedimentation rate, affected by oceanic (upwelling or geostrophic) currents, near the O2minimum layer. In some examples (Albian-Vraconian) the contemporaneous existence of a low-temperature submarine hydrothermalism cannot be excluded.