OOLITE STROMATOLITES AND THROMBOLITES, MIOCENE, SPAIN - ANALOGS OF RECENT GIANT BAHAMIAN EXAMPLES

At El Joyazo in Almeria, southeast Spain, closely packed thrombolite and stromatolite domes of Messinian age form a bioherm 12 m thick surrounded by oolite. Individual domes, up to 1.5 m high and 4 m across, are typically composite, with distinct stromatolitic and thrombolitic parts which are sharply juxtaposed. Both parts are composed of ooid and peloid fine-medium sand. Stromatolitic parts show good lamination and often contain small finger-like columns. Thrombolite is more abundant than stromatolite and has a distinct clotted fabric produced by irregular fenestrae. In size, general shape, internal structure, depositional environment (ooid shoal), grain size (fine-medium sand), and grain composition (ooids, peloids) the stromatolitic parts of the domes are closely comparable with Recent "giant" subtidal columnar stromatolites at Lee Stocking Island, Bahamas, and we regard them as analogues of these Recent examples. At present no modern analogues are known for the thrombolites. Previously, large Messinian stromatolites elswhere in southeast Spain have been attributed to hypersalinity caused by the Mediterranean "salinity crisis". The El Joyazo examples formed in current-swept ooid shoal environment, probably of normal marine salinity. We suggest that mobile sands, rather than hypersaline conditions, provided the stress to inhibit predators and competitors. These "giant" Messinian coarse-grained domes, like the Lee Stocking Island stromatolites, may have formed because of the presence of algae, such as chlorophytes and diatoms, capable of trapping large grains.