Subaqueous Siliciclastic Stromatolites: A Case History from Late Miocene Beach Deposits in the Sorbas Basin of SE Spain

Abstract

Agglutination of particles is a major process involved in microbial dome formation. Early lithification by biogenic precipitation or early cementation is also essential for dome accretion and preservation. Although it is possible for agglutinated grains to be siliciclastic, reports of siliciclastic microbial domes are scarce. Early lithification of microbial mats seems to have taken place only rarely in settings with terrigenous influx in marine environments. The few recorded examples of siliciclastic stromatolites and thrombolites occur, however, in a wide range of sedimentary environments, from coastal lakes to relatively deep subtidal settings, and from the Cambrian to the late Cenozoic.

We describe a new example of siliciclastic stromatolites in a Messinian (Late Miocene) post-evaporitic unit (the Sorbas Member) near Sorbas town in the Sorbas Basin of Almeria, southern Spain. These stromatolites occur in beach deposits. Stromatolite domes formed at the transition from the lowermost beach shore-face to the shelf. They are made up of dense, peloidal, clotted and bushy micrite, interpreted as microbial precipitates, together with siliciclastic particles which constitute up to 40% of the rock volume. Within a single stromatolite bed there is variation in dome composition and morphology. Proximal domes contain sand-grade siliciclastics, distinct lamination and high synoptic relief with steep sides. Downslope, they grade into large, flattened, silty stromatolites with gentle sides, intercalating with silt lenses. Microbial mats developed at the bottom of the lowermost part of the beach, but were inhibited by stronger wave energy at shallower settings. Deeper waters on the shelf were probably too dark for mat growth and dome formation.