CONTROLS ON MICROBIAL DOME FABRIC DEVELOPMENT ALONG A CARBONATE-SILICICLASTIC SHELF-BASIN TRANSECT, MIOCENE, SE SPAIN

In the late Miocene (Messinian) of the Sorbas Basin, SE Spain, marine microbial domes forming biostromes and bioherms can be traced virtually continuously from the mid-shelf, down a steep paleoslope, to the basin Poor. On the shelf they developed in fan-deltas and oolite shoals. The fan-deltas spilled sediment downslope which was also colonized by and incorporated into the deeper water domes. The domes show macrofabric variations from laminated (stromatolite), to clotted (thrombolite), and structureless (leiolite, new term; Gk leios = uniform or smooth, lithos = stone: microbial deposit with structureless macrofabric). The stromatolites and thrombolites vary in the distinctness of their macrofabric, from distinct, to crude and diffuse. These macrofabrics relate to environment. Leiolites appear only in the mid-shelf and thrombolites at the shelf-break. Stromatolites are present throughout the transect, but dominate the slope and basin. Macrofabric types do not relate directly to either dome components or microfabrics. Both components and microfabrics are relatively uniform throughout the transect and reflect formation by a combination of agglutination of allochthonous grains and of in situ microbial calcification. Thus, accretion processes were similar throughout the transect, Macrofabric instead reflects variation in accretion style (regularity and evenness) in response to environmental controls, particularly water movement and illumination. Mid-shelf leiolite hemispherical dome formation was dominated by more-or-less continuous trapping of ooids, creating grain-dominated microfabrics. Episodic trapping of mainly siliciclastic grains produced thrombolite and stromatolite dome accretion a; the shelf-break and downslope. Where accretion was even, it resulted in laminated (stromatolite) macrofabric. Uneven accretion resulted in. clotted (thrombolite) macrofabric. On the shelf-edge high energy and good illumination promoted development of tall, mainly thrombolitic domes with crude to diffuse macrofabrics. Lower energy and illumination resulted in. regular and continuous, distinct-to-crude stomatolitic layering in lower relief slope and basin domes.