ABSTRACT

Large coarse-grained siliciclastic stromatolite and thrombolite domes, forming biostromes and bioherms, occur in a marine fan-delta environment of Messinian age in the Sorbas Basin, Spain. Individual domes are up to 3 m in height and have estimated synoptic relief up to 1.5 m. They occur on a shelf-to-basin transition in the Terminal Complex, a mixed siliciclastic-carbonate unit which grades from coarse conglomeratic fan-delta and oolite deposits on the shelf, to basinal sands and silts of the Sorbas Member. Paleoslopes up to 35 degrees, extending downward for several hundred of meters to original water-depths of 70 m or more, mark the transition to the basin. Normal marine conditions are indicated by coralline algae and Porites within the domes. Thrombolites are restricted to he shelf and upper slope and are closely associated with stromatolites. The domes of the mid-slope and deeper environments are exclusively stromatolitic.

Medium to coarse siliciclastic sand is common in domes at all depths except on the inner shelf, where ooids are dominant. Quartz and metamorphic rock pebbles and granules are locally present in the thrombolites and stromatolites of the shelf-edge and upper slope, and boulders form nuclei of some of these domes. Carbonate grains, mainly ooids and peloids, are present in all domes. However, the dominant carbonate component is mainly micrite in the form of micro-bushes and clotted fabrics.

Important controls in the formation of these siliciclastic stromatolites and thrombolites were: 1) supply of siliciclastic grains, and 2) early carbonate cementation to maintain the relief of the domes during formation and to preserve primary fabrics after burial. Depth-limited microbial components of the mats, probably algae, may have assisted in trapping coarse sediment. They were also possibly responsible for limitation of the thrombolites to the shallow shelf environment.