Archaeocyathan buildups within an entirely siliciclastic succession: New discovery in the Toyonian Lalun Formation of northern Iran, the Proto-Paleotethys passive margin of northern Gondwana

Meter-scale buildups constructed exclusively by archaeocyaths have been recognized within the uppermost Lower Cambrian siliciclastic succession of Eastern Alborz in northern Iran. They are the only known Toyonian reefs in Iran and adjacent countries and occur in the lower part of the Shale unit of the Lalun Formation. The reefs are of limited lateral extent, reach a maximum thickness of 2.5 m and consist of several reef complexes containing cabbage- or sack-shaped buildups surrounded by well-bedded, colored shale. Each reef complex in the main reef zone consists of meter-scale individual and kalyptrate (compound) buildups that are overgrown by laminated stromatolite. The individual buildups are 12 to 75 cm thick and 5 to 50 cm in diameter, but the compound buildups are up to 2 m thick and their diameter ranges from 75 to 120 cm. In contrast to most Lower Cambrian reefs, these compound buildups demonstrate a complete ecological succession including pioneer and climax phases. Archaeocyaths in the buildups are solitary and colonial types showing great diversity of growth forms, with vase-, bowl- and cup-shaped and cylindrical forms present. The cups range from less than 1 mm to 1.5 cm in diameter and are up to 4 cm tall in the majority of the skeletons, but they may reach up to 4 cm in diameter in the massive colonial forms or in the branching forms of the upper outer part of the compound bioherms. The limited lateral extent of the reef horizons, and lateral facies changes in the colored shale toward various reefs supports deposition in drowned tidal channels in an estuarine depositional environment. A change of the individual bioherms to larger compound buildups suggests lateral depth variation of the tidal channels. Partial infilling of the primary inter-biohermal cavities by large reef clasts and cross-laminated siltstone to very fine sandstone suggests occasional disturbances by storms and periodic influx of coarse siliciclastics. In striking contrast to other Lower Cambrian reefs, the archaeocyathan buildups of the Alborz flourished in a completely siliciclastic setting and lack the skeletal calcimicrobes which dominated the Lower Cambrian reefs. Abrupt lateral and vertical facies changes to colored shale and fluvial red beds, the presence of infiltrated shale between the skeletons, and in central cavities and the intervallum of the archaeocyaths suggest highly turbulent and turbid incoming water with abnormally high concentrations of fine siliciclastic material during reef development. High concentrations of fine, suspended siliciclastics could well have prevented the light penetration which was necessary for calcimicrobial growth. In addition, the relatively small size of the archaeocyaths and absence of reef dwellers is very likely a consequence of the high terrigenous mud content and, perhaps, below-normal salinity of the ambient sea water. In the absence of calcimicrobes and suspension feeder metazoans in the dark and muddy water of the mid-estuarine setting, archaeocyathans became the only bioconstructors of the Iranian buildups. Abnormally high concentration of nutritious fine siliciclastics suggests that photosymbionts and oligotrophic conditions were not needed by archaeocyaths; particular hydrodynamic conditions along with high nutrient flux, rather than light, were essential for archaeocyathan communities.