The lower part of the Shale unit of the uppermost Lower Cambrian Lalun Formation in Tuyeh-Darvar section of Eastern Alborz include two reef horizons composed of reef complexes containing cabbage-or sack-shaped individual and compound (individual bioherms stacked one upon the other) buildups. The reefs are constructed by a consortium of archaeocyathids/sponges and coralomorphs and are surrounded by well-bedded colored shale. They are unique in that they lack the skeletal calcimicrobes, which dominated the Early Cambrian archaeocyaths reefs. The bioherms of the Lalun Formation are the only known Toyonian reefs in Iran and adjacent countries. The reefs flourished in turbid waters associated with high siliciclastic input in the high energy abandoned tidal channels of the middle estuarine depositional setting. The regressive Top Quartzite of the Lalun Formation which underlies the L-M Cambrian regional unconformity indicates a major Toyonian regression corresponding to the Hawke Bay extinction event that resulted in total destruction of the Lower Cambrian metazoan reef builders. Following post Early Cambrian reef crisis, only thrombolite and stromatolite reefs developed in the lower members of the Middle Cambrian Mila Formation
