ARCHAEOCYATHAN REEFS WITHIN A SILICICLASTIC SUCCESSION: NEW DISCOVERY IN THE CAMBRIAN (TOYONIAN) LALUN FORMATION OF NORTHERN IRAN, THE PROTO-PALEOTETHYS PASSIVE MARGIN OF NORTHERN GONDWANA

During late Early Cambrian, possibly due to the Toyonian regression, the pre-existing carbonate platforms of Iran and adjacent countries that bounded the Proto-paleotethys passive margin of northwest Gondwana became the site of deposition of a thick (over 1000 meters) red fluvial and marginal marine siliciclastics. In Iran these siliciclastics comprise the Zagoon and Lalun Formations. The Lalun Formation is divided into the lower Sandstone, the middle Shale, and the upper Top Quartzite units.

Two resistant reef horizons with well-defined syndepositional relief are present in the lower part of the Shale unit. They occur as reef complexes containing cabbage- or sack-shaped individual and compound buildups, surrounded by well-bedded colored shale and volumetrically less important pebbly archaeocyathid rudstone. The largest reef complex found in the study area is 1.7 meters high and extends laterally for 20 meters. Archaeocyathids, with fairly well-preserved skeletons, appear to be the primary constructors in the majority of buildups. Constructed by archaeocyathids or laminated stromatolite and archaeocyathid association, the individual buildups are 0.12 to 0.75 meters thick and 0.15 to 0.50 meters in diameter. The compound buildups consist of stacked archaeocyathid bioherms (Kalyptrate buildups), which are 1 to 2 meters thick and 0.75 to 1.2 meters in diameter. Sparry calcite cement is absent and the typically reddish-colored mud of the overlying shale fills the voids between the skeletons. In some cases, cross-laminated red siltstone containing reef debris (up to several centimeters in diameter) to very fine sandstone partially fills the primary inter-biohermal cavities in a few compound buildups.

The red siliciclastics of the Zagoon Formation and the lower Sandstone unit of the Lalun Formation were deposited in a fluvial (meandering river) environment. The Shale unit, including the reef horizons, records the inner to middle estuary and the regressive Top Quartzite unit represents an outer estuary/shoreface depositional setting, respectively. Lateral facies changes of the colored shale to various reef facies were apparently related to lateral depth variations within the drowned tidal channels of an estuarine depositional setting.