The lower-middle Cambrian boundary transition in Iran comprises the upper lower Cambrian Shale and Quartzite units of the Lalun Formation (nearly all siliciclastis) and the overlying lower middle Cambrian Member 1 carbonates (dominantly carbonates) of the Mila Formation, whose facies and stratigraphy reflect deposition on an extensive ramp platform in the northern passive margin of Gondwana. This paper focuses on facies and sequence stratigraphic analyses of the boundary interval to document the unconformable boundary on the Quartzite unit that may record the late early Cambrian global Hawke Bay (Toyonian) regressionand to define depositional sequences for regional and global correlation. The Shale unit unconformably overlies the fluvial red beds of the Lalun Sandstone unit. The unconformity is marked by a pebbly chert arenite containing black chert clasts and reworked caliche pisoids in places, which is coeval with the black chert conglomerate at the base of the Shale unit equivalent in east-central Iran. The Shale unit conformably underlies the Quartzite unit the base of which is marked by a change in depositional trend, but no evidence for an unconformity is recognized. This unit includes two reef horizons composed of dolomitized individual and compound metazoan buildups capped by planar- to wavy stromatolite. The upper contact of the Quartzite unit marks a regional unconformity and the abrupt appearance of shallow marine carbonates of the Mila Formation. The unconformity is characterized by a distinctive dark reddish brown to red-weathering horizon in which most of the sand grains are altered to a hematitic matrix and the sand content decreases toward the top of the profile. Close to the unconformable boundary, in a short stratigraphic interval (about 3 m), an open marine thrombolite reef zone capped by oolithic limestone is recognized near the base of the Mila Formation. The basal Mila boundary thrombolites, widespread development of stromatolite reefs, oncoids
