The lower–middle Cambrian transition and the Sauk I-II unconformable boundary in Iran, a record of late early Cambrian global Hawke Bay regression

The lower–middle Cambrian boundary transition in Iran consists of the upper lower Cambrian Shale and Quartzite units of the Lalun Formation (nearly all siliciclastics) and the overlying lower middle Cambrian Member 1 carbonates (­dominantly carbonates) of the Mila Formation, the facies and stratigraphy of which 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, which may record the late early Cambrian global Hawke Bay (Toyonian) regression, and 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, and it 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 where 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 (~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, and ooids, and absence of metazoan reefs in the middle Cambrian Member 1 of the Mila Formation may indicate a stressed ecosystem in the aftermath of the Hawke Bay extinction event and reef crisis similar to basal Triassic deposits. Four depositional sequences (third-order cycles) are recognized in the late ­early Cambrian–early middle Cambrian interval that are similar to the south China sequences. There is a good correlation between the unconformable boundaries recognized in this study and the major global sea-level falls reported for the late early Cambrian and early middle Cambrian intervals. The unconformity-bounded upper lower Cambrian fluvial red bed succession that overlies the lower Cambrian shallow-marine carbonates signifies the first pause in the Cambrian transgression. It correlates with the major global lowstand at the base of the Toyonian Stage and is here referred to as the early Toyonian Hawke Bay regression. The conspicuous unconformity on the Quartzite unit that underlies the basal transgressive package and the boundary thrombolite reef zone near the base of the Mila Formation indicate a major hiatus that is interpreted to correspond to the culmination of the global late early Cambrian Hawke Bay regression (Sauk I-II unconformity) and the Hawke Bay extinction event. The event resulted in the demise of metazoan reef builders and the emergence of microbialite reefs, oncoid, and ooid facies in the middle Cambrian basal members of the Mila Formation.