Lower-Middle Cambrian Boundary in Iran
|Category||Paleontology and Stratigraphy|
|Location||The 25 Symposium of Geosciences|
|Author||Yaghoob Lasemi & Hadi Amin-Rasouli|
|Holding Date||01 July 2007|
The Lower-Middle Cambrian boundary interval in Iran۱:country-region>۱:place> includes the Shale and Top Quartzite units of the Lalun Formation and the lower part of the Mila Formation. Due to lack of guide fossils in this interval, there have been conflicting views on the Lower-Middle Cambrian boundary position of Iran۱:country-region>۱:place>. Recent discoveries of metazoan reefs in the Shale unit of the Lalun Formation and thrombolite bioherms at the base of Member ۱ carbonates of the Mila Formation have provided evidence for global correlation and recognition of the Lower-Middle Cambrian boundary position in Iran۱:country-region>۱:place>. In additions, facies analysis and sequence stratigraphy of the Cambrian sections of Iran۱:country-region>۱:place> have revealed the presence of a regional unconformity on the Top Quartzite unit of the Lalun Formation. The unconformity is traceable in many parts of the world. In Tuyeh area of east Central Alborz, the Shale units unconformably overlies the lower Sandstone unit of the Lalun Formation and gradationally underlies white mature sandstone of the Top Quartzite unit. In this section two dolomitized metazoan reef horizons are present near the base of the Shale unit. In the Shahmirzad section, about ۶۰ kilometers west of Tuyeh section, a thrombolite reef zone occurs at the base of Mila Formation, about ۳ meters above the Top Quartzite unconformable boundary. The thrombolites are domical and their structure is dominated by clotted and spar filled fenestral fabrics.
The fluvial red beds of the uppermost Lower Cambrian succession (Zaigoon Formation and the lower Sandstone unit of the Lalun Formation), which are bounded by two major unconformities signify a pause in the Cambrian transgression and most likely corresponds to early Toyonian regression. The well-documented unconformity on the upper surface of the Top Quartzite unit indicates a late Toyonian regression that resulted in the demise of the Lower Cambrian metazoan reef builders. Following late Toyonian event, the Hawke Bay۱:place> extinction even, only microbialite (stromatolite/thromboloite) reefs appear in the Middle Cambrian successions worldwide. In the Middle Cambrian deposits of Iran۱:country-region>۱:place> (e. g., Members ۱ and ۲ carbonates of the Mila Formation of the Alborz), too, only thrombolite and stromatolite reefs are recognized. The Top Quartzite of the Lalun Formation, similar to regressive Hawke Bay Quartzite of North America and equivalent intervals in other parts of the world, conformably overlies inter-bedded shale, sandstone and carbonates (including metazoan reefs) and unconformably underlies microbialite bearing Middle Cambrian carbonates suggesting that the upper unconformable boundary of the Top Quartzite marks the Lower-Middle Cambrian boundary position in Iran.