Late Cambrian climate change recorded by a shift from an arid carbonate platform to a storm‐dominated cool‐water platform at the Gondwana margin (Alborz Zone, Iran)

Abstract

The Deh-Molla Formation of the Alborz Zone in northern Iran is a key unit to understanding global sea-level fluctuations, and likely climate changes during the Furongian Epoch. The sedimentological characteristics of the formation indicate that the succession was deposited on a gently sloping epeiric platform on the northern margin of Gondwana. The Paibian Member 1 of the Deh-Molla Formation was deposited on a carbonate platform with stable environmental conditions, probably during a (semi) arid climate condition, as indicated by microbial-sponge patch reefs, thick-bedded units formed in broad facies belts, and a thick dolomite unit formed under evaporative conditions. The overlying Member 2, considered to span the Jiangshanian-Age 10, includes thin-bedded wavy, and rippled strata known as ribbon rocks. This sequence records considerable environmental fluctuations. Thin-bedded strata with flat-pebble conglomerates, hummocky cross-stratification, lenticular, flaser, and wavy bedding, ripple cross-lamination, and graded bedding signify that deposition of Member 2 took place in a depositional environment repeatedly affected by tidal currents, and storm waves. A comparison of the Deh-Molla Formation, and particularly its ribbon rocks with coeval strata in Laurentia and North China reveals secular controls on their development. In contrast to the Furongian strata of the Great American Carbonate Bank, deposited under tropical conditions, the ribbon rocks of the Deh-Molla Formation suggest deposition in a temperate latitude zone with dominant cool-water conditions. However, the Deh-Molla platform was seasonally subjected to monsoons that brought warm tropical waters to the region, and led to rapidly changing environments. Ooids, peloids, stromatolites, and other criteria suggestive of a tropical climate are frequent in Laurentian successions, but absent in the Deh-Molla Member 2. However, both Laurentian and Gondwanan examples share characters that indicate comparable controls on their development. Significant environmental changes during the Furongian are indicated by the SPICE and HERB events, and the latter is taken as showing biotic and physico-chemical changes in the ocean and atmosphere, possibly concurrent with the formation of ribbon rocks on different continental blocks.