Formation of the Upper Cretaceous cherts in northeastern Sinai, Egypt

Abstract

The Upper Cretaceous cherts in northeast Sinai, Egypt are found as nodules, tabular sheets and continuous beds within the carbonate dominated successions. They occur in the Halal, Wata and Matulla Formations and become a very conspicuous constituent in the Sudr Chalk. The chert framework is typical of all interstratal structures and is of two types: spotted and brecciated forms. The chert is classified into a fossiliferous and nonfossiliferous variety. The first is likely either to form packstone-grainstone fabrics or to form wackestone fabrics. The packstone-grainstone fabric is interpreted as replacing platform carbonate deposits at relatively lower energy but in an oxygenated environment while the wackestone fabric chert replaces low energy deep water carbonates. This Upper Cretaceous silica cycle was dominated by inorganic reactions involving dissolved silica, and there is much evidence of secondary diagenetic silicification. This process would have started in early diagenesis as opal-A, opal-C and opal-CT precipitated from interstitial waters. Quartz represents the end product of recrystallisation. This transformation from metastable to stable silica phases is explained as a solid-solid diagenetic reaction as emphasised by δ 18O. The nodular cherts have formed in coastal mixing zones with opal-CT and quartz supersaturation and calcite undersaturation. The source of silica of the deep water cherts cannot be explained by this mixing zone model and needs further study. On the other hand, there is no evidence of deposition of layered amorphous silica in either shallow or deep environments.