Sprecher
Beschreibung
Vitrinite Reflectance (VRo) determination is the standard method used to estimate the thermal maturity of sedimentary basins but vitrinite (derived from ‘woody’ plant tissues) is absent in pre-Devonian rocks. Acritarch Palynomorph Darkness Index (PDI) can be used as a proxy for VRo in pre-Devonian marine strata. In principle, miospore (trilete spore) PDI can be used as a proxy for VRo in pre-Devonian nonmarine strata but the stratigraphic record of miospores only extends back into the latest Ordovician and they are often too rare in latest Ordovician-Silurian samples for this technique to be applied successfully. Cryptospores are common in pre-Devonian strata with a record extending back to the Mid Ordovician. These palynomorphs were produced by eophytes and their walls have a similar chemical composition to those of trilete spores. We tested the equivalence of cryptospore and trilete spore PDI by calculating PDI on co-occurring specimens from the same Late Ordovician to Early Devonian samples. PDIs of Miospores (trilete spores) and Cryptospores (hilate monads, permanent dyads and permanent tetrads) have been determined in latest Ordovician-Early Devonian samples from China, Saudi Arabia and the UK, where specimens of these palynomorph groups co-occur in samples of varying levels of thermal maturity. Elucidation of the correlation of Cryptospore PDI to Miospore PDI enables the former to be used to estimate the thermal maturity of nonmarine or nearshore marine sequences within the Late Ordovician-Silurian of Saudi Arabia. These include sequences of the Qalibah Formation (Qusaiba and Sharawra members) and the Tawil Formation.