Chitinozoan reflectance in the Lower Palaeozoic of the Welsh Basin

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Abstract
Vitrinite (woody matter) and spores are generally uncommon or absent in pre‐Devonian sediments. To determine organic maturity in the Lower Palaeozoic alternative methods, such as chitinozoan reflectance, must be employed. Chitinozoa are marine, vase‐shaped organic‐walled microfossils of unknown affinity. The reflectance of incident light from polished chitinozoa walls can provide reliable and accurate maturity data and, due to their relative facies‐independency, a high degree of resolution to regional maturity studies. Calibration of the chitinozoan reflectance scale with vitrinite reflectance enables palaeotemperature estimates to be made from pre‐Devonian sediments.The range of chitinozoan reflectance for the Welsh Basin is from under 1% to 8%. This corresponds to estimated palaeotemperatures of under 100°C to over 300°C. The regional spatial distribution of organic maturation recorded by chitinozoan reflectance in the Welsh Basin is attributed to palaeobasin form. The increase in reflectance with age in the rocks of the Myddfai Steep Belt and associated areas allows an estimation of a palaeogeothermal gradient. In the Silurian sequences around Llandovery the gradient is calculated to be in the region of 50°C km−1. Similar gradients are derived from other studies (clay minerals, metabasite mineral assemblages, conodont colours). The results from Myddfai Steep Belt suggest the thermal peak occurred during the Acadian event.Chitinozoan reflectivity demonstrates that, with respect to hydrocarbon generation, mature conditions occur on the palaeoplatform, where the sedimentary sequences are relatively thin. Overmature conditions are prevalent in the palaeobasin where there is a thick sedimentary pile which has been subsequently deformed.