The use of organic fades for refining palaeoenvironmental interpretations: A case study from the Otway Basin, Australia

Four depositional settings interpreted from sedimentary fades associations are present in the non‐marine Early Cretaceous sequence of the Otway Basin: high energy fluvial, low energy fluvial, lacustrine and fluvio‐deltaic facies associations. Organic‐petrological investigation of Otway Group coals delineated four coal types with distinct assemblages of maceral subgroups and individual macerals. The coal types, designated A to D, occur with distinct assemblages of dispersed organic matter (dom) in associated clastics. Specific coal type assemblages in the Otway Group are closely related to changes in sedimentary facies, depositional energy exerting a primary control on the organic facies. High energy deposits of the Pretty Hill Formation and the upper part of the Eumeralla Formation are characterized by a single organic facies. Organic facies in the lower energy lacustrine deposits of the Casterton Formation and the lower part of the Eumeralla Formation appear to be sensitive to subtle changes in environmental settings. Additional environmental information derived from the organic facies study has refined the depositional settings interpreted from sedimentary facies associations, particularly when applied to the deposits of floodplain‐ and lake‐dominated low energy depositional systems. The organic facies concept is thus applicable in a wider basin studies context and has potential to become an additional tool for interpretation of depositional environment in non‐marine settings.