High-frequency sequence stratigraphy in the Lower to Middle Triassic Narrabeen Group, Sydney Basin

The Narrabeen Group, in the Sydney Basin, Australia, exhibits repetitive sedimentary intervals from generally less than 10 m in thickness, but up to a little more than 20 m in thickness, that are regarded in sequence stratigraphy terms as high-frequency, fourth-order sequences. All these basic building blocks have recognisable features such as erosional sequence boundaries, transgressive surfaces and maximum flooding surfaces. Gamma and neutron geophysical logs help define trends in grainsize that may not be apparent from written lithological logs. Each fourth-order sequence defines an alluvial plain that grades seawards to an estuarine coastal plain and an estuarine central-bay facies. Changing sea-levels affected the rate of change in accommodation space that influenced sedimentation and accounted for the lithological variation and stacking patterns observed in fourth-order sequences. For example, slow rates of provision of accommodation space limited vertical accumulation and therefore encouraged very extensive sandy braidplains to develop, whereas more rapid provision of accommodation space favoured thicker floodplain deposits with localised, anastomosing streams. In the Central Coast Narrabeen Group, fourth-order sequences stack to constitute nine third-order sequences (1–9). The pattern of fourth-order sequence stacking was largely controlled by third-order sea-level changes, and the stacking pattern of third-order sequences was controlled by longer second-order sea-level changes, that stacked the Narrabeen Group sequences 1–6 in a retrogradational sequence set. Similarly, second-order sea-level changes controlled the stacking pattern of third-order sequences 7–10 in a progradational sequence set that constitutes the upper Narrabeen Group and the overlying Hawkesbury Sandstone. This study is based on an extensive coal exploration drilling program of over 350, mostly fully cored, boreholes covering some 250 square kilometres of the NSW Central Coast near Wyong.