Sequence Stratigraphy And Gross Seismic Facies Mapping Of The Mid To Late Eocene Traralgon Formation, Gippsland Basin
Geoscience Honours Degree, 2011
University of Adelaide
A sequence stratigraphic framework has been constructed integrating both well and 2D seismic reflection data encompassing the Middle to Late Eocene Traralgon Formation in the onshore Gippsland Basin. Four sequence boundaries have been age calibrated with the global coastal onlap curve using palynological zone age data. A seismic facies scheme was developed and applied between the four chronostratigraphically defined horizons. Seismic facies were then interpolated between seismic lines before being calibrated with lithofacies and depositional environments interpreted from tied wells. Calibrations of seismic facies with lithofacies and depositional environments generally gave consistent correlations within and between defined intervals. The sequence stratigraphic scheme has been interpreted between a number of regional wells and the corresponding intervals used as control points for both lithology and depositional environment mapping.
This new sequence stratigraphic scheme, which integrates both seismic and well data, has resulted in a proposed change to the Middle to Late Eocene Traralgon Formation sequence stratigraphic interpretation, with the identification of an upper and lower T0 sequence and circumstantial evidence for an upper and lower T1 sequence. Depositional environment mapping performed over the three chronostratigraphically defined intervals has confirmed the existence of a shoreface system in the northern extent of the study area which is largely absent in the south. First marine ingress into the study area has been constrained to the 38Ma sequence boundary, with marine influence growing through time up to the major marine transgression at the top of the Traralgon Formation. The occurrence of a robust sand barrier system offshore in the south of the study area is implied over the 38-37Ma and 37-36Ma intervals, with marine and shoreface deposits in these areas suggesting much lower depositional energy, consistent with a back barrier or restricted embayment setting. These observations support previous studies that have proposed similar depositional morphologies.
Finally, the changes in distribution of both lithofacies and depositional environments observed between the three intervals indicate the large effect of synsedimentary deformation. Depositional environment maps indicate the greater accommodation which was available in the Seaspray and Alberton Depressions due to inversion on bounding faults by the greater marine ingress into these areas through time. Seismic lines over the Won Wron Anticline, the major bounding fault of the Alberton Depression, indicate the accelerating magnitude of this movement by the thickening of the intervals into the inverted faults, with later packages exhibiting more thickening. Although no seismic data was available to confirm this occurrence over the Rosedale Fault in the Seaspray Depression, lithofacies and depositional environment maps suggest that it was similarly a major control on deposition in this area.