Reconstruction of Upper Strzelecki Stratigraphy based on Vitrinite Reflectance and Palynology Analyses, and New Ideas on the Amount of Erosion and Present Day Thickness.

Abstract

Strzelecki Group is the first appeared Mesozoic formation within the Gippsland Basin, as one of the largest Australian oil and gas basins, mainly consists of interbedded mudstones, sandstones and coal beds. This group plays role of basement in eastern part of the basin, however, it experienced uplift to the west that provides spectacular outcrops especially along the coastline near the township of Wonthaggi. There still questions exist regarding the present day thickness of this group in west Gippsland area and the amount of erosion during late Cretaceous uplift. Present study mainly focused on unique coastal outcrops of Strzelecki Group between Harmers Haven and Inverloch, and tried to reconstruct stratigraphic column of this highly faulted area in order to interpret the sedimentation history and estimate the present day and eroded section thicknesses. Vitrinite reflectance measurments combined with palynology have been used for correlation of fluvial sediments and reconstruction of stratigraphy. During the study correlation of measured logs faced number of problems such as unknown lateral extension of units, empty gaps because of either no access or not exposed area, effect of multi directional cross-cutting faults and high amount of erosion. The outcrops were mapped and logged, extensive numbers of samples were collected from rich carbonaceous sediments and at the end the stratigraphy was reconstructed based on palynology, vitrinite reflectance data and a geothermal gradient of 50 oC/km. The potential errors for this approach are possible effects of thermal events such as dykes on vitrinite reflectance; and also limitation in range of dating by palynology study of spore and pollens. Based on the reconstructed stratigraphic column, the Strzelecki Group has been classified into three distinct categories with range of eroded section from 1.3 km to 2.1 km in three different rifting blocks. Finally the sedimentation style within rift basin, effect of syn-depositional faults and the thickening of sediments toward south has been shown in a restored model prior to erosion.

AAPG Datapages/Search and Discovery Article #90194 © 2014 International Conference & Exhibition, Istanbul, Turkey, September 14-17, 2014