Abstract

Preliminary analysis of gravity anomalies across the Bonnet Plume Basin indicates that the gravity method can be used as a relatively inexpensive method of establishing basin structure and approximating formation thickness in coal basins with little to no subsurface control, providing that density contrast between the coal-bearing strata and underlying rocks is sufficiently high. In the Bonnet Plume Basin gravity measurements recorded along an east–west profile give Bouguer anomalies which vary from −42 to −82 mGal and place constraints on geological models which can be applied to the clastic fill of the Cretaceous–Tertiary basin. Within these constraints the Bonnet Plume Formation can be modelled as an eastward, gently dipping (1–9°) sequence of coal-bearing rocks which attains a maximum thickness of between 760 m and 4.7 km in a region 7 km west of Knorr Fault. The unconformity between the Bonnet Plume Formation and underlying Proterozoic rocks, east of this zone of maximum thickness, dips to the west at between l0and 44°. Accurate estimates of both the thickness and dip of the sequence cannot be made due to inadequate knowledge of the subsurface distribution of the upper (low density) and lower (high density) members of the formation. Application of a two-layer model for the Bonnet Plume Formation would indicate that the upper, lignite-bearing member of the formation may be more restricted than previously suspected and that the greater proportion of the basin to the south of the line of profile is underlain only by the lower member, containing subbituminous coals.

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