NON-STANDARD GRAVITY ANOMALIES OVER SEDIMENTARY STRUCTURES

Calculations of the gravity effect of sedimentary structures can now be easily and rapidly made using digital computers. When density variations are normal, they generate gravity profiles which are easily predictable. However, if the general rule regarding increase of density with depth is not fulfilled then anomaly variations can be unexpected. The standard profile for a fault is a change in Bouguer anomaly level, but in some circumstances it can be marked by a positive (or negative) anomaly and no overall change in level. The rather unexpected type of gravity anomaly generated from fairly standard geological sections emphasises the need to carry out modelling calculations based on the known or suspected geology.A gravity anomaly across the boundary between the Galilee and Drummond Basins in central Queensland is analysed. Part of the structure has been determined seismically, and a thickening sedimentary sequence appears to have caused a reverse density contrast with the basement resulting in a positive gravity anomaly. However, the flank of this anomaly has been affected by the negative contribution of a granite body, resulting in a displaced gradient, a minor low which is also displaced, and another minor high and low which could be mistakenly attributed to near-surface structure. A further complication is that a topographic feature occurs at this point of interaction, strongly affecting the multiple density Bouguer profiles, and thus the interpretation. The successful interpretation of this type of situation requires accurate gravity data, its accurate positioning relative to the seismic section, and gravity modelling facilities which allow a rapid trial-and-error approach to modelling.