Arumpo bentonite deposits: distinctive indicators of past volcanic events in the Murray Basin, southeastern Australia

The Latest Geological/Geophysical Interpretation of the NSW Murray Basin Basement

“…The modelled depth-to-top values of 200 to 500 m indicate that the diatremes do not intrude into the overlying Cenozoic Murray Basin sediments. This conflicts with the Gardam et al (2008) interpretation which proposes the diatremes are the source of 3 mya bentonite deposits. Gardam's interpretation requires the source of the small circular magnetic anomalies to have a much shallower source at approximately 26 m. The magnetic field of a source at this depth would not match the observed aeromagnetic data.…”

“…With this interpretation the depthto-top of the source body should be similar to the maximum depth of the bentonite deposits (approximately 26 m). Figure 1 shows that the resulting magnetic anomaly from this model does not match the aeromagnetic data; the Gardam et al (2008) interpretation creates an anomaly with steeper flanks. Figure 1, A profile over a circular magnetic anomaly with a peak of 165 nT.…”

“…Approximately 10% of the anomalies have a reversely magnetised source. Gardam et al (2008) have interpreted these anomalies as the source of the Arumpo bentonite deposits, which lie less than 10 km north of the swath of anomalies, depicted as a red star in Figure 4. Bentonite is an alteration product of volcanic ash and Gardam et al (2008) have dated the deposit as forming approximately 3 mya from an intermediate volcanic ash source.…”

“…Gardam et al (2008) have interpreted these anomalies as the source of the Arumpo bentonite deposits, which lie less than 10 km north of the swath of anomalies, depicted as a red star in Figure 4. Bentonite is an alteration product of volcanic ash and Gardam et al (2008) have dated the deposit as forming approximately 3 mya from an intermediate volcanic ash source. This interpretation would imply that the intermediate volcanic source bodies have an emplacement age of approximately 3 mya and that they intrude the Cenozoic Murray Basin sediments.…”

“…In addition, this paper also examines the results of magnetic modelling of total magnetic intensity (TMI) anomalies interpreted as being produced by diatremes. These diatremes have been interpreted by Gardam et al (2008) to be the source of the Arumpo bentonite deposits. The Arumpo bentonite deposits, situated in the Murray Basin sediments, are typically 5 m thick, covered by 5-20 m of clay and sand overburden, are derived from volcanic ash, and were deposited at about 3 Ma.…”