LINDQVIST, JON K.

Outcrop, core, and wireline log studies of unusually radioactive sandstone reveal that the gamma radiation coincides with concentrations of uranium and thorium-rich heavy minerals zircon and monazite. K-feldspar and mica also contribute to the radiation. Heavy mineral laminae are found in hummocky and swaley stratified intervals and particular Turritella-rich shell lags that form the basal 1-20 cm of typical shellbed- laminated sandstone- Ophiomorpha-burrowed sandstone cycles.

Six 2-4 m thick high gamma radiation zones with peaks up to 650 gamma radiation units span the upper 45-60 m of Wangaloa Formation. Characteristic wireline log signatures provide bed-to-bed correlations between coal exploration holes drilled 1-2 km apart onshore.

Wangaloa Formation records storm-influenced ocean shoreface deposition of well sorted sand near the base of a Late Cretaceous-Eocene transgressive succession. Overlying greensand, nodular phosphorite, and mica-rich mudstone beds are less or equally radioactive at the base-line level.

Radioactive heavy minerals are interpreted to have been derived by long shore drift from southern granitic sources. Great South Basin offshore is considered a frontier oil and gas exploration area. This study, part of broader research on transitional non-marine - marine sequences, indicates that standard gamma log interpretation procedures (such that low gamma radiation equates with clean, clay-poor sands) should not be used, without core control, for the identification of well sorted sand intervals in Great South Basin nearshore facies. 

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid