Sequence Stratigraphic Significance of Uranium Anomalies in Carbonate Rocks, ODP Leg 194

ODP Leg 194 drilled two transects on the Marion Plateau carbonate platform, offshore northeast Australia. Different patterns in high natural gamma radiation detected in core samples can be related to exposure surfaces and submarine hardgrounds. The calibration of the log responses to the different events enables to use the continuous record of the gamma log for the reconstruction of times of regressive and transgressive sedimentation. Downhole logs show that the gamma radiation is mainly related to uranium enrichment and different curve shapes are indicative for certain events in the sea level history. For example, a gradual increase in uranium is observed towards exposure surfaces, while submarine hardgrounds show usually a distinct peak with an abrupt increase at the bottom. The gradual increase below exposure surfaces indicates that water migrated upwards to the exposed surface, and uranium was precipitated when the water evaporated. Above the exposure surface the uranium concentration usually remains high, possibly related to a subsequent flooding event. While the normal background uranium content fluctuates between 1 and 4 ppm it reaches values between 7-15 ppm below exposure horizons and 10-12 ppm in transgressive beds. Hardgrounds on the Marion Plateau have a more diverse gamma-ray signature as result of their accumulation in a variety of uranium-bearing material like phosphate nodules, glauconite and to a lesser degree to (rare) organic matter. In addition, facies-related lateral variability is observed along the transect. Two platform sites (1196 and 1199) and two slope sites (1194 and 1195) show different patterns in the uranium log in correlative seismic sequences. Despite the variability the uranium signal provides a high-resolution record of sea-level related sedimentation on these Miocene platforms.