Investigating Increases in Spectral Gamma Ray Signals Observed From Wireline Logging Through Correlation With Core Geochemical Analyses: Integrated Ocean Drilling Program (IODP) Expedition 310, Tahiti Sea Level

The last deglacial reef sequence in Tahiti consists of a series of successive reef terraces seaward of the living barrier reef. IODP Expedition 310 recovered core from 37 boreholes which shows that the reef terraces are composed of two major lithological units (i) a last deglacial carbonate sequence and (ii) an older Pleistocene unit. Each sequence comprises of a series of distinctive coral assemblages, with microbialites locally forming the major component of the reef. High levels of U in carbonates have previously been linked to diagenetic processes, subaerial exposure, higher levels of organic material and the presence of red algae. Spectral gamma ray logs collected during Expedition 310 show a significant interval of elevated gamma ray counts in Hole M0005D. This ~20 m thick interval is located in the Pleistocene succession a few metres below the boundary with the last deglacial sequence. Analysis of the individual elements contributing to the gamma counts show that U is the major contributor to the higher counts in the upper part of this interval. This interval comprises the coralgal- microbialite reef frameworks that dominate much of the reef terraces and there is no apparent link between U and lithology. Higher counts in the lower section result from raised Th and K concentrations and correlate with silt dominated lithology at these depths. We selected 104 core samples from the high-U intervals for standard geochemical analyses (XRF). Major element correlation diagrams including our new data and published data on Tahiti igneous rocks imply that igneous fragments may have been incorporated during carbonate precipitation. Detailed thin section analyses reveal small volcanic clasts within some carbonates supporting this interpretation. The geochemical analyses of the core (both discrete samples and continuous core) also indicate that an interval of increased Mg correlates with the increase in U observed from the logs. We infer from high CaO (48 to 52 wt.%) and moderate MgO (2 to 6 wt.%) observed in this interval the presence of dolomite, thus implying a potential diagenetic influence. Mass balance calculations using major element oxides demonstrate that this is compatible with the geochemical data. These preliminary findings from combining spectral gamma ray observations with analyses performed on core samples provide insight into the processes operating in the Tahiti reefal carbonates. Establishing the potential causes of U increases and identifying these elsewhere may provide a tool to identify other regions of the reef affected by similar processes.