Why does not lithology correlate with gamma-ray spikes in the shaley source rocks of the Upper Jurassic Alge Member (southwestern Barents Sea)?

The Middle Jurassic to lowermost Cretaceous in the SW Barents Sea: Interplay between tectonics, coarse‐grained sediment supply and organic matter preservation

“…A second thickness map was made using the seismic interpretation between the tops near MFS3 and BCU, representing approximately the time thickness of sequence 4. Finally, three palaeogeographic maps were created based on the interpretation presented in this study, integrated with previously published core descriptions (Braut, 2018; Bugge et al., 2002; Helleren, et al, 2020; Sandvik, 2014). To constrain the palaeogeographic interpretations, in particular, the uplift of the Finnmark Platform in the southern part of the study area, we include a list of reworked Jurassic fossils identified in two samples from the Cretaceous Knurr Formation from well 7019/1‐1 (2232.55 and 2224.25 m).…”

“…Organic-rich rocks usually exhibit high GR log values due to high uranium content. However, uranium is a mobile element, therefore, organic-rich rocks do not always correlate with high GR values (Abouelresh & Slatt, 2012;Bowker & Grace, 2010;Rider, 1996;Helleren, et al, 2020). 3.…”

“…The Alge Member has been interpreted as deposited in a marine shelf mostly under anoxic sea floor conditions (Mørk et al, 1999). Distal gravity flow deposits have been described in well 7219/8-1s, located to the west of the Loppa High (Figures 1,2; Helleren, et al, 2020). The Krill Member is composed of shale (TOC between 2% to 6%; Henriksen et al, 2011), siltstone and minor sandstone units of Kimmeridgian to Ryazanian age (Braut, 2018;Georgiev et al, 2017;Mørk et al, 1999).…”

“…However, this work focused on the geophysical response and did not present further interpretations of these TOC trends. Helleren, Marín, Ohm, Augustsson, & Escalona, (2020) analysed the geochemistry of two of the maximum flooding surfaces (MFSs) defined in this current study. This study concluded that variations in uranium, as a response of fluctuations in sea level and redox conditions, controlled the intensity in the GR spikes at the location of MFSs.…”

“…Organic‐rich rocks usually exhibit high GR log values due to high uranium content. However, uranium is a mobile element, therefore, organic‐rich rocks do not always correlate with high GR values (Abouelresh & Slatt, 2012; Bowker & Grace, 2010; Rider, 1996; Helleren, et al, 2020). Organic‐rich rocks are fine grained and mostly deposited in anoxic sea floor conditions, usually without clear indicators of palaeobathymetry.…”