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

Abstract

Prominent high-value gamma-ray spikes in the maximum flooding surfaces of the Middle Oxfordian-Kimmeridgian succession (Alge Member) display stratigraphic and regional variations in the southwestern Barents Sea. These variations suggest heterogeneities in the composition of the Alge Member, thus, challenging the common perception that this interval is a homogenous succession of organic-rich black shale. This study explores the overall variation in; clay mineral composition, source rock quality, minerals and macerals, core logs and seismic facies of the Alge Member. Gamma-log fluctuations were found to be independent of these variations. Uranium precipitates on sea bottom sediments when conditions change from oxic to anoxic. Fluctuations in sea level may result in such changes and, hence, guide gamma-ray responses. Sea level fluctuations also lead to a changing distance to the shoreline, which again influences sediment types, grain sizes, and source rock quality. Thus, indirectly sea level appears to be the common denominator, which sometimes matches source rock richness with spikes on gamma logs. Presence and absence of gamma-ray spikes are, hence, an additional tool to reveal the redox conditions in the SW Barents Sea, which has implications for the understanding of the paleogeography (or paleobathymetry) during deposition.