Hyposaline to hypoxic foraminiferal facies and stratigraphy of Middle Jurassic to basal Cretaceous deposits of the Trøndelag Platform, mid-Norwegian shelf

The study is based on sample material and log data from the commercial well 6610/7-1 drilled on the Trøndelag Platform. The Melke and Spekk formations of the well contain a Bajocian to Ryazanian foraminiferal succession, here used for stratigraphic zonation and assessment of environmental conditions. Seven interval zones are established, with age estimates primarily based on foraminiferal stratigraphy using regional ranges of taxa. The foraminiferal assemblages are strongly dominated by agglutinated taxa and show low species diversities reflecting the effects of restricting water mass properties. Through most of the Melke Formation, morphogroup analysis of foraminifera reveals nearly equal amount of sediment-dwelling and surficial components suggesting that the bottom waters were sufficiently oxygenated for an infaunal mode of life. Through this formation, hyposaline waters low in carbonate are regarded the dominant factors controlling the development of foraminiferal assemblages. The organic-rich shales of the Spekk Formation with high gamma activity are typified by very low to extremely low species diversities and high dominance of the surface-dwelling morphogroup mainly composed of small-sized species of Trochammina, suggesting hypoxia as the main restricting factor. Here, CO2 release from organic matter degradation led to seawater acidification contributing to creation of inhospitable conditions for calcareous foraminifera. Restricted basinal conditions with stagnant bottom waters and high organic productivity are assumed crucial for this development. Comparison between foraminiferal assemblages of the Trøndelag Platform and the paleo-Arctic seas reveals close similarities illustrating that the mid-Norwegian shelf belonged to the Boreal Realm in accordance with a direct marine communication. •Seven foraminiferal interval zones of Bajocian to Ryazanian age are documented.•Low diversity agglutinant-dominated assemblages suggest restricted environments.•Faunal features suggest change from dominantly hyposaline to hypoxic environments.•Organic sedimentation lead to carbonate depleted waters and low pH.•Direct communication with the paleo-Arctic ocean created Boreal type faunas.