HOW DEEP DOES “TERRESTRIAL” WEATHERING EXTEND INTO CRYSTALLINE BASEMENT? EXAMPLES FROM THE MESOZOIC UTSIRA HIGH, NORWEGIAN NORTH SEA BASIN (Invited Presentation)

Interpreting the "depth of terrestrial weathering" (DTW) has long-challenged researchers and is important to Earth systems Critical Zone research, including biochemical models, as well as understanding early terrestrial life records. DTW studied in paleoweathering profiles formed on crystalline basement integrates micromorphological, mineralogical, geochemical and geochronological methods. Normally DTW methods rely upon quantifications of mineral weathering and chemical depletion profiles evaluated from bulk geochemistry, compared against the presumed protolith. Controls include biological activity, degree of rock fracturing, and rock lithology. Paleoclimate (MAP (CIA-K): 1152 mm yr^-1; (PPM1.1): 771-1072 mm yr^-1, MAT (PPM1.1): 10.4-11.4 ^oC) and paleoatmospheric chemistry (pCO₂) are especially important. Saprolites obtained from drillcores and formed in weathered crystalline basement from the Utsira High in the North Sea basin are late Triassic in age, based on recent K-Ar dating of illitic material, which likely represents the onset of weathering. In well 16/1-25S, illite/smectites dominate in the most highly weathered zones, whereas kaolinite dominates in the less-intensely weathered intervals. But how were the Late Triassic signatures preserved if the basement was subaerially exposed throughout the later Mesozoic, and why do the clay mineral assemblages vary inversely? Fungal mycelial structures observed in thin sections extend to depths of 20-30 m beneath the eroded tops of the Utsira profiles, which suggest possible deep terrestrial effects of Mesozoic near-surface rooting and vegetation; however, based on some recent research on Precambrian aquatic fungus-like mycelia, these Mesozoic mycelial structures might not be uniquely terrestrial. They could have formed after Jurassic-Cretaceous transgression, drowning, and submergence of the paleoweathering surfaces. Regolith profiles could be polygenetic and traces of several alteration episodes may be preserved. Intervals showing the most plagioclase dissolution and concomitant clay formation correspond to biotite-rich intervals, which seems plausible as the expansion of biotite during hydrolysis facilitates the percolation of fluids. This study demonstrates the complexity of buried Critical Zones in the paleo-record.