High-resolution platinum group elements and C-isotope analyses across the KT boundary in the Denver Basin

Two nearby (<10 m) locations, labeled Phone Booth and Bowring Pit sections of the continental CretaceousTertiary (KT) sequences from the West Bijou Site, in the Denver Basin are analyzed at high resolution for their concentrations in siderophile and Platinum Group Elements (PGE) as well as delta13C values. This studied sequence was deposited in a coalswamp to lacustrine setting and contains lignite, coal seams and fine to coarse mudstones. By refining the existing palynostratigraphy, the study aims to accurately pinpoint the precise location of the KT boundary. Repetitive ash layers, which can be dated by isotopic methods, characterize the West Bijou Uppermost Cretaceous and Lowermost Paleocene sequences, opening the possibility to precisely constrain the succession of event preceding and following the KT boundary impact. The concentrations of Ni, Co, and Cr were determined by ICPMS after acid digestion; PGE and Au abundances by using nickelsulfide fire assay combined with inductively coupled plasma mass spectrometry (ICP?MS). Continuous Flow Isotope Ratio Mass Spectrometry was used to analyze organic matter delta13C. The results show that the elemental abundance patterns across the two studied KT sections vary significantly. The first, the Phone Booth section, is characterized by confined PGE enrichments at the level of the KT boundary clay, based on palynology, and a sharp decrease in ?13C values immediately above this bed. In contrast, the Bowring Pit section displays highly remobilized PGE signatures and associated diluted PGE enrichments. Differential post?impact remobilization among PGEs, as well as input of eroded Ir rich material within the lacustrine depositional setting, is offered as explanation for these complex PGE abundance patterns across the Bowring Pit section. Also, a gradual decrease in ?13C values through the K?T boundary and Lowermost Paleocene was observed at this section. The close agreement between the observed PGE enrichments and the sudden delta13C shift, two independent indicators of the KT event, at the Phone Booth section, confirmed the exact placement of the K?T level in the West Bijou Strata. Finally, the markedly differing PGE abundance profiles across both studied sections reflect the importance of local conditions in determining the PGE post?depositional mobility.