The history of calcite diagenesis and origin of exceptionally negative oxygen isotope values in chalks of the Niobrara Formation, Denver Basin, USA

The Niobrara Formation of north-east Colorado, USA, has anomalously negative ?18O values compared to all other Cretaceous chalks. These unique have been attributed elevated heat flow and/or freshening the Western Interior Seaway. This work utilises clumped isotopes calcite (?47), peak burial temperatures estimated from pyrolysis data, and strontium neodymium carbonate re-evaluate origin calcite's 18O-depletion. Peak indicate lateral variability in geothermal gradients ca 20°C/km at tens kilometre scale, corroborate prior studies proposing locally palaeotemperatures. Greater insight is provided by numerical models recrystallisation oxygen isotope evolution that are constrained measured ?47-derived temperatures, inferences 87Sr/86Sr ?Nd values. (1) sea water seaway had normal marine ?1 (VSMOW) except on eastern margin basin where some freshwater dilution yielded ?2 ?3‰ water, (2) main driver anonymously was a semi-open hydrologic system few percent pore volume meteoric groundwater derived post-Laramide recharge into basin. Minor contributions were Laramide-aged pulse related underlying Colorado Mineral Belt, thermal insulating effects now eroded coals, small flux compaction-driven evolved smectite dehydration. However, those three factors alone insufficient drivers calcites' 18O depletion. High interpreted caused reordering least one well, but cannot yield observed study illustrates attributes Niobrara's diagenetic results its anomalous values, reaffirms value unravelling history chalk systems.