Isotopic Evolution of the Niobrara Chalk and the Implications to Water Mass Mixing in the Cretaceous Western Interior Seaway

Abstract

Niobrara (and Greenhorn) chalk-marl strata, deposited between ~80 and 90 ma in the Cretaceous Western Interior Seaway, are now substantial unconventional targets. Historically, these chalky packages were assumed to undergo a “standard” sequence of burial diagenesis: mechanical compaction in early burial followed by continuous chemical compaction and pressure dissolution, recrystallization, and cementation in deeper burial. However, calcite oxygen stable isotope data (δ18Ocal) from the Niobrara of the Denver Basin that have been invoked in support of this diagenetic model are anomalously depleted (~ -5.5 ‰ VPDB excess) relative to numerous other Cretaceous chalk formations across the globe. Either elevated temperatures in the Denver Basin or non-marine pore fluids present during diagenesis could explain the depletion, but neither has previously been systematically tested. Calcite clumped isotope temperatures, Sr/Nd ratios, and numerical models of bulk oxygen isotopic equilibrium demonstrate that elevated temperatures in the Denver Basin likely contributed to the overall trajectory of δ18Ocal evolution, but they cannot be solely responsible for the gross depletion relative to other chalks. Instead, water-mass mixing between Cretaceous marine fluid and an isotopically depleted fluid source is the most likely cause of ubiquitous and long-lived δ18Ocal depletion. The spatiotemporal extent of anomalous δ18Ocal suggests that depleted fluid must have been introduced either during deposition, or during very early burial, when porosity and permeability were still high enough to facilitate basin-scale advection. Sr/Nd ratios suggest no hydrothermal influence, and there are no other likely sources of early diagenetic fluid that are volumetrically significant enough to modify the pore fluid by an additional -5.5‰. These results revive the question of water mass mixing within the Cretaceous Western Interior Seaway itself, and may be evidence of significant meteoric input from the Sevier hinterland.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019