Revisiting the Upper Cretaceous Niobrara Petroleum System in the Rocky Mountain Region

Abstract

Studies of Niobrara depositional environments done during the 1980s and 1990s relied on what was understood about the processes controlling deposition in ~300 ft (~100 m) water depth at that time. A common idea was that the chalkier beds formed as carbonate-rich marine “snow” settled slowly to the sea floor to form blanket-like deposits that could be easily correlated across tens to hundreds of miles. The dominant control on chalk vs. marl deposition was thought to be relative sea level with highstands favoring chalkier deposition versus lowstands that favored the influx of some clays causing more marly deposition. Relatively recent studies of the deep-sea floor in some settings, however, have dramatically changed insights into deposition of the very fine grained (clay and silt-sized) hemipelagic deposits. Instead of a vertical rain of sediments, dynamic marine currents at depths of hundreds of feet (>100 m) can reign supreme and are now known to form scour and drift features that redistribute the sediments laterally into broadly lenticular sea-floor bars and channels that are themselves tens to hundreds of feet thick. The long (>3000 mi; 5000 km), relatively narrow (<400 mi; 650 km), north-south trend of the Western Interior Seaway between adjacent land masses made it particularly susceptible to a complex set of marine currents during Niobrara deposition that redistributed both the chalky and marly deposits. New evidence for the importance of marine currents in the Seaway during Niobrara deposition versus the traditional idea of fluctuating sea level is six-fold: 1) well-documented interfingering of the chalk and marl facies on a scale of centimeters or less, which is far too thin to be controlled by sea-level fluctuations; 2) a lack of evidence for chalk-related highstands along the seaway’s margins (e.g., in Utah and Kansas); 3) abrupt lateral changes in the thickness of chalkier deposits over distances of a mile (<2 km) or less; 4) thin (<2-inch [5 cm]) organic-rich (>15 weight % TOC) marly “kerogenites” within the clean chalk benches that are too thin to be the product of sea-level changes; 5) color-filled gamma-ray cross sections built with relatively closely spaced wells that clearly show the large-scale scour and drift features; and 6) study of modern ocean current flow patterns on deep-water hemipelagic deposits off New Zealand’s South Island and in the Mediterranean that have yielded bedforms seen on high-resolution seismic lines similar to those seen on color-filled gamma-ray cross sections of the Niobrara in the Denver Basin. Furthermore, subtle topographic features on the sea floor such as the paleo-Hartville Uplift apparently influenced current flow patterns and impacted deposition. The previously underappreciated role of currents must be accounted for when characterizing not only the deeper marine deposits of the Niobrara but also for many other marine deposits in the Western Interior Seaway. The impact of similar currents was probably also significant in other epeiric seaways around the world.