X-ray Fluorescence Quantification of Mixed Carbonate/Siliciclastic Chalks and Marls in the Upper Cretaceous Niobrara Formation, Laramie County, Wyoming

The Upper Cretaceous (Turonian-Campanian) Niobrara Formation in southeast Wyoming is ~100 m thick and was deposited over 6.2 my ± 0.5 my. This gives an average rate of deposition of 1.5 cm/1000 yrs, but based on total organic carbon content and trace elements such as molybdenum and vanadium as determined by X-ray fluorescence (XRF), the chalky intervals accumulated much more rapidly than the marly intervals. XRF also reveals a strong inverse correlation (R-squared = 0.93) between calcium and silicon in 98 m of continuous Niobrara core from Laramie County, and a similar strong inverse correlation between calcium and aluminum (R-squared = 0.91). Strong positive correlations occur between aluminum and potassium (R-squared = 0.91) as well as between aluminum and titanium (R-squared = 0.94). These relationships indicate that the Niobrara is a classic mixed bimodal carbonate and siliciclastic package of calcite and clay minerals in which the percentage of clays decreases nearly perfectly as the percentage of calcite increases.

The calcite component of the Niobrara is mainly in the form of coccolith-rich copepod fecal pellets with common planktonic foraminifers and scattered oyster shells, many of which were originally attached to floating vegetation. These carbonate components rained down onto the sea floor perhaps a hundred meters or more below the thriving pelagic community. The clay component in the Laramie County core is dominantly detrital with approximately 50% clay-sized quartz particles and 50% clay minerals in which mixed layer illite/smectite predominates over illite and mica with relatively minor amounts of kaolinite and chlorite. These clays commonly form distinct beds within the chalk-rich intervals and become more common and more dispersed in the marly intervals. Contacts between the chalky intervals and the marls tend to be fairly sharp, but with cyclical increases and decreases in carbonate content representing time intervals estimated at 20,000 to 200,000 years. Bentonites are most common in the middle chalk bench and may have provided a source of iron that fertilized the sea allowing both the coccoliths and copepods to flourish.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California