--> Integrated Stratigraphy of the Famennian Three Forks Formation, Williston Basin: A Study Using Physical, Biological, Sr, and S Isotopic Stratigraphic Signatures

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Integrated Stratigraphy of the Famennian Three Forks Formation, Williston Basin: A Study Using Physical, Biological, Sr, and S Isotopic Stratigraphic Signatures

Abstract

Sequence stratigraphy, biostratigraphy, strontium, and sulfur isotopes are used to constrain the relationship between the siliciclastic enriched Famennian Three Forks Formation deposits of the Williston basin and the chronostratigraphic equivalent deposits of the Wabamun carbonate ramp system. Seven allogeneic third-order transgressive-regressive cycles, correlated throughout the region, are biostratigraphically constrained as equivalents between the Three Forks and Wabamun formations. This biostratigraphic model allows for precise comparison of measured strontium isotope values to the secular curve. Three Forks 87Sr/86Sr values are consistently more radiogenic than the secular curve while following its trend. Authigenic evaporites from the Williston basin are slightly more radiogenic. Siliciclastic components of dolomudstone samples are even more radiogenic than the secular curve. This indicates that compared to the well-mixed ocean signatures, increased riverine run-off into the Williston basin occurred, especially associated with increased influx of detrital siliciclastics. Sulfur isotope values reveal a distinct partitioning between the two formations. Values are consistently enriched by three to five per mil δ34S in the Wabamun deposits compared to the Three Forks deposits. This distinction corroborates the Sr isotopic results, confirming elevated continental run-off in the Williston basin compared to the Western Canadian Sedimentary basin. No evaporite phases except anhydrite have been identified in the Three Forks, while halite and anhydrite have been identified in the Wabamun. The Williston basin salinities were thus buffered by fluvial inputs and rarely exceeded penesaline conditions.