Oxygen and hydrogen isotope geochemistry of Cretaceous bentonites and shales from the Disturbed Belt, Montana

Abstract

The mineralogy, δO 18, and δD of the <0.1 μm fraction of 22 Cretaceous bentonites and the mineralogy and δO 18 of the < 0.1 μm fraction of 14 adjacent shales collected from outcrops in the Sweetgrass Arch and Disturbed Belt, Montana, have been determined. Mixed-layer illite/smectite (I/S) is the dominant mineral in the bulk bentonite and usually the only mineral in the < 0.1 μm fraction. I/S is also the major clay mineral in the shales. The diagenetic grade in bentonite is qualitatively given by the percentage of illite layers in I/S, which varies from 2 to 25 (Sweetgrass Arch) to as high as 95 (Disturbed Belt). δO 18 of < 0.1 μm bentonite generally decreases from about +20%. to about +13%. with increasing diagenetic grade. On a plot of δD versus δO 18, data for the < 0.1 μm bentonite define a field that generally parallels, but falls on the meteoric water line side of the smectite-water line (Savin and Epstein, 1970). δO 18 of bulk bentonite is 1 to 3%. more negative than the δO 18 of the < 0.1 μm fraction, due to the presence of volcanic quartz and feldspar. δO 18 of several size fractions of clay-sized quartz separated from the bentonite varies from +11%. to +24%., and, in a given bentonite, generally increases with decreasing grain size. Among the different bentonites, the δO 18 range of the different grain sizes decreases as the percentage of illite layers in the coexisting I/S increases. The δO 18 of 0.1–0.5 μm shale quartz is generally 1 to 4%. more positive than clay-sized quartz from an adjacent bentonite, and the δO 18 of < 0.1 μm I/S concentrate of shales is generally < 1 to 4%. more negative than the < 0.1 μm I/S from an adjacent bentonite. Isotopic temperatures, interpreted to be maximum burial temperatures, range between about 160°C (shale), to about 250°C (bentonite). The isotopic data can be interpreted using the stages: 1. 1) deposition of volcanic glassy ash containing some quartz and feldspar; 2. 2) devitrification into mostly 100% expandable smectite with a δO 18 of about 20%. and δD of −60 to −80%.; 3. 3) tectonic burial beneath thrust sheets in the Disturbed Belt region which induced elevated temperatures that allowed oxygen and hydrogen isotope reequilibration as the percentage of illite layers in I/S increased and diagenetic quartz formed; and 4. 4) exhumation of the strata with preservation of O 18 O 16 and D/H attained during burial.