ISOTOPIC CHARACTERIZATION OF THE LATE CRETACEOUS FOX HILLS-HELL CREEK ESTUARY

Well-preserved biogenic carbonates (predominantly mollusks) from the Fox Hills-Hell Creek estuary (Late Maastrichtian riverine, estuarine, and marine sediments) have been analyzed for d¹⁸O, d¹³C values and ⁸⁷Sr/⁸⁶Sr ratios to constrain the composition of the distal portions of the Hell Creek distributary system where it emptied into the Fox Hills Sea.  d¹⁸O and d¹³C values of biogenic carbonates define a linear mixing trend for these systems (from (0, 1.5 ‰) to (–21, -4 ‰, PDB)) that blended river water whose source was high-altitude precipitation/snow pack (d¹⁸O = -20 ‰ SMOW) with seawater (d¹⁸O = -1 ‰ SMOW). Mixing is further constrained by ⁸⁷Sr/⁸⁶Sr ratios of marine, estuarine, and freshwater, biogenic carbonates.

Exceptionally low d¹⁸O values of Hell Creek river water were maintained over the entire length of this river system (~750 km) and appear to have been relatively undiluted by local precipitation, suggesting that the amount of local precipitation and/or runoff was low in eastern Montana and the Dakotas.  In contrast, abiotic carbonates from this region suggest that regional groundwater had a d¹⁸O value of approx. –8 ‰ (SMOW).  Freshwater mollusks from the Paleocene Sentinel Butte Fm. of North Dakota also have very low d¹⁸O values (–24 ‰, PDB) and indicate that these isotopically distinct river systems continued to flow without significant modification after the K-T boundary. We conclude that the hydrodynamic regimes of the Laramide headwaters were somewhat cooler and/or higher during the early Paleocene.

Freshwater ⁸⁷Sr/⁸⁶Sr ratios increase dramatically up-section from the upper Fox Hills Fm. (0.70771) through the Hell Creek Fm. (0.70798) and into the Paleocene Slope, Bullion Creek, and Sentinel Butte Fms. (0.70960), suggesting that the ⁸⁷Sr/⁸⁶Sr ratio of rock material weathering in the Hell Creek-Sentinel Butte drainage systems was increasing as the drainage system evolved.  This evolution may be explained by initial weathering of volcanic fields (low ⁸⁷Sr/⁸⁶Sr ratios) in western Montana and later exposure and weathering of rocks of Paleozoic and older ages.  The distinctive isotopic composition of Hell Creek river waters provides an excellent means of differentiating the paleo-environments of fossil materials and a possible chemostratigraphic tool.