The spatial and temporal distributions of vertebrate fossil assemblages at the terrestrial Cretaceous/Paleogene (K/P) boundary in eastern Montana and western North Dakota are constrained by the meandering fluvial depositional system in which they are preserved. Vertebrate fossils are clasts within that system, and thus their positions within the sedimentary sequence are subject as much to hydrodynamics as to chronostratigraphy or evolution. Laterally discontinuous facies, incomplete stratigraphic sequences, facies-dependent fossil distributions, and reworking distort original patterns of faunal number and diversity through time. The sedimentary effects on vertebrate fossil distributions, however, can be assessed by the adoption of a facies-based biostratigraphy and the utilization of a chronostratigraphy that is concordant with the resolution afforded by the K/P rocks of eastern Montana and western North Dakota.

Abstract Studies of several active and proposed surface coal-mining sites in western North Dakota have resulted in the development of a hydrogeochemical framework which accounts for the chemical characteristics of groundwater in coal- bearing strata. Data from surface mining sites in other western states and provinces indicate much regional similarity in the hydrogeochemical characteristics of shallow aquifers. SO 4 = and HCO 3 - are commonly dominant anions. Na + and Ca ‡ are generally the dominant cations. Mg ‡ is occasionally dominant. The pH of the groundwater normally ranges from 7 to 9. The electrical conductance of the groundwater ranges from 500 to 4500 /μS. Groundwater at depth is generally of the Na—H0O 3 type. Critical hydrogeochemical processes include pyrite oxidation, carbonate mineral dissolution, gypsum precipitation and dissolution, cation exchange, and sulfate reduction. The key components of this framework are mineralogical variables in the sediments. These variables are, in turn, largely dependent upon fluctuations in the fluvial depositional settings of the sediments. During surface mining operations unweathered materials are commonly emplaced in the zone of active weathering. Hydrochemical data from postmining landscapes suggest that severe degradation of groundwater quality is possible in some settings. The degree of weathering, dissolution, and ion exchange, as in premining settings, is largely dependent upon mineralogical variables in the overburden sediments. Degradation of groundwater quality in postmining landscapes can be minimized if these factors are understood and integrated within the framework of mine design.

The Golden Valley Formation consists of claystone, mudstone, siltstone, micaceous sandstone, and lignite deposited under predominantly fluvial conditions during late Paleocene and early Eocene time in the Williston basin of North Dakota. The maximum thickness of the formation in the scattered erosional remnants in which it still persists is 55 m (180 ft). Lithologically the formation is divisible into two members: a lower unit, from 1.5 to 20 m (5 to 65 ft) thick, of light gray or brightly colored kaolinitic strata herein designated the Bear Den Member and an upper unit, as much as 45 m (150 ft) thick, of yellow to tan illitic to montmorillonitic strata herein designated the Camels Butte Member. The bright coloration and high kaolin content of the Bear Den Member, the uniform presence of mica in the sandstone of the Golden Valley Formation, the typical scarcity of lignite throughout the formation, and the presence of the unique floating fern Salvinia in the Camels Butte Member make the Golden Valley Formation distinguishable from adjacent rock-stratigraphic units. At its basal contact the unit is conformable on the Fort Union Formation except where channeling has occurred. All evidence indicates continuous deposition across the lower boundary of the formation with a change in clay-mineral input during deposition of the Bear Den Member. In weathered outcrops this member typically develops a tripartite color zonation made up of a basal gray zone, a middle orange zone, and a somber-colored carbonaceous zone at the top. The orange zone is a conspicuous marker of the formation over most of its outcrop area. The Bear Den Member becomes increasingly fine grained and carbonaceous upward. A thin bed of lignite (the Alamo Bluff lignite) or its lateral equivalent, silicified siltstone or freshwater limestone (the Taylor bed), marks its upper boundary. The orange weathering color of the midportion of the member is virtually eliminated beneath the Taylor bed. The Camels Butte Member, consisting of cross-bedded, predominantly sandy channel deposits and parallel-bedded, finer grained overbank deposits, lacks marker horizons of general extent. The proportion of channel-facies deposits increases steadily upward. Beneath the overlying, angularly unconformable White River Group, a zone of weathering and leaching extends downward as much as 23 m (75 ft) into the Camels Butte Member. No evidence of a marginal facies could be found at the present geographic limits of the Golden Valley Formation. Leaf architectural analysis, developed largely as a result of these investigations, played an essential role in the identification of the megafossil plants described in this report. The Bear Den Member yielded a megaflora of 41 species, closely resembling the late Paleocene part of the Fort Union flora. The most widespread vegetational community represented in the unit is inferred to have been a lowland forest whose numerically most important forms were Metasequoia , Corylus , Meliosma , “ Cocculus ,” Cercidiphyllum , and Chaetoptelea . Swamp and aquatic vegetation becomes increasingly abundant and more widely distributed upward in the Bear Den Member at the expense of lowland forest representatives. Eocene plant species—including Salvinia preauriculata , Lygodium kaulfussi , Platycarya americana , and Hemitelia magna —first appear at the level of the Alamo Bluff lignite and gradually replace Paleocene forms upward through the formation. The Camels Butte Member contains 37 species, the most numerous of which are Lamanonia , Meliosma , Salvinia , and Ternstroemites . As a unit the Camels Butte florule shows little relation to any previously described fossil flora. Fossil plant evidence, corroborated by that of associated vertebrates and invertebrates, indicates a warm temperate climate with an estimated mean annual temperature of 15°C (59°F) in late Paleocene time. The climate changed to subtropical with a mean annual temperature of 18.5°C (65°F) in early Eocene time. A major increase in the average temperature of the coldest month may have contributed significantly to floral change across the Paleocene-Eocene boundary. Stratigraphic position and fossils of the Bear Den Member indicate a late Paleocene age, and the Camels Butte Member is assigned an early Wasatchian age. These findings represent a revision of the previous assignment of an early Eocene age to the entire formation. Transport from an intensively weathered igneous and metamorphic terrane lying at a moderate distance from the basin of sedimentation is inferred from the composition and texture of the Golden Valley sedimentary deposits. Although previous workers accepted the occurrence of the “widespread, uniform kaolin blanket” of the Bear Den Member as evidence of a lacustrine origin, the predominance of channel-facies sandstone lenses interbedded with typical overbank and backswamp deposits throughout both members indicates a fluvial origin. Tectonic forces affecting the source and depositional areas, rather than local conditions, are inferred to have caused the interval of fine-grained, swampy sedimentation at the Bear Den-Camels Butte boundary as well as the abrupt changes in clay-mineral composition above and below the Bear Den Member and the increase in channel-facies sediment toward the top of the formation.