Bear Lake is a large alkaline lake on a high plateau on the Utah-Idaho border. The Bear River was partly diverted into the lake in the early twentieth century so that Bear Lake could serve as a reservoir to supply water for hydropower and irrigation downstream, which continues today. The northern Rocky Mountain region is within the belt of the strongest of the westerly winds that transport moisture during the winter and spring over coastal mountain ranges and into the Great Basin and Rocky Mountains. As a result of this dominant winter precipitation pattern, most of the water entering the lake is from snowmelt, but with net evaporation. The dominant solutes in the lake water are Ca 2+ , Mg 2+ , and HCO 3 2‒ , derived from Paleozoic carbonate rocks in the Bear River Range west of the lake. The lake is saturated with calcite, aragonite, and dolomite at all depths, and produces vast amounts of carbonate minerals. The chemistry of the lake has changed considerably over the past 100 years as a result of the diversion of Bear River. The net effect of the diversion was to dilute the lake water, especially the Mg 2+ concentration. Bear Lake is oligotrophic and coprecipitation of phosphate with CaCO 3 helps to keep productivity low. However, algal growth is colimited by nitrogen availability. Phytoplankton densities are low, with a mean summer chlorophyll a concentration of 0.4 mg L ‒1 . Phytoplankton are dominated by diatoms, but they have not been studied extensively (but see Moser and Kimball, this volume). Zooplankton densities usually are low (<10 L ‒1 ) and highly seasonal, dominated by calanoid copepods and cladocera. Benthic invertebrate densities are extremely low; chironomid larvae are dominant at depths <30 m, and are partially replaced with ostracodes and oligochaetes in deeper water. The ostracode species in water depths >10 m are all endemic. Bear Lake has 13 species of fish, four of which are endemic.

Radiocarbon analyses of pollen, ostracodes, and total organic carbon (TOC) provide a reliable chronology for the sediments deposited in Bear Lake over the past 30,000 years. The differences in apparent age between TOC, pollen, and carbonate fractions are consistent and in accord with the origins of these fractions. Comparisons among different fractions indicate that pollen sample ages are the most reliable, at least for the past 15,000 years. The post-glacial radiocarbon data also agree with ages independently estimated from aspartic acid racemization in ostracodes. Ages in the red, siliclastic unit, inferred to be of last glacial age, appear to be several thousand years too old, probably because of a high proportion of reworked, refractory organic carbon in the pollen samples. Age-depth models for five piston cores and the Bear Lake drill core (BL00-1) were constructed by using two methods: quadratic equations and smooth cubic-spline fits. The two types of age models differ only in detail for individual cores, and each approach has its own advantages. Specific lithological horizons were dated in several cores and correlated among them, producing robust average ages for these horizons. The age of the correlated horizons in the red, siliclastic unit can be estimated from the age model for BL00-1, which is controlled by ages above and below the red, siliclastic unit. These ages were then transferred to the correlative horizons in the shorter piston cores, providing control for the sections of the age models in those cores in the red, siliclastic unit. These age models are the backbone for reconstructions of past environmental conditions in Bear Lake. In general, sedimentation rates in Bear Lake have been quite uniform, mostly between 0.3 and 0.8 mm yr ‒1 in the Holocene, and close to 0.5 mm yr ‒1 for the longer sedimentary record in the drill core from the deepest part of the lake.

Bear Lake is a long-lived lake filling a tectonic depression between the Bear River Range to the west and the Bear River Plateau to the east, and straddling the border between Utah and Idaho. Mineralogy, elemental geochemistry, and magnetic properties provide information about variations in provenance of allogenic lithic material in last-glacial-age, quartz-rich sediment in Bear Lake. Grain-size data from the silici-clastic fraction of late-glacial to Holocene carbonate-rich sediments provide information about variations in lake level. For the quartz-rich lower unit, which was deposited while the Bear River flowed into and out of the lake, four source areas are recognized on the basis of modern fluvial samples with contrasting properties that reflect differences in bedrock geology and in magnetite content from dust. One of these areas is underlain by hematite-rich Uinta Mountain Group rocks in the headwaters of the Bear River. Although Uinta Mountain Group rocks make up a small fraction of the catchment, hematite-rich material from this area is an important component of the lower unit. This material is interpreted to be glacial flour. Variations in the input of glacial flour are interpreted as having caused quasi-cyclical variations in mineralogical and elemental concentrations, and in magnetic properties within the lower unit. The carbonate-rich younger unit was deposited under conditions similar to those of the modern lake, with the Bear River largely bypassing the lake. For two cores taken in more than 30 m of water, median grain sizes in this unit range from ~6 μm to more than 30 μm, with the coarsest grain sizes associated with beach or shallow-water deposits. Similar grain-size variations are observed as a function of water depth in the modern lake and provide the basis for interpreting the core grain-size data in terms of lake level.

Sediments deposited over the past 220,000 years in Bear Lake, Utah and Idaho, are predominantly calcareous silty clay, with calcite as the dominant carbonate mineral. The abundance of siliciclastic sediment indicates that the Bear River usually was connected to Bear Lake. However, three marl intervals containing more than 50% CaCO 3 were deposited during the Holocene and the last two interglacial intervals, equivalent to marine oxygen isotope stages (MIS) 5 and 7, indicating times when the Bear River was not connected to the lake. Aragonite is the dominant mineral in two of these three high-carbonate intervals. The high-carbonate, aragonitic intervals coincide with warm interglacial continental climates and warm Pacific sea-surface temperatures. Aragonite also is the dominant mineral in a carbonate-cemented microbialite mound that formed in the southwestern part of the lake over the last several thousand years. The history of carbonate sedimentation in Bear Lake is documented through the study of isotopic ratios of oxygen, carbon, and strontium, organic carbon content, CaCO 3 content, X-ray diffraction mineralogy, and HCl-leach chemistry on samples from sediment traps, gravity cores, piston cores, drill cores, and microbialites. Sediment-trap studies show that the carbonate mineral that precipitates in the surface waters of the lake today is high-Mg calcite. The lake began to precipitate high-Mg calcite sometime in the mid–twentieth century after the artificial diversion of Bear River into Bear Lake that began in 1911. This diversion drastically reduced the salinity and Mg 2+ :Ca 2+ of the lake water and changed the primary carbonate precipitate from aragonite to high-Mg calcite. However, sediment-trap and core studies show that aragonite is the dominant mineral accumulating on the lake floor today, even though it is not precipitating in surface waters. The isotopic studies show that this aragonite is derived from reworking and redistribution of shallow-water sediment that is at least 50 yr old, and probably older. Apparently, the microbialite mound also stopped forming aragonite cement sometime after Bear River diversion. Because of reworking of old aragonite, the bulk mineralogy of carbonate in bottom sediments has not changed very much since the diversion. However, the diversion is marked by very distinct changes in the chemical and isotopic composition of the bulk carbonate. After the last glacial interval (LGI), a large amount of endogenic carbonate began to precipitate in Bear Lake when the Pacific moisture that filled the large pluvial lakes of the Great Basin during the LGI diminished, and Bear River apparently abandoned Bear Lake. At first, the carbonate that formed was low-Mg calcite, but ~11,000 years ago, salinity and Mg 2+ :Ca 2+ thresholds must have been crossed because the amount of aragonite gradually increased. Aragonite is the dominant carbonate mineral that has accumulated in the lake for the past 7000 years, with the addition of high-Mg calcite after the diversion of Bear River into the lake at the beginning of the twentieth century.

A continuous, 120-m-long core (BL00-1) from Bear Lake, Utah and Idaho, contains evidence of hydrologic and environmental change over the last two glacial-interglacial cycles. The core was taken at 41.95°N, 111.31°W, near the depocenter of the 60-m-deep, spring-fed, alkaline lake, where carbonate-bearing sediment has accumulated continuously. Chronological control is poor but indicates an average sedimentation rate of 0.54 mm yr ‒1 . Analyses have been completed at multi-centennial to millennial scales, including (in order of decreasing temporal resolution) sediment magnetic properties, oxygen and carbon isotopes on bulk-sediment carbonate, organic- and inorganic- carbon contents, palynology; mineralogy (X-ray diffraction), strontium isotopes on bulk carbonate, ostracode taxonomy, oxygen and carbon isotopes on ostracodes, and diatom assemblages. Massive silty clay and marl constitute most of the core, with variable carbonate content (average = 31 ± 19%) and oxygen-isotopic values (δ 18 O ranging from ‒18‰ to ‒5‰ in bulk carbonate). These variations, as well as fluctuations of biological indicators, reflect changes in the water and sediment discharged from the glaciated headwaters of the dominant tributary, Bear River, and the processes that influenced sediment delivery to the core site, including lake-level changes. Although its influence has varied, Bear River has remained a tributary to Bear Lake during most of the last quarter-million years. The lake disconnected from the river and, except for a few brief excursions, retracted into a topographically closed basin during global interglaciations (during parts of marine isotope stages 7, 5, and 1). These intervals contain up to 80% endogenic aragonite with high δ 18 O values (average = ‒5.8 ± 1.7‰), indicative of strongly evaporitic conditions. Interglacial intervals also are dominated by small, benthic/tychoplanktic fragilarioid species indicative of reduced habitat availability associated with low lake levels, and they contain increased high-desert shrub and Juniperus pollen and decreased forest and forest-woodland pollen. The 87 Sr/ 86 Sr values (>0.7100) also increase, and the ratio of quartz to dolomite decreases, as expected in the absence of Bear River inflow. The changing paleoenvironments inferred from BL00-1 generally are consistent with other regional and global records of glacial-interglacial fluctuations; the diversity of paleoenvironmental conditions inferred from BL00-1 also reflects the influence of catchment-scale processes.

Abstract The Cretaceous Western Interior Seaway Drilling Project was begun in 1991 under the auspices of the U.S. Continental Scientific Drilling Program. It was intended to be a multidisciplinary study of Cretaceous carbonate and siliciclastic rocks in cores from bore holes along a transect across the Cretaceous Western Interior Seaway. The study focuses on middle Cretaceous (Cenomanian to Campanian) strata that include, in ascending order, Graneros Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation. The transect includes cores from western Kansas, eastern Colorado, and eastern Utah. The rocks grade from pelagic carbonates containing organic-carbon-rich source rocks at the eastern end of the transect to nearshore coal-bearing units at the western end. These cores provide unweathered samples and the continuous depositional record required for geochemical, mineralogical, and biostratigraphic studies. The project combines biostratigraphic, paleoecological, geochemical, mineralogical, and high-resolution geophysical logging studies conducted by scientists from the U.S. Geological Survey, Amoco Production Company, and six universities.

Abstract The Upper Albian-Coniacian section cored in the Amoco No. 1 Rebecca K. Bounds well in Greeley County western Kansas, serves as a reference section for the timing of depositional events in the Western Interior Seaway. Chronostratigraphy of this section was calibrated by a multidisciplinary study of nannofossils, dinoflagellates, spores, pollen, foraminifers, and mollusks. Range data of the biota in the Bounds core were compared by graphic correlation to a global composite standard that includes key reference sections in Europe and North Africa. The basal Upper Albian sequence boundary is overlain by transgressive facies of the Purgatoire Formation dated as 102.8 Ma. The upper Upper Albian sequence boundary between the Purgatoire and Dakota Formations marks a hiatus in deposition from 99.4 to 98.2 Ma. The Albian-Cenomanian intra-Dakota sequence boundary spans from 96.0 to 94.1 Ma. The Turonian-Coniacian sequence boundary between the Carlile and Niobrara Formations spans from 89.9 to 88.3 Ma. Maximum flooding is documented within the Purgatoire at 101.4 Ma and in the Graneros Shale at 93.7-92.8 Ma. The Albian-Cenomanian boundary defined by European ammonites and correlated by dinoflagellates is placed at the intra-Dakota unconformity. Graphic correlation is an independent method of measuring the durations of Milankovitch-scale depositional cycles and can separate climatic cycles from longer tectono-eustatic cycles. Four orders of depositional cycles are recorded by lithological changes, and their durations are constrained by graphic correlation. The longest cycles range from 2.0 to 3.4 My and are found in the sequences defined by the Purgatoire Formation, the lower part of the Dakota Formation, the upper Dakota and Graneros Formations, and the Greenhorn and Carlile Formations. The next lower order comprises transgressive-regressive subcycles of about 0.5 My long in the Purgatoire. The third-scale cycles include sandstone-mudrock cycles in the Dakota, limestone-marl cycles in the lower part of the Greenhorn, and cyclical strata in the Fort Hays Limestone Member of the Niobrara Formation that are about 100 ka long. The shortest cycles are limestone-marl couplets in the upper Greenhorn that are about 41 ka long.

Abstract Calcareous nannofossil assemblages were investigated in two cyclic stratigraphic intervals from the Late Cretaceous Western Interior Seaway to determine the causes of lithologic variations. Relative abundance data were collected from the Bridge Creek Limestone Member (Cenomanian-Turonian) of the Greenhorn Formation and the transition between the Fort Hays Limestone and Smoky Hill Chalk Members (Coniacian-Santonian) of the Niobrara Formation from two cores, the No. 1 Amoco Rebecca Bounds Core, western Kansas, and the USGS No. 1 Portland Core, central Colorado. Stable-carbon and -oxygen isotopic analyses of fine (<38pm) fractions were carried out on samples from the interval with the best preserved nannofossils, the Fort Hays Limestone-Smoky Hill Chalk transition in the Bounds core. Preservation of nannofossil assemblages varies in the two cores. Correlations between CaCO 3 and the abundance of nannofossil species, which are used as proxies for organic productivity, are rarely significant, which indicates an inconsistent relationship between fertility and lithology. Fine-fraction, oxygen-isotopic values from the Fort Hays Limestone-Smoky Hill Chalk transition of the Bounds core correlate highly with nannofossil fertility markers, indicating a close relationship between organic productivity and the amount of run-off into the basin. However, the lack of consistent correlation between organic productivity (as seen in nannofossil assemblages) and lithology suggests that lithology was influenced by a variety of complex processes including variations in carbonate productivity and dilution with clastic material. Carbonate productivity was likely influenced by multiple water masses, including run-off from mountainous regions to the west, warm waters from the Tethys, and cooler waters from the Arctic. The interplay of the water masses and the additional dilution signal render the lithologic cycles out of phase with the periodicities that control organic productivity.

Abstract The Cenomanian-Santonian calcareous nannofossil biostratigraphy of the Rebecca Bounds, Portland, and Escalante cores was investigated. Zonal markers were used to correlate the Cenomanian-Turonian boundary interval in the cores with outcrop sections in the Western Interior basin. However, it is difficult to apply existing zonations in the remainder of the section. We determined several new biohorizons that are useful for drawing correlations between the cores. These biohorizons are combined with some of the standard zonal markers in defining informal zonal units. Environmental factors appear to have reduced nannofossil diversity along the western margin of the basin and led to the premature extinction of several nannofossil markers in the Cenomanian-Turonian boundary interval.

Abstract Foraminifera in shales and mudrocks of the Greenhorn Cycle (late Cenomanian-middle Turonian age) in the Cretaceous Western Interior Basin were strongly influenced by sea level change. This long-term record of third-order sea level rise and fall is superposed by fourth-order relative sea level cycles as delimited by carbonate and sedimentological data. The study interval includes the Cenomanian-Turonian boundary and the early Turonian record of the highest stand of sea level in the western interior. We document stratigraphic variations in foraminiferal assemblages and their response to changing sea level for one drill core through the Tropic Shale (Escalante, Utah) and two outcrop sections of the Mancos Shale (Lohali Point, Arizona; Mesa Verde, Colorado) from the Colorado Plateau. The three sections record deposition along the southwestern margin of the Greenhorn Sea and provide a temporal and spatial framework for interpretations of paleoecology and paleoceanography. Earlier studies demonstrate that fluctuations in planktic foraminifera and calcareous and agglutinated benthic foraminifera track the transgression and regression of the Greenhorn Cycle. Results of assemblage analyses presented here show that benthic taxon dominance also correlates to fourth-order sea level changes, and to the type of systems tract. Assemblages of calcareous benthic foraminifera are dominated by two species, Gavelinella dakotensis and Neobulimina albertensis. Neobulimina, an infaunal taxon, dominated during the late transgression and highstand of the Greenhorn Sea (early Turonian) when warm, normal salinity, oxygen-poor Tethyan waters advanced northwards into the seaway. In contrast, the epifaunal/shallow infaunal taxon Gavelinella proliferated briefly during times of water mass renewal and when deposition of organic matter increased at the transition between fourth-order cycles. Peaks in abundance of other calcareous benthic species delimit transgressive pulses prior to the spread of oxygen-poor Tethyan water masses. These broad-based correlations may result from an intricate relationship among changing water masses, flux of terrestrial and marine organic matter, sedimentation rates, and benthic oxygenation. Regression of the Greenhorn Sea resulted in a greater restriction of oceanic circulation and in the withdrawal of Tethyan waters that were replaced by cooler, lower salinity water masses of Boreal affinity. An abrupt change to dominance by agglutinated benthic foraminifera and loss of nearly all planktic foraminifera marks this paleoceanographic event. Enhanced biological productivity accompanied regression in south-central Utah. Depauperate benthic foraminiferal assemblages reflect the stress of low-oxygen conditions despite an abundance of food. Enhanced salinity stratification during later stages of regression may have reduced ventilation on the seafloor and led to dysoxic bottom waters. Sea level change helped produce distinctive assemblages of benthic foraminifera that can be used to delimit successive systems tracts. Foraminiferal assemblages also provide insight into their evolutionary responses to rapidly changing paleoenvironments. Our results indicate no evolution in the foraminiferal biota of the study sections, which we think points to evolutionary stasis.