Abstract Upper Cretaceous reefs were concentrated in low- to mid-latitude regions in the Northern Hemisphere between the Americas and the Arabian Peninsula. Rudist bivalves, scleractinian corals, sponges, stromatoporoids, and algae were the dominant biota. Most Late Cenomanian through Santonian reefs occurred in low paleolatitudes (0–30° N) and were dominated by rudist bivalves. North of 30°, reefs constructed of corals, stromatoporoids, and siliceous sponges outnumbered those of bivalves. Campanian through Maastrichtian reefs occurred between the equator and 30° N and were also dominated by bivalves, whereas corals and bryozoans dominated the northern occurrences. The distribution of Upper Cretaceous reefs was analyzed with respect to paleogeography, surface current circulation patterns, sea level, and sea-water chemistry. Considering the paleogeographic setting of the Late Cretaceous, westward-flowing surface currents accounted for the low- to mid-latitude distribution patterns of reefs, whereas northward surface currents could account for northern occurrences in the European and North American regions, especially during sea-level highstands when shelfal areas were flooded. There is a global correspondence between the development of Upper Cretaceous reefs and the first-order sea-level highstand of Haq et al. (1987) , but there is only a regional, not global, correlation between reefs and second-order sea-level fluctuations; some reefs were associated with third-order and fourth-order fluctuations. We found no direct correspondence between the global distribution of Upper Cretaceous reefs and oceanic anoxic events, salinity, aragonite-calcite seas, or sea-surface temperature, although links still need to be investigated for geographic regions and subdivisions of the Late Cretaceous. Numerical analyses of the PaleoReef database allowed for an assessment of the biological and physical attributes of reefs. From this database, Upper Cretaceous reefs representing the Upper Zuni 111 supersequence (Late Cenomanian-Santonian) can be characterized by rudists of the constructor guild. Other biota are also prominent. Biostromes and reef mounds in shallow intraplatform or platform-margin settings have large amounts of micrite and a moderate debris potential. Reefs representing the Upper Zuni IV supersequence (Campanian-Maastrichtian) can be characterized by rudists and oysters of the constructor guild. Other biota areprominent. Biostromes and reef mounds in a marginal marine setting have large to moderate amounts of micrite and a moderate debris potential.

Abstract The Western Interior Cretaceous Basin of North America evolved as a complex foreland basin between Late Jurassic and Cretaceous time in response to accelerated plate spreading, convergence, and subduction along the western margin of North America. Westward translation of compressive forces episodically deformed this basin, and divided it into six discrete structural provinces: (1) A western zone of pluton-ism, volcanism, and thrusting - the Cordilleran thrust belt; (2) a foreland basin which subsided rapidly in response to thrust and synorogenic sediment loading; (3) a discontinuous forebulge zone east of the foreland basin; (4) a broad axial basin characterized by subsidence at rates greater than sedimentation during structurally active periods; (5) a tectonic hinge zone on the east side of the axial basin, at its junction with the North American craton; and (6) a broad eastern stable cratonic zone. This basin was flooded by shallow Cretaceous epicontinental seas from the north and south beginning in Barremian-Aptian time. These discrete arms of the seaway joined in Late Alb-ian, and after a brief latest Albian separation across the Transcontinental Arch, the basin remained flooded for 38 Ma until late Middle Maastrichtian time. Initial flooding of the Western Interior Basin, and dynamic changes in the shape, size, and depth of this epicontinental sea reflected the complex interaction of episodic basin tectonics and first- through fourth-order tectonoeustacy, reflecting plate tectonic (especially seafloor spreading) activity levels. The Western Interior Cretaceous Seaway was highly dynamic throughout its history; its temperature, chemistry, circulation patterns and levels, stratification characteristics, shape and size varied greatly, and episodically in concert with tectonic, eustatic, and climatic factors and their interaction. Consequently, the sedimentologic record of basin evolution in the Western Interior Seaway is complex, and reflects both local, autocyclic and, more predominantly, regional to global allocyclic processes. Eustatic and climatic cycles were the prevailing controls of cyclic sedimentation, which is well defined in the basin center (e.g. Pueblo), and more complex in its marginal basin development. High resolution event-stratigraphy and assemblage zone biostratigraphy provide independent means of correlating diverse aspects of basin evolution, through graphic correlation, into a coherent basin model. This model suggests a two-phase evolution of the basin. Times of active plate spreading, ridge-building, and thus eustatic rise are generally coeval with active plutonism, volcanism, thrusting, and rapid subsidence in the Western Interior Basin of North America, preserving thick transgres-sive sediment sequences. Times of plate quiescence are also times of tectonic quiescence and filling of the basin associated with eustatic fall and regression.

Abstract Many shales, marls and carbonates show a rhythmic oscillation in carbonate content, resulting in alternate carbonate-richer and carbonate-poorer beds (bedding couplets ). Spacing of couplets may be even, irregular, or patterned ( bundled ), and bundles may be grouped into superbundles . Along with spacing, couplet sequences may also be modulated by changes in composition. These cycles record depositional response to cyclic, climatically induced changes in the rate of carbonate supply ( productivity cycles ), in rate of detrital supply ( dilution cycles ), and in carbonate removal ( dissolution cycles ). Systems vary in sensitivity, some recording every beat of the cycles, others only the strongest ones. The question of diagenetically introduced cycles remains in debate. Nearly a century ago Gilbert suggested that such oscillations in the Cretaceous of Colorado reflect climatic oscillations caused by the Earth’s orbital variations. Varve data suggest that these variations have influenced depositional systems for at least 250 ma, and that they have maintained much the same periods over that time span. Segments of a Cretaceous pelagic sequence in Italy show this hierarchy of cycles. Here the couplets reflect the precession, the bundles represent the short cycle of eccentricity, and the superbundles correspond to the long cycle of eccentricity. Couplets and bundles in the Niobrara Formation probably are of this origin. In other sequences couplets, generally unbundled, seem to represent the 41 Ka obliquity cycle. The Greenhorn cycles are of this type. Most long pelagic sequences are complex: some segments show the simple oscillations suggestive of the obliquity signal, others show the precession-eccentricity hierarchy, and yet others appear chaotic. There are indications that high latitudes such as northern Germany may yield long and relatively uncomplicated record of the obliquity cycle. Hope for sorting out complex sequences lies largely in instrumental scans of cores and in time-series analysis. The cycle geochronometry envisioned by Gilbert is not yet practicable but may come into reach. Cycle studies raise important questions for sedimentology and paleoceanography. For stratigraphy, they provide ultra-fine correlation within the cyclic facies. Furthermore, they imply that in some facies complexes the more limestone-rich parts of the shale facies are age equivalent to the more shaly segments of the limestone facies.

Abstract The Cretaceous was a time of decreased freeboard, when large portions of all continents were flooded by shallow marine waters. Suggested causes for this decrease in freeboard are a decrease in the volume of the ocean basins caused by more rapid sea floor spreading rates during the Cretaceous (Kominz, 1984) and a thermal bulge in the Pacific during this time (Schlanger et al., 1981). Kominz (1984) has analysed the possible errors involved in attempting to determine the volume of the ridge crests during the Cretaceous, and came to the conclusion that sea level was 228 m higher than today 80 m.y. ago. This included an estimate for the amount of continental ice present today, which was assumed to he absent during the Cretaceous. Schlariger et al. (1981) estimated a sea level rise of 80 m 70 m.y. ago, by studying thermal subsidence of areas in the Pacific which were known to show very large volcanic activity between 110 and 70 m.y. ago. A recent compilation of the causes of sea level change, in which ridge crest volume, thermal subsidence, ice volume, thermal shrinking of the water column, increased sediment load today compared with the Cretaceous, and increased oceanic area due to continental collision following closure of Tethys were all considered, gave a sea level increase 80 m.y. ago of 278 in (Harrison, 1985). Of particular interest to the readers of this volume are the sea level changes which produce the transgressive-regressive cycles seen in the sedimentary deposits of the Western

Abstract Pelagic limestone units were deposited in the North American Western Interior seaway during two major Cretaceous transgressive episodes. The Bridge Creek Limestone Member of the Greenhorn Formation, deposited during the Late Cenomanian-Early Turonian transgression, and the Smoky Hill Member of the Niobrara Formation, deposited during the overall Early Coniacian-Early Campanian transgression, are both enriched in organic-carbon and exhibit smallscale carbonate cycles representing periodicities in the range 20 to 40 ky. The distinct periodicity and overall unusual depositional milieu of both units are reflected in their sedimentary structures, mineralogy, and geochemistry. The Bridge Creek Limestone at Pueblo, Colorado, averages 78% CaCO 3 and 1.75% organic carbon with ranges of 42-96% and 0.06-6.97%, respectively, across small-scale cycles. High concentrations of Al, Fe, Mg, K, Ti, Na, Cr, Ni, V; higher Sr/Ca and lower Si/Al ratios; and lighter δ 18 0 in CaCO 3 in dark-colored clay-rich beds all suggest periodic influx of terrestrial clay minerals during times of peak fresh water runoff from uplifted highlands to the west. Higher Sr/Ca ratios in marlstone beds than in limestone beds suggest that the marlstone beds have undergone less diagenetic removal of Sr. Higher concentrations of organic carbon, hydrogen, and sulfur, and preservation of some lamination in the clay-rich beds also suggest that the times of enhanced runoff may have induced stable salinity stratification in the water column, which led to gradual depletion of dissolved oxygen in the bottom waters and enhanced preservation of organic carbon in sediments. The geochemistry also suggests that a significant change in sedimentation occurred at the Cenomanian-Turonian boundary. The geochemical characteristics of the Niobrara Formation near Fort collins, Colorado, are very similar to those of the Bridge Creek Limestone at Pueblo, suggesting similar depositional conditions and source of clastic materials. However, the small scale cycles are present but more subdued in the Niobrara Formation than in the Bridge Creek Limestone, and the Niobrara Formation in the Fort Collins area has not been as altered by diagenesis.

Abstract The Greenhorn Limestone is a remarkable pe1agic/hemipe1agic carbonate-rich unit that marks late Cenomanian-midd1e Turonian peak and near-peak transgressional conditions in the Western Interior Sea. Parts of the section consisting of or equivalent to the Hart-land and Jetmore Members comprise a sequence of generally laminated chalky, marly, or calcareous shale beds that alternate with bioturbated chalk, chalky limestone, or hard brittle limestone beds. Progressive westward lithologic change to less calcareous facies reflects influence of the Sevier orogenic belt as a major source for Greenhorn terrigenous detritus. Limestone beds of the Hartland-Jetmore sequence are time parallel and most can be traced across vast expanses of the study area. Any two of these marker beds serve to define a 1ithochronozone within which, or within a succession of which, regional variations of biota, facies, strati-graphic thickness, remanent magnetism, and geochemistry can be investigated with chronologic precision not afforded by other methods. Few such analyses have been carried out but the marker beds have been useful in demonstrating the widespread effects of short-term events, an example being that which produced the foraminiferal calcarenite that lies between marker beds JT-1 and JT-2 at many Greenhorn localities between north-central Kansas and western New Mexico. Analysis of thickness variations between marker beds has shown the presence of condensed Greenhorn sections, commonly rich in calcarenite, that apparently were deposited over topographic highs on the sea floor. General correspondence of condensed sections with structural features strongly suggests that Greenhorn deposition was influenced by contemporaneous tectonism. Precise regional correlation of time-parallel Greenhorn limestone beds also makes possible detailed regional comparisons of geochemical data not only for a given lithochronozone but also through a succession of precisely correlated stratigraphic intervals. This approach will permit rigorous testing of models that have been proposed to account for the strati-graphic rhythmicity that characterizes much of the Greenhorn section in the central Great Plains and Southern Rocky Mountains.

Abstract Variation in the carbon isotopic composition of organic matter in the Greenhorn Limestone are described for Black Mesa, Arizona; Pueblo, Colorado; Bunker Hill, Kansas; Ponca State Park, Nebraska; and Cone Hill, Montana. There is a 2.5 to 3.5 o/oo positive (heavy) excursion in carbon isotopic values spanning the Cenomanian-Turonian boundary at each of these localities. Carbon and oxygen isotopic compositions of inoceramid bivalve shells and whole-rock carbonate are reported for the Black Mesa and Pueblo sections. Oxygen isotopic values of whole-rock carbonate generally become more positive upward in the Greenhorn from the lower Lincoln Limestone Member through the Hartland Shale Member and into the lower Bridge Creek Limestone Member. This trend is inferred to reflect generally increasing salinity of Western Interior seawater associated with increasing water depths and less restricted oceanic connections as the Greenhorn transgression progressed. Superimposed on this overall trend are intervals with marked isotopic and geochemical fluctuations suggesting rapidly changing proportions of oceanic and riverine water or rapidly changing relative rates of evaporation and input. A particularly severe and rapid paleoenvironmental change is inferred to have occurred just prior to the Cenomanian-Turonian boundary defined by macrofossil extinctions.

Abstract Field measurement of gamma-ray spectra on outcrops of Upper Cretaceous marine shale and limestone that were deposited in the Western Interior Seaway in Utah, New Mexico, and Colorado provides accurate estimates of total gamma radiation and potassium, uranium, and thorium contents. Spectrometer profiles of measured sections were combined with lithostratigraphic and biostratigraphic data to show that during Early Turonian time, potassium and thorium contents, total gamma radiation, Th/U, and K/U decreased offshore. Oxygenated bottom waters predominated within 100 km of the western paleoshoreline, whereas oxygen-depleted bottom waters were dominant more than 600 km offshore. During late Middle Turonian time, well-oxygenated bottom water may have extended across the study area. Peak transgressions are characterized by low Th/U and K/U and can be identified in outcrops and on well logs. Anomalously high Th/U and K/U near the Cenomanian-Turonian boundary and in the Coniacian may correspond to “oceanic anoxic events.” Lithologic/geochemical facies are identified on Th-U plots.

Abstract The distribution of foraminifera in the Greenhorn Western Interior seaway reflects the influence of water masses from both the north and south. Three distinct benthic biofacies (arenaceous, mixed, calcareous) are recognized. As the seaway deepened a benthic fauna composed of arenaceous species was succeeded in the center of the seaway by a zone barren of benthic foraminifera and in fringing environments shoreward by a mixed biofacies. The northward expansion of the barren zone, which indicates a deoxygenated benthos, coincided with the influx of diverse planktonics of southern affinities. The distributions of the benthic biofacies and of planktonic diversities probably reflect bathymetry. The appearance of a calcareous benthic fauna at the base of the Bridge Creek Limestone Member of the Greenhorn Formation represents oxygenation of the sea floor due to increased vertical circulation. The alternation of limestones and marlstones in the Bridge Creek reflects alternations in the productivity of calcareous plankton. The limestones represent episodes of dry climate at low latitudes when large volumes of warm saline bottom water formed, stratification of the oceans weakened, and widespread upwelling boosted productivity. Marlstones represent wet climates at low latitudes when little WSBW formed, stratification of the oceans intensified, and productivity fell. The Bridge Creek foraminifera do not support the idea that a brackish lid developed periodically on the seaway.

Abstract The Lower Cretaceous (Aptian (?) - Albian) Dakota Group, as exposed along the Skyline Drive, near Canon City, Colorado, and along the Arkansas River, near Pueblo, Colorado, is divided into four distinct lithologically-sitnilar and genetically related formations. In ascending order, these are: (1) the Lytle Formation; (2) Plainview Formation; (3) Glencairn Formation; and (4) Muddy Sandstone. The Lytle Formation is composed of white to light gray, friable, poorly sorted, cross-stratified, pebbly sandstone, deposited by braided fluvial systems draining the Mogollon Highlands and Sevier Orogenic belt. The Plainview Formation is composed of interbedded brown-weathering fluvial sandstone and carbonaceous shale. The Clencairn Formation is composed of five upward-coarsening, marine shale to sandstone, progradational shoreface sequences, separated by regional transgressive disconformities. The Muddy Sandstone, a massive, brown-weathering, cross-stratified fluvial sandstone, that grades upward into interbedded fluvial and marginal marine sandstone and shale, represents aggradation of a previously eroded paleotopography during base level rise. The Dakota Group records two major third-order tectonoeustatic sealevel changes: the Kiowa-Skull Creek and Greenhorn transgressive-regressive cycles. The Plainview and Glencairn Formations were deposited during the Kiowa-Skull Creek cycle, representing coastal plain aggradation, followed by transgression of, and small-scale fluctuations of the Kiowa-Skull Creek strandline as the cycle approached peak transgression. The fourth progradational sequence represents peak transgression. The Kiowa-Skull Creek regression is recorded as a widespread, regional erosional unconformity, which developed on top of Glencairn-Kiowa-Skull Creek deposits. The initial transgression of the Greenhorn seaway into the Canon City - Pueblo areas is recorded by the Muddy Sandstone, which aggraded this erosional paleotopography.

Abstract The Graneros Shale at Pueblo is typical of the formation throughout the southern Colorado Front Range. It is comprised of three members: The lower shale member, middle Thatcher Limestone Member, and upper shale member. The Graneros is generally represented by dark gray, slightly to moderately silty, well-laminated, non-calcareous clay shale containing scattered low diversity benthic faunas. Many parts of the Graneros lack faunas or bioturbation. These features, and relatively high levels of organic carbon (1 - 4.2 %/wt.) suggest a predominance of quiet water, oxygen-depleted (dysaerobic), benthic environments in proximal to medial offshore settings below wave base. The Graneros Shale preserves abundant event marker beds, enhancing precise regional correlation; bentonites are especially abundant and reflect the waning stages of the largest Cretaceous (Lake Albian) volcanic episode. High-resolution stratigraphic analysis of the Graneros Shale demonstrates dynamic environmental and paleoceanographic changes during its deposition, as follows: Well-circulated, oxygenated benthic substrates within reach of storm wave base (Facies 1; base of lower shale member) grade upward into; (2) quiet water, oxygen-depleted benthic environments below wave base (lower source rock interval); to (3) moderately well-circulated and partially oxygenated substrates in quiet water settings, preserving little organic carbon at the top of the lower shale member; to (4) a rapid incursion of Subtropical, oxygenated, normal marine waters with accelerated eustatic rise; calcareous shales and pelletoid limestone characterize this interval and contain a moderately to highly diverse warm water benthic and pelagic biota (the Thatcher Limestone Member); to (5) quiet water, oxygen-depleted benthic environments preserving abundant organic material in medial offshore settings (upper shale member); and at Pueblo (6), a recirculation event in the seaway associated with incursion of warm southern waters, normalization of oxygen and salinity, and development of diverse benthic faunas. This event marks the first phase of a long pelagic carbonate-producing episode that characterizes the overlying Greenhorn Formation. Fourth-order eustatic fluctuations are thought to be the major controlling factor in major facies changes during deposition.

Abstract The Lincoln Limestone Member of the Greenhorn Limestone is characterized by calcareous shale and calcarenite beds deposited during the transgressive phase of the Greenhorn Cyclothem. At Rock Canyon anticline the Lincoln Member is conformably underlain by the Graneros Shale and overlain by the Hartland Shale Member of the Greenhorn Limestone. A detailed study of the Lincoln divides the member into a lower and an upper interval, based on lithostratigraphic, geochemical and paleobiologic criteria. The lower unit is characterized by laminated, weakly calcareous shales, relatively high organic carbon content (C org ∼ 2.5%), and a macrofaunal assemblage transitional between the Graneros Shale and overlying Greenhorn deposits. The upper Lincoln interval is characterized by frequent alternation of laminated shale and ripple-bedded calcarenite beds with associated high C org values in the shale beds (>2.0%) and lower C org values in the calcarenite beds (<1.0%). Faunal composition through this upper interval records development of the first typical Greenhorn biota in the calcarenite beds. In the upper interval, the interbedding of contrasting lithofacies and biofacies suggests a dynamic paleooceanographic system poised between a relatively quiet water, oxygen-deficient environment during shale deposition, and well-circulated, well-oxygenated benthic conditions during calcarenite deposition. The Lincoln Limestone Member thus represents one complete 4th order marine cycle within the Greenhorn Cyclothem beginning with lithofacies and biofacies characteristic of a stillstand event in the lower Lincoln interval, progressing to deposits characteristic of an overall transgressive pulse in the upper Lincoln interval.

Abstract The Hartland Shale Member of the Greenhorn Limestone is predominantly composed of finely laminated calcareous shale with high levels of organic carbon and low levels of fossil diversity and abundance, suggesting a Cretaceous epicontinental anoxic event. At Rock Canyon anticline the Hartland member is conformably underlain by the Lincoln Limestone Member and conformably overlain by the Bridge Creek Limestone Member of the Greenhorn Limestone. A detailed lithostratigraphic, geochemical and paleobiologic study of this interval has identified characteristic lithofacies and biofacies which subdivide the member into three units and suggests a cyclic pattern of oceanographic processes during Hartland deposition. An alternating transgressive pulse/stil1 stand model is proposed to account for these patterns of cyclic sedimentation and oxygen-deficient biofacies distribution.

Abstract The Bridge Creek Limestone Member of the Greenhorn Formation records maximum eustatic rise of the Greenhorn Marine Cycle, and the maximum aerial extent reached by the Western Interior Seaway. Limestone-shale couplets reflecting Milankovitch-style climatic cycles, and abundant laterally persistent bentonites, within a highly refined biostratigraphic framework based primarily on rapidly evolving cosmopolitan molluscan taxa, allows the development of a very detailed system of event-isochron stratigraphy divided into intervals of 100,000 yr or less. The Bridge Creek Limestone Member can be divided into three informal subdivisions (lower, middle, and upper) based on overall lithologic characteristics and slope forming profile. These precisely correlate to the upper Hartland, Jetmore, and Pfeifer members in central Kansas and reflect changes in marine environments and sedimentation style corresponding to the periods just prior to, during, and after maximum transgression respectively. The development and preservation of cyclic sedimentation during deposition of the Bridge Creek Member is largely the result of slow rock accumulation rates of 0.5 to 1.0 cm/1000 years across the central Western Interior. The combination of widespread deposition, numerous chronostratigraphic marker-beds, and a highly refined regional and global biostratigraphy make the Bridge Creek Limestone Member a magnificent natural laboratory for detailed ecologic, evolutionary, biogeographic, and oceanographic studies.

Abstract Ammonites are listed from 25 beds of limestone and calcareous shale in the Upper Cretaceous Bridge Creek Member. Brief descriptions and faunal content are given for 14 distinctive beds of limestone. The lowest fourth of the member is late Cenomanian in age, the highest fourth is middle Turonian in age, and the rest is early Turonian. The greatest diversity of ammonites (9 species) occurs in one of the limestone beds of early Turonian age.

Abstract Seventeen samples bracketing the Cenomanian -Turonian stage boundary in the Bridge Creek Limestone (Greenhorn Formation), Pueblo, Colorado, were analyzed for planktonic and benthic foraminifera. A major turnover in the biostratigraphically important, deep-dwelling planktonic foraminifera occurred across this boundary. The Cenomanian rotaliporids were succeeded by the Turonian globotruncanids, perhaps due in part to dynamic global oceanographic developments including the expansion of an oceanic oxygen-minimum zone. Shallow-dwelling planktonic foraminifera on the other hand, were strongly influenced by regional factors (e.g. salinity) affecting the uppermost water column of the Greenhorn sea.

Abstract Preliminary biostratigraphic investigation of the Greenhorn Limestone at Pueblo, Colorado indicates that the Lincoln, Hartland, and lower Bridge Creek members are part of the Lithraphidites acutum calcareous nannofossil Zone. The middle and upper part of the Bridge Creek Member is assigned to the Quadrum gartneri Zone. Subdivision of these zones is difficult due to the scarcity of several biostratigraphically important taxa. Biohorizons based on secondary taxa do, however, offer promise for intrabasinal and intercontinental correlation. Paleobiogeographic analysis using three time-slices across the basin suggests that fresh-water influx from the basin margins was an important factor in restricting nannofossil occurrence and diversity during the initial stages of the invasion of oceanic surface water masses into the basin (middle Cenomanian). By maximum transgression, however, the effects of fresh-water influx appear to be minimal in the eastern and central portions of the basin.

Abstract The Cenomanian-Turonian boundary extinctions, as observed near Pueblo, Colorado, are represented by a series of events reflecting accelerated macrofaunal evolutionary and extinction rates. Four main events are recognized near Pueblo which are cumulatively responsible for 80% species-level and about 30% genera-level macrofaunal extinction. The two main extinction events near Pueblo occured about 250 and 200 Ka before the terminal extinctions at the stage boundary. In addition to the evolutionary and extinction related faunal turnover, fluctuations in the relative abundances of inoceramid bivalves, oysters and ammonites appear to be related to substrate and inferred habitat changes reflected by the climatically controlled lithologie cyclicity. The increased rates of evolution and extinction observed through the Cenomanian-Turonian boundary interval may have been driven by increased biotic stress resulting from eustatic highstand and increased climatic variability influencing temperature, salinity, and benthic oxygenation within the Western Interior seaway and globally.

Abstract The Greenhorn Cyclothem has become the model for eustatically-generated, third-order, cyclic sedimentation in Cretaceous epicontinental settings. The Rock Canyon Anticline sequence near Pueblo, Colorado, is the standard reference section for this cyclothem. The Greenhorn transgressive hemicyclothem (including the Muddy Sandstone, Mowry Shale, Graneros Shale, and the Greenhorn Formation through the middle Bridge Creek Limestone Member) has been the subject of detailed stratigraphic and paleontologic study for many years. In contrast, the Greenhorn regressive hemicyclothem (including the upper Bridge Creek Limestone Member, Greenhorn Formation, and the Fairport Shale, Blue Hill Shale, and Codell Sandstone Members of the Carlile shale) has received only general stratigraphic study. In this paper, high-resolution lithostratigraphic, geochemical, event-stratigraphic, and paleobiologic data for the Greenhorn regressive hemicyclothem are presented and analyzed in terms of facies and depositional history. The regressive sequence at Pueblo is nearly complete, and in general displays a graded, upward-coarsening suite of facies cut by rare disconformities. Cyclically bedded pelagic carbonates (Bridge Creek Limestone Member) grade upward through chalky and calcareous shales (Fairport Member) to dark non-calcareous shales becoming more silty and sandy upward (Blue Hill Shale Member), and the sequence terminates in lower, middle, and upper shoreface sands of the Codell Sandstone Member. The top of the cyclothem is truncated on middle-upper shoreface sandstones of the Codell by a regional transgressive unconformity at the base of the Niobrara Cyclothem. A second regional disconfor-mity may occur at the Fairport - Blue Hill member contact. Although the Greenhorn regressive facies sequence generally mirrors that of the transgression (the symmetrical cyclothem model), significant differences were recorded in this study which suggest that the Western Interior Basin was tectonically passive during eustatic fall and regional regression; subsidence did not outstrip or equal sedimentation rates and the basin filled; shallow water environments predominated. Thus, the regressive sequence of the Greenhorn Cyclothem is coarser-grained, more bioturbated, less laminated, and contains lower percentages of organic carbon than equivalent transgressive facies. Greater current circulation and higher levels of benthic oxygenation are implied during regression. Volcanic ash (bentonite) beds are extremely common in transgressive facies, and comparatively rare in regressive facies; this suggests a major slowdown in tectonism and associated volcanism during eustatic fall and regional regression. The number and extent of disconformities cutting the regressive sequence is far less than during either the Greenhorn or the Niobrara transgression; eustatic fall and regression were more continuous than the highly punctuated transgressive events.