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 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.

Abstract The Niobrara Transgressive Hemicyclothem at Pueblo and to the south and west comprises four discrete lithostratigraphic members, separated by four and possibly five regional disconformities. The sequence disappears to the east and north as disconformities expand, anastomose, and become a single regional unconformity, bringing pelagic Fort Hays limestones, representing maximum transgression and eustatic rise, into erosional contact with maximum regressive shoreface sandstones (Codell) of the underlying Greenhorn Cyclothem. Eight molluscan biozones are defined within the complex Niobrara transgressive sequence in the Axial Basin. These biozones progressively disappear eastward within anastomosing disconformities. Near Pueblo only three molluscan biozones are recognized within the pre-Niobrara part of the cyclothem. A sequence of four foraminiferal biofacies are defined at Pueblo and along the Colorado Front Range which reflect rapid development of clear, warm, well circulated waters early in the transgressive history. These biofacies rise stratigraphically to the west, associated with eustatic rise in sealevel, and grade laterally along any time-line from diverse open water assemblages to environmentally stressed, less diverse, assemblages westward. This lateral gradation, and the rise of foraminiferal biofacies westward, is thought to reflect the westward migration of turbid watermasses associated with the active margin of the seaway during eustatic rise of the Niobrara Marine Cycle.