Bridge Creek Limestone Member Research Articles

Bayesian integration of astrochronology and radioisotope geochronology

Abstract. Relating stratigraphic position to numerical time using age–depth models plays an important role in determining the rate and timing of geologic and environmental change throughout Earth history. Astrochronology uses the geologic record of astronomically derived oscillations in the rock record to measure the passage of time and has proven to be a valuable technique for developing age–depth models with high stratigraphic and temporal resolution. However, in the absence of anchoring dates, many astrochronologies float in numerical time. Anchoring these chronologies relies on radioisotope geochronology (e.g., U–Pb, 40Ar/39Ar), which produces high-precision (<±1 %), stratigraphically distributed point estimates of age. In this study, we present a new R package, astroBayes, for a Bayesian inversion of astrochronology and radioisotopic geochronology to derive age–depth models. Integrating both data types allows reduction in uncertainties related to interpolation between dated horizons and the resolution of subtle changes in sedimentation rate, especially when compared to existing Bayesian models that use a stochastic random walk to approximate sedimentation variability. The astroBayes inversion also incorporates prior information about sedimentation rate, superposition, and the presence or absence of major hiatuses. The resulting age–depth models preserve both the spatial resolution of floating astrochronologies and the accuracy as well as precision of modern radioisotopic geochronology. We test the astroBayes method using two synthetic datasets designed to mimic real-world stratigraphic sections. Model uncertainties are predominantly controlled by the precision of the radioisotopic dates and are relatively constant with depth while being significantly reduced relative to “dates-only” random walk models. Since the resulting age–depth models leverage both astrochronology and radioisotopic geochronology in a single statistical framework they can resolve ambiguities between the two chronometers. Finally, we present a case study of the Bridge Creek Limestone Member of the Greenhorn Formation where we refine the age of the Cenomanian–Turonian boundary, showing the strength of this approach when applied to deep-time chronostratigraphic questions.

Testing the astronomical time scale for oceanic anoxic event 2, and its extension into Cenomanian strata of the Western Interior Basin (USA)

The development of integrated astronomical and radioisotopic time scales from rhythmic strata of the Western Interior Basin (WIB) has played a fundamental role in the refi nement of Late Cretaceous chronostratigraphy. In this study, X-ray fl uorescence (XRF) core scanning is utilized to develop a new elemental data set for cyclostratigraphic investigation of CenomanianTuronian strata in the WIB, using material from the Aristocrat-Angus-12-8 core (northcentral Colorado). The XRF data set yields the fi rst continuous 5-mm-resolution analysis of lithogenic, biogenic, and syngeneticauthigenic proxies through the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member, including oceanic anoxic event 2 (OAE 2). The 40 Ar/ 39 Ar ages from ashes in three biozones, including a new age from the Dunveganoceras pondi biozone (uppermost Lincoln Limestone Member), provide geochronologic constraints for the cyclostratigraphic analysis. Astrochronologic testing of the 5-mm-resolution XRF weight percent CaCO3 data via average spectral misfi t analysis yields strong evidence for astronomical infl uence on climate and sedimentation. Results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the U.S. Geological Survey #1 Portland core (central Colorado), and identifi cation of an astronomical signal in the underlying Hartland Shale Member now permits extension of the WIB astrochronology into the earlier Cenomanian, prior to OAE 2. High rates of sedimentation in the Angus core during the interval of OAE 2 initiation, as compared to the Portland core, allow recognition of a strong precessional control on bedding development. As a consequence, the new results provide a rare high-resolution chronometer for the onset of OAE 2, and the timing of proposed hydrothermal trace metal enrichment as observed in the 5 mm XRF data.

Bombers and Bivalves: First North American Occurrence of the RudistDuraniasp. (Bivalvia: Radiolitidae) in the Upper Cretaceous (Cenomanian) Greenhorn Limestone of Southeastern Colorado

A colonial monospecific cluster of rudist bivalves from the lowermost Bridge Creek Limestone Member, Greenhorn Limestone (Upper Cenomanian) are attributable to Durania cf. D. cornupastoris. This discovery marks only the eighth recorded pre-Coniacian occurrence of rudist bivalves in the Cretaceous Western Interior and the only Cenomanian record of rudist Durania in North America. Discovered in 2011, the specimen was unearthed by aerial bombing at a training facility utilized during World War II. The appearance of rudist bivalves at mid-latitudes coincident with marked change in marine sediments likely represents the onset of mid-Cretaceous global warming.

Of Bombers and Bivalves: North American Cenomanian Occurrence of the Rudist Durania sp. (Bivalvia: Radiolitidae) in the Upper Cretaceous Greenhorn Limestone of Southeastern Colorado

A colonial monospecific cluster of rudist bivalves from the lowermost Bridge Creek Limestone Member, Greenhorn Limestone (Upper Cenomanian) are attributable to Durania cf. D. cornupastoris. This discovery marks only the eighth recorded pre-Coniacian occurrence of rudist bivalves in the Cretaceous Western Interior and the only Cenomanian record of rudist Durania in North America. Discovered in 2011, the specimen was unearthed by historic aerial bombing during World War II. The appearance of rudist bivalves at mid-latitudes coincident with marked change in marine sediments likely represents the onset of mid-Cretaceous global warming.

Quantification of deep-time orbital forcing by average spectral misfit

Quantification of Milankovitch orbital cyclicity within ancient strata has become a principal tool for refinement of the geologic time scale. However, accurate characterization of the orbital signal in deep time paleoclimate records is commonly challenged by inadequate radiometric time constraints for calibration of the spatial rhythms to temporal periods. This problem can potentially introduce large errors into derivative orbital timescales. In this study we develop a new method for the identification and calibration of orbital cyclicity in cyclostratigraphic records. The method (average spectral misfit, or ASM) yields an objective estimate of the optimal sedimentation rate for a stratigraphic interval that preserves a record of orbital forcing. The technique also provides a formal statistical test for rejecting the null hypothesis (no orbital signal). Application of the method to assess orbital cyclicity in the upper Bridge Creek Limestone Member (Turonian) of the Western Interior Basin highlights the utility of this new cyclostratigraphic tool, and provides a means to independently evaluate conflicting interpretations of the lithologic cycles. Importantly, ASM offers a new consistent standard by which orbital timescales may be compared. Hence, the quality of an orbital timescale can be formally qualified by reporting its average spectral misfit and null hypothesis significance level. This technique will permit improvement of Mesozoic/Cenozoic orbital timescales and extension of orbital time scale development into the Paleozoic, as the method is not dependent upon well-constrained radiometric age data.

The Global Boundary Stratotype Section and Point for the base of the Turonian Stage of the Cretaceous: Pueblo, Colorado, U.S.A.

Following the recommendation of the International Commission on Stratigraphy (16 votes Yes [94%], 1 abstention, 2 votes not received), the Global boundary Stratotype Section and Point (GSSP) for the base of the Turonian Stage of the Cretaceous System is defined as the base of bed 86 of the Bridge Creek Limestone Member of the Greenhorn Limestone Formation at the western end of the Denver and Rio Grande Railroad cut near the north boundary of the Pueblo Reservoir State Park Recreation Area, west of Pueblo, Colorado, USA. This GSSP horizon is also exposed and protected in the adjacent state recreation area. It coincides with the first occurrence of the ammonite Watinoceras devonense, is in the middle of a global positive excursion in Carbon-13 isotopes, and is bracketed by widespread bentonites that have yield edages of 93 to 93.5 Ma.

Detection, quantification, and significance of hiatuses in pelagic and hemipelagic strata

Temporal gaps in the stratigraphic record have important implications for the interpretation of stratigraphically rapid geochemical and paleobiologic changes, for the development of high-resolution time scales (e.g., orbital time scales), and for deciphering the response of depositional systems to climatic change and basin evolution (e.g., accommodation space vs. sediment supply). In ancient pelagic and hemipelagic strata, the identification of relatively short hiatus (103–106 years) is commonly challenged by the resolution limits of geochronologic techniques. In this study, a quantitative method is developed to identify and quantify hiatuses in strata where Milankovitch orbital cycles are documented. The new method is based on the interpretation of evolutive harmonic analysis (EHA) results and permits estimation of minimum durations for the temporal gaps. Application of this technique to an optical densitometry data series derived from the Cenomanian/Turonian (C/T) Bridge Creek Limestone Member permits quantification of hiatus within the unit and addresses important questions about abrupt changes in biogeochemical proxy records across the boundary interval. Integration of the new method with stratigraphic data and geochemical burial flux estimates permits a detailed reconstruction of the controls on sediment delivery and hiatus generation, including a refined analysis of the orbital-scale linkages between climate and sedimentation.

Integrated Quantitative Stratigraphy of the Cenomanian-Turonian Bridge Creek Limestone Member Using Evolutive Harmonic Analysis and Stratigraphic Modeling

ABSTRACT Secular changes in bulk sedimentation have a significant impact on the preservation of climatic signals in rhythmically bedded strata. In this study a quantitative cyclostratigraphic methodology is developed to use these signal distortions as a means to reconstruct sedimentation. The method exploits a moving-window Fourier technique that we designate evolutive harmonic analysis (EHA). We calibrate this methodology to a specific set of geologic conditions (i.e., changes in bulk sedimentation) using a series of stratigraphic models and then apply the technique to the Cenomanian-Turonian Greenhorn Formation of the Western Interior basin. Application of EHA to strata that preserve a record of orbital forcing allows the reconstruction of a high-resolution sedimentation history in which bulk sedimentation rates and hiatuses are quantified. Using the data on bulk sedimentation rate, rates of accumulation for geochemical proxies representing detrital, biogenic, and authigenic contributions to the sediment can be calculated. By integrating these quantitative assessments of siliciclastic dilution, primary production, and depositional redox conditions within a high-resolution chronostratigraphic framework, we are able to deconvolve secular trends in hemipelagic sedimentation during the Late Cenomanian-Early Turonian in the Western Interior basin. This integrated quantitative stratigraphic approach allows a reevaluation of hypotheses for the accumulation of excess organic matter in the Western Interior basin during the interval representing Oceanic Anoxic Event II.

Evidence for Milankovitch Periodicities in Cenomanian-Turonian Lithologic and Geochemical Cycles, Western Interior U.S.A.

ABSTRACT The limestone/marlstone bedding couplets of the Bridge Creek Limestone Member, Cenomanian-Turonian Greenhorn Formation, were analyzed by applying spectral techniques to high-resolution lithologic and geochemical data from a core. The results suggest that the Bridge Creek contains a complex record of orbital cyclicity. The dominant signal appears to be obliquity, but signals corresponding to precession and eccentricity were also observed. The development of the bedding couplets is interpreted to have resulted from a combination of factors, including insolation-controlled changes in higher-latitude precipitation leading to dilution/redox cycles, and in lower-latitude evaporation, leading to changes in surface water conditions and productivity cycles in the calcareous plankton. The data nterpreted to reflect redox cycles appear to be more strongly influenced by obliquity, and show a weak precessional signal. In contrast, trends in the carbonate record show the opposite response. The complex bedding pattern observed in the Bridge Creek Limestone is interpreted to result from the competing influences of different orbital cycles expressed through different pathways of the depositional system, and was also affected by changes in sedimentation rates related to relative sea level fluctuations, aperiodic dilution by volcanic ash, and changes in organic-matter production and redox conditions related to a global oceanic anoxic event. These factors complicate cycle analysis in the lower part of the member but leave a relatively undisturbed record in the upper Bridge Creek Limes one.

A note of caution concerning the application of quantitative palynological data from oxidized preparations

Abstract. Kerogen extracted from rock samples for palynological studies frequently requires oxidative treatment in order to liberate palynomorphs from amorphous organic matter (AOM). Treatment with nitric acid (70% HNO3) is sometimes adequate though many Mesozoic organic-rich samples require more severe oxidation to break up clumped AOM. A widely used method involves the repeated oxidation of a preparation in Schulze’s solution (70% HNO3 supersaturated with KClO3), followed each time by rinsing with 2% potassium hydroxide solution (KOH) (Jones, 1994).The mid-Cretaceous Greenhorn Formation, exposed near Pueblo, Colorado, USA, is composed of interbedded bioturbated limestones and laminated calcareous shales (Cobban & Scott, 1972). In the middle part of the Bridge Creek Limestone Member, kerogen extracted from these two lithologies is markedly different. Diverse, predominantly gonyaulacineaen dinocyst assemblages are found in kerogen from the bioturbated limestone samples. These either require five minutes of nitric acid treatment or no oxidation at all. Most of the AOM is finely disseminated and passes through a standard 10 μm sieve mesh.Dinocysts in the >10μm kerogen fraction from the laminated calcareous shale samples are outnumbered by clumped AOM at a ratio of between 10 and 1000:1. Prior to oxidation, gonyaulacineaen and peridiniineaen dinocysts are seen to be present. However, if Schulze’s solution and KOH are applied until most of the clumped AOM has disintegrated (a process which takes from 1 to 48 hours), Gonyaulacineae are rare (<5%) in resulting dinocyst assemblages (which are dominated by well-preserved Peridiniineae). . . .

Cyclostratigraphy of the Cenomanian-Turonian Bridge Creek Limestone Member of the Greenhorn Formation in the U.S. Western Interior: ABSTRACT

Cyclostratigraphy of the Cenomanian-Turonian Bridge Creek Limestone Member of the Greenhorn Formation in the U.S. Western Interior: ABSTRACT

The significance of palynofacies fluctuations in the Greenhorn Formation (Cenomanian-Turonian) of the Western Interior Basin, USA

In the Greenhorn Limestone Formation of the Pueblo Section, Colorado, the palynofacies in the Hartland Shale Member is remarkably homogeneous. This is in marked contrast to that of the overlying Bridge Creek Limestone Member, in which the alternating limestones and marlstones are characterized by rhythmic variations in the terrestrial phytoclasts; such variations appear to be related to Milankovitch cycles, as well as to both the dilution and productivity models. The marine palynofacies includes three dinocyst associations and Scytinascia (foraminiferal linings). Changes in the marine palynofacies do not appear to be related to the cyclical limestone-marlstone deposits, however, the alternating dinocyst associations suggest mesotrophic-oligotrophic rhythms in the surface waters. The discrepancy between the dinocyst cycles and the lithological couplets could reflect the modulation of the sea-level rises in the Milankovitch frequency band by a barrier during deposition of the Hartland Shale Member within the second-order (T6) transgression of Kauffman. The discrepancies between the foraminifer, nannofossil and dinocyst fluctuations are interpreted as reflecting specific responses by each planktic group to global oceanographic changes and diagenesis.

Contrasting Styles of Fault Control on Position of Pre-Dakota Unconformity and on Depositional Environments of Dakota Sandstone, Northern San Juan Basin, Colorado: ABSTRACT

Detailed subsurface analyses of depositional environments and sandstone geometry of rocks from the upper part of the Upper Jurassic Morrison Formation through the Upper Cretaceous Dakota Sandstone indicate that faulting was a major control on distribution and preservation of lithofacies in the northern San Juan basin. The faults have a predominant northwest trend and were episodically active from Late Jurassic through early Late Cretaceous time. The main effect of faulting on the Morrison Formation and Lower Cretaceous Burro Canyon Formation was in the preservation and/or erosion of rock. These formations were uniformly deposited across the study area and then locally removed across horsts. Although depositional thinning of the Morrison occurs from west to east across the northern part of the basin, thickness differences (as much as 200 ft) between the Bridge Creek Limestone Member of the Mancos Shale and the Todilto Limestone of the Wanakah Formation cannot be solely attributed to regional thinning. Among closely spaced drill holes (1-3 mi apart) erosion is evidenced by thinning or local absence of the Burro Canyon and/or loss of section at the top of the underlying Morrison. The unconformity below the Dakota is not a simple regionally inclined surface from north to southmore » across the San Juan basin. Rather, its development in the northern part of the basin is more intense on uplifted areas among a series of parallel northwest-trending vertical faults.« less

New Early Silurian rugose corals from the Panuara area, central New South Wales

Cystipaliphyllum conspersum sp. nov., Amplexoides gephyra sp. nov. and Cantrillia webbyi sp. nov. are described from the Bridge Creek Limestone Member, of late early to early middle Llandoverian age, in the Panuara area of central New South Wales. This represents the first record in Australia of the genera concerned. The general Asian affinities of the Llandoverian and younger Silurian rugose corals of New South Wales are reviewed and related to a Silurian palaeogeographic reconstruction.

Widespread, Synchronously Deposited, Burrow-Mottled Limestone Beds in Greenhorn Limestone (Upper Cretaceous) of Kansas and Southeastern Colorado

In central Kansas the Hartland Shale and Jetmore Chalk Members of the Greenhorn Limestone contain 16 burrow-mottled, ledge-forming, chalky limestone beds that can be traced across the entire outcrop. The same beds are recognized in equivalent strata of the Bridge Creek Limestone Member of westernmost Kansas, and all but one or two can be recognized in a Bridge Creek exposure in southwestern Pueblo County, Colorado. For a distance of nearly 450 mi some of these beds lie on, below, or close to equally persistent bentonite seams. The parallelism of bentonite seams and adjacent or nearly adjacent beds of limestone, and the uniform relative spacing of all the limestone beds prove that some of the latter are time parallel and suggest most convincingly that each of these widespr ad beds is time parallel. In contrast to intervening beds of laminated shaly chalk, the limestone beds lack internal stratification because of activity of a highly mobile infauna. The shaly chalk beds contain higher percentages of terrigenous detritus, organic carbon, and pyrite than the limestone beds, and are interpreted as reflecting greater rates of terrigenous detrital influx and an interstitial reducing environment that was inimical to development of a burrowing infauna. Smaller organic carbon and pyrite content of the limestones is believed to reflect a lower clay content of original sediments, and at least a partly oxidizing interstitial environment, as suggested by the high degree of bioturbation. Despite substrate differences, the aqueous environment directly above the sediment-water interface did not change significantly as sediment type alternated from detritus-rich to detritus-poor, because the shelly epifauna is essentially the same in adjacent be s of shaly chalk and chalky limestone. The widespread, apparently time-parallel limestone beds are believed to be the result of regionally manifested changes in volume of terrigenous detrital influx, coming principally from the west, that were superimposed on a more or less continuous accumulation of carbonate sediments. The noncrushed condition of macrofossils and fecal pellets suggests that limestone beds suffered early lithification; this process was probably influenced by initially greater purity of limestone-forming muds, slower sedimentation, and interstitial circulation resulting from bioturbation.