Gregory A. Ludvigson

Recognizing the Albian-Cenomanian (OAE1d) sequence boundary using plant carbon isotopes : Dakota Formation, Western Interior Basin, USA.

Analysis of bulk sedimentary organic matter and charcoal from an Albian-Cenomanian fluvial-estuarine succession (Dakota Formation) at Rose Creek Pit (RCP), Nebraska, reveals a negative excursion of ∼3‰ in late Albian strata. Overlying Cenomanian strata have δ13C values of −24‰ to −23‰ that are similar to pre-excursion values. The absence of an intervening positive excursion (as exists in marine records of the Albian-Cenomanian boundary) likely results from a depositional hiatus. The corresponding positive δ13C event and proposed depositional hiatus are concordant with a regionally identified sequence boundary in the Dakota Formation (D2), as well as a major regressive phase throughout the globe at the Albian-Cenomanian boundary. Data from RCP confirm suggestions that some positive carbon-isotope excursions in the geologic record are coincident with regressive sea-level phases. We estimate using isotopic correlation that the D2 sequence boundary at RCP was on the order of 0.5 m.y. in duration. Therefore, interpretations of isotopic events and associated environmental phenomena, such as oceanic anoxic events, in the shallow-marine and terrestrial record may be influenced by stratigraphic incompleteness. Further investigation of terrestrial δ13C records may be useful in recognizing and constraining sea-level changes in the geologic record.

Evidence for an Albian Hudson arm connection between the Cretaceous Western Interior Seaway of North America and the Labrador Sea

Numerous researchers have alluded to the existence of a Cretaceous Hudson Arm connection between the Labrador Sea and the Western Interior Seaway of North America. However, the evidence for this marine connection has been circumstantial. In this paper we present sedimentary geochemical data that indicate a marine influence in the Albian Mattagami Formation of the Moose River basin, James Bay Lowlands, Ontario. The facies associations between dinoflagellate-bearing laminated mudstones, fluvial sandstones, and early pyrite mineralization are interpreted to indicate deposition in the central basin of an estuary. We use the facies association between the estuarine fill and coeval kaolinitic paleosols in the Moose River basin, and in similar deposits in Quebec and Labrador, to reconstruct a southern shoreline of the Albian Hudson Arm to the Cretaceous Western Interior Seaway. We suggest that development of the Hudson Arm connection between the Labrador Sea and the Cretaceous Western Interior Seaway may be related to a regional extensional regime associated with rifting between Labrador and Greenland, and the passage of eastern North America over Cretaceous hotspots.

Isotopic composition of low-latitude paleoprecipitation during the Early Cretaceous

The response of the hydrologic cycle in global greenhouse conditions is important to our understanding of future climate change and to the calibration of global climate models. Past greenhouse conditions, such as those of the Cretaceous, can be used to provide empirical data with which to evaluate climate models. Recent empirical studies have utilized pedogenic carbonates to estimate the isotopic composition of meteoric waters and calculate precipitation rates for the AptianAlbian. These studies were limited to data from mid- (35°N) to high (75°N) paleolatitudes, and thus future improvements in accuracy will require more estimates of meteoric water compositions from numerous localities around the globe. This study provides data for tropical latitudes (18.5°N paleolatitude) from the Tlayua Formation, Puebla, Mexico. In addition, the study confi rms a shallow nearshore depositional environment for the Tlayua Formation. Petrographic observations of fenestral fabrics, gypsum crystal molds, stromatolitic structures, and pedogenic matrix birefringence fabric support the interpretation that the strata represent deposition in a tidal fl at environment. Carbonate isotopic data from limestones of the Tlayua Formation provide evidence of early meteoric diagenesis in the form of meteoric calcite lines. These trends in δ 18 O versus δ 13 C

Stable isotope chemostratigraphy in lacustrine strata of the Xiagou Formation, Gansu Province, NW China

Abstract Two sections from Early Cretaceous lacustrine strata of the Xiagou Formation from the Changma Basin in Gansu Province, China, are correlated based on their carbon isotopic compositions of bulk sedimentary organic matter and carbonate, as well as carbonate oxygen-isotopic compositions. The samples were collected from fossiliferous strata, which contain well-preserved Cretaceous bird remains. The sections are primarily correlated based on a two-step increase in δ13Corg with an overall magnitude of c. 12.5‰. The stratigraphic variations in carbon isotopes within the two lacustrine sections are correlated with global carbon isotope variations C3–C7 based on marine carbon isotope records. This correlation places the Xiagou lacustrine strata in this locality within the early Aptian Stage, specifically, the Selli Equivalent, which is associated with Ocean Anoxic Event 1a.

Anatomy of an embayment in an Ordovician epeiric sea, Upper Mississippi Valley, USA

The integration of stratigraphic, geochemical, and biostratigraphic data from Middle Ordovician carbonates and shales indicates that the North American epeiric sea was partitioned into shelf areas with distinct characteristics. The Upper Mississippi Valley part of the epeiric sea was appraised by using regionally traceable and geochemically “fingerprinted” K-bentonites, as well as detailed lithologic correlation. In the Midcontinent, the Decorah Formation records a time of high clastic sediment influx and abundant freshwater runoff from the Transcontinental Arch that created a salinity-stratified water column and led to episodic dysoxia. Later, relative flooding of the clastic source areas greatly reduced both the clastic sediment and freshwater runoff. As a result, the salinity stratification broke down, oxygenating the seafloor and permitting carbonates to form. Associated with this change, clarity of the water improved and the photic zone expanded, allowing seasonal blooms of Gloeocapsomorpha prisca to occur, resulting in increased burial of organic matter. The increase in G. prisca and total organic carbon coincided with, but lagged behind, a regional δ 13 C excursion. In addition, the timing of the initiation of the isotopic anomaly is different across the studied area, suggesting that local environmental conditions influenced the isotopic record. Data presented in this study support the partitioning of distinct areas within epeiric seas and the importance of this setting in storing inorganic and organic carbon and recording environmental and biological changes.

Aggradation of gravels in tidally influenced fluvial systems: upper Albian (Lower Cretaceous) on the cratonic margin of the North American Western Interior foreland basin

Alluvial conglomerates were widely distributed around the margin of the Early Cretaceous North American Cretaceous Western Interior Seaway (KWIS). Conglomerates, sandstones, and lesser amounts of mudstones of the upper Albian Nishnabotna Member of the Dakota Formation were deposited as fill-in valleys that were incised up to 80 m into upper Paleozoic strata. These paleovalleys extended southwestward across present-day northwestern Iowa into eastern Nebraska. Conglomerate samples from four localities in western Iowa and eastern Nebraska consist mostly of polycrystalline quartz with lesser amounts of microcrystalline (mostly chert), and monocrystalline quartz. Previous studies discovered that some chert pebbles contain Ordovician‐Pennsylvanian invertebrate fossils. The chert clasts analyzed in this study were consistent with these findings. In addition, we found that non-chert clasts consist of metaquartzite, strained monocrystalline quartz and ‘vein’ quartz from probable Proterozic sources, indicating that parts of the fluvial system’s sediment load must have travelled distances of 400‐1200 km. The relative tectonic stability of this subcontinent dictated that stream gradients were relatively low with estimates ranging from 0.3 to 0.6 m/km. Considering the complex sedimentologic relationships that must have been involved, the ability of low-gradient easterly-sourced rivers to entrain gravel clasts was primarily a function of paleodischarge rather than a function of steep gradients. Oxygen isotopic evidence from Albian sphaerosiderite-bearing paleosols in the Dakota Formation and correlative units from Kansas to Alaska suggest that mid-latitude continental rainfall in the Albian was perhaps twice that of the modern climate system. Hydrologic fluxes may have been related to wet-dry climatic cycles on decade or longer scales that could account for the required water supply flux. Regardless of temporal scale, gravels were transported during ‘high-energy’ pulses, under humid climatic conditions in large catchment areas. An overall rising sea level during the late Albian created accommodation space for the gravelly lithofacies equivalent to the Kiowa-Skull Creek rocks. As Western Interior sea level rose, regional stream gradients were reduced, resulting in regional fluvial aggradation. The conglomeratic lower parts of the Nishnabotna Member of the Dakota Formation formed the transgressive systems tract within an upper Albian sequence that is defined by two unconformities that can be traced from marine Kiowa strata in western Kansas northeastward into western Iowa (Brenner et al., 2000). Mud-draped cross-bedded sandstone bodies, laminated mudstone intervals, and vertical burrows in the lower strata of the Nishnabotna Member indicate that estuarine conditions existed at the mouths of the river system, and tidal effects were transmitted at least 200 km inland from the interpreted late Albian coast. These observations suggest that estuarine conditions stepped up the incised valleys as fluvial sediments aggraded in response to regional transgression that continued through the Late Albian. 2003 Elsevier Ltd. All rights reserved.

Use of multiple oxygen isotope proxies for elucidating Arctic Cretaceous palaeo-hydrology

Abstract Stable oxygen isotope analysis of siderite and dinosaur tooth enamel phosphate from the Campanian–Maastrichtian Prince Creek Formation, Alaska, USA, are analysed to determine the palaeohydrology of the ancient Colville Basin north of the Ancestral Brooks Range. δ18O of freshwater siderites relative to V-PDB ranges between −14.86 and −16.21‰. Dinosaur tooth enamel δ18O from three different sites (Kikak–Tegoseak, Pediomys Point, Liscomb) range between +3.9‰ and +10.2.0‰. δ18Ometeoric water are calculated from δ18Osiderite that formed at seasonal temperatures ranging from −2 to 14.5 °C, with a mean annual temperature of 6.3 °C. At 6.3 °C, the δ18Ow calculated from siderite ranged between −22.23 and −20.89‰ V-SMOW. Ingested water compositions are estimated from dinosaur teeth assuming body temperatures of 37 °C and local relative humidity of 77.5%, resulting in values ranging from −28.7 to −20.4‰ V-SMOW, suggesting consumption of meteoric water and orographically depleted runoff from the Brooks Range. The ranges in calculated δ18Ometeoric water are compatible between the two proxies, and are mutually corroborating evidence of extremely 18O-depleted precipitation at high latitudes during the Late Cretaceous relative to those generated using general circulation models. This depletion is proposed to result from increased rainout effects from an intensified hydrological cycle, which probably played a role in sustaining polar warmth. Supplementary material: Parameters used for generation of equations compared to Kohn (1996) can be found at http://www.geolsoc.org.uk/SUP18642

Terrestrial Carbon Isotope Chemostratigraphy in the Yellow Cat Member of the Cedar Mountain Formation: Complications and Pitfalls

Organic carbon (OC) isotope profiles from four sections of the Early Cretaceous continental Yellow Cat Member (YCM) of the Cedar Mountain Formation are presented to explore the constraints of studying and correlating continental sections. A significant body of research demonstrates that some continental chemostratigraphic profiles record global perturbations of the carbon cycle and specifically globally correlative carbon isotope excursions that can be correlated with well-constrained marine carbon isotope records to give some chronostratigraphic constraint. However, this is not always straightforward. Here we present our findings of four sections of the YCM and discuss some caveats of using continental OC isotope records, with an insight into some of the possible solutions. In this study a regionally extensive calcrete holds the key to resolving correlation of poorly reproducible carbon isotope profiles. Here, we tentatively correlate the YCM to the Barremian-lower Aptian.