Geochronology of late Albian−Cenomanian strata in the U.S. Western Interior

Since the publication of 40Ar/39Ar dates from Cretaceous bentonites in the Western Interior Basin by J.D. Obradovich in 1993 and in Japan by J.D. Obradovich and colleagues in 2002, improvements in the 40Ar/39Ar method have included a shift to astronomically calibrated ages for standard minerals and development of a new generation of multi-collector mass spectrometers. Thus, the 40Ar/39Ar chronometer can yield results that are synchronous with U-Pb zircon dates and astrochronologic age models for Cretaceous strata. Ages determined by Obradovich have ± 2σ analytical uncertainties of ± 400 ka (excluding J value or systematic contributions) that have been used to discriminate stratigraphic events at ca. 1 Ma resolution. From among several dozen sanidine samples, 32 of which were dated by Obradovich in 1993, we present new multi-collector 40Ar/39Ar ages that reduce the average analytical uncertainties by nearly an order of magnitude. These new ages (where the uncertainties also include the contribution of the neutron fluence J value) include: • Topmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 Ma • Clay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 Ma • Arrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 Ma • Upper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 Ma • Middle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 Ma • Shell Creek Shale, Bighorn Basin, Crow Reservation, Wyoming, 99.62 ± 0.07 Ma • Shell Creek Shale, Bighorn Basin, Greybull, Wyoming, 99.67 ± 0.13 Ma • Shell Creek Shale, Bighorn Basin, Lander, Montana, 100.07 ± 0.07 Ma • Muddy Sandstone, Wind River Basin, Wyoming, 101.23 ± 0.09 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 101.36 ± 0.11 Ma • Vaughn Member, Blackleaf Formation, Sweetgrass Arch, Montana, 102.68 ± 0.07 Ma • Taft Hill Member, Blackleaf Formation, Sweetgrass Arch, Montana, 103.08 ± 0.11 Ma • Base of the Skull Creek Shale, Black Hills, Wyoming, 104.87 ± 0.10 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 106.37 ± 0.11 Ma A new U-Pb zircon age of 104.69 ± 0.07 Ma from the Skull Creek Shale at Dinosaur Ridge, Colorado, USA, is close to the new 40Ar/39Ar age of the Skull Creek Shale in the Black Hills, Wyoming, but 5 m.y. is missing in the unconformity between the Skull Creek Shale of the Black Hills and the overlying Newcastle Sandstone. Considering the average total uncertainties that include decay constant and standard age or tracer composition for the 40Ar/39Ar (± 0.19 Ma) and the U-Pb (± 0.13 Ma) ages does not alter this finding. Moreover, the lower Thermopolis Shale in the Bighorn Basin is 1.5 Ma older than the Skull Creek Shale in the Black Hills. The 100.07 ± 0.07 Ma Shell Creek Bentonite in Montana is close to the Albian−Cenomanian boundary age of 100.2 ± 0.2 Ma of Obradovich and colleagues from Hokkaido, Japan, and 100.5 ± 0.5 Ma adopted in the 2012 geological time scale of J.G. Ogg and L.A. Hinnov. Our findings indicate that correlations based on similarity of lithology, without independent radioisotopic ages or detailed biostratigraphic constraints, can be problematic or invalid. There is much more time missing in unconformities than has been previously recognized in these important, petroleum-bearing reservoir strata.

Modeling anisotropic static elastic properties of soft mudrocks with different clay fractions

We have quantified the effects of clay fraction and fabric on the static elastic properties of soft mudrocks with emphasis on microlevel mechanisms. Soft mudrocks are treated as a mixture of nonclay minerals and clay-water composites. We have devised a simplified approach to estimate the fabric orientation distribution of soft mudrocks based on measured parameters such as clay fraction and porosity. A single parameter (fabric angle) that characterizes the fabric orientation distribution of soft mudrocks is related to the void ratio of clay-water composites. The static transversely isotropic (TI) elastic properties of soft mudrocks are modeled using an anisotropic differential effective medium approach. The effect of variation in fabric orientation distribution on the TI elastic parameters of clay-water composites is studied by applying the Voigt approximation. With an increase of clay fraction, soft mudrocks have decreasing trends in the deformation moduli because some nonclay minerals are replaced by clay-water composites. However, the deformation moduli of clay-water composites could increase when there is more anisotropy in the fabric due to an increase in the clay fraction. Thus, the correlations between anisotropic elastic moduli and volume fraction of clay-water composites will display some fluctuations. Such nonlinear relationships are validated against published experimental data on Colorado shale samples from the Western Canadian Sedimentary Basin.

Transversely isotropic stiffness parameters and their measurement in Colorado shale

Drained stress path triaxial tests and confined torsion tests were conducted on Colorado shale core samples to investigate transversely isotropic stiffness parameters at small strain deformation. Nonlinear plastic behaviour occurred along the primary loading path even at strains less than 0.01%. Nonlinear, hysteric behaviour was only observed during the loading–unloading path. However, the shale material displayed transverse isotropy in deformation, and very small or nondetectable Poisson's ratios in vertical and horizontal directions. This special response alludes to the postulate that the Colorado shale could be approximated by a transversely isotropic elasticity model at small strain levels with negligible yielding.Key words: anisotropy, elasticity, stiffness, triaxial test, torsion test, shale.

Foraminifera of the lower part of the Sully Formation (upper Albian), northeastern British Columbia

An extensive microfauna of over 25 genera and 40 species of arenaceous foraminifera occurs in the lower Sully Formation on the Sikanni Chief River and consists of Bathysiphon spp., Hyperammina sp., Saccammina spp., Lituotuba? sp., Glomospira sp., Ammodiscus sp., Reophax spp., Haplophragmoides spp., Bigenerina sp., Ammobaculites spp., Ammotium sp., Haplophragmium spp., Trochammina spp., Ammobaculoides sp., Textulariopsis spp., Gaudryina sp., Orientalia sp., Eggerella sp., Gravellina sp., Verneuilinoides sp., Uvigerinammina sp., Pseudobolivina sp., Dorothia sp., Miliammina spp., and Psamminopelta sp.The suite occurs below the fish-scale marker bed of the Cretaceous Colorado Shale within the uppermost Albian portion of the Neogastroplites Zone. The assemblage defines the Haplophragmium swareni Subzone of the Miliammina manitobensis Zone of the Colorado Group. It is correlative with the middle Mowry Shale fauna of Wyoming.The suite is deep neritic (200 m) with some suggestion of salinity lessened slightly below normal marine. It is accompanied by an extensive microflora of pollen, spores, and dinoflagellates. The suite represents biofacies intermediate between those previously found at similar stratigraphic horizons within the Cruiser Shale of the Moberly Lake area of British Columbia (a shallower fauna) and within the type Shaftesbury Formation of Peace River, Alberta (deeper, or farther offshore).