Jacqueline E. Huntoon

Repetitive stratal patterns in a foreland basin sandstone and their possible tectonic significance

Two distinctive, repetitive stratal patterns within members of the Upper Cretaceous Blackhawk Formation in Utah are recognized by comparison of (1) progradational distances of individual parasequences within each member and (2) the position of the updip termination of marine facies for each parasequence. The oldest parasequence within each member always progrades substantially farther than the younger parasequences within the same member, regardless of whether these parasequences are separated by sequence boundaries. The pattern of progradational distances results in an initial progradational to later aggradational stacking pattern within each member. This pattern is observed in all members of the Blackhawk Formation where there is sufficient outcrop exposure to trace the nonmarine-to-marine transition, which is used to establish the progradational distances. These stratal patterns are interpreted to define the path of relative change of sea level through time within the Cretaceous foreland basin and may provide information characterizing thrust-sheet emplacement. Flooding events bounding parasequences may represent periods of punctuated thrusting, with the main flooding surface (member boundaries) representing the period of greatest thrust-sheet movement.

Modeling of Cretaceous foreland-basin parasequences, Utah, with implications for timing of Sevier thrusting

Computer modeling suggests that thrusting order can be discriminated on the basis of foreland basin geometry and parasequence stacking patterns. The position of sequential thrust sheets relative to the basin affects the amount and distribution of subsidence, source terrain elevation and distance from the shoreline, and sediment transport into the basin. For this study, a forward numerical model was constructed to quantitatively simulate the interaction of geologic processes active in the foreland basin setting, including thrusting and orogenic development, lithospheric flexure, orogenic denudation, sediment transport across the coastal plain, and nearshore deposition. In order to isolate the effects of tectonics and sediment supply on nearshore deposition, eustatic sea level is held constant. Model runs simulate two different thrusting sequences: foreland stepping (in sequence) and hinterland stepping (out of sequence). When in-sequence thrusting is modeled, sediment is trapped in intra-orogen basins. Because sediment is prevented from reaching the shoreline, the basin is underfilled. When out-of-sequence thrusting is modeled, sediment is transported from the orogen to the shoreline. Episodes of thrusting result in subsidence and marine flooding surfaces, and resulting accommodation space allows for parasequence development. Comparing model output to stratigraphy observed in eastern Utah suggests that Late Cretaceous out-of-sequence uplift and thrust loading resulted in aggradational and progradational parasequence stacking in the Campanian Blackhawk Formation. This result supports the theory that the Pavant thrust system was active prior to deposition of the Blackhawk Formation and that the location of thrust shortening shifted toward the hinterland prior to or during Blackhawk Formation deposition.

Particle sizes of andesitic ash fallout from vertical eruptions and co-pyroclastic flow clouds, Volcán de Colima, Mexico

We report particle-size distributions for andesitic ash fall from vertical eruptions and clouds above block-and-ash fl ows at Volcan de Colima, Mexico, during 2004‐2006. We analyzed 17 samples using laser diffraction: 11 from vertical eruptions and 6 from block-and-ash fl ows (copyroclastic fl ow ash). Vertical eruptions produce well-sorted fall deposits, whereas co-pyroclastic fall deposits are poorly sorted, with high proportions of very fi ne grained ash (<30 µm). Statistical analysis shows particle-size distributions of vertical eruptions are more leptokurtic (peaked) than co-pyroclastic samples. Deconvolution of grain-size histograms shows that copyroclastic samples have at least one subpopulation with a mode of 8.3‐8.7 φ (2.4‐3.1 µm). Estimates of the number of particles in different size ranges show that co-pyroclastic samples contain much greater numbers of very fi ne particles than vertical eruption samples. Our results provide no direct evidence that milling or comminution produces hazardous fi ne ash particles in pyroclastic fl ows, but are consistent with that interpretation.

Marine origin of paleotopographic relief on eolian White Rim Sandstone (Permian), Elaterite Basin, Utah

The tar-bearing Permian White Rim Sandstone in the Elaterite basin near Canyonlands National Park, Utah, is an example of an exhumed paleotopographic high (paleohigh) with approximately 250 ft (76 m) of relief. Accumulations of heavy hydrocarbons are concentrated along the axis of the high. Eolian processes controlled deposition of most of the White Rim Sandstone, and the action of erosional marine processes modified preexisting dune relief at the top of this sandstone. The interaction of these depositional processes is evidenced by the distinction of two informal units within the White Rim Sandstone. The lower unit is an eolian facies that comprises most of the formation. This unit is characterized by prominent, large-scale, high-angle cross-stratification; ripples with high ripple indices; and eolian ripple (translatent) strata. The upper unit is a thin marine facies with oscillation (wave) ripples, fluid-escape structures, megapolygons, and rip-up clasts of the lower unit. The eolian and marine facies are separated by a sharp erosional boundary. The basic form of the paleohigh is the result of sedimentation within an eolian dune field, but much of the study areas unusual paleotopographic relief is attributed to erosion and reworking of the lower unit during a major marine transgression. The upper marine facies of the White Rim was also deposited during this transgression. Six different wave-cut terrace levels on the flanks of the elongate high record a discontinuous rise of sea level during the transgression. Megapolygons within the upper unit resulted from desiccation and expansive salt-crystal growth when the area became subaerially exposed during a regression at the end of the Permian. The paleohigh was eventually mantled by fine-grained sediments of the Triassic Moenkopi Formation. The Moenkopi cover effectively protecte the delicate features of the paleohigh now exposed by Holocene erosion in the Elaterite basin. The depositional and preservational model for the White Rim Sandstone proposed here may be a useful analog to other stratigraphic traps associated with paleotopographic relief.

Thermal History of the 1.1-Ga Nonesuch Formation, North American Mid-Continent Rift, White Pine, Michigan

The Nonesuch Formation, a black siltstone, was deposited in the Lake Superior portion of the mid-continent rift system during middle Proterozoic rifting. Despite its age of nearly 1.1 Ga, the Nonesuch Formation contains liquid petroleum and solid pyrobitumen. Numerical modeling techniques were used in this study to constrain the thermal history of the Nonesuch Formation at White Pine, Michigan. Thermal modeling addresses two issues: (1) the utility of illite-smectite expandability as a limiting parameter for describing the thermal history of ancient (Proterozoic) sedimentary rocks, and (2) the potential for hydrocarbon maturation within the Nonesuch Formation at White Pine. A range of potential burial histories for the Nonesuch Formation was constructed based on geologica evidence. Time-temperature histories were calculated for each burial history model assuming one-dimensional, transient, conductive heat flow. Results of numerical time-temperature models were then evaluated on the basis of organic and inorganic thermal maturity indicators including Rock-Eval® pyrolysis data and biomarker indices (correlated to equivalent vitrinite reflectance values), in addition to illite-smectite expandability. Illite-smectite expandability values appear generally consistent with measured organic thermal maturity values. The preferred set of cases also produces calculated illite-smectite expandability and vitrinite reflectance values consistent with thermal maturity indicators (that do not include vitrinite) collected in the field. Modeling results demonstrate tha a combination of elevated basal heat flow and rapid burial during the Proterozoic is required to produce the observed thermal maturities at White Pine, Michigan. More specifically, modeling indicates that maximum temperatures for the Nonesuch Formation, 110 to 125°C, were reached at about 1075 Ma, coincident with a maximum burial depth of about 6 km, although 4 km represents a more plausible value. The results support a thermal history consistent with in-situ oil generation at White Pine; however, thermal modeling alone cannot rule out the possibility that oil was generated and expelled elsewhere and migrated into the area with fluids expelled during an episode of postrift compression.

Thermal Evolution of the Newark Basin

A one-dimensional conductive thermal model is used to calculate the transient thermal history of the Newark basin, a Triassic-Jurassic continental rift basin in the eastern United States that formed during the separation of North America and Africa. The model accounts for deposition, erosion, igneous activity, lithology-dependent variations in thermal conductivity, depth-dependent radiogenic heat production, and changes in heat flow through time. A burial and erosion history for the Newark basin is constructed for the modeling, including changes in heat flow through time, emplacement of Jurassic lava flows at the surface, and emplacement of the Palisades sill at depth. Vitrinite-reflectance values and apatite and zircon fission-track ages, for units of both Triassic and Jurassic age, are used to constrain the models. Use of two different data sets greatly limits the number and types of models that can reproduce the observed data. Modeling results indicate that initial formation of the Newark basin is not coincident in time with a thermal event. Elevated heat flow (on the order of 130 mW/m² in the models) did affect the basin during its evolution, however, and was associated with igneous activity (at approximately 201-199.5 Ma in the models). Results of the modeling also indicate that the original sedimentary package in the Newark basin was approximately 2.5 km thicker than today.

Determination of the Influence of Wind on the Keweenaw Current in the Lake Superior Basin as Identified by Advanced Very High Resolution Radiometer (AVHRR) Imagery

Abstract The Keweenaw Current is a warm coastal current in Lake Superior that flows northeastward along the northern shore of Michigans Keweenaw Peninsula. This study focuses on the fate of the current at the tip of the Keweenaw Peninsula. Results of this study suggest that the path of the current beyond the peninsula is primarily controlled by wind. In this study, eleven surface temperature maps derived from Advanced Very High Resolution Radiometric (AVHRR) data using computer assisted image processing techniques were used to identify the Keweenaw Current. Wind data from two moored data buoys, a Coastal Marine Automated Network (CMAN) fixed buoy, and three airport weather stations, collected on the same day as each of the images and for the two days preceding the image date, are used to determine whether wind direction and speed influence the path of the Keweenaw Current past the tip of the peninsula. In nine images the currents path is similar to the surface Ekman transport direction predicted from wind data. All eleven images show a similarity between the currents actual path and a path calculated when net Ekman transport is assumed. Results of this study also show that there may be a possible lag time of one day between a change in wind direction and the currents adjustment to that change.

Log-curve amplitude slicing: Visualization of log data and depositional trends in the Middle Devonian Traverse Group, Michigan basin, United States

Well-log-curve shapes and amplitude trends are routinely used to correlate and map formations and reservoirs across petroleum basins or fields. However, the pattern-matching methods typically employed for correlation fail to make full use of the vertical resolution of well-log curves. A new technique, log-curve amplitude slicing, facilitates correlation by automating visualization of spatial trends in log-curve amplitudes. This new technique can be used with any type of log data that is sampled at a regular interval between two distinct correlated surfaces. The method generates a series of subhorizontal slices through the log curves that can be gridded and contoured to show trends and patterns in log-curve amplitudes in map view. The slices represent approximate time lines and are relative chronostratigraphic surfaces. When appropriate logs are used, the slices show the inferred distribution of lithofacies at the time of deposition. The evolution of an area can be investigated by stepping through a series of slices. Application of the log-curve amplitude-slicing technique facilitates correlation because it highlights trends that are not apparent using traditional methods to display and compare log curves.As an example of the log-curve amplitude-slicing technique, gamma-ray log data are used in this article to analyze the evolution of the Michigan basin (United States) during deposition of the Middle Devonian Traverse Group. The Traverse Group consists mostly of carbonates, but the location, relative timing, and extent of significant fine-grained clastic influx to the basin is readily identifiable when the basins history is reconstructed using the log-curve amplitude-slicing technique.

“Hidden” threats to science education

Many readers of Eos are involved with education. Most would agree that what happens at precollege levels will ultimately affect the geoscience profession; after all, future scientists are todays precollege students. While a growing number of scientists are working to improve the quality of precollege programs, only a few are addressing what we term the “hidden” threats to science education. Hidden threats have nothing to do with scientific content; rather, they result from social, political, and bureaucratic forces operating within and outside of schools and universities.

Changes in Rate of Transgression Across the Permian White Rim Sandstone, Southern Utah

ABSTRACT A change in the rate of transgression of the Permian Kaibab sea across southern Utah is proposed on the basis of stratal relations and preserved paleotopography in the Permian White Rim Sandstone. As the Kaibab sea transgressed southern Utah, across a generally west-dipping continental margin, the rate of transgression increased. Interbedded coastal dune sandstones (White Rim Sandstone) and marine carbonates (Kaibab limestone) reflect slow transgression across what is now south-central Utah. Preserved paleotopography and the absence of intertonguing of carbonates and sandstones in southeastern Utah reflects a more rapid transgression, indicating that the rate of sea-level rise increased with time. Throughout the transgression, relative rise in sea level was discontinuous, and punctuat d by stillstands. In general, intervening stillstands decreased in duration as the transgression progressed. This change in transgression rate can be interpreted in one of two ways: rate of regional sea-level rise across southern Utah increased with time, or previously unrecognized and localized subsidence occurred in eastern Utah during Kaibab transgression. Although regional tectonism is important, we favor the former interpretation because no independent evidence of local tectonic movement can be found.