Charles W. Rovey

An isochron method for cosmogenic-nuclide dating of buried soils and sediments

We describe an improved method for dating buried paleosols using measurements of the cosmic-ray-produced radionuclides 10Be and 26Al in quartz grains, and apply it to a sequence of intercalated tills and paleosols in central Missouri, USA, that record Plio-Pleistocene advances of the Laurentide Ice Sheet. A buried paleosol implies a period of surface exposure and nuclide accumulation, followed by burial and a halt to nuclide production. If the paleosol is formed in a sedimentary unit such as till, this unit may also have been emplaced with unknown 26Al and 10Be concentrations inherited from past surface exposure. If the inherited nuclide concentrations are the same at all depths in the soil—as is true for well-mixed sediments such as till—then the 26Al and 10Be concentrations at different depths in the paleosol will show a linear relationship. The slope of this line depends on the duration of burial of the paleosol, but not on the inherited nuclide concentrations or on the sample depths. Thus, one can date strata overlying buried paleosols by measuring 26Al and 10Be at multiple depths in the paleosol and calculating the burial age of the paleosol from the resulting isochron. We focus on applying this approach to till-paleosol sequences, but the basic idea of forming an 26Al-10Be burial isochron with a set of samples that share the same burial age, but differ in other aspects of their exposure history, applies to other stratigraphic settings as well. The method yields ages for four tills in Missouri that are stratigraphically consistent, agree with paleomagnetic age constraints, and show that ice advanced into Missouri near 1.25 Ma, near 0.8 Ma, and twice between ca. 0.4 and 0.2 Ma.

Absolute chronology for major Pleistocene advances of the Laurentide Ice Sheet

We used a 26 Al- 10 Be burial isochron method to date the glacial stratigraphic section in Missouri, USA, that records the largest advances of the Laurentide Ice Sheet. This permits an improved comparison of terrestrial and marine records of glaciation. The first recorded advance of the Laurentide Ice Sheet reached 39°N, near the extreme southern limit of North American glaciation, 2.4 Ma. The next advance to this latitude took place near the beginning of the mid-Pleistocene transition, 1.3 Ma, and three more took place from 0.75 to 0.2 Ma. There is no evidence that the Laurentide Ice Sheet advanced south of ∼45°–47°N between 2.4 and 1.3 Ma. This chronology: (1) shows that North American continental glaciation postdated Cordilleran alpine glaciation; (2) is consistent with the hypothesis that both of these events were threshold responses to tropical cooling; (3) is consistent with the hypothesis that the first advance of the Laurentide Ice Sheet was glaciologically anomalous due to the presence of deformable preglacial regolith; (4) is not consistent with the hypothesis that this deformable regolith persisted until the mid-Pleistocene transition; and (5) indicates that the increase in global ice volume at the mid-Pleistocene transition was at least in part the result of a more extensive Laurentide Ice Sheet.

Wellskins and slug tests : where's the bias?

Pumping tests in an outwash sand at the Camp Dodge Site give hydraulic conductivities (K) approximately seven times greater than conventional slug tests in the same wells. To determine if this difference is caused by skin bias, we slug tested three sets of wells, each in a progressively greater stage of development. Results were analyzed with both the conventional Bouwer–Rice method and the deconvolution method, which quantifies the skin and eliminates its effects. n nIn 12 undeveloped wells the average skin is +4.0, causing underestimation of conventional slug-test K (Bouwer–Rice method) by approximately a factor of 2 relative to the deconvolution method. In seven nominally developed wells the skin averages just +0.34, and the Bouwer–Rice method gives K within 10% of that calculated with the deconvolution method. The Bouwer–Rice K in this group is also within 5% of that measured by natural-gradient tracer tests at the same site. In 12 intensely developed wells the average skin is <−0.82, consistent with an average skin of −1.7 measured during single-well pumping tests. n nAt this site the maximum possible skin bias is much smaller than the difference between slug and pumping-test Ks. Moreover, the difference in K persists even in intensely developed wells with negative skins. Therefore, positive wellskins do not cause the difference in K between pumping and slug tests at this site.

Summary of Early and Middle Pleistocene Glaciations in Northern Missouri, USA

Abstract Northern Missouri, USA was glaciated five times prior to the Illinoian (penultimate) glaciation. Cosmogenic-isotope burial dates show that tills with reversed magnetic polarity were deposited at about 2.4 and 1.3 Ma during the Early Pleistocene. Three younger tills with normal magnetic remanence reached the same approximate latitude of 39° during the Middle Pleistocene. The oldest of these normal-polarity tills has a burial age that is indistinguishable from the Matuyama/Bruhnes transition at 0.78 Ma. Two younger tills were deposited between ~0.2 and 0.4 Ma, but the uncertainty in these dates remains large

The nature and origin of gleyed polygenetic paleosols in the loess covered glacial drift plain of Northern Missouri, USA

Abstract At four study sites in northern Missouri the Farmdale and Sangamon paleosols, which are developed in loess, are welded to the underlying Yarmouth paleosol, developed in till. Upon initial examination, however, they appear to be a single, thick, continuous gleyed horizon (Gleysol). Along flat tabular drainage divides this welded solum is gleyed and is identical in all respects with the historical definition of gumbotil. The solum may exceed 4 m in thickness, but has little discernable horizonization. The profiles are nearly homogeneous, containing approximately 50% clay throughout most of the solum, of which 70–80% are expandable clay minerals. Variations in texture, clay mineralogy and weathering indices are typical of a welded solum which forms a complex, polygenetic profile. The base of the solum retains abundant slickensided joints, and upon exposure it rapidly develops shrink-swell features typical of Vertisols. This behavior, coupled with the remarkable homogeneity within the solum, indicates that these soils underwent significant mixing, primarily by pedoturbation, a characteristic of modern Vertisols.

Monitoring and Numerical Modeling of Shallow CO{sub 2} Injection, Greene County, Missouri

The project titled Monitoring and Numerical Modeling of Shallow CO{sub 2} Injection, Greene County, Missouri provided training for three graduate students in areas related to carbon capture and storage. Numerical modeling of CO{sub 2} injection into the St. Francois aquifer at the Southwest Power Plant Site in Greene County, Missouri indicates that up to 4.1 x 10{sup 5} metric tons of CO{sub 2} per year could be injected for 30 years without exceeding a 3 MPa differential injection pressure. The injected CO{sub 2} would remain sequestered below the top of the overlying caprock (St. Francois confining unit) for more than 1000 years. Geochemical modeling indicates that portions of the injected CO{sub 2} will react rapidly with trace minerals in the aquifer to form various solid carbonate mineral phases. These minerals would store significant portions of injected CO{sub 2} over geologic time scales. Finally, a GIS data base on the pore-fluid chemistry of the overlying aquifer system in Missouri, the Ozark aquifer, was compiled from many sources. This data base could become useful in monitoring for leakage from future CO{sub 2} sequestration sites.

Llandoverian to Ludlovian Barrier Reef Complex in Southeast Wisconsin: ABSTRACT

Subsurface exploration in the Michigan basin established that a carbonate bank and barrier reef complex prograded basinward during the late Wenlockian to early Ludlovian, but the corresponding Niagaran Series is generally undifferentiated. In southeast Wisconsin the series is well exposed; thus, a better record of depositional history is available. Until now, reefs in the Racine formation of southeast Wisconsin (upper Wenlockian through lower Ludlovian) were interpreted as patch reefs built landward of the barrier complex. However, the following criteria are consistent with an extension of Michigans northern barrier complex beneath Lake Michigan to southeast Wisconsin: (1) Ubiquitous presence of reef facies along a southwest to northeast trend. This trend is coincident with thickening and a facies change indicative of a deep to shallow water transition, (2) similarity in depositional sequence of the overlying Salina Group in Wisconsin and Michigan. The Salina sediments surround, but are absent over, structures interpreted as pinnacle reefs and form a feather edge against the thicker belt interpreted as a barrier complex. Hence, the Racine reefs are reinterpreted as a barrier complex. Hence, the Racine reefs are reinterpreted as a barrier and pinnacle reef complex. Similar facies changes are also present in older formations. Intraformational truncationmorexa0» surfaces in the underlying Waukesha Dolomite (upper Llandoverian to lower Wenlockian) clearly indicate the presence of a nearby carbonate slope. Therefore, the carbonate buildup originated prior to the Wenlockian and migrated further basinward than previously believed.«xa0less