John C. Hathaway

U.S. Geological Survey Core Drilling on the Atlantic Shelf

The first broad program of scientific shallow drilling on the U.S. Atlantic continental shelf has delineated rocks of Pleistocene to Late Cretaceous age, including phosphoritic Miocene strata, widespread Eocene carbonate deposits that serve as reflective seismic markers, and several regional unconformities. Two sites, off Maryland and New Jersey, showed light hydrocarbon gases having affinity to mature petroleum. Pore fluid studies showed that relatively fresh to brackish water occurs beneath much of the Atlantic continental shelf, whereas increases in salinity off Georgla and beneath the Florida-Hatteras slope suggest buried evaporitic strata. The sediment cores showed engineering properties that range from good foundation strength to a potential for severe loss of strength through interaction between sediments and man-made structures.

Methane-Derived Marine Carbonates of Pleistocene Age

In some calcium carbonate-bearing sandstones from the edge of the continental shelf off the northeast United States, the δC13 range is from -30 and -60 per mil for both aragonite and high-magnesium calcite. The δC13 of co-existing shells of Modiolus sp. is normal (+ 1.7 to -2.7 per mil). The δO18 values of around + 3.5 per mil in all samples suggest deposition at temperatures around 0�C. Quaternary methane oxidized either chemically or microbiologically to carbon dioxide is the probable source of carbon in these carbonates.

Studies of Some Vermiculite-Type Clay Minerals*

X-ray and electron-diffraction studies, and differential thermal analyses of the clay fraction of some soils from Virginia, New Jersey, and Wisconsin indicate an abundance of minerals resembling vermiculite. Measurements of the 060 spacing for these minerals give values lower than those obtained for trioctahedral vermiculites. It is suggested that the minerals studied are dioctahedral forms, which have resulted from the weathering of muscovite-type micas.

Mixed-Layered Structures in Vanadium Clays1

A group of claylike silicates from zones of vanadium mineralization in the sandstone-type deposits of the Colorado Plateau have been examined by x-ray powder diffraction methods to determine their mineralogic composition. The materials studied were separated by water elutriation, bromoform separation, or both, and oriented aggregates as well as randomly oriented powders were prepared. A comparison of the x-ray diffractometer patterns of these specimens with roscoelite from Coloma, California (AMNH 13,565) shows that most of the nine samples examined differ from roscoelite in two important ways: by interstratification of mica layers with montmorillonite, and by variation in octahedral substitution of V for A1. The mixed layering of mica-montmorillonite is revealed by distinct changes .in position and intensity of the first-order basal spacings on treatment with ethylene glycol and on heating of the samples to 400 ~ C and 500 ~ C. Variations in octahedral substitutions are inferred from variations in the intensity of the second-order basal spacings relative to the first and third orders. Low relative intensity of the second order is interpreted as indicative of high electron density in the octahedral positions caused by substitution of V for A1. One sample (ALB-34-54) from Placerville, Colorado, shows x-ray characteristics very similar to the roscoelite from California and a chemical composition which gives a sum of 1.969 for octahedrally coordinated cations, indicating that it is a dioctahedral mineral. Two samples from Rifle, Colorado, also show characteristics closely resembling roseoelite although one of these, from an oxidized zone, exhibits a relatively stronger second-order basal spacing, and the suggestion is made that the amount of vanadium substitution is less than in the Coloma, California, or the Placerville, Colorado, material. The rest of the samples show mixed layering to various degrees, with the first-order basal spacing of the untreated material ranging from 10.1 A to 10.5 A. Chlorite is present in at least seven of the samples and is the dominant mineral in at least two of them. Only in the materials from Coloma, California, and Placerville, Colorado, was chlorite absent. The chlorites in two samples show mixed layering with expanding material, although one of these--from Thompson, Utah may be more closely allied to vermiculite than to chlorite. This indeterminant sample differs considerably from the other materials examined in that the mixed-layered mica-montmorillonite as well as the vermiculitic (?) mineral do not survive heating to 500 ~ C. The nonmixed-layered chlorites are characterized by almost equal intensities of the first four basal

Distribution of Clay Minerals and Ion-Exchange Capacity in Some Sedimentary Materials1

The cation-exchange capacity of the clay fraction was determined in 67 samples of various types of sedimentary materials, and the clay minerals were identified from x-ray diffraetometer patterns of oriented aggregates. The probable quantitative clay mineral composition was established for each sample assuming a comparable degree of crystallinity for the clay minerals in the sample. Agreement was poor between the ion-exchange capacities determined and those predicted from the clay mineral composition. The results suggest that the figure obtained for the ion-exchange capacity of a sample of sedimentary material may be misleading as a guide to the types of clay minerals present.