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J. S. Gilmore

Publications and source records attributed to J. S. Gilmore.

6 recordsLinked to original sources
USGS2012

Late Eocene impact microspherules: Stratigraphy, age and geochemistry

Recent discoveries of microtektite and related crystal bearing microspherule layers in deep-sea sediments of the west equatorial Pacific DSDP Sites 292, 315A and 462, off-shore New Jersey in Site 612 and in southern Spain have confirmed the presence of at least three microspherule layers in Late Eocene sediments. Moreover, these discoveries have extended the North American strewn field from the Caribbean and Gulf of Mexico region to the northwest Atlantic, and have established a third strewn field in western equatorial Pacific and Indian Ocean which may extend to the Mediterranean. Stratigraphically the oldest microspherule layer occurs in the planktonic foraminifer Globigerapsis semiinvoluta Zone about 0.5 m.y. prior to the closely spaced crystal bearing microspherule layer and North American microtektite layer in the Globorotalia cerroazulensis Zone. Major element composition of the G. semiinvoluta Zone layer and the crystal bearing microspherule layer overlap, but there is a clear trend towards higher Al 2 O 3 and FeO values in SiO 2 equivalent microspherules of the latter layer. The G. semiinvoluta Zone microspherules also contain a higher percentage of non-crystalline spherules (microtektites) than the crystal bearing microspherule layer, but lower than the North American microtektite layer. Excess iridium due to an abrupt increase in supply is associated with the middle crystal bearing microspherule layer and to a lesser extent with the other two layers. But, Ir excess due to concentration as a result of carbonate loss was also observed at two sites (462, 612). The three late Eocene microspherule layers do not precisely coincide with planktonic foraminiferal species extinctions, but a major faunal assemblage change is associated with the G. semiinvoluta Zone layer. Abundant pyrite is present in the North American microtektite layer of DSDP Site 612 suggesting reducing conditions possibly due to a sudden influx of biologic matter (dead bodies) to the ocean floor, and the crystal bearing microspherule layer coincides with five radiolarian extinctions. All three microspherule layers are associated with decreased carbonate possibly due to sudden productivity changes, increased dissolution as a result of sea-level and climate fluctuations, or the impact events.

New Jersey↗
USGS1984

Trace element patterns at a non-marine cretaceous-tertiary boundary

At the fossil-pollen-defined Cretaceous-Tertiary boundary in the Raton Basin of New Mexico and Colorado, an iridium abundance anomaly and excess scandium, titanium, and chromium are associated with a thin ash or dust fallout bed (now kaolinitic clay) that was preserved in freshwater coal swamps. ?? 1984 Nature Publishing Group.

Nature↗
USGS1984

Disruption of the terrestrial plant ecosystem at the Cretaceous-Tertiary boundary, western interior

The palynologically defined Cretaceous-Tertiary boundary in the western interior of North America occurs at the top of an iridium-rich clay layer. The boundary is characterized by the abrupt disappearance of certain pollen species, immediately followed by a pronounced, geologically brief change in the ratio of fern spores to angiosperm pollen. The occurrence of these changes at two widely separated sites implies continentwide disruption of the terrestrial ecosystem, probably caused by a major catastrophic event at the end of the period.

North America western interior↗
USGS1982

Iridium abundance measurements across the Cretaceous/Tertiary boundary in the San Juan and Raton Basins of northern New Mexico

During the past year we have been measuring trace element abundances and searching for anomalously high iridium (Ir) concentrations in continental sedimentary rocks that span the Cretaceous-Tertiary boundary in the Raton and San Juan Basins of northern New Mexico and southern Colorado. Using neutron activation and radiochemical separations, we have identified anomalous concentrations of Ir in samples from two sites in the Raton Basin: in a drill core at York Canyon, about 50 km west of Raton, New Mexico, and in a road cut near the city of Raton. In both cases the anomaly occurs essentially at the base of thin coal beds, across a thickness span of only a few cm and at the same level at which several species of Cretaceous pollen become extinct and the ratio of angiosperm pollen to fern spores drops sharply. The Ir surface density ranges from 8 to 40 × 10 −9 g cm −2 . In the York Canyon core the Ir concentration reaches a value of 5.6 × 10 −9 g/g of rock over a local background of about 10 −11 g/g; the Pt abundance distribution is similar to that for Ir, while Au reaches its maximum concentration about 10 cm below the Ir peak. Se, V, Cr, Mn, Co, and Zn are about two-fold more abundant at the anomaly zone than in adjacent zones, and mass spectrometric 244 Pu analysis showed the 244 Pu/Ir atom ratio ⩽ 1 × 10 7 . In the San Juan Basin we have located a small Ir spike (55 × 10 −12 g/g over a local background of 8 × 10 −12 g/g) that is accompanied by high concentrations of Co and Mn. It is thought to be due to geochemical enrichment processes.

Colorado, New Mexico↗
USGS1981

An iridium abundance anomaly at the palynological Cretaceous-Tertiary boundary in northern New Mexico

An iridium abundance anomaly, with concentrations up to 5000 parts per trillion over a background level of 4 to 20 parts per trillion, has been located in sedimentary rocks laid down under freshwater swamp conditions in the Raton Basin of northeastern New Mexico. The anomaly occurs at the base of a coal bed, at the same stratigraphic position at which several well-known species of Cretaceous-age pollen became extinct.

New Mexico↗