Nancy D. Naeser

The Application of Fission-Track Dating to the Depositional and Thermal History of Rocks in Sedimentary Basins

Fission tracks are zones of intense damage formed when fission fragments travel through a solid. One of the isotopes of uranium, 238U, is the only naturally occurring isotope with a fission half-life sufficiently short to produce a significant number of fission tracks through geologic time. Uranium occurs in trace amounts in many minerals, and, because the spontaneous fission of 238U occurs at a known rate, it is possible to calculate the age of a mineral by determining the number of fission tracks and the amount of uranium it contains.

Provenance studies by fission-track dating of zircon-etching and counting procedures

Abstract In sedimentary rocks that have not been heated to high enough temperatures to anneal fission tracks in zircon (greater than ≈ 160°C), fission-track ages of individual detrital zircon grains provide valuable information about the source rocks eroded to form the sediments. The success of such studies depends, however, on the degree to which the ages determined from the detrital suite accurately portray the range of grain ages that are present in the suite. This in turn depends to a large extent on using counting and, in particular, etching procedures that permit proper sampling of grains with a wide range of age and uranium concentrations. Results are reported here of an experimental study of a ‘detrital’ zircon suite manufactured from several zircon populations of known age. This study suggests that multiple etches are required when a complete spectrum of ages in a zircon suite is desired.

Thermal History of Sedimentary Basins: Introduction and Overview

Interest in the thermal histories of sedimentary rocks and basins has grown rapidly since 1970 and is now intense. The main reason behind this acceleration is the increasing awareness that the natural processes responsible for generating oil and gas from kerogens of petroleum source rocks depend essentially on burial heating. Debates about the relative roles of other factors (time, heating rates, kerogen types, specific kerogen components, natural catalysis, and so forth) go on, but geochemists and virtually all petroleum geologists agree that heating of preserved sedimentary organic detritus is essential for oil and gas generation, and that burial in sedimentary basins or depocenters is required to achieve sufficient heating for commercial accumulations to occur.

Thermal history of rocks in southern San Joaquin Valley, California; evidence from fission-track analysis

The theory of the fission-track method and its application to sedimentary basin analysis is illustrated by a case study in the southern San Joaquin Valley, California. Fission tracks provide a powerful tool for studying the thermal history of sedimentary basins because the two minerals most commonly used in fission-track studies, apatite and zircon, occur as detrital constituents in many sedimentary rocks, and their annealing temperatures span the main temperature range for oil generation. Fission tracks also provide information on the sedimentation record and provenance of rocks in a basin. We have used fission-track analysis to study the thermal and depositional history of the subsurface Tertiary sedimentary rocks on both sides of the active White Wolf reverse fault in the southern San Joaquin Valley. The distinctly different thermal histories of the rocks in the two structural blocks are clearly reflected in the apatite fission-track data, which suggest that rocks in the rapidly subsiding basin northwest of the fault have been near their present temperature for only about 1 m.y. compared with about 10 m.y. for rocks southeast of the fault. These estimates of heating time agree with previous estimates for these rocks. Zircon fission-track data indicate that the Tertiary sediments were derived from parent rocks of more than one age. However, from at least the Eocene to late Miocene or Pliocene, the major sediment source was rocks related to the youngest Sierra Nevada Mesozoic intrusive complexes, which are presently exposed east and south of the southern San Joaquin Valley.

Contrasting tectonothermal domains and faulting in the Potomac terrane, Virginia–Maryland—discrimination by 40Ar/39Ar and fission-track thermochronology

New 4 0 Ar/ 3 9 Ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the Potomac terrane in northern Virginia, thus confirming previous interpretations that it is a composite terrane. The rocks of the Potomac terrane were examined along the Potomac River, where it has been previously subdivided into three units: the Mather Gorge, Sykesville, and Laurel Formations. In the Mather Gorge Formation, at least two metamorphic thermal domains were identified, the Blockhouse Point and Bear Island domains, separated by a fault active in the late Devonian. Early Ordovician (ca. 475 Ma) cooling ages of amphibole in the Bear Island domain reflect cooling from Taconic metamorphism, whereas the Blockhouse Point domain was first metamorphosed in the Devonian. The 4 0 Ar/ 3 9 Ar data from muscovites in a third (eastern) domain within the Mather Gorge Formation, the Stubblefield Falls domain, record thrusting of the Sykesville Formation over the Mather Gorge Formation on the Plummers Island fault in the Devonian. The existence of two distinctly different thermal domains separated by a tectonic boundary within the Mather Gorge argues against its status as a formation. Hornblende cooling ages in the Sykesville Formation are Early Devonian (ca. 400 Ma), reflecting cooling from Taconic and Acadian metamorphism. The ages of retrograde and overprinting muscovite in phyllonites from domain-bounding faults are late Devonian (Acadian) and late Pennsylvanian (Alleghanian), marking the time of assembly of these domains and subsequent movement on the Plummers Island fault. Our data indicate that net vertical motion between the Bear Island domain of the Mather Gorge complex and the Sykesville Formation across the Plummers Island fault is east-side-up. Zircon fission-track cooling ages demonstrate thermal equilibrium across the Potomac terrane in the early Permian, and apatite fission-track cooling ages record tilting of the Potomac terrane in the Cretaceous or later with the west side up at least 1 km.