Title

Petrology and geochemistry of the Ordovician Blountian and Taconic foreland basin sequences: Implications for terrane accretion history

Date of Award

1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Bryce Hand

Keywords

Blountian basin, Taconic foreland basin

Subject Categories

Geology

Abstract

The Paleozoic terrane accretion history of the eastern United States is one of the more intriguing tectonic problems in Appalachian geology. The colliding terranes and the timing of their collision are poorly constrained due to multiple tectonic events during the Paleozoic. Two classic examples of this problem are the Ordovician Blountian and Taconic foreland basins. Sedimentary rocks of these foreland basins, which formed as the result of terrane-continent collision, may hold the key to understanding terrane accretion history. However, past studies of foreland basin sandstones indicate that sandstones do not accurately record sedimentary provenance, especially volcanic sediment sources associated with the colliding terrane. Accurately identifying sediment sources is a significant problem because foreland basins are commonly the only record of terrane accretion. In many cases, the foreland basin sedimentary sequence holds the only information about accretion of the colliding terrane.

The dominant lithology of the Blountian and Taconic foreland basins is mudstone, a lithology rarely used in the study of sedimentary provenance. The clay mineral, chemical, and neodymium isotopic compositions of the mudstones were determined to constrain changes in sedimentary sources. Chemical composition of the mudstones distinguishes felsic and mafic sources and neodymium isotopic composition distinguishes continental (cratonic) and juvenile (volcanic) sources. Clay mineral composition appears to be controlled more by early diagenetic processes that are controlled by burial rate.

All three compositional parameters show a distinct change within both foreland basin sequences that clearly records a significant influx of sediment with a volcanic provenance. Regional geology constrains the primary source of foreland basin sediment to the colliding terranes. Early in accretion history, the accretionary wedge provides recycled continentally derived sediment to the foreland basin. As collision proceeds, and the forearc basin closes, sediment eroded off the volcanic arc bypasses the accretionary wedge and is deposited in the foreland basin. Compositional differences between the two basins suggest that (1) arc sources provided different proportions of sediment to the two basins, or (2) rocks of the accretionary wedge had different compositions, or (3) the Blountian arc had a more juvenile composition than the Taconic arc.

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