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Oxygen isotope studies of some sedimentary and metasedimentary rocks of the central and northern Appalachian Mountains, the Colorado Plateau, and the Ouachita Mountains

Citation

Burt, Emelia Anna (1993) Oxygen isotope studies of some sedimentary and metasedimentary rocks of the central and northern Appalachian Mountains, the Colorado Plateau, and the Ouachita Mountains. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:11292012-100814038

Abstract

Terrigenous sedimentary rocks from the Colorado Plateau show a relatively uniform bulk silicate δ^(18)O of +14.8 with an SEM of 0.32. Shales and calcilutites in this region have a mean bulk silicate δ^(18)O of +17.7 which is significantly heavier than the mean for interbedded sandstones and siltstones. Bulk silicate δ^(18)O is decoupled from carbonate δ^(18)O due to differences in mode of deposition and diagenetic behavior.

Central Appalachian terrigenous sedimentary rocks show a surprisingly uniform bulk silicate δ^(18)O of +14.8 with an SEM of 0.1. The mean bulk silicate δ^(18)O for all shales (+15.2) is only 0.3 per mil heavier than the mean for all sandstones and siltstones (+14.9). The oxygen isotope uniformity of Central Appalachian sedimentary rocks is mainly a primary depositional feature that is the result of thorough, grand-scale mixing of terrigenous sediment in the Appalachian geosyncline, probably involving several cycles of sedimentation, uplift, erosion, and reworking extending over hundreds of millions of years during the Paleozoic era. The bulk silicate δ^(18)O of siltstones and shales shows a significant (P ˂ 0.05) correlation with conodont color alteration index, which is a measure of diagenetic temperature. As a result of isotopic exchange with porewater during diagenesis, the bulk silicate δ^(18)O of shales and siltstones can apparently be lowered by as much as 2.5 to 4.0 per mil. These diagenetic effects contributed to the overall homogeneity of these sedimentary rocks because the shales started out at higher δ^(18)O.

A reconnaissance ^(18)O/^(16)O study of 14 samples of terrigenous sedimentary rocks from the Ouachita Mountains suggests more inherent isotopic variation in these samples, perhaps in part as a result of greater heterogeneity of source regions. Some of the isotopic variation also seems clearly attributable to diagenetic effects. A significant (P ˂ 0.05) correlation was found between mean vitrinite reflectance, also a measure of diagenetic temperature, and the bulk silicate δ^(18)O difference between shale-sandstone pairs in three different sedimentary formations.

Northern Appalachian metasedimentary rocks show a decrease in bulk silicate δ^(18)O at garnet grade and higher. The terrigenous facies metamorphic rocks have been depleted in ^(18)O by about two per mil relative to their unmetamorphosed counterparts in the Central Appalachians, except where they are adjacent to carbonate-rich sections. Carbonate facies metasedimentary rocks are 5 to 6 per mil higher than interbedded terrrigenous facies rocks, but at the margins of that formation there is a distinct lowering of bulk silicate δ^(18)O and carbonate δ^(18)O due to influx of metamorphic hydrothermal fluids from the adjacent terrrigenous rocks. This is attributed to the involvement of isotopically light fluids during metamorphism. Further work is need to elucidate the differences between metamorphic processes in pelitic and calcareous sediments.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Geology
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geology
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Taylor, Hugh P. (advisor)
  • Rossman, George Robert (advisor)
Thesis Committee:
  • Rossman, George Robert (chair)
  • Epstein, Samuel
  • Wyllie, Peter J.
  • Burnett, Donald S.
Defense Date:8 January 1993
Record Number:CaltechTHESIS:11292012-100814038
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:11292012-100814038
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7295
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:29 Nov 2012 19:21
Last Modified:26 Dec 2012 04:46

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