Terrigenous sedimentary rocks from the Colorado Plateau show a relativelyuniform bulk silicate δ^(18)O of +14.8 with an SEM of 0.32. Shales and calcilutites in thisregion have a mean bulk silicate δ^(18)O of +17.7 which is significantly heavier than themean for interbedded sandstones and siltstones. Bulk silicate δ^(18)O is decoupled fromcarbonate δ^(18)O due to differences in mode of deposition and diagenetic behavior.
Central Appalachian terrigenous sedimentary rocks show a surprisingly uniformbulk silicate δ^(18)O of +14.8 with an SEM of 0.1. The mean bulk silicate δ^(18)O for allshales (+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 ismainly a primary depositional feature that is the result of thorough, grand-scale mixing ofterrigenous sediment in the Appalachian geosyncline, probably involving several cycles ofsedimentation, uplift, erosion, and reworking extending over hundreds of millions of yearsduring the Paleozoic era. The bulk silicate δ^(18)O of siltstones and shales shows asignificant (P ˂ 0.05)correlation with conodont color alteration index, which is a measure ofdiagenetic 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 2.5to 4.0 per mil. These diagenetic effects contributed to the overall homogeneity of thesesedimentary rocks because the shales started out at higher δ^(18)O.
A reconnaissance ^(18)O/^(16)O study of 14 samples of terrigenous sedimentary rocksfrom 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 isotopicvariation also seems clearly attributable to diagenetic effects. A significant (P ˂ 0.05)correlation was found between mean vitrinite reflectance, also a measure of diagenetictemperature, and the bulk silicate δ^(18)O difference between shale-sandstone pairs in threedifferent sedimentary formations.
Northern Appalachian metasedimentary rocks show a decrease in bulksilicate δ^(18)O at garnet grade and higher. The terrigenous facies metamorphic rocks havebeen depleted in ^(18)O by about two per mil relative to their unmetamorphosed counterpartsin the Central Appalachians, except where they are adjacent to carbonate-rich sections.Carbonate facies metasedimentary rocks are 5 to 6 per mil higher than interbeddedterrrigenous facies rocks, but at the margins of that formation there is a distinct lowering ofbulk silicate δ^(18)O and carbonate δ^(18)O due to influx of metamorphic hydrothermal fluidsfrom the adjacent terrrigenous rocks. This is attributed to the involvement of isotopicallylight fluids during metamorphism. Further work is need to elucidate the differencesbetween metamorphic processes in pelitic and calcareous sediments.
