Remarkably uniform bulk silicate δ^(18)O values of terrigenous sedimentary rocks from the Central Appalachian and Ouachita geosynclines

Abstract

The whole-rock δ^(18)O values of terrigenous sedimentary rocks in the Appalachian Mountains are surprisingly uniform, irrespective of mineralogy, age, lithology, environment of deposition, or grain size. The bulk silicate (i.e. non-carbonate) portions of 75 Paleozoic shales, siltstones, and sandstones covering an age span from Ordovician to Permian range in δ^(18)O only from +13.2 to +17.6, with a mean δ^(18)O of +14.9. Non-marine samples have slightly lower mean δ^(18)O (+14.7) and are isotopically somewhat more uniform than marine samples (+15.3). In spite of the overall homogeneity, a systematic decrease in bulk silicate δ^(18)O is observed in shales and siltstones going from the SW (Kentucky, Tennessee) to the NE (central Pennsylvania). This geographic trend in δ^(18)O correlates with conodont color alteration index (CAl, a measure of organic metamorphism) but not with age, depositional environment, or source region, and there is no correlation of bulk silicate δ^(18)O with conodont CAl for the sandstones. These systematics are interpreted to be the result of oxygen isotope alteration during diagenesis; exchange with diagenetic pore fluids lowered the bulk silicate δ^(18)O of siltstones and shales by as much as 2.5 to 4.2 per mil, bringing these finer-grained rocks closer to the δ^(18)O values of the sandstones. These diagenetic changes partly account for the overall oxygen isotopic homogeneity of the Ctntral Appalachian geosynclinal rocks (together with the thorough, grand-scale mixing of terrigenous sediment that occurred in the Appalachian geosyncline over several cycles of sedimentation, uplift, erosion, and reworking of sediments extending over hundreds of millions of years during the Paleozoic era). An analogous study of 14 terrigenous sedimentary rocks from the Ouachita Mountains (Oklahoma, Arkansas) shows a similar range of δ^(18)O (+13.4 to +16.9, mean = +15.3), but suggests greater inherent oxygen isotopic heterogeneity of source areas compared with the Central Appalachians. This complicates analysis of variation in bulk silicate δ^(18)O due to diagenesis. However, a significant correlation (r = 0.95) is found between vitrinite reflectance (another measure of organic metamorphism) and the difference in bulk silicate δ^(18)O between interbedded shale and sandstone pairs. Thus, because of their finer grain size, the bulk silicate δ^(18)O values of the shales were shifted downward more than those of the sandstones during diagenesis.

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