Cambrian-Ordovician strata of the Mississippi Valley have been extensively modified by potassic diagenesis. Ordovician vitric tuffs are altered to K-feldspar and illite-rich, mixed-layer illite-smectite (I/S), both of which have been dated by K/Ar. Rb/Sr dates for K-feldspar support its reliability as a K/Ar clock, despite a dislocation density of 109-1010 cm-2. Age data document three Paleozoic episodes of potassic diagenetis, which have mean ages of 396 Ma (Early Devonian) for K-feldspar of the upper Mississippi Valley (UMV), 362 Ma (Late Devonian) for I/S off the UMV, and 265 Ma (Permian) for I/S of Missouri.

Devonian dates from the UMV correspond to times of wide-spread epeirogeny in the U.S. midcontinent, and diagenetic episodes are attributed to regional flow of basinal brines caused by ground water recharge on uplifted arches. Potassium for K-feldspar may have come from the Michigan basin, and K for illite may have come from the east-central Iowa basin. K-feldspar and I/S of the UMV are in oxygen-isotopic disequilibrium, indicating that the K-feldspar crystallized from fluid that was either warmer or had lower δ18O values than that of the I/S. Permian illitization apparently records northward movement of warm saline fluid as a result of the Ouachita orogeny.

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