delta O 18 values for 73 chert samples from the Arkansas Novaculite (Devonian-Mississippian) range from + 17.8 per thousand to 33.7 per thousand . Much of this variation is related to the effects of local metamorphism and isotopic impurities such as detrital quartz and clay minerals. Pure, unrecrystallized samples from the western outcrops in Oklahoma range from 30.3 per thousand to 33.7 per thousand , whereas samples from Arkansas range from 25.1 per thousand to 29.7 per thousand . As a group, western outcrops of the Novaculite are distinctly richer in O 18 . Petrographic and SEM examination of the O 18 -rich western samples reveal well preserved early diagenetic textures including abundant relict opal-CT lepispheres (now replaced by microquartz). Radiolaria and sponge spicules show well preserved morphologies. One western sample yielding the highest delta -value is microbrecciated, contains abundant dolomite rhombs and is laced with quartzine (length-slow chalcedony) suggesting silicification under evaporitic conditions. Samples from eastern localities display no evidence of early diagenetic textures, and radiolaria and sponge spicules are preserved only as petrographic ghosts. The isotopic and petrographic data can be interpreted in terms of a siliceous biogenic ooze deposited in environments ranging from supratidal to open marine. Shallow (platform?) deposition occurred in western areas with early silicification contributing to the preservation of diagenetic textures. Assuming that delta O 18 of the diagenetic waters was within 1 per thousand of seawater the higher delta -values in the west reflect chert crystallization at isotopic temperatures of 20 degrees to 29 degrees C. The lower delta -values in the east suggest chert crystallization during deeper burial at isotopic temperatures of 31 degrees to 53 degrees C, similar to burial diagenesis in Tertiary deep sea cherts.