THE ISOTOPE CHEMISTRY OF QUARTZ AND ORGANIC MATTER IN SILICIFIED WOOD, PETRIFIED FOREST NATIONAL PARK, ARIZONA

The aspect ratio of silicified logs in the late Triassic Chinle Formation at Petrified Forest National Park in Arizona indicates that silica mineralization of the wood took place rapidly, at least within the first several meters of burial. This study investigates the isotope chemistry of quartz and occluded organic matter in petrified wood for indicators of environmental conditions during Chinle deposition.

Silicified logs, roots, and organic mats (silicretes) were collected from 9 stratigraphic horizons in the Chinle Formation spanning 225 to 207 m.y.a. Cell structure preservation in silicified wood varies from well preserved thick black cell walls to thin brown traces that are barely identifiable as tracheids. Cell structure is not observable in roots and organic mats. Organic carbon content in petrified wood ranges from 0.01 to 4 wt. %. The carbon isotopic composition of the organic matter in petrified wood follows the stratigraphic trend of bulk paleosol organic matter although it is offset to smaller delta values consistent with differences between tree parts (boles versus leaves).

The mineralogy of silicified organic matter is dominated by alpha-quartz although small amounts of calcite and smectite are often present. Quartz occurs as chalcedonic masses that fill lumens as well as masses of mega-quartz that occur in areas with wood rot or in fractures. Quartz occurring in areas with variable cell preservation has oxygen isotope delta values that vary between +26 to 29 per mil (SMOW). Quartz in wood with no cell preservation and in one of the silcretes (within the Mesa Redondo Member) has a second generation of quartz with an oxygen isotope ratio of +23 per mil and this quartz appears to be sourced from hydrothermal waters.

Quartz in silicified wood with the largest delta values could have precipitated at temperatures of 15 to 20 °C from waters with oxygen isotope delta values of -4 to -5 per mil SMOW. This would be consistent with orographically influenced precipitation on an elevated landscape. These temperatures are also consistent with temperature proxies from adjacent paleosols. The many samples of silicified wood with oxygen isotope ratios less than +29 per mil may have recrystallized at depth or may contain variable amounts of the second generation of quartz.