PALEOMAGNETICS AND MAGNETOSTRATIGRAPHY OF THE PETRIFIED FOREST AND POLEO FORMATIONS, UPPER TRIASSIC CHINLE GROUP, CHAMA BASIN, NORTH-CENTRAL NEW MEXICO

As indicated by vertebrate biostratigraphy and palynostratigraphy, the Upper Triassic Chinle Group spans most of the Late Triassic. The Chinle Group was deposited by a fluvial system and consists predominantly of red to purple mudstones with some red to orange siltstones and red to buff sandstones. The Chama basin, north-central New Mexico, contains excellent exposures of both lower and upper Chinle Group strata that can be used to develop a magnetic reversal chronology for the Late Triassic. Our sampling methods concentrated on mudrocks, using an intricate block sampling technique. Specimens typically carry a well-defined, well-grouped magnetization dominated by pigmentary hematite that is unblocked below about 660ºC. For example, a single horizon, with six independent samples, yielded D=185.5º, I=0.3º, α₉₅=6.6º and k=102.6. Sandstones and siltstone contain both detrital hematite and some magnetite. An initial reversal chronology has been developed for the Poleo and Petrified Forest formations in the Chama basin. The Petrified Forest Formation is characterized by fairly regular reversals, and all acceptable sites in the Poleo Formation are of reverse polarity.