LUCAS, SPENCER G. New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104. - Upper Triassic megafossil plant biostratigraphy, Chinle Group, western United States.
Chinle Group strata are siliclastic red beds that record fluvial,
lacustrine and minor eolian deposition in a Late Triassic basin that
extended from West Texas-Oklahoma to eastern Nevada, southeastern
Idaho and Wyoming. Megafossil plants have been known from the Chinle
Group since 1850, and they have been used to construct a
biostratigraphy of three floral zones (ascending order):
Eoginkgoites, Dinophyton and Sanmiguelia zones.
This biostratigraphy can be evaluated by stratigraphically ordering
Chinle plant localities using lithostratigraphic and
vertebrate-fossil-based correlations, which are independent of the
plant biostratigraphy. This reveals that the three zones are
stratigraphically successive, and that the bulk of the Chinle
paleoflora (more than 50% of the taxa) is from theDinophyton
zone. Eoginkgoites is confined to basal Chinle strata, but
co-occurring genera restricted to these strata are rare and/or known
from a single locality. The Sanmiguelia zone is represented by
scattered localities in the middle-upper Chinle and cannot be
characterized except by the presence of Sanmiguelia, which is
known from less than 10 localities and endemic to Chinle strata.
Correlation of Chinle Group plant zones with Newark Supergroup plant
sites is consistent with vertebrate-fossil-based correlations.
However, Chinle plants only provide a useful biostratigraphy of upper
Carnian strata (Eoginkgoites and Dinophyton zones). The
Norian-Rhaetian paleoflora of the upper Chinle (Sanmiguelia
zone) needs further collecting and study before it can be of much
Key words: biostratigraphy, Chinle Group, megafossil plant, Triassic, United States