The Chinle Group - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 131

(both formerly members of the Chinle
Formation; 133 ). The Petrified Forest
Formation now consists of three
members, in ascending order, the Blue
Mesa, Sonsela, and Painted Desert
members. Fossil vertebrates and other
biostratigraphically significant fossils
indicate that the Chinle Group is of
late Carnian-middle Norian age, about
212-222 million years old, with the
Carnian-Norian boundary at or near the
base of the Sonsela Member.
The Blue Mesa Member includes strata
previously known as the 'Lower Petrified
Forest Member', and consists of up to
83 m (270 ft) of noncalcareous, bentonitic
mudstones with interbedded cross-bedded
sandstones. The Sonsela Member consists
of 20 m (65 ft) of bench-forming cross-

bedded sandstones and chert-pebble
conglomerates which form the mesa tops
in much of the Petrified Forest. The base
of the Sonsela Member marks a major
change in lithology; the contact shows
evidence of channeling and erosion and is
unconformable.
The Painted Desert Member includes
strata previously known as the 'Upper
Petrified Forest Member'. It overlies the
Sonsela Member conformably and consists
of up to 147 m (480 ft) of reddish-brown,
calcareous, bentonitic mudstones with
persistent cross-bedded sandstone beds
in the lower part forming persistent
ledges. It is this member that includes
the important Black Forest Bed (or
Black Forest Tuff), a prominent, white,
tuffaceous sandstone and conglomerate

133

Wingate
Sandstone

conglomerate

Hite Bed

sandstone (massive)

sandstone (trough cross-bedded)

Rock Point
Formation

sandstone (ripple laminated)

siltstone

mudstone (with calcrete nodules)

mudstone

Owl Rock Formation
Painted
Desert
Member

calcrete horizon

Sonsela Mbr

Blue
Mesa
Member

fossiliferous
horizon

Cameron Formation

20 m (66 ft)

Shinarump Formation

Moenkopi Formation

133 Diagram to show the stratigraphy of the Chinle Group after Heckert et al., 1999.