PLIENSBACHIAN (LOWER JURASSIC) BIOSTRATIGRAPHY AND ...

Figure 4. Inferred stratigraphic relationships between the Spatsizi Group strata, the Cold Fish volcanics, and
other volcanic units in the northern Spatsizi area. (M odified fro m Thom son et al., 1986.)
from field studies by H.W. Tipper (pers. comm., 1986)
suggest the following biostratigraphic relationships in
this area. Sedimentary beds containing M iltoceras sp.
and a single specimen of a serpulid (PI. 2, fig. 6) were
found at the lowest levels examined in the volcanic
sequence and represent the lowest stratigraphic level of
the Pliensbachian in the Spatsizi area. Higher in the
section, Tropidoceras sp. and Metaderoceras sp. aff.
M. talkeetnaense n. sp. suggest a distinctly higher
stratigraphic level. At or near the highest level of the
Pliensbachian volcanic sequence, Acanthopleuroceras
sp. aff. A . stahli and Metaderoceras evolutum (Fucini)
were found along with a single specimen of a
phylloceratid (PI. 3, fig. 5). In the Joan Lake map
area, A . sp. aff. A . stahli w'as found at the base of
Section 2 (Fig. 7) overlying the Cold Fish volcanics.
Spatsizi Group
The Spatsizi Group comprises five formations: the
Joan, W olf Den, Melisson, A bou, and Quock
formations (in ascending order), together attaining a
thickness of up to 750 m (Thomson et al., 1986). The
formations are defined from exposures around Joan
Lake where they were initially mapped at a scale of
1:25 000 over an area of about 80 km2 (Figs. 5, 6).
Each formation reflects deposition in a different
marine sedimentary environment affected by varying
degrees of volcanic (epiclastic or pyroclastic) input.
The sequence is composed, in order of decreasing
abundance, of shale, siltstone, sandstone, conglomerate,
and limestone (see Figure 5 for a general
lithological description of each formation). The
Pliensbachian is represented by the Joan Formation
and lowermost Wolf Den Formation, described below
(Figs. 7-10).
Joan Formation
In the map area (Fig. 5), the base of the Joan
Formation rests unconformably on the uppermost flow
of the Cold Fish volcanics; a basal conglomerate less
than 10 m thick containing the Lower Pliensbachian
ammonite Acanthopleuroceras sp. aff. A . stahli
(Oppel) is present above the contact. The conglomerate
grades upward into the siltstone that makes up the bulk
of the Joan Formation and which attains a maximum
thickness of about 60 m. Lower Pliensbachian
a m m o n ite s, in clu d in g D u b a r ic e r a s f r e b o l d i
Dommergues et al. and several species of the genus
M etaderoceras, range throughout the Joan Formation
siltstones. Three specimens of the nautiloid Cenoceras
sp. were also found in the formation (PI. 3, fig. 1).
The Joan Formation contains a diverse bivalve
fauna that indicates a relatively shallow to moderately