PLIENSBACHIAN (LOWER JURASSIC) BIOSTRATIGRAPHY AND ...

Figure 2. Geological m ap o f northern Stikinia
a n d su rro u n d in g terra n es, o u tlin in g th e
distribution o f the m ajor Triassic and Jurassic
rock units and tectonic features. (M odified
fro m Thom son et al., 1986.)
The Spatsizi area is bounded to the north by the
Stikine Arch, a major east-west trending tectonic
feature that began to rise in the Late Triassic and
exerted a fundamental control on the volcanic and
sedimentary depositional patterns of the Spatsizi region
into the late Mesozoic.
Ammonites examined in this study were collected
from the Cold Fish volcanic rocks and the Spatsizi
Group sedimentary rocks distributed along the
southern flank of the Stikine Arch (Fig. 3). The Cold
Fish volcanics are probably genetically related to the
Stikine Arch, and the Spatsizi Group sedimentary
rocks represent the basinward sedimentary equivalents
of the Cold Fish and other volcanic rocks (Thomson
et al., 1986). The southward transition from volcanic
to sedimentary rocks, illustrated schematically in
Figure 4, is not clearly understood due to structural
complications. It is not known whether the volcanic
rocks thin and grade basinward into sedimentary
rocks, or whether the volcanic rocks end abruptly. The
Spatsizi Group is exposed from beneath the Bowser
Lake Group in an erosional inlier (Figs. 5, 6). The total
extent of the Spatsizi Group is unknown due to the
otherwise unbroken cover of the Bowser Lake
sediments. It is assumed that the Spatsizi Group
continues for a considerable distance below the Bowser
Figure 3. Geological m ap o f the northern Spatsizi
area showing the distribution o f the Cold Fish
a nd other volcanic units, and the Spatsizi
G roup strata. (M odified fro m Gabrielse and
Tipper, 1984, and Sm ith et al., 1984.)
Lake Group and that Lower to lower Middle Jurassic
shale underlies much, if not all, of the Bowser Basin
(Thomson et al., 1986).
STRATIGRAPHIC SUMMARY
Cold Fish volcanics
In the vicinity of the Stikine Arch, much of the
Lower to Middle Jurassic sequence comprises volcanic
rocks, but farther south, where Spatsizi Group
sedimentary rocks occur, volcanic rocks account for
only the lower portion of the sequence (Smith et al.,
1984; Thomson et al., 1986). The stratigraphy of the
volcanic rocks west of Cold Fish Lake is poorly
understood, but is the subject of ongoing studies and
will be described later by others (H. Gabrielse,
pers. comm., 1986). In this paper, the term “ Cold Fish
volcanics” is used informally to refer to volcanic rocks
of Pliensbachian age.
Lower Pliensbachian ammonites obtained from
strata interbedded with the Cold Fish volcanics were
collected from scattered localities to the south and
southwest of Cold Fish Lake (Fig. 1). Relative
stratigraphic positions of the collections determined