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STRATIGRAPHIC AND STRUCTURAL OVERVIEW OF
THE BOWSER AND SUSTUT BASINS
C.A. Evenchick
Project workshop, Calgary, Feb. 27, 2004
mapping...
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Natural Resources Ressources Naturelles
Canada
Canada
Northern
Resource
Development
Program
CAE Feb 27/04
REGIONAL GEOLOGICAL FRAMEWORK - central BC geology
 basins are Jurassic and Cretaceous clastic rocks
 overlie Triassic and lower Jurassic volcanic arc rocks (Stuhini, Hazelton groups)
 deformed in Cretaceous
to form Skeena Fold Belt
that terminates on the NE
in a frontal triangle zone
within Sustut Basin strata
 intruded on west by
early Tertiary plutonic
rocks (Coast Belt), and
locally in southeast by
Cretaceous and early
Tertiary plutons
 overlain by late Tertiary
and Quaternary volcanic
rocks (minor)
modified from Evenchick and Thorkelson, GSC Bull. 577, 2003/in press
CAE Feb 27/04
… a quick history of research
1944: Bowser and Sustut
basins not known to exist,
although exploration in the
Groundhog Coalfield dates
back to the early 1900’s
1962: parts of northern
Bowser and Sustut basins
recognized in 1956 GSC
mapping;
2 wells drilled 1969, 1972

Green and blue outlines
are current basin outlines
CAE Feb 27/04
STRATIGRAPHIC FRAMEWORK
Stratigraphy of Spatsizi River
area is typical of other
regions in the vicinity of the
Bowser and Sustut basins:
Table of formations, Spatsizi River map area.
Time scale of Harland et al. (1990).
MESOZOIC BASINS
Cretaceous Sustut Basin;
nonmarine
Jurassic-Cretaceous Bowser
basin; marine and nonmarine
MESOZOIC ARCS
Triassic and lower Jurassic volcanic
arc rocks (plutons, volcanic rocks,
related sedimentary rocks)
STIKINE ASSEMBLAGE
Paleozoic metavolcanic rocks,
metaplutonic rocks, carbonates
modified from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press
CAE Feb 27/04
STRATIGRAPHIC FRAMEWORK - BOWSER LAKE GROUP
Lithofacies assemblages
new stratigraphic approach to better deal with the Bowser Basin on regional
scale and provide conceptual tools for interpreting basin architecture and
depositional history (when integrated with regional fossil distribution)
modified from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press
CAE Feb 27/04
STRATIGRAPHIC FRAMEWORK - BOWSER LAKE GROUP
Lithofacies assemblages
integrating distribution of lithofacies assemblages with fossil data gives a
general picture of depositional history of the basin on a regional scale
modified from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press
modified from Evenchick etal., 2001, GSC Open File 3956
CAE Feb 27/04
STRUCTURAL FRAMEWORK – THE SKEENA FOLD BELT
REGIONAL RELATIONSHIPS
Location of cross-section
Strata of the 3 main clastic successions,
and underlying Stikinia, are folded and
thrust faulted. Contractional structures
define the Skeena Fold Belt, a thin
skinned fold and thrust belt of Cretaceous
age. The dominant fold trend is northwest,
but domains of northeast trending
structures occur locally on the west side
of the fold belt .
modified from Evenchick, 2001
CAE Feb 27/04
STRUCTURAL FRAMEWORK - SKEENA FOLD BELT
FIRST ORDER FEATURES - GEOMETRY AND SCALE
 dominant structures are folds; they are close to tight, and upright to inclined to the
northeast; they have wavelengths of hundreds of metres to 1 km.
NE
NE
Sustut Group
Bowser Lake Group
 wide range of scales and styles!!.
NE
Sustut Group
Bowser Lake
Lake Group
Group
Bowser
SE
CAE Feb 27/04
STRUCTURAL FRAMEWORK - SKEENA FOLD BELT
FIRST ORDER FEATURES - GEOMETRY AND SCALE
 larger wavelength folds are associated with structural culminations of competent
volcanic rocks of Stikinia.
SW
km
2
0
km
2
0
from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press
Comparison of scales of folding near basement culminations (green in
top diagram) and distant from culminations (bottom)
CAE Feb 27/04
STRUCTURAL FRAMEWORK - SKEENA FOLD BELT
FIRST ORDER FEATURES - GEOMETRY AND SCALE
 thrust faults are present but, unless cut-offs are recognized, they are difficult
to recognize because Bowser Lake Group lacks distinctive regional stratigraphic
markers
SE
Hazelton Gp clastic rocks
CAE Feb 27/04
STRUCTURAL FRAMEWORK - SKEENA FOLD BELT
FIRST ORDER FEATURES - GEOMETRY AND SCALE
 contractional structures affect underlying successions of Stikinia
NE
JHSu
JHSu
JHCu
JHCu
from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press
CAE Feb 27/04
STRUCTURAL FRAMEWORK - SKEENA FOLD BELT
FIRST ORDER FEATURES - MAGNITUDE OF SHORTENING
 the fold belt accommodated a minimum of 44% horizontal shortening by folds and thrust faults
 distinctive map units in the northeastern fold belt permit construction of balanced cross sections
NE
modified from Evenchick 1991
Tectonics vol 10
 in the Bowser Lake Group, excellent exposure
permits estimation of minimum shortening
by measuring bed-lengths of local markers
NE
from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press
CAE Feb 27/04
STRUCTURAL FRAMEWORK - SKEENA FOLD BELT
FIRST ORDER FEATURES - BOUNDARIES
 the fold belt terminates to the northeast in a
triangle zone within the Sustut Group
 it is interpreted to root to the west in the Coast
Belt, where there are contracrtional structures of
similar age
Triangle zone
NE
KTC
KBP
NE
NE
KBP
KBP
KTC
KTC
KBP
CAE Feb 27/04
STRUCTURAL FRAMEWORK - SKEENA FOLD BELT
FIRST ORDER FEATURES - TIMING
Relationships between structures and stratigraphic units indicate that orogenic
shortening began prior to Albian time, and continued into the Maastrichtian or later.
The youngest rocks deformed below
the sub-Sustut angular unconformity
are Oxfordian. The youngest deformed
rocks in the fold belt are late Early
Maastrichtian age (Brothers Peak Fm).
KTC
KBP
The Sustut Group and Devils Claw Formation are
inferred to be synorogenic clastic basin fill associated
with formation of the Skeena Fold Belt (e.g. Evenchick,
2000); the Sustut Group formed in the foreland (basin)
to the Skeena Fold Belt, whereas the Devils Claw
formed in a piggy back basin within the fold belt.
CAE Feb 27/04
STRATIGRAPHIC AND STRUCTURAL OVERVIEW OF
THE BOWSER AND SUSTUT BASINS
Conclusions
The Bowser Basin consists of a huge volume of clastic rocks, divided into
lithofacies assemblages which interfinger and overlap in age.
All elements of clastic depositional systems are represented, from nonmarine to
distal marine, and present a wide range of potential stratigraphic traps.
Folds and thrust faults in these rocks accommodated a large amount of horizontal
shortening, and present a wide range of structural traps.
The combination of structural complexity, lack of regional stratigraphic markers,
and general similarity of individual lithologies throughout the Bowser Lake Group,
pose a challenging mapping problem !!!
CAE Feb 27/04