CAE Feb 27/04

STRATIGRAPHIC AND STRUCTURAL OVERVIEW OF THE BOWSER AND SUSTUT BASINS C.A. Evenchick

Project workshop, Calgary, Feb. 27, 2004

mapping...

structural analysis...

basin analysis...

Natural Resources Canada

Ressources Naturelles Canada

energy resource studies...

NorthernResourceDevelopmentProgram

CAE Feb 27/04

basins are Jurassic and Cretaceous clastic rocks

overlie Triassic and lower Jurassic volcanic arc rocks (Stuhini, Hazelton groups)

REGIONAL GEOLOGICAL FRAMEWORK - central BC geology

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

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

… a quick history of research

Green and blue outlines are current basin outlines

CAE Feb 27/04

STRATIGRAPHIC FRAMEWORKTable of formations, Spatsizi River map area.Time scale of Harland et al. (1990).

Stratigraphy of Spatsizi River area is typical of other regions in the vicinity of the Bowser and Sustut basins:

STIKINE ASSEMBLAGEPaleozoic metavolcanic rocks, metaplutonic rocks, carbonates

MESOZOIC ARCSTriassic and lower Jurassic volcanic arc rocks (plutons, volcanic rocks, related sedimentary rocks)

MESOZOIC BASINSCretaceous Sustut Basin; nonmarineJurassic-Cretaceous Bowser basin; marine and nonmarine

modified from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press

CAE Feb 27/04

STRATIGRAPHIC FRAMEWORK - BOWSER LAKE GROUPLithofacies 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 GROUPLithofacies assemblagesintegrating distribution of lithofacies assemblages with fossil data gives a general picture of depositional history of the basin on a regional scale

modified from Evenchick etal., 2001, GSC Open File 3956

modified from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press

CAE Feb 27/04

REGIONAL RELATIONSHIPS

STRUCTURAL FRAMEWORK – THE SKEENA FOLD BELT

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 .

Location of cross-section

modified from Evenchick, 2001

CAE Feb 27/04

FIRST ORDER FEATURES - GEOMETRY AND SCALE

STRUCTURAL FRAMEWORK - SKEENA FOLD BELT

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.

SENE

NE

Bowser Lake Group Bowser Lake Group

Sustut Group NE Sustut Group

SEBowser Lake Group

wide range of scales and styles!!.

CAE Feb 27/04

FIRST ORDER FEATURES - GEOMETRY AND SCALE

STRUCTURAL FRAMEWORK - SKEENA FOLD BELT

larger wavelength folds are associated with structural culminations of competent volcanic rocks of Stikinia.

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)

SW

km

0

2

km0

2

CAE Feb 27/04

FIRST ORDER FEATURES - GEOMETRY AND SCALE

STRUCTURAL FRAMEWORK - SKEENA FOLD BELT

thrust faults are present but, unless cut-offs are recognized, they are difficult to recognize because Bowser Lake Group lacks distinctive regional stratigraphic markers

Hazelton Gp clastic rocks

SE

CAE Feb 27/04

FIRST ORDER FEATURES - GEOMETRY AND SCALE

STRUCTURAL FRAMEWORK - SKEENA FOLD BELT

contractional structures affect underlying successions of Stikinia

JHCu

JHCu

JHSu

JHSu

NE

from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press

CAE Feb 27/04

FIRST ORDER FEATURES - MAGNITUDE OF SHORTENING

STRUCTURAL FRAMEWORK - SKEENA FOLD BELT

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

modified from Evenchick 1991Tectonics vol 10

in the Bowser Lake Group, excellent exposurepermits estimation of minimum shorteningby measuring bed-lengths of local markers

from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press

NE

NE

CAE Feb 27/04

FIRST ORDER FEATURES - BOUNDARIES

STRUCTURAL FRAMEWORK - SKEENA FOLD BELT

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

NE

NE

NE

KBPKTC

KBP

KBPKBP

KTC

KTC

Triangle zone

CAE Feb 27/04

FIRST ORDER FEATURES - TIMING

STRUCTURAL FRAMEWORK - SKEENA FOLD BELT

Relationships between structures and stratigraphic units indicate that orogenic shortening began prior to Albian time, and continued into the Maastrichtian or later.

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.

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

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 !!!