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

STRATIGRAPHIC AND STRUCTURAL OVERVIEW OF THE BOWSER AND SUSTUT BASINS C.A. Evenchick Project workshop, Calgary, Feb. 27, 2004. energy resource studies. mapping. basin analysis. structural analysis. Northern Resource Development Program. Natural Resources Canada.

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

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  1. STRATIGRAPHIC AND STRUCTURAL OVERVIEW OF THE BOWSER AND SUSTUT BASINS C.A. Evenchick Project workshop, Calgary, Feb. 27, 2004 energy resource studies... mapping... basin analysis... structural analysis... Northern Resource Development Program Natural Resources Canada Ressources Naturelles Canada CAE Feb 27/04

  2. 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) CAE Feb 27/04 modified from Evenchick and Thorkelson, GSC Bull. 577, 2003/in press

  3. … a quick history of research 1962: parts of northern Bowser and Sustut basins recognized in 1956 GSC mapping; 2 wells drilled 1969, 1972 1944: Bowser and Sustut basins not known to exist, although exploration in the Groundhog Coalfield dates back to the early 1900’s  Green and blue outlines are current basin outlines CAE Feb 27/04

  4. STRATIGRAPHIC FRAMEWORK Table 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: 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

  5. 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

  6. 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

  7. 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

  8. 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 Sustut Group SE SE Bowser Lake Group NE Bowser Lake Group Bowser Lake Group • wide range of scales and styles!!. CAE Feb 27/04

  9. 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 2 km 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

  10. 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

  11. STRUCTURAL FRAMEWORK - SKEENA FOLD BELT FIRST ORDER FEATURES - GEOMETRY AND SCALE • contractional structures affect underlying successions of Stikinia NE JHSu JHCu JHSu JHCu from Evenchick and Thorkelson, GSC Bull. 577, 2004/in press CAE Feb 27/04

  12. 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

  13. 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 KBP KTC CAE Feb 27/04

  14. 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

  15. 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

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