1. Project funded by the B.C. Dept. of Energy and Mines, and the Geological Survey of Canada Apatite Fission Track (AFT) thermochronology of the Northern Bowser and Sustut Basins: Constraints on the Latest Cretaceous through Cenozoic thermal history Apatite Fission Track (AFT) thermochronology of the Northern Bowser and Sustut Basins: Constraints on the Latest Cretaceous through Cenozoic thermal history Paul O’Sullivan - Apatite to Zircon, Inc. Fil Ferri - British Columbia Dept. Energy and Mines Carol Evenchick - Geological Survey of Canada Kirk Osadetz - Geological Survey of Canada Ray Donelick - Apatite to Zircon, Inc.

2. Overview of Presentation - Introduce some general background concerning apatite fission track (AFT) analyses and interpretation of AFT data - Introduce some regional geologic constraints within the Bowser Basin - these help explain why samples were originally collected for AFT analysis - Present representative AFT results from study region - Summarize the Late Cretaceous to Tertiary thermal history of the study region based on the AFT results* * NOTE - I will leave the geologic implications of these results to those most familiar with the regional geology - Introduce some general background concerning apatite fission track (AFT) analyses and interpretation of AFT data - Introduce some regional geologic constraints within the Bowser Basin - these help explain why samples were originally collected for AFT analysis - Present representative AFT results from study region - Summarize the Late Cretaceous to Tertiary thermal history of the study region based on the AFT results* * NOTE - I will leave the geologic implications of these results to those most familiar with the regional geology

3. ThermochronometersThermochronometers Temperature (°C) Argon/Argon Fission Track U/Th-Helium 0 100 200 300 400 500 600 700 800 Topography Surface processes Sedimentary Basins Burial/Metamorphism Igneous processes MetamorphismMid-crustalprocesses Hornblende Zircon Sphene Apatite Sphene Apatite K-Spar Muscovite Zircon

4. Apatite Fission-Track (AFT)Thermochronology Fission-track age area density of fossil tracks induced fission tracks (U conc) Fission-track lengths mean track length distribution of lengths Fission-track kinetics measure Dpar for each grain analyzed Fission-track age area density of fossil tracks induced fission tracks (U conc) Fission-track lengths mean track length distribution of lengths Fission-track kinetics measure Dpar for each grain analyzed Constrains maximum paleotemperatures plus timing and style of cooling from maximum paleotemperatures Constrains maximum paleotemperatures plus timing and style of cooling from maximum paleotemperatures “time” of cooling “time” of cooling “rate” & “style” of cooling “rate” & “style” of cooling 1250x ‘confined’lengths‘confined’lengths annealing “resistivity”

5. Number of tracks provides a measure of time over which tracks have accumulated Both grains have same Uranium concentration Pretty young Bloody old Apatite Fission-Track (AFT)Thermochronology - Fission-Track Age - 2000x 220 Ma

6. Length of a track today is determined by the maximum paleotemperatures since that track formed Apatite Fission-Track (AFT)Thermochronology - Fission-Track Lengths - 2250x “Confined” tracks Prettycool Bloodyhot

7. Dpar (µm) Apatite Fission-Track (AFT)Thermochronology - Fission-Track Kinetics - 2250x 2000x “Confined” track Etch pit Spontaneoustracks Dpar is essentially a measure of apatite solubility in a grain. Solubility in apatite is related to the grain’s composition C-axis

8. Little variation between grains AFTSolve: Measure kinetic variation (Dpar) Significant variation between grains Dpar is essentially a measure of apatite solubility in a grain. Solubility in apatite is related to the grain’s composition Totalresetting~100°C Totalresetting~115°C~145°C~160°C

9. Oilgeneration Wet gas generation Potential oil generation AFTSolve: Modeled Thermal History (not reset) AFT reset AFT Age 103±6 Ma Onset of cooling 88±4 Ma Stratigraphic Age ~130 Ma

10. Oilgeneration Wet gas generation Lack of control prior to ~56 Ma AFTSolve: Modeled Thermal History (reset) AFT reset real / predicted AFT Age = 53±4 Ma / 53±4 Ma M.L. = 14.3±0.2 µm / 14.2±0.2 µm Onset of cooling = 56±5 Ma Stratigraphic Age ~180 Ma

11. Bowser Basin (Skeena FB) - regional stratigraphic framework - Bowser Basin (Skeena FB) - regional stratigraphic framework - Sustut Group coarsening upward deltaic to fluvial succession Bowser Lake Group coarsening upward deep marine to deltaic succession Hazelton Group volcanic and volcaniclastic strata Cretaceous Jurassic 0 1000 2000 3000 meters ~200 my ~175 my ~155 my ~144 my ~138 my ~75 my Tertiary Plutonic Rocks - Tertiary sedimentary rocks have been eroded from the Skeena Fold Belt - thus the majority of the post-Cretaceous geologic record has been deleted - thus the majority of the post-Cretaceous geologic record has been deleted - requires regional relationships and/or thermochronology to help - requires regional relationships and/or thermochronology to help constrain geologic history since ~75 Ma constrain geologic history since ~75 Ma

12. At least 2 distinct episodes of folding recognized: - BLG rocks folded pre-Sustat Basin deposition - older folding 95 Ma - Sustat Basin rocks themselves are folded - so younger episode of folding <75 Ma - Folds in Sustat Basin rocks truncated a by 53 Ma pluton - folds are >53 Ma Coast Plutonic Belt North American Craton Stikinia EW Intermontane Belt Skeena Fold Belt Bowser Basin SustatBasin Omineca Belt Rocky Mountain Fold & Thrust Belt Subducting Farallon/Kula plate beneath the Coast Complex Bowser Basin (Skeena FB) 1) regional relationships Bowser Basin (Skeena FB) 1) regional relationships - Diving force for deformation within the basin related to subduction of Pacific plates beneath North American Craton beneath North American Craton - Skeena Fold Belt has experienced ~50% horizontal shortening (initially >360 km wide)

13. Coast Plutonic Belt North American Craton Stikinia EW Intermontane Belt Skeena Fold Belt Bowser Basin SustatBasin Omineca Belt Rocky Mountain Fold & Thrust Belt Subducting Farallon/Kula plate beneath the Coast Complex Rapid cooling at some time between: ~100-80 Ma ~65-60 Ma ~45-35 Ma ~10-8 Ma Rapid cooling at some time between: ~100-80 Ma ~65-60 Ma ~45-35 Ma ~10-8 Ma Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma Bowser Basin (Skeena FB) 2) regional AFT results Bowser Basin (Skeena FB) 2) regional AFT results ??

14. AFT Samples - locations - On GSC Thermal Maturity Base* Maturity Base* - locations - On GSC Thermal Maturity Base* Maturity Base* * GSC Report 4343 Evenchick et al. (2002) 1) Triangle Zone & Upper Sustut Basin Upper Sustut Basin 2) Groundhog coalfield 3) Mount Ritchie 1) Triangle Zone & Upper Sustut Basin Upper Sustut Basin 2) Groundhog coalfield 3) Mount Ritchie

15. Triangle Zone / Upper Sustut BLG BPF BLG

16. AFT Age 59±3 Ma Onset of Cooling 62±3 Ma AFT Age 59±3 Ma Onset of Cooling 62±3 Ma AFT Age 40±4 Ma Onset of Cooling 46±4 Ma AFT Age 40±4 Ma Onset of Cooling 46±4 Ma AFT Age 40±3 Ma Onset of Cooling 44±3 Ma AFT Age 40±3 Ma Onset of Cooling 44±3 Ma - Significant Eocene cooling - Note recent cooling Paleocenecooling Triangle Zone / Upper Sustut Notallreset

17. GroundhogGroundhog BLG BLG BLG

18. GroundhogGroundhog AFT Age 49±4 Ma Onset of Cooling 59±4 Ma AFT Age 49±4 Ma Onset of Cooling 59±4 Ma AFT Age 35±2 Ma Onset of Cooling 44±4 Ma AFT Age 35±2 Ma Onset of Cooling 44±4 Ma AFT Age 45±3 Ma Onset of Cooling 48±4 Ma AFT Age 45±3 Ma Onset of Cooling 48±4 Ma - Significant Eocene cooling - Note recent cooling

19. Mt. Ritchie BLG BLG a-3-J BLG

20. a-3-J AFT Age 38±3 Ma Onset of Cooling 41±3 Ma AFT Age 38±3 Ma Onset of Cooling 41±3 Ma AFT Age 41±5 Ma Onset of Cooling 43±4 Ma AFT Age 41±5 Ma Onset of Cooling 43±4 Ma AFT Age 46±5 Ma Onset of Cooling 49±5 Ma AFT Age 46±5 Ma Onset of Cooling 49±5 Ma - Significant Eocene cooling - Note recent cooling

21. Mt. Ritchie No known tectonic mechanism to justify higher paleogeothermal gradients - thus we prefer the constant gradient solution (A)

22. TimingTiming Western Bowser Northern Bowser Constant Values: ~65-60 Ma ~45-30 Ma ~10-5 Ma Constant Values: ~65-60 Ma ~45-30 Ma ~10-5 Ma

23. Coast Plutonic Belt North American Craton Stikinia EW Intermontane Belt Skeena Fold Belt Bowser Basin SustatBasin Omineca Belt Rocky Mountain Fold & Thrust Belt Subducting Farallon/Kula plate beneath the Coast Complex Bowser Basin (Skeena FB) - regional AFT results - Bowser Basin (Skeena FB) - regional AFT results - Rapid cooling sometime: ~65-60 Ma ~45-30 Ma ~10-5 Ma Rapid cooling sometime: ~65-60 Ma ~45-30 Ma ~10-5 Ma Rapid cooling at some time between: ~100-80 Ma ~65-60 Ma ~45-35 Ma ~10-8 Ma Rapid cooling at some time between: ~100-80 Ma ~65-60 Ma ~45-35 Ma ~10-8 Ma Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma ?? Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma ?? Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma ?? Rapid cooling at some time between: ~65-60 Ma ~45-40 Ma ??

24. So what do AFT results suggest? - AFT results are interpreted to record rapid cooling from paleotemperatures ≥110-160°C (range based on different annealing resistivities from Dpar) - Cooling is interpreted to have occurred in association with km-scale denudation throughout the region during the Paleocene, Eocene, Miocene - Major episodes of denudation occurred within the Skeena Fold Belt after the last known phase of contractional deformation (between 75-53 Ma) - Results do not discount the possibility that minor reactivation within the fold belt might have occurred more recently than previously documented - AFT results are interpreted to record rapid cooling from paleotemperatures ≥110-160°C (range based on different annealing resistivities from Dpar) - Cooling is interpreted to have occurred in association with km-scale denudation throughout the region during the Paleocene, Eocene, Miocene - Major episodes of denudation occurred within the Skeena Fold Belt after the last known phase of contractional deformation (between 75-53 Ma) - Results do not discount the possibility that minor reactivation within the fold belt might have occurred more recently than previously documented Remember: - At least 2 distinct episodes of folding within the Skeena Fold Belt have been recognized: 1) >95 Ma, but 95 Ma, but <150 Ma - BLG rocks folded prior to Sustat Basin deposition 2) >53 Ma, but 53 Ma, but <75 Ma - Sustat Basin rocks folded, trucated by 53 Ma pluton

25. Furthermore:Furthermore: - Incorporating regional thermal maturity data (%VR) with the AFT results and paleogeothermal gradient information derived from the a-3-J well, suggests that at least 7.0 to 4.4 km of sedimentary section has been removed over sections of the Bowser Basin (assuming geothermal gradients of 31°C/km and 45°C/km respectively) - Hydrocarbon generation from potential source beds ceased with rapid cooling/denudation sometime between latest Cretaceous and Paleocene times - Petroleum generation ceased earlier in overmature regions - Incorporating regional thermal maturity data (%VR) with the AFT results and paleogeothermal gradient information derived from the a-3-J well, suggests that at least 7.0 to 4.4 km of sedimentary section has been removed over sections of the Bowser Basin (assuming geothermal gradients of 31°C/km and 45°C/km respectively) - Hydrocarbon generation from potential source beds ceased with rapid cooling/denudation sometime between latest Cretaceous and Paleocene times - Petroleum generation ceased earlier in overmature regions

26. Bowser Basin / Skeena Fold Belt

28. Temperature (°C) Time (Ma) 12 3 3 2 1 Number of tracks Track length (µm) Final distribution (at time 3) Fission Track Methodology - System Response 1 - Small Dpar TA* Large Dpar TA* * TA = Total Annealing (resetting)

29. Number of tracks Track length (µm) Final distribution (at time 3) 12 3 Fission Track Methodology - System Response 2 - Temperature (°C) Time (Ma) 3 2 1 Small Dpar TA* Large Dpar TA* * TA = Total Annealing (resetting)

30. Bowser Basin / Skeena Fold Belt Lunch? Time for a Canadian, eh?