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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. Paul O’Sullivan - Apatite to Zircon, Inc. Fil Ferri - British Columbia Dept. Energy and Mines

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Project funded by the B.C. Dept. of Energy and Mines, and the Geological Survey of Canada

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  1. 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. Project funded by the B.C. Dept. of Energy and Mines, and the Geological Survey of Canada

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

  4. Apatite Fission-Track (AFT)Thermochronology Constrains maximum paleotemperatures plus timing and style of cooling from maximum paleotemperatures • 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 “time” of cooling ‘confined’ lengths “rate” & “style” of cooling annealing “resistivity” 1250x

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

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

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

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

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

  10. AFTSolve: Modeled Thermal History (reset) Stratigraphic Age ~180 Ma Oil generation AFT reset Wet gas generation 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 Lack of control prior to ~56 Ma

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

  12. Bowser Basin (Skeena FB) 1) regional relationships W E Intermontane Belt Skeena Fold Belt Bowser Basin Omineca Belt Coast Plutonic Belt Rocky Mountain Fold & Thrust Belt Sustat Basin Stikinia North American Craton Subducting Farallon/Kula plate beneath the Coast Complex - Diving force for deformation within the basin related to subduction of Pacific plates beneath North American Craton - Skeena Fold Belt has experienced ~50% horizontal shortening (initially >360 km wide) At least 2 distinct episodes of folding recognized: - BLG rocks folded pre-Sustat Basin deposition - older folding <150 Ma and >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

  13. W E Intermontane Belt Skeena Fold Belt Bowser Basin Omineca Belt Coast Plutonic Belt Rocky Mountain Fold & Thrust Belt Sustat Basin Stikinia North American Craton Subducting Farallon/Kula plate beneath the Coast Complex Bowser Basin (Skeena FB) 2) regional AFT results 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 ??

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

  15. Triangle Zone /Upper Sustut BPF BLG BLG

  16. AFT Age 59±3 Ma Onset of Cooling 62±3 Ma Triangle Zone /Upper Sustut Not all reset Paleocene cooling AFT Age 40±4 Ma Onset of Cooling 46±4 Ma AFT Age 40±3 Ma Onset of Cooling 44±3 Ma - Significant Eocene cooling - Note recent cooling

  17. Groundhog BLG BLG BLG

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

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

  20. Mt. Ritchie AFT Age 38±3 Ma Onset of Cooling 41±3 Ma AFT Age 41±5 Ma Onset of Cooling 43±4 Ma a-3-J 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. Timing Northern Bowser Constant Values: ~65-60 Ma ~45-30 Ma ~10-5 Ma Western Bowser

  23. W E Intermontane Belt Skeena Fold Belt Bowser Basin Omineca Belt Coast Plutonic Belt Rocky Mountain Fold & Thrust Belt Sustat Basin Stikinia North American Craton Subducting Farallon/Kula plate beneath the Coast Complex Bowser Basin (Skeena FB) - regional AFT results - Rapid cooling at some time between: ~100-80 Ma ~65-60 Ma ~45-35 Ma ~10-8 Ma Rapid cooling sometime: ~65-60 Ma ~45-30 Ma ~10-5 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? Remember: - At least 2 distinct episodes of folding within the Skeena Fold Belt have been recognized: 1) >95 Ma, but <150 Ma - BLG rocks folded prior to Sustat Basin deposition 2) >53 Ma, but <75 Ma - Sustat Basin rocks folded, trucated by 53 Ma pluton - 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

  25. 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

  26. Bowser Basin / Skeena Fold Belt

  27. 1 2 Final distribution (at time 3) 3 Fission Track Methodology- System Response 1 - Time (Ma) 1 3 Temperature (°C) 2 Small Dpar TA* Large Dpar TA* Number of tracks *TA = Total Annealing (resetting) Track length (µm)

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

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

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