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Discovering the Franciscan Complex in Grindstone Creek

Explore the exposed Franciscan Complex in the Grindstone Creek area, including unique geological formations and sediment gravity flows. Witness the diversity of volcanic origin and conglomerate layers in this Work in Progress study by Tom MacKinnon.

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Discovering the Franciscan Complex in Grindstone Creek

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  1. Franciscan complex exposed in Grindstone Creek Work in Progress Tom MacKinnon tom.mackinnon@comcast.net

  2. Grindstone Creek from Longpoint trail

  3. Grindstone Creek area

  4. Next slide

  5. ppsm 313 Ybc? Willows 1:100,000 Geologic map ppv2 Hardin Hole Ridge Upstream area ppv2 Long Point ppv3

  6. Graywacke and Argillite

  7. Conglomerate • ~ 325 meters thick • 100% exposure • Deposited by sediment gravity flows • Sediment is mainly of volcanic origin • ~10-15% is graywacke/argillite

  8. Near the base of the conglomeratic section

  9. Basal conglomerate

  10. Bulk of section consists of this lithology, may be red or green

  11. Poorly sorted pebbly siltstone/mudstone with a few clasts to 10 cm

  12. Typical conglomerate bed

  13. Top of Conglomeratic section Graywack- argillite Fault contact

  14. Volcanic sequence • ~ 400 meters thick • 100% exposure • Seamount origin? • Chert at top

  15. Entering the gorge at the base of the volcanic section

  16. Volcanic section

  17. Volcanic section

  18. Volcanic section Pillow lava?

  19. Chert at top of Volcanic section

  20. Pebble Conglomerate

  21. Pebble Conglomerate Pebble Conglomerate

  22. Pebble Conglomerate

  23. Pebble Conglomerate

  24. Hinge zone

  25. Isoclinal fold hinge zone

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