Notes on Blacktail Canyon Lecture

Introduction

• Location: Blacktail Canyon, Grand Canyon, Northern Arizona
• Presenter: Geology Professor Sean Bilsey
• Focus: Geologic features, specifically the Great Unconformity

Overview of Blacktail Canyon

• Narrow canyon carved by flash floods
• Contains impressive scenery and geology
• Access to hallmark geologic features

Tapeats Sandstone

• Main Unit: Tapeats Sandstone
  • Color: Beige to buff
  • Age: Cambrian (about 530 million years old)
  • Composition: Grains of sand, some pebbles
  • Depositional Environment: Coastal setting where rivers fed into the ocean
  • Features: Possible small crossbeds indicating river channel deposits

Vishnu Schist and Zoroaster Granite

• Underlying Unit: Vishnu Schist
  • Age: About 1.8 billion years old
  • High-grade metamorphic rock
• Intrusions: Zoroaster Granite
  • Intrudes into Vishnu Schist

The Great Unconformity

• Definition: Contact between Tapeats Sandstone and Vishnu Schist
  • Represents a significant period of erosion or missing rock layers
  • Known as the Great Unconformity (1.3 billion years of geological history)
• Implications:
  • Metamorphic rocks indicate formation at great depths beneath mountains
  • Erosion smoothed the landscape to allow for deposition of Tapeats Sandstone
  • Represents a drastic change in geological settings

Geological Processes

• Formation of Vishnu Schist: Created through heat and pressure deep within the Earth
• Erosion: Period needed to planar off mountains to allow for sedimentary deposits
• Importance of understanding how these processes shaped the Grand Canyon

Visual Observations

• Foliation in Schist: Vertical layering cut by quartz veins or dikes
• Contact View: Tapeats Sandstone directly on Vishnu Schist
• Sand Lens: Observed within the base of the Tapeats Sandstone
• Additional Remarks: Folded or bent foliation observed in some metamorphic rocks

Rafting and River Dynamics

• Flash floods push rock and debris into the Colorado River, creating rapids
• Water constriction increases flow speed, contributing to river channel irregularities

Conclusion

• Blacktail Canyon offers an exceptional view of the Great Unconformity
• Importance of preservation for geological education
• Encouragement for further support and engagement with geology education through donations and social media engagement.