Lecture Notes: Stratigraphic Principles and Geological History

Key Example: Capitol Reef, Utah

• Demonstrates geological layers
• Layers visible as bottom, middle, and top
• Sequential order: Bottom layer (A) is oldest, followed by middle (B), then top (C)

Stratigraphic Succession

• Records chronological geological history in vertical strata
• Includes sedimentary and extrusive igneous rocks (e.g., basalt in Columbia River Basalts)
• Columbia River Basalts: Multiple lava flows visible in layers

Laws of Stratigraphy

Original Horizontality

• Sediments are deposited in horizontal layers
• Assumes undisturbed horizontal bedding

Superposition

• In undisturbed sequences, each layer is younger than the one beneath it
• Example: Grand Canyon, where higher layers are younger

Law of Inclusions

• Rock containing an inclusion is younger than the inclusion
• Inclusion must pre-exist to be part of another rock
  • Example: Xenoliths in sills

Cross-Cutting Relations

• Disturbances like faults, dikes, and sills
• Cutting feature is younger than the rock it penetrates
  • Example: Dikes penetrate pre-existing rock

Baked Contacts

• Thermal metamorphism from igneous intrusion
• Baked rock must pre-exist the intrusion
• Example: Pluton intruding and cooling, creating baked contacts

Principle of Lateral Continuity

• Layers extend laterally until interrupted by erosion or other features
• Example: Grand Canyon, where matching layers are separated by erosion but remain continuous

Examples and Applications

• Grand Canyon: Demonstrates superposition and lateral continuity
• San Diego: Field examples of cross-cutting relations

These notes cover the key principles of stratigraphy, relating to both sedimentary and igneous rock formations, and their implications for interpreting geological history.