The Rock Record: Determining Relative Age Chapter 8

Rocks Tell a Story!!!

How do we know what happened? Relative Dating – tells which one is older, not actual age Absolute Dating – actual age

Determining Relative Age James Hutton discovered that the present is the key to the past. Principle of Uniformitarianism:

Layers of rock called strata tell the story of what came first. A relative age (older vs. younger) can be determined with a few simple laws.

Law of Original Horizontality Sedimentary rocks left undisturbed will remain in horizontal layers.

Law of Inclusions Rock fragments (in another rock) must be older than the rock containing the fragments.

...granite is older. ...sandstone is older.

Law of Crosscutting Relationships A fault or an intrusion is always younger than the rock layers it cuts through.

UNCONFORMITIES Gaps in the geologic record when deposition stopped, rock was eroded away, and then deposition restarted. There are 3 types: 1. Angular Unconformity 2. Disconformity 3. Nonconformity

Angular Unconformity Boundary between a tilted layer and a horizontal layer

Disconformity Nearly horizontal Boundary between layers of rock that have not been deposited continuously A large time gap exists.

Nonconformity Boundary where stratified (layered) rock rests on unstratified rock

Possible Solution 1. Deposition of shale 2 Possible Solution 1. Deposition of shale 2. Deposition of conglomerate 3. Deposition of sandstone 4. Deposition of lithic sandstone 5. Deposition of limestone 6. Deposition of quartz sandstone 7. Deposition of limey shale 8. Deposition of mudstone 9. Intrusion of granite. This may have occurred earlier. However, as granites are emplaced at depth, it is assumed all the sequence was deposited and lithofied prior to emplacement. 10. Uplift and erosion to form current land surface. 11. Deposition of alluvium

Possible Solution 1. Deposition of conglomerate 2 Possible Solution 1. Deposition of conglomerate 2. Deposition of sandstone 3. Deposition of limestone 4. Deposition of shale 5. Deposition of mudstone 6. Faulting 7. Intrusion of gabbro

Possible Solution 1. Deposition of Quarry Shale 2 Possible Solution 1. Deposition of Quarry Shale 2. Deposition of Cave Bluff Limestone 3. Deposition of Redin Sandstone 4. Deposition of Tinely Ashstone 5. Deposition of Clipton Volcanics 6. Folding 7. Intrusion of basalt dyke 8. Faulting

Possible Solution 1. Deposition of Davin Shale 2 Possible Solution 1. Deposition of Davin Shale 2. Deposition of Hutt Ashstone 3. Deposition of Cabin Limestone 4. Deposition of Lower Coal Seam 5. Deposition of Ravin Sandstone 6. Deposition of Blackman Shale 7. Deposition of Fossil Bed Limestone 8. Folding 9. Uplift and erosion (forming unconformity surface) 10. Deposition of Basin Conglomerate 11. Deposition of Log Creek Mudstone 12. Deposition of Loren Shale 13. Deposition of Jackson Sandstone 14. Intrusion of basalt dyke 15. Tilting 16. Erosion to form present landscape.