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Relative Dating Unit B Geologic Time Part I: Relative Dating Unit B Unit A James Hutton conceived of the Principle of Uniformitarianism at Sicar Point, Scotland in 1785. Think about how much time would pass to create the rock record shown above. What sequence of events would occur to produce the above record? Unit B Unit A 1. Deposition of sediment comprising Unit A (Sedimentation rates in the deep ocean range from .0005 - .001 mm per year). How long would it take to accumulate 200 meters of sediment?) 2. burial compaction & lithification of Unit A, 3. deformation (folding) of Unit A, 4. uplift and erosion of Unit A, 5. Deposition of sediment (Unit B), 6. burial compaction & lithification of Unit B, 7. deformation (folding) of Unit B, 8. uplift and erosion of Unit B. These geologic events require millions of years of time! Principle of Original Horizontality: Layered sedimentary rock is typically deposited horizontally, but can be deformed by tectonic processes. Can you think of a depositional environment where sedimentary layers are not deposited horizontally? Geologists use sedimentary structures to determine whether sedimentary layers or beds are right-side up, vertical or overturned. The Principle of Uniformitarianism states that the “present is the key to the past.” Those processes operating at the earth’s surface today are inferred to have operated in the past, such as the mudcracks forming in a playa lake today. Note the blowing sand in the background will fill in the cracks. The mudcracks shown above formed over 1.1 billion years ago in a dry lake basin are now found in Glacier National Park, Montana. Raindrop impressions are preserved in 1.1 billion year old sedimentary rocks in the Belt Group rocks of Montana. Why do you think raindrop impressions were preserved in this sediment? Think about the life record on Earth at this time. The Principle of Superposition states that younger sedimentary rock layers will overlie older sedimentary rock layers, providing the rock unit has not been overturned. The principle of Cross-Cutting Relations states that any structure or rock unit cross- cutting another structure or rock unit will be younger. The fault line shown above cross-cuts a glacial moraine. The seismic event that cause uplift along the fault post- dates the formation of the moraine? Based on this same logic which event is older, the offset along the fault or the stream deposition that covers the fault. C B A Which formed first and last? A. The micaceous schist, B. the plagioclase dike or C. the fault that offsets the dike? Use the principle of cross-cutting relations to figure out this mess! If you can numerically date the dikes, you can reconstruct the deformation history of this region of Washington. Which is older? The roof pendant comprised of contact meta-sedimentary rock or the plutonic rock? A B C The Principle of Inclusion states that the included body will be older than the rock containing the inclusion (in this case the clasts within the conglomerate). Which clast age will be the closest constraining age for the conglomerate? A (55 Ma), B (75 Ma), or C (100 Ma). Ma (million years). The Principle of Faunal Succession utilizes evolutionary changes in the preserved life record to assign relative ages to rock units. Trilobites were dominant during the Cambrian Period (543-490 Ma) and went extinct by 245 Ma. Radiometric dating techniques have been used to assign numerical ages to evolutionary time periods. Ferns first evolved during the Carboniferous Period (363-290 Ma) and were dominant terrestrial plants. Conifers, flowering plants and grasses evolved later. During the Mesozoic Era dinosaurs flourished on the continental land masses. This Allosaurus footprint impression is found in Jurassic aged rock. Diplodocus was the largest land animal to walk the surface of the earth. It measured 90 feet long from head to tail. It lived during the Jurassic Period. Horses evolved 55 million years ago during the Eocene Epoch of the Tertiary Period. Warm-blooded mammals dominate the Cenozoic Era. Geologic Time Scale Do not memorize the geologic time scale. Note that the major subdivisions such as eons represent major developments in the Earth’s history. The three eras (Paleozoic, Mesozoic and Cenozoic) are all contained within Phanerozoic eon (largely defined by fossil preservation in the life record). Periods are defined by major evolutionary changes, such as the evolution of early fish in the Ordovician period (505-435 Ma ago) of the Paleozoic era (545-245 Ma). Why do you think that the more recent portion of the geologic time scale has greater detail (defined by epochs)? Unconformities represent breaks or missing time in the rock or geologic record. Unconformities, such as the angular unconformity shown on the left, form because of erosional hiatuses or long periods on non-deposition between depositonal events. Think about Mercer Island. When where our glacial sediments deposited? About 15,000 years ago. When do you expect to see more sediment deposited over University Hill? 60,000 years in the future during the next glacial cycle. That’s 75,000 years between depositional events! Three major types of unconformities: unconformities represent missing time between the two rock units. 1. Nonconformity: A break separating older metamorphic or intrusive igneous rocks from younger sedimentary layers. 2. Angular Unconformity: Tilted or folded sedimentary rocks under more flat- lying layers 3. Disconformity: mostly horizontal break in sedimentary record: both layers above and below the unconformity are relatively horizontal. This type of unconformity can be very hard to recognize in the field. How could you determine in the field the difference between a nonconformity and an igneous intrusion contacting country rock? Disconformity Angular Unconformity Stages of formation of a disconformity and an angular unconformity. Formation of a Nonconformity A.Magma intruded into pre-existing sedimentary “country rock. Note that a contact metamorphic zone will form where the magma contacted and reheated the country rock. B. Uplift and erosion of country rock exposing the intrusive body to the surface of the earth. Note that country rock has been eroded. C. Deposition of new sedimentary layers over the eroded surface of the igneous (granitic) intrusion or metamorphic rock. D. How would you differentiate a nonconformity from a contact metamorphic zone (both have plutonic igneous rock adjacent to sedimentary rocks)? What type of unconformity is shown above? How do you recognize it? What type of unconformity exists at Sicar Point, Scotland, where James Hutton developed ideas related to the Principle of Uniformitarianism? Draw the unconformity. What type of unconformity exists at Sicar Point, Scotland, where James Hutton developed ideas related to the Principle of Uniformitarianism? Draw the unconformity. Is this where you would map it? What type of unconformity exists at the base of the Grand Canyon. The lower rock unit is crystalline plutonic and metamorphic. The rock unit lying above is layered meta-sedimentary deposited in coastal setting. You have already determined why disconformities are the most difficult unconformities to recognize in the field. What types of geologic evidence might you use to recognize disconformities? Fossil evidence or buried paleosols (old soils or weathering zones that require time to form prior to burial). The Grand Canyon has several unconformities recognized within its rock record. Five well-known unconformities are shown on the above cross-section of the Grand Canyon. Relative dating techniques are based on stratigraphic principles can be used to differentiate the relative age rock units and landforms. Relative dating techniques by themselves cannot be used to assign independent numerical ages. Numerical dating allows geologist to assign actual ages to a deposit or landform. Many of the methods are based on the ratio radioactive isotopes and their respective decay products measured in a material to be dated. In many cases relative dating techniques are used in tandem with numerical dating techniques to constrain the age of a landform or deposit. Unit A (225 m.y. old) Unit B (how old is it?) Unit C (275 m.y. old) .
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