Relative dating methods and the time divisions of Earth history Geology 103

Two types of dating Relative dating asks “Is a given event older or younger than another event?” Numerical (absolute) dating asks “How many years ago did an event take place?”

What is an “event”? A discrete occurrence that can be inferred from the rock or fossil record Examples: the deposition of a sedimentary layer, or the death of an organism

Relative dating Steno’s principles (1669) are all very common sense but do order strata from oldest to youngest Stratum = layer Strata = layers Stratigraphy = study of the order of events in sedimentary rocks The following three principles are grouped as “Steno’s principles”

Principle of original horizontality Gravity will “even out” any topographic highs and lows as sediment is deposited. Any distortion of sedimentary strata is due to deformation after the strata were deposited and lithified.

Principle of lateral continuity Strata are deposited in a basin and thus will be continuous from end to end Any breaks are due to erosion or deformation after deposition

Principle of superposition Younger strata overlie older strata True of all gravity- driven deposits

Principle of faunal succession Attributed to William Smith and George Cuvier (around 1800): There is an order to the fossils found in strata Smith and Cuvier did not think of evolution but the principle led Charles Darwin to think of how the order might arise

Principle of correlation Similar strata separated by gaps, were once part of a continuous layer Led to the idea of “formations”: a mappable unit of rocks sharing a common origin event Attributed to Smith in England, and Cuvier and Alexander Brongnairt in France.

First large-scale geologic map Though not the first geologic map, William Smith’s 1815 map covered a country-sized area and used fossils (as well as minerals and color) for correlating similar layers separated by erosion and distance. He even drew cross- sections to clarify where layers were in contact.

Principle of cross-cutting relationships Attributed to James Hutton, Theory of the Earth (1795) The body that cuts through is younger than the body that got cut through True for dikes, faults, joints and any non- concordant stratum

Principle of inclusions Attributed to Charles Lyell, Principles of Geology (1830) Sometimes called “xenoliths” The inclusion is older than the rock in which it is included

Geology’s big problem in the late 18 th century: where does granite come from? Neptunism (Abraham Werner): Deposited as sediment from oceans Plutonism (James Hutton): Erupted by ancient volcanoes

Which lead to the development of the geologic timescale This changed with Lyell, as geologists started using biological markers to demarcate time.

Modern geologic timescale

The modern geologic timescale Divided up by major changes in the fossils found in the strata (for the Phanerozoic Eon, anyway): mass extinctions mark the boundaries between time divisions Major chronological divisions: eon, era, period, epoch The basic problem is that rocks don’t care about life: that is, the same sediment will deposit before, during and after a mass extinction, as long as the depositional environment remains the same Thus, chronology is not lithostratigraphy, or one period ≠ one formation The end-Triassic extinction is in here, somewhere

Why it matters The diagram above shows a cross-section of a prograding (growing) delta: there are two depositional environments, the delta front (yellow) and the area in front of the delta front (brown). Each would be mapped as a separate formation but note that each contains within it several time divisions.

Unconformities An unconformity is a time gap in the rock record Classified into different types, but all represent significant missing time So if there can be many time divisions within one formation, is it possible that there are missing time divisions in a set of layered rocks? YES!

John Wesley Powell’s “Great Unconformity” at the Grand Canyon The angular unconformity between the tilted Grand Canyon Supergroup rocks (740 – 1200 Ma) and the horizontal layered Paleozoic rocks (<525 Ma)

But there’s an even larger unconformity! There’s a nonconformity between the schist (metamorphic rock) and the overlying sandstone (sedimentary rock)