GEOL 104 Dinosaurs: A Natural History

Fall Semester 2000
Deep Time: Dating the fossil record

“Deep Time”: analogy to “deep space”; the vast expanse of time in the (geologically ancient) past.

Many attempts at calculating age of the Earth:

Two different aspects of time to consider:

In the history of geology and paleontology, relative time was determined LONG before absolute time.

Sedimentary rocks, because they are deposited, naturally form horizontal layers (strata, singular stratum). Because of their layered form, strata allow geologists to determine relative time (that is, sequence of deposition of each layer, and thus the relative age of the fossils in each layer):

Using these principles, early geologists were able to figure out the sequence of events of deposition, the changing local environments, and the folding, faulting, igneous intrusions, etc. for any particular section of rock. However, how could they extrapolate the sequence at one section with the sequence at another?

In some cases, the particular rock type, color, sedimentary structures, and so on were the same in strata in nearby sections. These groups of strata were named formations, which represent units of rock produced by the same conditions (environment) and having the same history (produced over a particular sequence of time. Formations are given formal names (e.g., the Morrison Formation, the Hell Creek Formation, the Solnhofen Limestone, etc.). Sometimes groups of formations which lie directly on top of or next to each other are catalogued together as formal Groups, and sometimes groups which lie directly on top of or next to each other are placed into formal Supergroups.

By mapping out formations, groups, and supergroups, geologists could connect sequences of rocks across regions. But what about across continents and oceans?

Needed a method of correlation. Rock type doesn't work, because the same environment will produce the same rock type regardless of relative or absolute time. Fossils, however, were useful:

Fossils allowed correlation from continent to continent. Only certain types of fossils (called index fossils) were useful for correlation. To be a good index fossil, the species should:

Using index fossils, geologists were able to correlate across Europe, and then to other continents. Created a global sequence of events (based on the sequence of (mostly European) formations and the succession of fossils) termed the Geologic Column. Became a “calendar” for events in the ancient past: used to divide up time as well as rocks.

The Geologic Column is divided into a series of units. Each unit may contain smaller units, and may be part of larger units. The largest units are Eons: animal and plant fossils are mostly restricted to the last (most recent) Phanerozoic Eon (“visible life eon”). The Phanerozoic Eon is comprised of three Eras:

When these terms were named, no one had any idea how long (in numerical time) each era was. The boundaries between eras were defined by the disappearance (mass extinction) of many different groups of marine invertebrates.

Eras are divided into multiple Periods; periods are divided into multiple Epochs; and epochs are divided into multiple Ages.
The Mesozoic Era is divided into three periods:

Although the Geologic Column was developed as a relative time scale, geologists wanted to figure out the numerical age dates for Era-Era boundaries and other events.

Discovered radiometric dating.

Radiometric dates reveal the Paleozoic-Mesozoic boundary is 251±0.1 Ma (million years ago); the Triassic-Jurassic boundary is 200±0.4 Ma, the Jurassic-Cretaceous boundary is 142±2.0 Ma, and the Mesozoic-Cenozoic boundary is 65±0.1 Ma.

Other techniques are also used in global correlation:

Most effective approach in getting age dates for a fossil bed is to combine multiple techniques: get relative age relationships between local units, find index fossil ages for the sedimentary rocks, and radiometric and magnetic dates where possible.

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