GEOL 104 Dinosaurs: A Natural History

Fall Semester 20056
Deep Time I: Dating the fossil record

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

Two different aspects of time to consider:

Relative time was determined LONG before absolute time.

Sedimentary rocks naturally form horizontal layers (strata, singular stratum). 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):

Use these principles to figure out time sequence in any particular section of rock. BUT, how to 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:

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 Time Scale. Became a "calendar" for events in the ancient past: used to divide up time as well as rocks.

Geologic Column divided into a series of units: from largest to smallest Eons, Eras, Periods, Epochs, Ages.

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:

The Mesozoic Era is divided into three periods:

Deep Time II: Millions of years ago...

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 various techniques:

Radiometric dates reveal the Paleozoic-Mesozoic boundary is 2510.4 Ma (million years ago); the Triassic-Jurassic boundary is 199.60.6 Ma, the Jurassic-Cretaceous boundary is 1452.0 Ma, and the Mesozoic-Cenozoic boundary is 65.500.3 Ma.

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.

To Next Lecture.
To Previous Lecture.
To Syllabus.

Last modified: 14 July 2006