GEOL 104 Dinosaurs: A Natural History

Fall Semester 2018
Deep Time: How Old is That Fossil?

Strata from the Upper Jurassic Morrison Formation to the Upper Cretaceous Frontier Formation, Bighorn Basin, WY

Key Points:
•Relative time represents the sequence of events; numerical time is the statement of dates or durations in terms of actual measured units (years, etc.).
•Geologic time is an example of "deep time": the history of the Earth is incredibly long compared to our personal experience, being measured in millions and billions of years.
•Because they naturally form strata, the relative sequence of time in sedimentary rocks are relatively straight forward to work out.
•The physical stratigraphy (position of different strata of a given spot) allows one to figure out the sequence of oldest to youngest event at this spot.
•Correlation from one spot to another can be done by tracing out particular beds (formations), assuming the two spots are physically close; for correlation over longer distances, methods such as biostratigraphy are needed.
•Biostratigraphy uses the sequence of index fossils through different strata as markers of time.
•The geologic time scale was initially developed using index fossils. It divides up the history of the Earth into Eons, which are subdivided into Eras, which are broken up into Periods, which are divided into Epochs, which are spit into Ages (or Stages).
•The best estimates of numerical time come from radiometric decay. Some naturally occurring isotopes change from the parent material into the daughter product at a constant rate of decay; comparing the ratio of daughter to parent alows you to calculate the age of the rock.
•However, radiometric dating works only for igneous rocks. So you can use volcanic ash beds, lava flows, and igneous intrusions to bracket the age of the fossil bearing strata, but rarely directly date the fossils themselves.
•Other methods of determining the age of rocks include using marker beds (that document single widespread events) or tracing the flip-flop of the Earth's magnetic field over time.

Deep Time: Ruins of a Former World

"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 new 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:

No one region has a continuous sequence of time. Any given location has likely had periods of non-deposition or erosion, which would leave gaps in the geological and fossil record at any given spot.

An interactive project on geologic time, for those who want to explore in more detail.

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 252.17±0.06 Ma (million years ago); the Triassic-Jurassic boundary is 201.3±0.2 Ma, the Jurassic-Cretaceous boundary is 145.0 Ma, and the Mesozoic-Cenozoic boundary is 66.0 Ma. (These represetne recent recalibrations; many texts and figures show slightly different numbers for these based on pre-2013 calculations.)

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.

Here is a nice set of graphics to put the scale of geologic time in perspective.

A relevant video:

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Last modified: 9 August 2018

The boundary between the Jurassic Morrison Formation (lower) and the Cretaceous Yellow Cat Member of the Cedar Mountain Formation (upper) in Utah.