GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:
The Fossil Record of Vanished Worlds of the Prehistoric Past

Spring Semester 2017

Clocks in the Rocks: Geologic Time

Relative & Numerical Time
"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.

Steno, Hutton, & Physical Stratigraphy
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). 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). These form the basic Principles of Stratigraphy. The first three principles were developed by Niels Stensen (better known as Nicolaus Steno):

As Steno and others mapped out strata, they found that sometimes there were types of breaks (discontinuities) in the layers. These are called unconformities, and represent gaps in the rock record (periods of erosion and/or non-deposition). James Hutton studied these and recognized that they represented aspects of relative time.

From unconformities, Hutton added additional Principles of Stratigraphy:

(Here is a great blogpost about Siccar Point and Hutton's discoveries.)

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? That is, how could one correlate between locations?

Formations, Lithostratigraphy, and Regional Correlation
Packages of similar strata formed over a region are called formations. Each represents a unit of rock produced by the same conditions (environment) and having the same history (produced over a particular sequence of time). At any given spot, if we see a section through the bedrock, we can see the transition from one formation to another, representing a transition from one environment to another.

Recognizing and defining formations is one of the main tasks of the discipline of lihthostratigraphy. 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. In the other direction, formations may be subdivided into members and beds.

Mapping out formations, groups, supergroups, members, and beds, geologists could connect sequences of rocks across regions. By the principle of lateral continuity, the formation will extend out to the edge of the depositional basin (or at least as far as that set of environmental conditions extend.) This allows for regional correlations, but what about across continents and oceans?

Index Fossils, Biostratigraphy, and Global Correlation
Needed something that had a particular non-repeating, unique, global pattern.

William "Strata" Smith, creating first geologic maps of southern England (and expanded out to include the Continent) observed that the pattern of fossils through the strata was consistent from location to location. Developed this into a new stratigraphic principle:

In order to be an index (or guide) fossil, the organism used must have certain desirable features:

The method of using index fossils to correlate rocks is called biostratigraphy. Here is an excellent summary of biostratigraphic correlation.

In combination, the principles of stratigraphy were useful for determining a global relative time scale, but questions of numerical time were still unresolved.

The Geologic Timescale
Using index fossils, geologists were able to correlate across Europe, and then to other continents. During the 19th Century, geologists created a global sequence of events (based on the sequence of (originally 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 (or Stages). No initial understanding of the scale of numerical time for these different units.

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:

Expanding out the Phanerozoic, we can see the different Periods within these three Eras:

We are currently in the Quaternary Period of the Cenozoic Era, and the Holocene Epoch of the Quaternary Period.

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.

Radiometric Dating
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.

Various early scientific attempts to determine the age of the Earth,included such approaches as:

Additionally, attempts were made to date particular rocks (i.e., by comparing the sedimentation-rate based time scale or the cooling-based time scale, and estimating what percentage far back a given rock came from), or the duration over which deposits were generated (for environments with annual deposition, like some lakes and glaciers, this was a matter of counting.)

How to reconcile the sedimentation-based scale (needing 100s of millions of years) with the cooling-based scale (limited to <90 million years?. The discovery of radioactive decay at the dawn of the 20th Century gave the key:

Radiometric Dating: the single most important method of determining numerical rock ages:

Traditional radiometric dating needs some special conditions, however:

When possible, radiometric dates of different isotopes with different decay rates are calculated for same sample. If these converge, good support for that age.

In order to get around the issue the requirement of zero initial daughter product, mid-20th Century geochemists developed the actual version of radiometric dating that is currently used by geologists: the isochron method. We won't go into great detail for this in this particular course, but in brief it requires sampling not just the parent (P) and daughter (D) products of a radioactive decay series, but also a stable (non-radioactively-generated) isotope of the same element as D (Di). By comparing the ratios D/Di vs. P/Di, different minerals in the same rock will plot along a particular line (the isochron), the slope of which is scaled to the number of half-lives the rock has gone through. Scatter of plots around the line is a measure of how much contamination or loss there has been of materials, and thus the degree of confidence we have in the measure.

Using the radiometric methods of dating, geologists have estimated the ages of the various boundaries of the Geologic Timescale.

Radiocarbon Dating
Of special note is radiocarbon dating, a special case of radiometric dating. Carbon comes in three isotopes: the common stable 12C, the very rare stable 13C, and the radioactive 14C. 14C decays into 14N with a half-life of 5730±40 years, MUCH faster than the series typically used in geology. However, it has great utility in archaeology and in the very youngest parts of paleontology. It has the advantage over standard radiometric dating in that it actually dates the fossils themselves, and not simply the rocks in which they are deposited.

Living things take in 14C during life, but not when they die: from that point on, it will simply decay away.

Of special note with radiocarbon: studies showed that the amount of 14C in the atmosphere (and thus the amount that organisms take up) has varied over time, requiring a calibration curve to convert from the amount of radiocarbon to the number of calendar years.

Other Methods
Additional methods of relative dating have been developed, which can be incorporated into the Geologic Timescale:

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Last modified: 7 January 2017