Geology Reference
In-Depth Information
For example, the Cambrian Period is defined as the time
during which strata of the Cambrian System were deposited.
are oldest in the center at its base and progressively younger
as they move outward (principle of superposition). Using
techniques based on ratios of uranium 234 to thorium
230, geologists can achieve very precise radiometric dates
on individual layers of a stalagmite. This technique enables
geologists to determine the age of materials much older than
they can date by the carbon-14 method, and it is reliable
back to approximately 500,000 years.
A study of stalagmites from Crevice Cave in Missouri
revealed a history of climatic and vegetation change in the
midcontinent region of the United States during the interval
between 75,000 and 25,000 years ago. Dates obtained from the
Crevice Cave stalagmites were correlated with major changes
in vegetation and average temperature fl uctuations obtained
from carbon-13 and oxygen-18 isotope profi les, to reconstruct
a detailed picture of climate changes during this time period.
Thus, precise dating techniques in stalagmite studies
provide an accurate chronology that allows geologists to
model climate systems of the past and perhaps to deter-
mine what causes global climate changes and their duration.
Without these sophisticated dating techniques and others
like them, geologists would not be able to make precise
correlations and accurately reconstruct past environments
and climates. By analyzing past environmental and cli-
mate changes and their duration, geologists hope they can
use these data, sometime in the near future, to predict and
possibly modify regional climate changes.
Given the debate concerning global warming and its
possible implications, it is extremely important to be able
to reconstruct past climatic regimes as accurately as possible
(see Geo-Focus on pages 460 and 461). To model how
Earth's climate system has responded to changes in the past
and to use that information for simulations of future climate
scenarios, geologists must have a geologic calendar that is as
precise and accurate as possible.
New dating techniques with greater precision are
providing geologists with more accurate dates for when
and how long past climate changes occurred. The ability to
accurately determine when past climate changes occurred helps
geologists correlate these changes with regional and global
geologic events to see if there are any possible connections.
One interesting method that is becoming more common
in reconstructing past climates is to analyze stalagmites from
caves. Recall that stalagmites are icicle-shaped structures
rising from a cave floor and formed of calcium carbonate
precipitated from evaporating water. A stalagmite therefore
records a layered history because each newly precipitated
layer of calcium carbonate is younger than the previously
precipitated layer (
◗
Figure 17.29). Thus, a stalagmite's layers
Stratigraphic
Section
Biostratigraphy
Radiometric
Dates
more strata above
160m
140m
Baculites
eliasi
zone
Sandstone
120m
Baculites
jenseni
zone
Baculites
reesidei
zone
"Snakebite 1"
volcanic ash
100m
feldspar, biotite, and zircon crystals
K/Ar date: 72.5
±
0.2 Million years ago
U/Pb date: 72.4
0.4 Million years ago
Rb/Sr date: 72.54
±
0.18 Million years ago
feldspar, biotite, and zircon crystals
U/Pb date: - strongly discordant - samples
probably contain inherited radiogenic lead
Rb/Sr date: 73.65
±
concretion bed
80m
"Snakebite 2"
volcanic ash
Baculites
cuneatus
zone
60m
±
0.59 Million years ago
40m
Sandstone
Baculites
compressus
zone
20m
Mudstone
0m
Sandstone
more strata below
◗
Figure 17.28
Rocks and Fossils of the Bearpaw Formation in Saskatchewan, Canada The column
on the left shows formation and members that are lithostratigraphic units. Notice that the biozone
boundaries do not correspond with lithostratigraphic boundaries. The absolute ages for the two volcanic
ash layers indicate that the
Baculites reesidei
zone is approximately 72 to 73 million years old.
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