Download Link to Material for Relative Dating (Dating Principles)

UNIFORMITARIANISM
Pennsylvanian,
Lookout Mt.
Recent
Mid Ordovician,
Pigeon Mt.
Recent
Late Ordovician,
Taylor Ridge
Recent
“THIN SECTION”
GRAINS
+/- PORES
CEMENT
Precambrian Sandstone,
San Juan Mts, CO
Between Silverton and
Durango
These fossils were all
found within about 50
miles of Americus. All
have living relatives,
most of them very
close relatives.
Their relatives all live
in a particular
environment which is
not like the
environment that
presently exists in SW
GA.
Furthermore, no
relative of any of
these is physiologically
capable of living in the
sort of environment
presently occurring in
SW GA.
These two rocks are mined from Paleocene and Eocene strata near Andersonville in Sumter and
Macon Counties. Both require intense chemical weathering of aluminosilicate minerals like
feldspar to form. The required intensity only occurs in tropical rainforests. The closest tropical
rainforest is in Jamaica or Cuba. Explain.
INCLUSIONS
A Garden Spider (Argiope
aurantia) dispatching a wasp on
my front porch.
Solid piece of
acrylic
with a
solid Garden
Spider inside it.
Glossopetrae
(“tonguestones”) were well
known objects in Medieval
Europe. They were lumped
as “fossils”, which comes
from the Latin root that
translates “excavated”,
along with other curiously
shaped things dug from the
Earth. Well formed crystals,
for example, were “fossils”.
Tonguestones were thought
to be simply odd
concretions that had
formed in the
rocks in roughly the
shape of a tongue.
From Steno, 1669
We know a little more
about them now.
Shark tooth partially
excavated from a piece
of Eocene Limestone.
Morocco.
(Collector unknown).
Nicholas Steno (Nils Stensen)
was the first educated
European to see inside a
shark’s mouth and realize what
a “tonguestone” is. He
published his observations (and
a lot of others that sprang
from them) in 1669.
From Steno, 1669
Solid piece of
sandstone
with a
solid quartz grain
inside it.
Solid piece of
conglomerate
with
solid quartz
gravel inside it.
2.5cm (1”)
Moxie Bald, ME
Solid mass of
granite
with a
solid mass of baked
mudstone inside it
(a “xenolith”).
Near Elberton, GA
Solid mass of
granite
with a
solid mass of
gneiss inside it (a
“xenolith”).
(Water leaking from fracture porosity.)
Original Horizontality
Angle of repose:
Damp Sand
Dry Sand
Saturated Sand
Most sedimentary rocks are marine. Why?
(This means they were deposited saturated with water).
Canyonlands N.P., UT
Grand Canyon, AZ
These beds tilt underneath those beds!
Horizon
Bed Surface
(Not horizontal!)
UP
Grand Canyon, AZ
Lookout Mt, near Cooper Heights, Walker Co., GA
Franklin Mts, El Paso, Texas
New Creek, WV
You are looking directly at the top of a package of sandstone beds at
Seneca Rocks, near Franklin, WV. Which way is “up”?
Which way is “up”?
Pinto, MD
Vertical
Rocks like these are
widespread in PA,
MD, WV, and VA.
This easily
recognizable
package of
sediments is called
“Tonoloway
Limestone”, named
for Big Tonoloway
Creek in MD where
they were first
described.
Which way is “up”?
UNDERLYING Rose
Hill Sandstone lies in
this direction along
the outcrop.
Which way is up?
Recent stromatolites
dome upward.
Fossil stromatolites
presumably did as well
when they were alive.
The beds are
slightly
“overturned”.
horizontal
“Fold” between Huntersville and Minnehaha Springs, Pocahontas CO, WV.
Germany Valley, near Franklin, WV, ~50km (35 mi) north of Minnehaha Springs.
Tuscarora Sandstone
on
North Fork Mountain
The shape of asymmetric ripples
makes it easy to tell which way was
up when they formed.
You are looking at the tops of these
ripples.
Which way is “up”?
San Juan Mts, CO
Original Lateral Continuity
Navajo Sandstone (Jurassic)
Wingate Sandstone (Triassic)
Moenkopi Shale (Triassic)
Cutler Formation (Permian)
Canyonlands N.P., UT – Bed continuity has been disrupted by erosion.
Kaibab Limestone
Redwall Limestone
Tapeats Sandstone
Grand Canyon, AZ. The beds all along either side of the canyon match.
The erosion that created the canyon has “cut through” them.
Germany Valley, near Franklin, WV, ~50km (35 mi) north of Minnehaha Springs.
Originally a continuous layer, the Silurian
Tuscarora Sandstone
has been breached and removed by
erosion, along with some of the underlying
Ordovician rocks.
Erosion has truncated the beds
I-68 cut at Sideling Hill, near Hancock, MD. Photo by Chelsea Carter
Sideling Hill
Lookout Mt, near Cooper Heights,
Walker Co., GA
Pinto, MD
Faults (dashed lines) will
also disrupt the continuity
of (or “cut across) things
that were present when
the fault moved. Arrows
indicate beds that were
originally connected but
were offset by the faults.
?
Eocene basaltic dikes cutting across Cambrian limestone.
Highland Valley, VA
Basaltic dike of Triassic (Mesozoic) age cutting across Paleozoic Granite.
Near Elberton, GA
The same dike as in the previous photo cutting across a slightly older basaltic dike.
Superposition
6. Glue with blue dye was added to the coarse sand.
It infiltrated the pores and cemented the sand. (It
also seeped downward into the permeable fine
sand and partially stained that sand).
5. Coarse sand with fine gravel was added atop the
medium sand.
4. Un-dyed glue was added to the fine sand,
partially filling the pores and cementing it.
3. Medium grained white sand was added, forming
a flat layer.
2. Dilute glue was added, partially filling the pores
of the sandy clay and cementing the top.
1. Red sandy clay was added to the bottom of the
jar. Not surprisingly, it formed an ~flat layer.
Last formed layer
(youngest)
progressively
younger
First formed layer
(oldest)
Youngest
Kaibab Limestone (Permian)
Redwall Limestone (Mississippian)
Oldest
(Precambrian)
Grand Canyon, AZ
Tapeats Sandstone (Cambrian)
These beds tilt underneath those beds.
Therefore they are older!
Youngest
Oldest
Grand Canyon, AZ
MESOZOIC
PALEOZOIC
Grand Canyon, AZ
Younger
Younger
Younger
Younger
Younger
Younger
Cross-Cutting Relationships
The erosional surface cuts across the youngest (highest) rock in the canyon (Permian), so it must
be younger than that rock. In fact, the distant Mesozoic rocks are also eroded, so the surface
must be younger than that.
The limestone is Cambrian, so the dikes must be younger than that.
The granite is very late Devonian and/or very early Mississippian so the dikes must be younger
than that. The order of events is given on the diagram.
1
2
3
Basic Principles of Relative Dating:
• Uniformitarianism – The natural laws and processes in the universe are
constant through time. “The present is the key to the past”. (If it looks
like a ripple mark, it was a ripple mark).
• Original horizontality – Sedimentary rocks are laid down in essentially
horizontal layers. Lava flows, particularly those from fissure eruptions,
tend to follow this pattern as well. Most igneous rocks do not.
Metamorphosed sediments may preserve original sedimentary bedding.
• Original Lateral Continuity – Sedimentary layers typically do not form with
abrupt edges unless the sediment was filling a basin with a steep side. A
rock can thin to not thickness over some distance or it can gradually
change its character (from sand to mud, for example) but an abrupt
termination generally implies a later event (erosion, intrusion, or faulting)
that has truncated its original continuity.
Basic Principles of Relative Dating:
(continued)
• Inclusion – Any geologic feature or piece of a geologic feature that is
entirely included within a rock is necessarily older than the rock that
encloses it.
• The clasts (individual grains or pieces of sediment) in detrital rocks come
from parent rocks that are older than the sediment they are in.
• Xenoliths in an igneous rock are from the (pre-existing) host rock into
which the igneous rock was intruded or from some deeper, even older
layer through which the magma passed.
• Alterations of contact metamorphism can be thought of as the intrusions
heat “included in” the metamorphic rock.
Basic Principles of Relative Dating:
(continued)
• Superposition – Sedimentary rocks are deposited sequentially one atop
another. Therefore they are progressively younger toward the top of the
stack. Extrusive igneous rocks also follow this pattern.
• If rocks have been structurally shifted to a vertical position or overturned
then indicators of “original up” are helpful in age determination.
• Intrusive rocks may not conform. Original sedimentary layering in
metamorphic rocks may be recognizable, but the probability that they are
not still upright is quite high.
Basic Principles of Relative Dating:
(continued)
• Cross-Cutting Relationships – Erosional surfaces, igneous intrusions, and
faults all truncate the original continuity of sedimentary and other rocks.
Any of these that “cuts across” rocks and other geologic features is
younger than what it “cuts”.
• If one of these features does not cut a feature, but ends at that feature,
then the feature that fails to cut is older than what it fails to cut.
• Cutting features may cut sedimentary beds, igneous or metamorphic rock
bodies, or older cutting features.