Transcript Gamma Ray Logs

Stratigraphic Facies
and Geologic Time
Amantz Gressley, 1834, and the Jurassic Rocks of the
Jura Mountains between France and Switzerland
The Interpretation of Geologic History
Requires Knowledge of the Following
The Interpretation of Sedimentary Rocks
Requires Knowledge of the Following:
1. Sedimentary Rocks
1. Rock Classification
2. Igneous Rocks
2. Depositional Environments
3. Metamorphic Rocks
3. Sedimentary Structures
4. Origin and History of Life
4. Sedimentary Tectonics
5. Tectonics, including:
5. Sedimentary Facies and
Time
• Structural geology
• Plate tectonic theory
• Etc.
The core concept is tectonics since
nothing in geology makes sense except
in the light of tectonics
Abraham Gottlob Werner’s Geologic Time Scale
The Neptunist World View
Sea Level after deposition of the Primitive rocks
Sea Level after deposition of the Transition rocks
Stratified
Sea Level after deposition of the Stratified rocks
Transition
Primitive
Transported
Primitive – crystalline rocks, both igneous and metamorphic. Thought to represent first chemical
precipitates from a worldwide ocean.
Transition – stony, indurated stratified rocks such as graywacke, limestones, sills.
Stratified – obviously stratified fossiliferous rocks, thought to represent the first deposits after receding
of the worldwide oceans, formed by erosion of emergent mountains.
Transported – Poorly consolidated clays, sands and gravels. Thought to have been deposited after
final withdrawal of a worldwide ocean.
Volcanic – Younger lava flows associated with volcanic vents (added to the classification later as an
afterthought, lavas were thought to be local phenomena resulting from the burning of coal beds.
Layer Cake Stratigraphy
The study of rock strata, especially the
distribution, deposition, and age of
sedimentary rocks
P 126
Werner’s
theory
made
a
firm
prediction, that the same kinds of rocks
should have been laid down in the same
sequence all over the world.
The Facies Concept
It is not certain who first noticed that rocks were not layer cake.
Levoisier in 1789 is the earliest mention we have, but Amantz Gressley
coined most of the important concepts while working in the Jura
Mountains.
While describing the rocks he observed lateral changes in the
composition and described them with clarity calling these changes
facies. But, then later in his paper he spoke of facies changes “in the
vertical direction” meaning that the rocks were different vertically as
well as horizontally. This has led to ongoing confusion.
Perhaps a dozen different
concepts and definitions
about the facies have been
proposed. But, they all go
back to the two original ways
Gressley used the term – his
formal definition, and his
offhanded use of the term.
Two Facies Definitions
Definition One
The facies is the sum total of all the physical,
biological and chemical characteristics imparted to a
sedimentary rock at the time of deposition.
Definition Two
Facies are the many different sediments and
resulting rocks that form at the same time, but in
different depositional environments.
The Transition from Wernerian "Transition Rocks"
To the Lower Paleozoic Periods
By Sedgewich and Murchinson
Murchinson, 1835
Sedgewick, 1835
Charles Lapworth
1879
Ordovician
Cambrian
Sedgewick, 1835
Cambrian
overlap
Opps !
Silurian
Silurian
Murchinson, 1835
Pleistocene
Pliocene
Miocene
Eocene
Lyell 1833
D’Halloy 1822
Cretaceous
Gressley 1795
Jurassic
Alberti 1834
Triassic
Murchinson 1841
Williams 1891
Permian
Pennsylvan.
Williams 1891 Mississippian
Sedgewich & Murchinson 1839
Murchinson 1835
Devonian
Silurian
Lapworth 1879
Ordovician
Sedgewick 1835
Cambrian
Carbonif.
“Old Red ss”
Unstudied
Until
1830’s
The Transition from Wernerian "Transition Rocks"
To the Lower Paleozoic Periods
By Sedgewich and Murchinson
Adapted from Dott and Batten: Evolution of the Earth
Old Red Sandstone
Early Devonian fishes from the Old Red Sandstone of
Spitzbergen (Wood Ray Formation)
http://www.picturescape.co.uk/gallery%20page
s/gallery%20one/caldey%20sandstone.htm
http://www.picturescape.co.uk/gallery%20pages/gallery%20one/caldey%20sandstone.htm
Old Red Sandstone
The Old Red Sandstone exhibited many changes over short distances, with thinly layered
areas alternating with conglomerates and outstanding crossbedded sandstones.
http://virtual.yosemite.cc.ca.us/ghayes/Siccar%20Point.htm
Devonian Marine Rocks of Devon, England
The cliffs at Fremington are Devonian with Glacial beds on top of this, below the
Devonian beds follows the carboniferous beds. Both Upper and Lower Carboniferous
rocks have been found at Fremington, however it is suspected that some of these rocks
have drifted from up or down stream, this could explain why occasionally blocks of
Carboniferious limestone can be found.
http://www.ukfossils.co.uk/sec084c.htm
Devonian Marine Rocks of Devon, England
http://www.earthfoot.org/places/uk005.htm
After their work on the Cambrian and Ordovician – but before they had
their falling out over the overlap of their systems – Sedgewick and
Murchinson decided to tackle the problem of the Old Red Sandstone and
the marine bearing rocks of Devonshire exposed on opposite sides of
Bristol Bay.
Wales
Bristol Bay
Devonshire
http://www.camelotintl.com/heritage/counties/england/devon.html
http://www.devonshireheartland.co.uk/
Scenes of Devonshire, England
http://www.picturesofengland.com/Devon/pictures-1.htm
Scenes of Devonshire, England
http://www.picturesofengland.com/Devon/pictures-1.htm
The Problem
There are different rocks sandwiched between the
Silurian and Carboniferous rocks as found in Wales
and Devonshire.
Onlap (Transgressive) Sequences
Shifting Facies through Time
Time Rock Unit
Time Rock Unit
Time Rock Unit
Time Rock Unit
Time Rock Unit
Time Rock Unit
Far Shelf
limestone
Near Shelf
shale
Beach
sandstone
Beach moves farther away
Water gets deeper
Sediment becomes finer
FUS – Fining Upward Sequence
= Transgressive Sequence
Offlap (Regressive) Sequences
Shifting Facies through Time
Prograding Regression
Time Transgressive Rock Unit
Beach
sandstone
Near Shelf
shale
Far Shelf
limestone
Beach moves closer
Water gets shallower
Sediment gets coarser
CUS – Coarsening Upward Sequence
= Regressive Sequence
Transgressive Sequence
Far Shelf
limestone
Near Shelf
shale
Beach
sandstone
Beach moves farther away
Water gets deeper
Sediment becomes finer
Regressive Sequence
Prograding Regression
Time Transgressive Rock Unit
Beach
sandstone
Near Shelf
shale
Far Shelf
limestone
Beach moves closer
Water gets shallower
Sediment gets coarser
There are Facies, and then there are Facies
The facies is the sum total of all the
physical,
biological
and
chemical
characteristics imparted to a sedimentary
rock at the time of deposition.
Facies One
Facies are the many different sediments
and resulting rocks that form at the same
time, but in different depositional
environments.
A couple of hundred miles
The Problem
There are different rocks sandwiched between the
Silurian and Carboniferous rocks as found in Wales
and Devonshire.
FUS
CUS
FUS
CUS
FUS
CUS
http://instruct.uwo.ca/earth-sci/300b-001/
Transgressive Sequence in the
Grand Canyon of Arizona
TONTO GROUP
Cambrian Period, 500-520 Million Years Old, 1025 Feet Thick
Yellowish ledges on top, the Tonto Platform between, and brown cliff below
COARSENING
UPWARD
SEQUENCE
http://www.canyondave.com/TontoPg.html
Transgressive Sequence
Shown above is an example of a prominent transgressive surface, combined with a sequence
boundary. This surface separates underlying shallow subtidal carbonate from overlying deep
subtidal carbonate and mudstone. Note the pyritization, visible as a rusty stain, at this
surface. Photograph taken at the contact between the Upper Ordovician Carters Limestone
(below) and Hermitage Formation at the Nashville International Airport. This outcrop has
subsequently been removed and is no longer visible.
http://www.uga.edu/~strata/sequence/transgressivesurface.html
Transgressive Sequence
The next example of a transgressive surface separates underlying shallow
subtidal carbonate from overlying offshore mudstone. Photograph taken at
the basal contact of the Nolichucky Formation in southwestern Virginia.
Transgressive Sequence
http://www.bees.unsw.edu.au/future/geology.html
Regressive Sequence
Table mountain near Mitzpe Ramon, central Negev, Israel
http://www.geomorph.org/gal/mslattery/world.html
Regressive Sequence
http://www.geneseo.edu/~gsci/pages/department/information/brochure/brochure_department.html
Transgressive-Regressive
Sequences
http://www.geology.utoronto.ca/basinanalysis/photos.htm
The Fractal
Nature of
Transgression
and Regression
Universality
53
Properties of Complex Evolutionary Systems
Fractal Organization – Sea Level Changes
Relative Sea Level in Meters
Meter Changes Over 125,000 Years
pr esen t sea l ev el
0
-20
-40
-60
-80
-100
-20
-40
-60
-80
-100
-120
18
16
14
12
10
8
6
4
2
Time in Thousands of Years
-120
g l a cia t i on
100,000
50,000
en l a
Present
rge
to
Centimeter Changes Over 100 Years
Time in Years
8.0
0
rg
nla
et
o
4.0
Meter Changes Over 18,000 Years
-20
-40
-60
Sea Level in Centimeters
e
Relative Sea Level in Meters
Relative Sea Level in Meters
+20
Meter Changes Over 18,000 Years
0
annual mean
5 year running mean
0
-4.0
-8.0
-80
-100
-120
18
-12.0
1900
16
14
12
10
8
6
4
Time in Thousands of Years
2
1920
1940
Date
1960
1980
Universality
Properties of Complex Evolutionary Systems
Fractal Organization – Sea Level Changes
Centimeter Changes Over 100 Years
8.0
Sea Level in Centimeters
4.0
annual mean
5 year running mean
0
-4.0
-8.0
-12.0
1900
1920
1940
Date
1960
1980
patterns, within patterns, within patterns
Millimeter Changes Over 2 Years
15
_ 0.8mm/year
Average Rate = 3.9 +
Mean Sea Level in Millimeters
10
Periodic change
in mean sea level
5
0
-5
-10
-15
1993
1993.5
1994
Date
1994.5
1995
53
Hierarchy of Sequences
(All sequence orders may not be present in one section; depend on local
tectonics, depositional rates, etc.)
Order
Duration
Range
Probably Cause1 2
First Order
200 my
750 feet
Tectonic
Second Order
9-10 my
366 feet
Glacio-Eustatic
Third Order
1-2 my
200 feet
Glacio-Eustatic
Fourth Order
0.1-0.2 my
40 feet
Milkanovitch cycle
3
Fifth Order
.01-0.2 my
20 feet
Milkanovitch cycle
Graph to left takes up
Only this much time on the
Above graph
Relative Sea Level
Curves
Relative Sea Level Curves and
Constructive and Destructive
Interference
3rd order down, 4th order up;
muted sea level fall
3rd order up, 4th order down;
muted sea level rise
3rd order up, 4th order down;
muted sea level rise
Both curves go down;
exaggerated sea level fall
Sea Level Changes and Corresponding
Trangressions/Regressions are Fractal
Third Order Transgression . . . followed by . . . A Third Order Regression
Sea Level Changes and Corresponding
Trangressions/Regressions are Fractal
Third Order Transgression . . . followed by . . . A Third Order Regression
4th Order Regression . . . followed by . . . 4th Transgression. . . followed by . . .
4th Regression . . .
followed by . . .
4th Transgression
followed by . . .
4th Regression
Sea Level Changes and Corresponding
Trangressions/Regressions are Fractal
FUS
CUS
FUS
CUS
Figure 8 shows upward and seaward increase in depositional energy (yellow dotted and green areas), which is tied to increases in porosity and
permeability. The basal disconformity (wavy line) is the horizontal datum for the 3-D porosity and permeability models. The wedge shape of the
Sussex "B" interval results from reworking by currents of seaward margins of sand ridges, and landward redeposition of sediment. The blue-lined
areas are basal and landward low-depositional-energy facies; these exhibit low porosity, permeability, and petroleum production.
The disconformity at the top of the Sussex "B" sandstone is generally marked by a thin chert-pebble sandstone (figure 9A). Shading variation of the
quartz (figure 9B) results from fracturing of the grain in this cross-nicols photomicrograph view (light is transmitted differently due to rotation of the
crystal axes). Quartz grains that were incorporated from underlying sand-ridge sediments commonly exhibit early stages of diagenesis within marine
environments, primarily chamosite overgrowths under the quartz overgrowths. Grain-to-grain contacts within this facies are mainly point with lesser
long-straight contacts.
http://pubs.usgs.gov/dds/dds-033/USGS_3D/ssx_txt/depomod.htm
Correlation
Demonstrating the
Equivalency of
Stratigraphic Units
Equivalency may mean:
Lithologic:
Same rock unit
Paleontologic: Contain same fossils
Time:
Deposited at same time
Biostratigraphic Facies # 2
Facies are the many different sediments and resulting rocks that form at the same time, but in different depositional
environments.
Facies # 1
Facies are the many different sediments and resulting rocks that form at the same time, but in different
depositional environments.
1. Ways of Correlating - Lithologic
“Walking Out”
Physically tracing a bed from one place to another to insure it is in fact
the same rock unit; literally “walking it out.”
Or, tracing an outcrop down the highway. Can be done in many places in
the west where good exposure, and flat lying beds are easy to trace.
Grand Canyon
of Arizona
http://www.mongabay.com/external/grand_canyon_trouble.htm
http://www.ggl.ulaval.ca/personnel/bourque/s4/cambrien.pangee.html
http://www.raphaelk.co.uk/main/worldwonders.htm
http://www.jgk.org/maps/grand-canyon-large.html
The problem is, . . . Rocks are not always flat laying, and traceable at the
surface.
A cross section through the Harrisonburg and Bridgewater, Virginia area, showing a duplex “herd of horses.” The floor thrust is at the bottom of the
drawing just above the basement rocks. The North Mountain fault is the roof thrust. In between are a series of splay faults that isolate a series of
horses. Note the overturned anticline on the far left (west) side where the last ramp formed. From Gathright and Frischmann, 1986, Geology of the
Harrisonburg and Bridgewater Quadrangles, Virginia.
2. Ways of Correlating - Lithologic
“Key Beds”
Correlating by recognizing and identifying beds that are so distinctive you
always know them when you see them.
1. Distinctive lithology
2. Distinctive mineral assemblage.
3. Particular sedimentary structures.
“Key Beds”
The Chattanooga
Shale
http://www.uta.edu/paleomap/homepage/Schieberweb/summer_2000_field_work.htm
“Key Beds”
The iridium layer at the KT boundary
An analysis of the chemical
composition of this clay layer
shows that it contains a
relatively high concentration of
an element called iridium.
Iridium is rare in the Earth’s
crust, but more common
towards the Earth's centre, and
in space. It continually filters
down to earth from outer
space, and so a high
concentration of iridium is
usually an indication that the
sediment was deposited very
slowly, absorbing lots of iridium
over time.
http://www.bbc.co.uk/beasts/whatkilled/evidence/analyse1.shtml
http://c3po.barnesos.net/homepage/lpl/fieldtrips/K-T/day3/day3.html
http://www.athro.com/geo/trp/ktm/ktmain.html
The hill in the background of this photograph is known as Iridium Hill. The
bands on the side of the hill are layers of rock of different ages that span the
time of the extinction of the dinosaurs.
http://www.uhaul.com/supergraphics/crater/what-is-it2.html
http://www.student.oulu.fi/~jkorteni/space/boundary/
“Key Beds”
The Navajo Sandstone
http://www.mines.utah.edu/geo/about_ES/Geology/ZionGIFS/XbedSS.html
http://www.olympic.ctc.edu/class/dassail/CapReef.html
http://www.creationsafaris.com/crev07.htm
3. Ways of Correlating - Lithologic
“Position in Sequence”
Identifying a relatively nondescript formation, which could be confused
with other similar looking beds, by its relationship to other more distinctive
units.
Limestone
Cross Bedded Sandstone
Non-Descript Shale
Non-Descript Shale
Quartz Arenite
Arkose
4. Ways of Correlating - Lithologic
“Wire Line Well Logging”
Measuring geophysical properties of a rock as recorded by instruments
lowered down a well hole.
In logging the well four main types of equipment are
used: the downhole instrument (which measures the
data), the computerized surface data acquisition
system (to store and analyze the data), the cable or
wireline (which serves as both mechanical and data
communication link with the downhole instruments),
and the hoisting equipment to raise and lower the
instruments.
Resistivity Logs
Gamma Ray Logs
Acoustic Logs
http://www.bakerhughes.com/bakeratlas/about/log4.htm
“Wire Line Well Logging”
http://www.trianaenergy.com/ucwell/photos/march_26/march_26.htm
Geophysical logging involves lowering a series of probes
into drilled boreholes (or existing fractures or wells) as deep
as several thousands of feet into the ground. One type of
multiparameter probe that has been used in Maryland and
Delaware measures several characteristics of subsurface
properties, including natural gamma radiation, or a material’s
resistance to electric current, which is useful for finding a
good water-bearing sand aquifer for water-supply purposes.
Another type is an acoustic velocity probe, which works by
transmitting acoustic signals and recording the traveltime of
the acoustic wave from one or more transmitters to receivers
in the probe. The recorded information can be used to
measure porosity and calculate the material’s density. This
technique was used to determine the extent of jumbled
geologic strata caused by a crater impact at the mouth of the
Chesapeake Bay 30 million years ago. Another type of probe,
called an Acoustic Televiewer, transmits acoustic signals to
subsurface rock layers and uses state-of-the-art computer
software to convert the recorded data into an actual image of
the borehole. This image can be used to determine the
amount of water that could be extracted from individual
fractures in the rock formation.
Even though most of the parameters measured by these probes can only be determined in a newly
drilled “open” borehole, certain probes emit signals that can penetrate well casings, making it possible to
measure subsurface materials after a well is constructed. Gamma rays can travel through almost any type of
well casing, while an “induction” probe can measure conductivity electromagnetically through polyvinyl
chloride (PVC) casing. Other parameters, such as the borehole’s fluid temperature and conductivity, can also
be measured, making it possible to evaluate water quality. The flow direction of ground water can also be
determined with several types of probes. All of this equipment enables scientists to characterize the
properties of subsurface materials, improving our knowledge of what lies beneath the Earth’s surface.
http://md.water.usgs.gov/publications/fs-126-03/html/
A typical well logging arrangement and the resultant logs from two types of
tools, the GR and Resistivity Logs
http://www.brookes.ac.uk/geology/8345/8345welc.html#Wireline
Gamma Ray
Logs
One of the advantages of gamma ray
logs is that the gamma ray intensity
closely corresponds with texture of the
rocks.
Typically, gamma ray radiation is
higher with shales (because they have
radioactive K40 in them which
undergoes K to Ar decay.) Sandstones
tend to have a lower gamma radiation.
Thus, we can use the gamma ray log
as a proxy for texture of the sediment,
and this allows us to read them like a
strip log, obtaining information about
the energy of deposition.
http://www.kgs.ukans.edu/Dakota/vol3/fy89/app_b.htm
Gamma Ray Logs and Strip Logs
Low
Radioactivity
SANDSTONE
High
Radioactivity
SHALE
Very
rapid
FUS
Gamma Ray
Trace from
Well log
Rapid FUS is a rapid rise in
sea level. They are
parasequence boundaries
used for correlation.
Converted into a
Stratigraphic
Strip log
Coarsening
Upward
Sequence
Observe that gamma ray
strip logs are the mirror
image of a regular strip
log where texture
increases to the right.
Gamma Ray Strip Logs
Vary with Depositional Environment
Shoreface
Rapid FUS is a
parasequence boundary
used for correlation.
Tidal Shoreline
Subtler FUS is a
parasequence boundary
used for correlation.
Subtler FUS is a
parasequence boundary
used for correlation.
Subtler FUS is a
parasequence boundary
used for correlation.
Rapid CUS is a
parasequence boundary
used for correlation.
Rapid CUS is a
parasequence boundary
used for correlation.
Gamma Ray Correlation
Followed by Facies Correlation
Overall
CUS
Coastal Plain
Shoreface
Offshore Shelf
Sea level rises affect large parts of the depositional basin, and their effects are therefore widespread making
them ideal for correlations.
5. Ways of Correlating - Lithologic
“Reflection Seismicity”
Seismic surveys use low frequency acoustical energy generated by
explosives or mechanical means. These waves travel downward, and
as they cross the boundaries between rock layers, energy is reflected
back to the surface and detected by sensors called geophones. The
resulting data, combined with assumptions about the velocity of the
waves through the rocks and the density of the rocks, are interpreted
to generate maps of the formations.
Seismic surveys are usually performed using multiple geophones
set at known distances from the energy source. Early seismic surveys
used mechanical plotters to record the received signals, and were
restricted to a few geophones. These surveys placed the source and
geophones in a straight line, with the interpretation of the resulting
data producing a 2-D cross section of the formation under that line.
The interpretations were subject to error, which increased the difficulty,
and cost, of accurately locating hydrocarbon-bearing formations.
Today, the development of digital recording systems allow the
recording of data from more that 10,000 geophones simultaneously,
greatly speeding data collection. Sophisticated computer programs
develop highly accurate 3-D models of rock structures. These models
are more accurate than past 2-D maps, and increase the likelihood of
accurately identifying hydrocarbon-bearing formations.
http://www.bakerhughes.com/bakeratlas/about/log2.htm
Dark lines are seismic reflection surfaces. Detailed study shows they are essentaily
time lines corresponding also with lithologic contacts.
http://www.geocities.com/jtvanpopta/seismic_reflection.html
http://www.bgr.de/b322/index.html?/b322/text/d_sunda.htm
Seismic profile across the Cocos and North
American Plates adjacent to Costa Rica. Single
and double-headed arrows delineate structural
fabric in the crust and mantle
http://www.mala.bc.ca/~earles/hydrated-mantle-sep03.htm
http://www.gfz-potsdam.de/pb4/pg3/projects/3-D_structural_modelling_CEBS/content_en.html
http://www.niwa.cri.nz/pubs/wa/11-3/images/news4_large.jpg/view
Ways of Correlating –
Biostratigraphic
Biostratigraphic Correlation is based on the work of William Smith and
George Cuviere who established the two principles by which geologic
maps are drawn.
1. Principle of Faunal Succession
The Subversive Fossil
2. Principle of Faunal Correlation
It had been towards the end of the seventeenth century that
the first very few and very bold observers raised (albeit timidly)
the ultimate heretical thought: the possibility that perhaps, just
perhaps, these objects actually were what collectors and
scientists and countrymen had long been loath to consider
admitting - the organic remains of the very creatures that they
looked like.
The same strata are always found in the same
order of superposition, and they always contain
the same peculiar fossils.
Basis of Biostratigrapic Correlation
Zone:
A body of rock characterized, recognized and identified
by one or more of the fossils it contains.
Range Zone: based on the entire vertical range of a single
species.
Assemblage Zone: based on the entire vertical range of a
community of species.
Teil Zone: “part zone” defined locally by only part of the
known total range of a particular species.
Peak Zone: based on the greatest abundance (the
abundance peak) of a species.
Species 2
Species 1
Species 12
Species 11
Species 10
Species 13
Species 9
Species 8
Species 7
Species 6
Species 5
Fossil 6 disappears
Species 3
Species 4
Defining Biostratigraphic Zones
Biostratigraphic
Zone Based
On Species 13
Biostratigraphic
Biostratigraphic
Zone
Zone Based
Based
On
On Species
Species 56
Fossil 6 first appears
Assemblage Zone
With Fossil 6
The Index Fossil
Not all fossils are equally useful for correlation.
Lingula, the inarticular brachiopod, for
example appears in the record about 540
million years ago, and is still living today.
The best knowledge we get from Lingula
is that the rock was deposited between 450
million years ago and today. Not very
useful.
On the other hand, Lingula prefers to live in
tidal systems and so does provide us with
paleoenvironmental information.
The Index Fossil
The fossils that are most useful for correlation possess the following
characteristics:
1. Abundant – no one wants to spend hours looking for the index
fossil. They should be easy to find.
2. Rapidly Evolving – want species that evolve and diversify rapidly
so that small stratigraphic intervals can be distinguished.
3. Widely dispursed – the best index fossils are swimmers or
floaters since their remains tend to show up in many different
environments. Facies fossils, those living on the bottom in restricted
habitats, are not as useful.
There are abundant practical problems associated with
biostratigrapic correlation. Requires the work of specialists who
have done the technical, nit-picking, careful, highly detailed work that
is necessary.
The Relationship Between Lithologic and
Biostratigraphic Correlation
Local
Section
#1
Local
Section
#2
Biostratigraphic Correlation between
local sections
Time
Fossil Zone
Lirthologic Correlation between local
sections
Hundreds of Miles
http://www-odp.tamu.edu/publications/198_IR/chap_05/c5_f6.htm
http://www-odp.tamu.edu/publications/183_SR/002/images/02_f02.gif
Diastems and Unconformities
Gaps in the Record
Premise 1 – We want a complete history of the Earth.
Premise 2 – The Record is preserved only in the rocks.
Premise 3 – The Rock Record is incomplete, being
destroyed by weathering and erosion, or lack of deposition.
Therefore – A complete history of the Earth is not possible.
Consequenctly, in order to understand the Earth’s history we
must understand the gaps in the record, what is missing, why
it is missing, and how we know.
Nonconformity
Angular
Disconformity
Angular Unconformity
The Taconic Unconformity, an angular unconformity between
the vertical beds of the Ordovician Austin Glen Formation and
the overlying, but steeply dipping, Late Silurian Rondout
Formation.
http://3dparks.wr.usgs.gov/nyc/parks/loc26.htm
Angular Unconformity
http://www.gly.uga.edu/railsback/FieldImages.htm
Angular Unconformity at Siccar Point
James Hutton’s Famous Unconformity
http://www.gly.uga.edu/railsback/FieldImages.html
Angular Unconformity
http://geology.asu.edu/~sreynolds/glg103/relative_age_principles.htm
Nonconformity
http://www.geowords.com/lostlinks/c19/nonconformity.htm
Nonconformity
Along U.S. Highway 67 south of Farmington, Missouri we came to a road cut which
featured a very weathered section of granite (probably the Knob Lick granite) which is
overlain by a sandstone layer (presumably the Lamotte). Shown in the image on the
left, the granite layer is the white weathered debris on the bottom and the sandstone
unit is the layered rock on top.
http://www.pittstate.edu/services/scied/Teachers/Field/Camp/Us67-1/us67-1.htm
Nonconformity
View from Hout Bay towards Chapmans Peak, showing the
nonconformity between the Cape Granite and strata of the Cape
Supergroup. Cutting the granite and unconformity is a dolerite
dyke
http://web.uct.ac.za/depts/geolsci/dlr/peninsula%20geology.html
Disconformity
Closer view of contact point between
Lykins formation (reds) and Canyon Spring
sandstone (whites). The greenish layer in
between is where iron has leached out of
the uppermost Lykins formation. A
disconformity exists here because approx.
70 million years of deposition is missing
here between the early Triassic Lykins
formation and the mid-to-late Juarssic
Canyon Spring sandstone.
http://www.paleocurrents.com/cert_classes/2003_03_15_5/HTML/img_8159.htm
Disconformity
http://www.gc.maricopa.edu/appliedscience/gjc-nsf/reldat/reldat26.html
http://rockhounds.com/grand_hikes/hikes/cape_solitude/index.shtml
Diastems - 1
Invisible Gaps in the Record
Erosion and Deposition on a
Relative Sea Level Curve
In this model a sea level rise
leads to deposition, and a sea
level fall to erosion and/or no
deposition – resulting in a gap
in the record.
The model assumes a simple
relationship: only sea level
rises not countered by a drop
result in a permanent record. A
sea
level
rise
with
a
corresponding drop at any time
in the future results in no
permanent record.
Diastems - 2
Nearly Invisible Gaps
in the Record
Parasequence
Rapid Rise in Sea Level
With sea level not changing much
sediment fills in the accommodation
resulting in a regression and a CUS
Sea Level Rise
Shoreline moves inland
Shore
Condensed
Section
Layer of black
shale only a few
mm or cm thick.
Hard to see or
find in ourcrop.
Distal basin receives little
sediment resulting in a
condensed section
Near Far
Shelf Shelf
Old Near Shelf
now becomes deep, distal far
shelf
Diastems - 2
Invisible Gaps in the Record
Low
Radioactivity
SANDSTONE
Condensed Sections in
Parasequences
Slowing of rise in sea
Level; deposition
Catches up to increase
In water depth
Rapid Rise in Sea Level
Condensed Section
Is only mm to a cm thick
But contains as much
Time as the rest of
The parasequence
Parasequence
A parasequence is a deposit laid
down between two pulses of sea
level rise.
High
Radioactivity
SHALE
Condensed Section
Rapid rise in sea level at
this point puts the section
distally away from the
source land. Sediment can
no longer reach it and only
a very thin deposit is laid
down, almost too thin to
notice.
Diastems - 3
Invisible Gaps in the Record
Episodic Depositional
Events
When we look at an outcrop of
rock is it easy to think that it
represents continuous
deposition. After all, we don’t
see any gaps or holes in the
outcrop. Yet, there are lots of
holes (gaps) and not all deposits
represent equivalent time.
Most of the beds we see in an outcrop
represent geologically instantaneous events.
They took at most a few hours or a few days to
be deposited. The shale beds in between
represent slow deposition over years of time.
The outcrop is a kaleidoscope of different
lengths of time – and they are fractal. Most of
the record is in fact gap.
We see the rocks, but we do not see the gaps.
Low
Radioactivity
SANDSTONE
High
Radioactivity
SHALE
But, this shale may represent
a year or years or decades of
time.
A few hours of time to
deposit this.