Chapter 3 – Paleontology and Geology as Sciences
Introduction
Paleontology and geology are
historical sciences
Deal with phenomena that occurred
millions of years ago
Similar to forensic science
Tools used in paleontology and geology
(see Tables 3.1 and 3.2)
Maps especially useful
Basic Principles of Geology
Relative age dating principles (see
Figures 3.2 to 3.5)
Original Horizontality - Sediment
originally deposited in nearly horizontal strata or beds
Superposition - The
oldest layer is at the bottom in undisturbed strata
Lateral Continuity - Strata
continue laterally until they encounter some barrier
Allows correlation of strata from
place to place
Nicolaus Steno clearly enunciated
during the last half of the 1600’s
Cross-Cutting Relationships - igneous intrusion or fault younger than rock intruded or cut
Inclusions - rock fragments included within a rock mass are older than the enclosing rock mass
Sir Charles Lyell articulated very well
during the early 1800’s as part of establishing uniformitarianism, as
opposed to catastrophism, as the guiding paradigm of historical geology
Biologic/(fossil) Succession - fossil assemblages succeed one another through time in a regular & determinable order
Index/(guide) fossils: fossil species that are geographically widespread, occur in many types of sediments, and are restricted to a narrow time interval
Geologic range: time interval that a fossil species occurs in the geologic rock record
Geologic ranges may be refined occasionally when new fossils are found
Example
Coelophysis
always found in strata older than either Allosaurus or Apatosaurus
Coelophysis is Late Triassic in age and Allosaurus or Apatosaurus are Late Jurassic in age
Note – the principle of biologic/(fossil)
succession was developed and used long before Darwin published The Origin of
Species by Means of Natural Selection,
although evolution and extinction of species controls their geologic range
Biostratigraphy: use of fossils to determine the age of strata
George Cuvier and William Smith
independently developed during the early 1800’s
Stratigraphic Terminology
Time (Geochronologic) Units
Period: Fundamental time unit
Time units are abstract units
modified into smaller units by late, middle & early
other time units: eon, era, epoch, age
Chronostratigraphic Units
System: Fundamental time-stratigraphic unit
Rocks deposited during a certain time interval
Defined by rocks of a particular area (stratotype or type section) & recognized elsewhere by fossil content
modified into smaller units by upper, middle & lower
other chronostratigraphic units: eonothem, erathem, series, stage
Equivalence of Chronostratigraphic & Geochronologic Unit Terms
Chronostratigraphic Time Examples
Eonathem Eon Phanerozoic
Erathem Era Mesozoic
System Period Cretaceous
Series Epoch Upper/Late
Stage Age Maastrichtian
Lithostratigraphic Units
Formation: Fundamental lithostratigraphic unit
Distinctive lithology with recognizable contacts with underlying & overlying units that can be traced laterally (can be correlated) with no regard for time boundaries
other lithostratigraphic units: supergroups, groups, members, beds
Formations can be & often are time transgressive
Transgression & regression
Movement of shoreline landward = transgression
Movement of shoreline seaward = regression
Produce distinct vertical sequences: transgression - nearshore strata overlain
by offshore strata; regression
- offshore strata overlain by nearshore strata
Also produce individual rock units that are time transgressive, that is, they become younger in a landward
(transgression) or seaward (regression) direction
Unconformities - surfaces of nondeposition or erosion encompassing significant amounts of geologic time
Hiatus = interval of time not represented by strata in an area
Diastems = relatively short hiatuses in a continuous, conformable sequence
Bedding Planes are essentially diastems
Nonconformity = unconformity cut into metamorphic or igneous rocks & overlain by sedimentary rocks
Angular unconformity = unconformity where overlying & underlying strata different dips
Disconformity = unconformity where overlying & underlying strata are parallel to each other
Miscellaneous notes
Overturned
strata – strata have been tilted
until they are upside down; simple application of superposition would yield the
wrong age relationships among the strata
Reworked
fossils – fossils that have been
eroded out of their original strata and redeposited into younger strata
Absolute age dating
Radioactivity
Spontaneous change (decay) in the
nucleus of an atom
Atomic nucleus - protons (p+, electrical charge = +1, mass = 1) &
neutrons (no, electrical charge = 0, mass = 1)
Atomic number (determines the element) = number of p+
Atomic mass = number of p+ + n˚
NOTE: n˚ = electron (e-, electrical charge = -1, mass = 0, called a beta particle) + p+
Isotopes = different types of an element differing in atomic mass
Forms of decay
Alpha decay (ejection of an alpha particle from a nucleus)
alpha particle = nucleus of a Helium atom (2 p+ + 2 n˚‚ [atomic # = 2, atomic mass = 4])
atomic # decreases by 2 & atomic mass decreases by 4
example: 92U238 - alpha particle -> 90Th234 (daughter product)
Beta decay (ejection of a beta particle from a nucleus)
atomic number increases by 1 & atomic mass remains the same
example: 37Rb87 - beta particle -> 38Sr87
Electron capture decay (capture of an e- [or beta particle] by a nucleus)
atomic number decreases by 1 & atomic mass remains the same
example: 19K40 + beta particle -> 18Ar40
Half-life of a radioactive element - time for one-half of any amount of a radioactive element to decay
Decay is exponential, that is it is faster earlier (see Figure 3.6)
Decay
rate is constant,
however
N = Noe-lt, where N is the number of atoms now and N is the original number of radioactive atoms, e is the exponential constant (~2.718), l is the decay constant and t is time
Decay rates have not been observed to vary and are considered to be as factual as gravity
Saying that decay rates may have been different in the past is analogous as saying that apples fell upwards in the past
Assumptions & Sources of Error
1. Half-lives don't change & are measured accurately (see above for decay constants)
"Concordant" ages (same age – 2 or more different decay series) confirms (see Table 3.4)
2. Mineral/rock is "closed" system (parent & daughter don't leave or move into [contaminate] system)
amount of parent remaining + daughter = amount of original parent
Isochron dating (comparing parent & daughter ratio with daughter & different daughter isotope) confirms
3. No daughter present initially
often can correct for any daughter present initially anyway
example: isotopes of lead (Pb) = Pb204, Pb206, Pb207, Pb208
Pb204 from original solar nebula only, while Pb206, Pb207 & Pb208 from both original solar nebula & decay of U238, U235 & Th232
obtain original solar nebula Pb206, Pb207, & Pb208 from amount of Pb204 using fixed ratio of Pb204:Pb206:Pb207:Pb208 in original solar nebula (from meteorites), then subtract to get radiogenic
4. Mass Spectrometer Measurement Error is ±0.2-2.0%
Principle Radiometric Timekeepers
Half-lives
Rb87 - 48.8 by K40 - 1.3 by
Th232 - 14.0 by U235 - 713. my
U238 - 4.47 by C14 - 5,730. years
Lead producers (U238 -> Pb206; U235 -> Pb207)
Potassium-Argon (K40 + beta -> Ar40 [11%])
K40 - beta particle -> Ca40 (89%), but can't correct for non-radiogenic
Rubidium-Strontium (Rb87 - beta particle -> Sr87)
Pb producers & Rb-Sr - used for plutonic igneous rocks; K-Ar - used for volcanic igneous rocks
Hard to
radiometrically date sedimentary rock directly – sedimentary particles are older than the deposit
Ash beds an exception (see Table 3.5)
C14 - for dating geologically young organic material (less than 100 ky)
C14 is created continously in the atmosphere (N14 + n˚ -> C14 + p+)
C14 - beta particle -> N14; ratio of C14 to all carbon gives age
Plate tectonics
Compositional Structure of the Earth
Strength Structure of the Earth
Mantle & crust (see diagram below)
Core - solid inner
core & fluid
outer core
Lithosphere divided into several large (Major)
& many smaller (Minor) PLATES (See Figure 3.7)
Indicated by the distribution of earthquakes & volcanoes
long, narrow belts coincident with MOR crest,
deep-sea trenches & long faults
plates move relative to each other, but deform only at their edges
7 Major
plates - North & South
American, Indian-Australian, Eurasian, Pacific, African, & Antarctic
Some important
minor plates - Nazca, Cocos, Juan de Fuca & Philippine [Pacific], Arabian [Indian], & Caribbean [Atlantic]
Alfred Wegener
First to assemble diverse data supporting continental drift (1912)
Late Paleozoic supercontinent (Pangea) surrounded by a superocean (Panthalassa) with an embayment of Panthalassa (Tethys) into Pangea
an extension of Tethys broke Pangea into Laurasia (N. America & Eurasia) & Gondwana (S. America, Africa, Antarctica, Australia, & India)
Evidence
Shape of continents ("real" edge of continent = continental slope)
Geology (Match up continental geology on either side of an ocean)
Mountain belts (Appalachians/Caledonides); sedimentary basins (S. Africa/Argentina); [radiometric age provinces (Brazil, W. Africa)]
Paleontology (Similarity/differences of fossils on various continents)
Similar Carboniferous & Permian flora (Glossopteris ) & fauna (including Mesosaurus ) in Gondwana continents
Paleoclimatology (Climate zones vary by latitude - distinct sedimentary deposits in each zone)
If the continents are fixed, how does one explain tillites in India & shallow-marine limestone in the Arctic & Texas in the Permian & tillites in the Sahara & shallow-marine limestones in Vermont in the Ordovician?
Wegener's mechanism for continental drift (continents plow through the oceans toward the poles due to the equatorial bulge) was not acceptable
Oceanography
Marine Geology
mapped
topography of seafloor (central Mid-Ocean Ridge [MOR] with rift valley at crest = extension in centers of oceans)
collected
sediment cores & rock
dredges (oldest sediments & rocks were only Mesozoic in age)
Harry
Hess (1962) - proposed the concept of Seafloor Spreading (convection in mantle with upwelling of hot mantle under MOR & creation of new seafloor; conveyor belt carrys continents
passively)
Marine Geophysics
magnetic
polarity reversals found in
ocean crust; marine
magnetic anomalies were recognized to result
from seafloor spreading combined with reversals of the Earth's magnetic field (magnetic polarity vs. width of seafloor)
seafloor spreading rates were calculated (~1 mm/yr to >17 cm/yr)
oldest seafloor - Jurassic in age (<200 m.y.)[oldest continental rocks - Archean in age {3.8 b.y.}]
extension of magnetic polarity time scale - marine magnetic anomalies extended beyond Gilbert Epoch
age of seafloor predicted
Deep Sea Drilling Project (DSDP) - drilled section across South Atlantic & confirmed predicted ages & seafloor spreading
oldest sediment
on top of basaltic oceanic crust got older away from MOR crest in predicted
manner; generated band
wagon effect
helps
absolute dating of geologic time scale for Mesozoic/Cenozoic
Types of Plate Motion
Divergent (= seafloor spreading) - plates move
away from each other
associated with Mid-Ocean Ridges & continental rift zones
new oceanic lithosphere is created at divergent plate boundaries
Convergent (= subduction) - plates move toward
each other
associated
with deep-sea trenches & island
arcs or marginal mountain
belts (Andes); continental
collision zones (Alps/Himalayas)
old
oceanic lithosphere is subducted
back into the aesthenosphere at
convergent plate boundaries along inclined seismic zones
Lateral (= transform or strike-slip) - plates
slide past each other
associated
with transform faults (like the San
Andreas) & fracture zones
Driving Forces
Mantle convection
"Push-pull" - plates pushed apart at MOR & pulled down at trench by cold, subducting slab
Hot Spots
Stationary "hot spot" in
aesthenosphere & voluminous
source of basaltic magma
MOR above
sealevel when at
divergent plate boundaries (Iceland)
Volcanic
plateaus or island chains in plate interiors (Hawaii)
Wilson Cycles
Continents alternately consolidate into large
supercontinents (like Pangea at the end of the Paleozoic) or disperse into
several continental masses (like we have today)
Affects
global environment, including
climate
Effect on organic evolution:
Life is relatively diverse during continent
dispersal - many geographic barriers
Life is less diverse during continent
consolidation - few barriers
Recovery/Collection and Preparation
of Dinosaur Fossils
Prospecting (not in textbook)
Where to look -
Sedimentary rocks of
Igneous rocks started out molten -no organisms, and
Metamorphic rocks are heated and compressed, processes that destroy bones
Triassic, Jurassic or Cretaceous age (~230 to 66 million years ago) [together known as the Mesozoic Era (245 to 66 million years)] that were
Dinosaur fossils are sometimes reworked into Cenozoic sedimentary rocks
Deposited in terrestrial environments
Although dinosaur carcasses were sometimes washed into lake and ocean environments
Chances of finding fossils are increased where
more surface area is exposed
Often badlands & deserts - dry, less vegetation
Collecting
Exposing, Cleaning and Hardening
Matrix (rock surrounding fossil) is removed
until fossil sits on pedestal (pillar of matrix under the fossil)
Glue applied that soaks into bone and then
hardens
Padding and Jacketing
Padding (to cushion fossil & prevent jacket
from sticking) often consisting of wetted toilet paper is applied
Jacketing (to protect fossil) often consists of
strips of burlap soaked in plaster
After the jacket has hardened, specimen is turned by separation at the base of the pedestal
A plaster cap is applied to open bottom of jacket
Transporting out of field can be easy or difficult
Preparing and Curating
Preparation = freeing from matrix and putting
fragmented fossils back together
Variety of techniques used
Curation = displaying and archiving
Fiberglass or resin casts used by most museums
now for display
Actual fossils archived in conditions best
suited for protection and study
Nests, Eggs and Tracks
Nests and Eggs treated like skeletal
material
Tracks mostly studied in field and non-destructive
casts made for later study and display