Chapter 2 Outline
STRATIGRAPHY
- STUDY OF SEDIMENTARY ROCK LAYERS (STRATA)
- CHRONOSTRATIGRAPHY (ALSO CALLED GEOCHRONOLOGY)
- STUDY OF ABSTRACT TIME
- regardless of whether sediments were deposited
- FUNDAMENTAL GEOCHRONOLOGIC UNIT: PERIOD
- modified into smaller units by late, middle &
early
- other geochronologic units: eon, era, epoch,
age (see below)
- RADIOMETRIC DATING TECHNIQUES
- Radioactivity - Spontaneous change (decay) in the nucleus of
an atom
- ATOMIC NUCLEUS - protons (p+, electrical charge = +1, mass =
1) & neutrons (n°, 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
- Assumptions & Sources of Error
- 1. HALF-LIVES DON'T CHANGE & are MEASURED ACCURATELY
- "CONCORDANT" AGES (same age - 2 different decay series) CONFIRMS
- 2. Mineral/rock is "CLOSED" SYSTEM (parent & daughter
don't leave system)
- amount of PARENT REMAINING + DAUGHTER = amount of original PARENT
- 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
| Isotope |
Half-life |
Isotope |
Half-life |
| Rb87 |
47. by |
U235 |
713. my |
| U238 |
4.51 by |
C14 |
5,730. years |
| K40 |
1.3 by |
|
|
- 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
- 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
- LITHOSTRATIGRAPHY
- STUDY OF ACTUAL ROCKS PRESERVING GEOLOGIC RECORD
- defined by rocks of a particular area (stratotype) &
recognized elsewhere by fossil content
- FUNDAMENTAL TIME-STRATIGRAPHIC UNIT: SYSTEM
- modified into smaller units by upper, middle &
lower
- SEDIMENTARY ENVIRONMENTS (in or near which dinosaurs lived)
- Nonmarine (continental) environments
- FLUVIAL - meandering rivers & braided streams
- BRAIDED STREAMS - coarse with horizontally-bedded gravel & cross-bedded
sand with little mud
- MEANDERING RIVERS - channel deposits (point bar) = cross-bedded sand
to gravel; levees = silt & fine sand, floodplains = clays; crevasse
= fine sand
- produce elongate sand bodies surrounded by mud
- DESERT
- SAND DUNES (eolian) = cross-bedded sand
- PLAYA LAKES = clays & evaporites
- ALLUVIAL FANS = coarse sediments from braided streams
- LACUSTRINE - lakes: fine-grained laminated sediments with freshwater
fossils
- SWAMP - fine-grained sediments with lots of organic matter
- Transitional (shoreline) environments
- DELTAS - river delivers sediment faster than marine processes
can redistribute it
- TIDAL FLATS - form where tidal ranges are large
(>2 m)
- BARRIER ISLANDS - form where tidal ranges are small
(<1 m)
- BEACHES, DUNES, & WASHOVERS = sand, mostly exposed
- RELATIVE AGE DATING
- Nicholas Steno recognized 2 principles:
- Superposition - the oldest layer is at the bottom in undisturbed
strata
- Original Horizontality - sediment originally deposited in nearly
horizontal layers
- BIOSTRATIGRAPHY
- BASED ON FOSSIL SUCCESSION
- fossil assemblages succeed one another through time in a regular
& determinable order
- FUNDAMENTAL BIOSTRATIGRAPHIC UNIT: BIOZONE
- recognizable assemblage of distinct fossils
- Biozones are often nearly time synchronous & are used
to establish time equivalency for geographically-separated lithostratigraphic
units
- INDEX FOSSILS: fossil species that are geographically widespread,
occur in many types of sediments, and are restricted to a
narrow time interval
- DEVELOPMENT OF THE GEOLOGIC TIME SCALE
- RELATIVE GEOLOGIC TIME SCALE - Based on PRINCIPLES of SUPERPOSITION,
ORIGINAL HORIZONTALITY, AND FOSSIL SUCCESSION
- yields sequence of events, but doesn't say how long ago or
of what duration
- developed mostly between 1822 & 1841 from rocks exposed
in Europe
- ABSOLUTE GEOLOGIC TIME SCALE - Based on RADIOMETRIC DATING TECHNIQUES
- gives ages in years before present
Geologic Time Scale for the Phanerozoic Eon
PHANEROZOIC ERAS |
PHANEROZOIC PERIODS |
AGE |
Cenozoic
(Age of Mammals) |
Quaternary |
0 my
65 my |
Tertiary |
Mesozoic
(Age of Reptiles) |
Cretaceous |
144 my
208 my
245 my |
Jurassic |
Triassic |
Paleozoic
(Age of Fish) |
Permian |
544 my |
Pennsylvanian |
Carboniferous |
Mississippian |
Devonian |
Silurian |
Ordovician |
Cambrian1 |
1Note: the Cambrian starts at 570 M.Y. in your text
- MESOZOIC PERIOD SUBDIVISIONS
| Cretaceous |
Late |
65 my
97 my |
| Early |
144 my |
| Jurassic |
Late |
157 my |
| Middle |
178 my |
| Early |
208 my |
| Triassic |
Late |
231 my |
| Middle & Early |
245 my |
PLATE TECTONICS
- COMPOSITIONAL STRUCTURE OF THE EARTH
- STRENGTH STRUCTURE OF THE EARTH
- CORE - SOLID INNER CORE & FLUID OUTER CORE
- MANTLE & CRUST (see diagram below)
- LITHOSPHERE DIVIDED INTO SEVERAL LARGE (MAJOR) & MANY SMALLER (MINOR)
PLATES
- 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]
- 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
- 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)
- EFFECT ON ORGANIC EVOLUTION:
- life is relatively diverse during continent dispersal
- many geographic barriers
- life is less diverse during continent consolidation - few barriers
- PLATES THROUGH TIME
- BEFORE DINOSAURS
- Oldest rocks - 3.96 billion years old
- EARTH VERY DIFFERENT
- CONTINENTAL CRUST ASSEMBLED INTO TRUE CONTINENTS (STABLE PARTS CALLED
CRATONS) ABOUT 2.5 BILLION YEARS AGO
- Cambrian - continents are well dispersed &
mostly moving apart (like today)
- 6 MAJOR CONTINENTS
- LAURENTIA (North America + Scotland)
- BALTICA (Scandinavia, northern Europe, Russia [west of the Urals])
- SIBERIA (east of the Urals + Mongolia = Northeast Asia)
- KAZAKHSTANIA (Central Asia)
- CHINA (including Southeast Asia)
- GONDWANA (South America, Africa, Antarctica, Australia, India, southern
Europe & Florida and south Georgia!)
- Devonian - Laurentia & Baltica collide to form
Laurussia (Old Red Sandstone continent in text)
- Late Carboniferous [Pennsylvanian] - Laurussia, Kazakhstania
& Siberia collide to form Laurasia
- Permian - Laurasia & Gondwana collide to form
Pangea (China is the only continent part of Pangea)
- PANTHALASSA IS THE OCEANIC COUNTERPART OF PANGEA
- TETHYS (AN EMBAYMENT OF PANTHALASSA) EXTENDS INTO EAST SIDE OF PANGEA
- DURING THE TIME OF DINOSAURS
- Mesozoic tectonics dominated by divergence as Pangea was dismembered
and the Atlantic & Indian Oceans opened
- Triassic - rifting in Pangea starts
- PANGEA STILL INTACT, HOWEVER
- EQUATOR BISECTS PANGEA
- Jurassic
- EARLY JURASSIC (190 MY) - INITIAL BREAKUP OF PANGEA BEGINS
- seafloor spreading in central North Atlantic off eastern North America
& northwest Africa as Tethys extends westward
- Pangea breaks into Laurasia & Gondwana (Florida & south Georgia
left with North America)
- MIDDLE JURASSIC (165 MY) - GONDWANA BEGINS TO BREAK UP
- seafloor spreading off eastern Africa & Antarctica
- Gondwana breaks into western Gondwana (South America/Africa), eastern
Gondwana (Antarctica/Australia/India/Madagascar) & Indonesia
- still a narrow connection between South America & Antarctica
- Epicontinental, or epeiric, seas (shallow-marine seas covering parts
of the continents) first appear
- result from higher global (eustatic) sea level caused by increased
width of mid-ocean ridges
- LATE JURASSIC - LOW EUSTATIC SEA LEVEL
- Cretaceous
- EARLY CRETACOUS (135 MY) - GONDWANA CONTINUES TO BREAK UP
- western Gondwana separates into South America & Africa; eastern
Gondwana separates into Antarctica/Australia & India/ Madagascar (now
part of Africa)
- connection between South America & Antarctica still remains
- epicontinental seas not common at beginning, but are extensive in mid-Cretaceous
time as eustatic sea level rises
- North America bisected by Western Interior Sea; Europe was an archipelago
- LATE CRETACEOUS (90 MY) - LAURASIA BEGINS TO BREAK UP
- Laurasia breaks into North America & Eurasia/Greenland
- North America connected to Eurasia in the Arctic (Bering Straits
& Greenland)
- India separates from Madagascar & accelerates toward Asia
- Epicontinental seas remain extensive
- North America still bisected by Western Interior Sea
MESOZOIC CLIMATE
- BASIC PRINCIPLES
- HEAT RETENTION
- Solids (continents) - heat up & cool down rapidly
- Fluids (oceans) - heat up & cool down slowly
- Triassic - continental effects would have dominated
because of the existence of Pangea
- Jurassic & Cretaceous - oceanic effects would have
dominated because of the breakup of Pangea & because
of epicontinental seas
- OCEAN CIRCULATION
- Caused by wind
- SEPARATE GYRES IN EACH OCEAN BASIN
- CIRCUM-POLAR CIRCULATION AROUND ANTARCTICA
- Jurassic & Cretaceous - Circum-equatorial currents
nearly circle the world (flow west through Tethys &
North Atlantic)
- ORIGINATED AFTER SEPARATION OF PANGEA INTO LAURASIA & GONDWANA
- CIRCUM-POLAR CIRCULATION OF TODAY DIDN'T ORIGINATE UNTIL THE EOCENE
- WIND (See Fig. 2-14) & RAIN
- Driven by Thermal Convection
- UNEQUAL HEATING OF EARTH'S SURFACE BY SOLAR RADIATION
- Moist, heated air rises at Equator, cools &
releases moisture, & moves poleward at high altitude;
at the poles, dry, cool air sinks & returns
to the Equator along the surface
- This simple convection system breaks down into 3 separate convection
cells in each hemisphere
- TROPICAL CELLS FROM THE EQUATOR TO 30 DEGREES NORTH & SOUTH
- Dry, cool air sinks at 30 degrees North & South
- TEMPERATE CELLS BETWEEN 30 & 60 DEGREES NORTH & SOUTH
- Warm, moist air rises during temperate cell storms
- POLAR CELLS POLEWARD FROM 60 DEGREES NORTH & SOUTH
- ROTATION OF THE EARTH CAUSES CORIOLIS DEFLECTION OF THE WINDS
- SURFACE WINDS IN TROPICAL & POLAR CELLS BLOW FROM THE EAST; THOSE
IN TEMPERATE CELLS ARE WESTERLIES
- Large continental masses disrupt (like Pangea or Gondwana or
Laurasia) this pattern and produce monsoonal (seasonal) winds
& rains
- Mountain ranges create rain shadows
- SUMMARY
- GENERALLY WARM DURING MESOZOIC
- Continents strattle the Equator & none are
at the poles
- Circum-equatorial currents
- DRIER IN TRIASSIC & JURASSIC; WETTER IN CRETACEOUS
- Pangea is so big that moisture cannot reach the
interior
- Mountain ranges resulting from formation of Pangea create
numerous rain shadows
- By Cretaceous time, the continents have dispersed significantly
& eustatic sea level is high
- LATE TRIASSIC & EARLY JURASSIC
- Hot & dry with strong seasonality
- Evidence:
- SAND DUNE DEPOSITS, EVAPORITES, REDBEDS, & CALICHE NODULES IN PALEOSOLS
- OXYGEN ISOTOPES
- MIDDLE & LATE JURASSIC
- Warm & dry with reduced seasonality
- Evidence:
- SAND DUNE DEPOSITS, EVAPORITES, REDBEDS, & CALICHE NODULES IN PALEOSOLS
- OXYGEN ISOTOPES
- DEEP OCEAN SEDIMENTS
- EARLY CRETACEOUS
- Warm & moist with reduced seasonality
- Evidence:
- COAL DEPOSITS
- OXYGEN & CARBON ISOTOPES
- DEEP OCEAN SEDIMENTS
- HIGH ATMOSPHERIC CO2 FROM 1) VOLCANIC ERUPTIONS ASSOCIATED WITH FAST
SPREADING RATES ALONG THE MOR & WITH VAST OUTPORINGS OF BASALT IN INDIA
(DECCAN TRAPS), & 2) LESS CO2 DISSOLVED IN WARMER CRETACEOUS OCEAN
WATER
- Air trapped in bubbles in 80 my old amber (fossil pine pitch) has high
(700 ppm) CO2 content
- What do you suppose the effect of increased CO2 content of the present
atmosphere (350 ppm & rising from 280 ppm prior to the Industrial Revolution
in 1750) resulting from human activities (burning of fossil fuels, deforestation)
will have on the Earth's climate?
- LATE CRETACEOUS
- Warm & moist with increased seasonality
- Evidence:
- FOSSIL LEAF SHAPES
- OXYGEN & CARBON ISOTOPES
- DEEP OCEAN SEDIMENTS