Download Chapter 15: The Neogene

Chapter 15 – The Neogene: The Iceman Cometh
15.1 Introduction to the Neogene
 During the Neogene the configuration of continents and oceans, as well as many
mountain ranges and other geologic features, reached their present form
 Climatic cooling continued, and northern hemisphere glaciation occurred
 The evolution and spread of humans took place, possibly under the influence of
glacial cycles
15.2 Tectonics and Sedimentation
 Europe, Asia, and Africa
o The Tethyan Seaway continued to close with the ongoing collision of
India and Asia, and the crushing of smaller plates between the African
plate and southern Europe
o Tectonic closure of the eastern portion of Tethys running through Asia,
and the closure of the Straits of Gibraltar (the connection between Tethys
and the Atlantic) turned the Tethys into a series of inland lakes by the Late
Miocene
o Today, these remnants are the Black, Caspian, and Aral seas, along with
the Mediterranean
o Other major Neogene tectonic features include the east African rift valleys
and the opening of the Red Sea and Gulf of Aden (all related in a triple
junction), and the Zagros Mountains, formed as Arabia collided with Iran
 Central and South America
o In Central America, the remnant of Tethys left between North and South
America was closed by the Isthmus of Panama about 3 to 3.5 million years
ago
o The Caribbean plate, originally a portion of Pacific Ocean crust was cut
off; in the process, what are now the islands of the Greater Antilles moved
from the southern end of the North American Corcillera into the
Caribbean Sea
o In South America, the Andes continued to rise, shedding sediment
eastward to fill the foreland basin (today the Amazon River, with its vast
rain forests, occupies this region)
 Atlantic and Gulf Coasts of North America
o Erosion and sedimentation continued into the Paleogene as the
Appalachians were again rejuvenated by isostatic uplift
o The Gulf of Mexico likewise served as a gigantic sediment trap for
sediment shed during the Laramide Orogeny
 Western North America
o The Rockies underwent simultaneous uplift and erosion; uplift extended
into the Colorado Plateau, with downward erosion producing features such
as the Grand Canyon
 West Coast of North America
o Subduction continued along the west coast as the North American plate
moved over the Farallon plate
© 2013 Jones and Bartlett Learning, LLC
o The Columbia River plateau basalts erupted in the Middle Miocene as the
North American plate moved over what may be a hotspot
o By Late Miocene and Pliocene the eruptions shifted eastward to form the
Snake River Plain and then Yellowstone
o Sediments were also shed from the rising Sierra Nevadas into the central
valley of California, forming basins rich in petroleum from the dead
remains of plankton
o To the south, subduction also formed the Coast Ranges of California
 Evolution of the San Andreas Fault System
o In the Late Oligocene the portion of the North American plate represented
today by central and southern California began to encounter a transform
(offset) in the East Pacific Rise spreading center
o Subduction of the Farallon plate ceased, as did volcanism further east
o The Farallon plate split into two plates: the Juan de Fuca (to the north) and
the Cocos (to the south)
o Eventually, as the Pacific plate began to encounter the North American
plate it was not subducted; instead, the northwest lateral movement of the
Pacific plate became dominant
o Thus the western and southwestern portions of California began to move
in a northwesterly direction relative to the North American plate; the
former transform fault was transformed into the San Andreas Fault, a right
lateral strike-slip fault
15.3 How Was the West Widened? Evolution of the Basin and Range
 Background
o The Basin and Range is an extensional tectonic regime, as indicated by a
series of north-south trending horst and graben structures
 Hypotheses for the Formation of the Basin and Range
o Early Hypotheses – Early (now rejected) hypotheses included crustal
extension caused by the overriding of the East Pacific Rise by the North
American plate; another involved the formation of an incipient back-arc
basin by the Farallon plate
o Involvement of the San Andreas Fault System – Another early idea was
that extension was caused by the right-lateral movement of the San
Andreas fault system being transferred to the Basin and Range, stretching
it apart
o Volcanism and Uplift – Yet another suggestion was that the crust of the
Basin and Range is mechanically weak due to widespread basaltis
volcanism in the region
o Mantle Uplift – This hypothesis suggests that the buoyancy of a warm
mantle plume might have caused uplift and extension of the crust, perhaps
related to the presume Yellowstone mantle plume
o Slab Gap Hypothesis – This is a variation on several of the preceding
ideas, that as the crustal block to the west of the Ancestral San Andreas
Fault moved northwest, it caused the Basin and Range province to expand
© 2013 Jones and Bartlett Learning, LLC
o Gravitational Collapse – A more recent model suggests that the great
compression and uplift of previous orogenies caused the ancient mountain
ranges of the Basin and Range to collapse on themselves and spread out
15.4 Climate, Ocean Circulation, and Chemistry
 Modern ocean circulation, chemistry, and climate regime also began to be
established during the Neogene
 The range of climate in different regions became more extreme during the
Neogene in response to glaciation and tectonism
 Ice caps began to grow over the North Pole late in the Neogene (around 2.8
million years ago)
o This follws at least several hundred thousand years after the rise of the
Isthmus of Panama, which blocked the flow of warm currents from the
Atlantic into the Pacific
o The diversion of water strengthened the Gulf Stream, leading to more
vigorous ocean circulation patterns and water masses such as the North
Atlantic Deep Water
15.5 The "Ice Ages": Evolution of a Theory
 Background
o Early 19th century natural historians took the presence of glacial erratics
and other features to suggest that mountain glaciers had once existed at
much lower elevations
o Other geologists maintained that Noah’s flood was responsible for these
observations (the “diluvial theory”)
o Lyell initially argued that floating icebergs had deposited debris when sea
level was much higher; ultimately he concluded that the evidence for
ancient ice sheets was overwhelming, and taccepted the glacial theory
 The Eccentricity of James Croll
o Croll reasoned that Earth warmed and cooled depending on its distance
from the Sun (i.e., the eccentricity of its orbit)
o This would account for the advance and retreat of glaciers about every
100,000 years
o At first this seemed to match the record, but a closer look showed that the
last glacial cycle ended about 10,000 to 15,00 years ago
o Thus eccentricity alone could not account for the growth and retreat of
glaciers
 Precession of the Equinoxes
o Croll looked for another astronomical mechanism and concluded that the
phenomenon known as the precession of equinoxes affects the amount of
solar radiation received by Earth
o Precession (wobble) has to do with where the seasons occur in Earth’s
orbit
 Milutin Milankovitch and Obliquity
o Milankovitch, an astronomer, made a mathematical theory of the climate
of Earth, Mars and Venus his life’s work
o He developed a mathematical theory that allowed calculation of the solar
flux for any given latitude and season
© 2013 Jones and Bartlett Learning, LLC
o Unlike Croll, who had decided that diminished radiation during winter at
high latitudes was the critical factor in prompting ice growth,
Milankovitch concluded that summer radiation was of more significance
o He also found that the obliquity (tilt) of Earth’s axis strongly affected his
calculations; thus there are three separate factors in the Milankovitch
theory: eccentricity, obliquity, and precession
o The Milankovitch theory was published in 1924, but fell out of favor
because the number of major tillites (four) did not match the predictions of
the theory
 Planktonic Foraminifera and the Oxygen Isotope Curve
o As support for the Milankovitch theory waned, studies of the planktonic
foraminifera and oxygen isotope records of deep sea cores began
o By the 1960s it was recognized that the oxygen isotope curve primarily
indicates ice volume rather than temperature
o Age calibrations using magnetic reversal stratigraphy in the 1970s helped
reestablish the link between glacial cycles and astronomical forcing, as
predicted by the Milanktvitch theory
o Eccentricity, obliquity and precession (referred to as “Milankovitch
cycles”) combine to produce all sorts of gradations of ice advance and
retreat, as well as shifts in character of the cycles (i.e., dominated by
precession rather than obliquity)
15.6 Neogene Life
 Marine Life
o Diversification of the Modern Fauna continues, although it is unclear if
this is real or an effect of better preservation of the fossil record in
younger materials
o Some plankton populations have become strongly associated with certain
water masses
o Other taxa have become restricted toward lower latitudes because of
glaciation and cooling of water masses near the poles
 Land Plants
o Grasses diversified into thousands of species as Earth dried out due to
glaciation
o A shift in carbon isotope values during the Late Miocene (6 million years
ago) may reflect the expansion of C4 grasses, which extract more of the
heavier isotope of carbon during photosynthesis compared to C3 grasses
o The rise of grasses, which contain microscopic phytoliths of opaline silica
may have helped supply this biolimiting nutrient to the sea, perhaps
contributing to the tremendous Neogene diversification of diatoms
 Terrestrial Vertebrates
o The expansion of grasses has a major effect on terrestrial communities and
the diversification of different types of vertebrates, including rodents,
frogs, songbirds, and snakes
o The changes in ungulates continued, as horse (and other perissodactyl)
diversity declined and artiodactyls diversified
© 2013 Jones and Bartlett Learning, LLC
o Mammoths evolved into grazers in the Pleistocene; carnivores (wild dogs,
saber-toothed cats, hyenas and bears) preyed upon the herbivores
 Evolution of Humans
o Evolutionary Relationships of Humans – Humans belong to the group of
hominoid apes called hominids; significant differences exist between
humans and other hominoids
o Hypotheses of Human Origins – Ever since Darwin, various hypothesis
have been proposed for the evolution of humans; these are almost
constantly being revised or disputer on often fragmentary fossil evidence
o Why Did Hominids Leave Africa? – Various factors possible responsible
for human migration out of Africa include bipedalism, climate change,
brain evolution, and evolution of behavior itself,
15.7 Extinction
 Large grazers underwent a significant turnover during the Late Miocene, probably
related to a change in vegetation as C4 grasses began to replace C3 taxa
o Species with lower crowned teeth were unable to adapt to the increase in
silica content of the C4 grasses
 The Great Fauna Interchange occurred when the Isthmus of Panama was
established as a land bridge in the Pliocene
o Many South American marsupials were replaced by placental mammals
 Mastodons and mammoths died out as recently as 25,000 years ago
o Extinction may have been due to climate change, predation by ancient
humans, or both
 All hominid species preceding Homo sapiens have persisted for a few hundred
thousand to roughly a million years before becoming extinct for reasons not understood
© 2013 Jones and Bartlett Learning, LLC