Download Correlation and Time`s Arrow

Correlation and Time’s Arrow
Building a global history
Johann Gottlob Lehmann
The structure of mountain belts
• Highest peaks/central parts show granite/ other
crystalline rocks. These Lehmann termed
primitive.
• Moving to surrounding, lower hills & mountains,
find sedimentary formations, generally
horizontal/ slightly tilted, overlying the granites.
• These Lehmann termed Floetzgebirge.
• Finally, moving still further away from the highest
peaks, these formations are covered by looser
materials (gravel, sand, silts) that Lehmann
called Alluvium.
The theory
• A Neptunian view:
– The primitive formation dates back to the universal
ocean, which deposited these rocks from a turbulent
suspension all over the earth.
– The floetz were deposited during Noah’s flood, which
caught up many living things and deposited their
remains along with the sediment of the flood.
– Finally, the alluvium was deposited on top by local
events, such as more recent floods, or the very last
stages of Noah’s flood.
Arduino
• A similar sort of generalization of local
stratigraphy:
• Primary or Primitive mountains: mineral-bearing
crystalline rocks.
• Secondary mountains: marble, limestone, few
mineral deposits, many fossils.
• Tertiary mountains: younger and lower, made of
gravel, sand and clay (often containing fossils),
and volcanic rocks.
• Finally, recent sediments washed down from the
mountains, the allluvium (again).
General observations
• Rock units described in terms of the types of
rock (crystalline vs. sedimentary of various types
vs. volcanic)
• Presence or absence of fossils noted.
• Superposition used to determine relative age.
• Note Burnet’s theory fails here: If mountains
come in different ages, then a single disastrous
flood event can’t explain them all.
Gotlob Werner
A directional history
• Hutton’s view of geology doesn’t include
systematic change over time.
• For Hutton, as far as geology is concerned
things have not changed. The cycles of erosion
and sedimentation followed by uplift & collapse
continue back into the past, and seem destined
to continue indefinitely into the future.
• Werner takes a different view: for him, there is a
‘temporal arrow’ in the earth’s history, viz. (as
with de Maillet) the withdrawal of the sea and the
resulting changes in geological conditions &
processes.
Background
• A serious interest in mineralogy; published
a book on mineral identification (using
colour, crystalline structure, hardness &
other features) at 25, in 1774.
• Took a job at a mining school that year
and stayed on until he died in 1817.
• A very successful teacher, attracted
students from all across Europe.
A modified version of Lehmann
• Four categories of rock:
• Primitive (urgebirge): crystalline rocks such as granite,
basalt, gneiss, serpentine, marble, quartzite. Fossils
absent, some minerals/ores.
• Floetz: Overlie the primitive rocks, with units of
sandstone, limestone, clay, chalk, coal, salt and
gypsum. Contain fossils, more ‘limely and clayey’.
• Volcanic: ‘True volcanics’ include surface lava, pumice,
ash, tuff (cemented ash). ‘Pseudo-volcanics’ form layers
in the floetz, but formed directly by combustion in coal
beds.
• Alluvial: Youngest, almost entirely re-worked materials
from primitive & floetz. Sand, gravel, peat, spread over
lowland plains.
A later addition
• Transitional rocks are inserted between
primitive and floetz.
• Largely crystalline, but includes some
fossiliferous sedimentary rocks.
The Narrative
• When the history of the earth is told, in effect, by a
sequence of rock-types characteristic of different
periods, what we need to know is (of course) just what
processes were going on, that produced these different
rocks at different times.
• Werner begins with a universal ocean, covering a
mysterious and hidden, irregular core.
• The ocean contained the materials of the primitive rocks
in solution and suspension. As the ocean calmed,
crystals precipitated and were deposited on the ocean
floor. The level simply followed the irregularities of the
core. Granite came first, with other rocks laid down later.
(But some intermingled with the granite.)
Sea level falls (again)
• Water was somehow decomposed, may have escaped into space.
• Solid particles begin to settle to the bottom to form rock too– here
we finally get to the transitional rocks, mixing crystalline and
sedimentary types.
• Islands emerge somewhere along here, but leave little trace.
• So both primitive and transitional rocks are considered universal.
• Continued lowering of water levels leads to floetz deposits: many
fossils, some of these deposits are local only, as land masses grew
in extent. Finally, alluvium was deposited as erosion on the surface
produced and moved gravels, sands etc. to lower elevations.
Qualifications
• In the field this doesn’t all work out perfectly– the lines
apply only in general, and there are exceptions/
intermediate cases.
• There were fluctuations in the water level from time to
time, laying younger layers on top of some primitive
layers that had been exposed and eroded.
• Steep folds and dips not due to tectonic movements, but
instead reflect the curve/tilt of the underlying ‘core’.
• Deformation of sedimentary rock (which should have
been horizontal when deposited) happened while the
rock was still soft. (Again, playing down tectonic forces/
earth movements, so important to Hutton.)
Response
• This general division due to Lehmann and
Werner was widely accepted in Europe up
to the early 19th century.
• Applied to the U.S. by McClure.
• Finer divisions developed in Europe,
distinguishing particular formations
(systems) by their superposition relations
and rock types.
Some details
• Von Humboldt used Jurassic to name a limestone
formation exposed in the Jura Mountains.
• This dips under sequences of chalky limestones to the
west, named Cretaceous (= Chalk).
• The Cretaceous formations are overlain in and around
Paris by sands, clays, marls, limestones and gypsums.
Arduino’s term, Tertiary, was applied to these.
• Alluvium over these formations came to be called
Quaternary. Triassic (limestone between clay and
sandstone) underlies the Jurassic formations along the
Rhine valley. See p. 123 f for more…
Wales and the older rocks of Britain
• Older ‘transition’ beds in Wales & along its border were
not sorted out by Smith.
• Too much folding & faulting– and fossils either rare or at
least hard to extract. A major project in field geology.
• Early 1830’s, Sedgwick and Murchison set to work on it.
• Murchison, in the south of Wales, identifies a system of
units underneath the Carboniferous (due to Conybeare
and Phillips); each associated with distinctive fossils.
Names it the Silurian, after a Welsh tribe.
• Sedgwick finds a three-part sequence of units in the
north of Wales, and names it the Cambrian, Latin for
Wales.
The conflict
• At first, both thought their two systems fit together, with the Silurian
lying directly on top of the Cambrian.
• Further study showed that the two actually overlapped.
• Neither would yield part of his system to the other’s.
• Long afterwards (around 1871) Lapworth showed that there were
distinctive fossils in the overlap, quite different from those in the
remains of the Cambrian below and the Silurian above.
• Lapworth therefore interposed the Ordovician (named for another
tribe in Wales) between the two.
• The Devonian was named by Murchison and Sedgwick before their
conflict– it overlies the Silurian, and underlies the Carboniferous
(and included the old red sandstone, previously attached to the
Carboniferous).
• In 1840/41 Murchison traveled to Russia where he named the
Permian, overlying the Carboniferous there, again distinct in its
fossils.
Geological names
• From place names (Jurassic).
• From rock types (Cretaceous).
• From order (Tertiary, Quaternary still in use even though
primary & secondary are gone…)
• From local names (tribes, towns, etc.).
• Distinguished by rock types and also by their
characteristic fossils (which were key to sorting out
disagreements and finding reasonable boundaries to
draw).
• Particular rock-systems are not universal, and are highly
variable in the sorts of rocks they include and their
thickness.
Some basic rules
• What the names refer to: both the rock units
and the times at which those units were laid
down.
• The rocks are called systems (collections of
systems are called terrains).
• The times are called periods (collections of
periods are called eras).
• Type or reference sections: The standard
examples of a particular rock system
(sometimes differing from where they were first
found).
Changes
• Interposing some (the Ordovician).
• Filling gaps (extending systems/periods so
that the top lies conformably with the
bottom of the next & vice versa.)
• Adjustments/ new information (an
ambiguous bed with no tell-tale fossils
may get re-assigned as new evidence
comes along).