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HISTORICAL GEOLOGY
LECTURE 1. STRATIGRAPHY
Stratigraphy:
Many past geologic events have resulted in strata or layers of
sediment which have formed a stacked vertical sequence of sedimentary
rocks. A good example is sediment in the form of mud and sand from a river
accumulating on a sea-floor. The sediment forms a layer which eventually
becomes hardened or lithified into a sedimentary rock.
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The importance of stratigraphic sequences to historical geology is that they
provide a rock record of past events e.g. a pebbly beach may be recorded as a
CONGLOMERATE, a sandy river may be recorded as a layer of
SANDSTONE; a clayey tidal flat as a layer of SHALE.
Pebbly beach
conglomerate
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Sandy beach
sandstone
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Muddy marsh
shale
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Stratigraphy is the study of these layered rocks, especially
their sequence, correlation from place to place, relative ages and
interpretation. Several important stratigraphic principles
emerged from the study of stratigraphy centuries ago by the
early founders of the science of geology:
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NICOLAUS STENO (1669)
1. Principle of Superposition: In any sequence of undisturbed strata, the
oldest layer is at the bottom and higher layers are successively younger.
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Connected to this is the idea of conformable and unconformable sequences.
A conformable sequence is one in which there has been continuous
deposition of sediment such that the resulting rocks grade one into another
via gradational contacts. An unconformable sequence is one in which
deposition has been interrupted by periods of erosion, causing removal of
some layers and resulting in erosional contacts. The unconformity
represents a GAP in the rock record. Various types of unconformities result,
including angular unconformities (sedimentary strata meet at an angle),
disconformities (essentially parallel strata) and nonconformities (younger
strata on older metamorphic/igneous rocks).
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Continuous deposition of sediment
layers.
Uplift and folding of conformable
sequence of sedimentary rocks.
Erosion forms an erosion surface.
Renewed deposition forms an
angular unconformity.
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2. Principle of Original Horizontality: Most sedimentary rocks formed
originally in close-to-horizontal layers (although many have since been moved
from their original position).
Undeformed strata
Deformed strata (tilted)
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3. Principle of Original Lateral Continuity: Originally sedimentary strata
extended in all directions until they either; thinned out, ended abruptly at
some kind of barrier or graded into a different kind of sedimentary rock.
This principle is important for correlating sedimentary rocks from place to
place i.e. across a valley. Stratigraphic correlation is the practice of
"matching up" equivalent bodies of rock from different locations. The
equivalence may be in terms of lithology, age or fossil content.
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JAMES HUTTON (1726 - 1797)
4. Principle of Uniformitarianism: "The present is the key to the past" i.e.
geologic processes operating today also operated in the past. e.g. river deposits
forming today have a similar composition and character as their ancient
counterparts; glacial erosional and depositional features are basically the same
today as in the past...etc.
Modern
sand dune
Ancient
sand dune
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Harry Williams, Historical Geology
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WILLIAM "STRATA" SMITH (1769 - 1839)
5. Principle of Biological Succession: Different kinds of plants and animals
succeed one another in time because life has evolved continuously; therefore only
rocks formed during the same age can contain similar assemblages of fossils.
Since these fossil assemblages are unique for particular periods of the past, they
can be used to:
a. correlate rocks from around the world, and to
b. order rock layers into a sequence of relative age (i.e. older …. newer).
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CHARLES LYELL (1830)
6. Principle of Cross-Cutting Relationships: Any geologic feature which cuts
across or penetrates another body of rock must be younger than the rock mass
penetrated.
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Sandstone
Older
Fault A
Conglomerate
Shale
Fault B
Batholith
Dyke B & sill
Dyke A
Younger
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7. Principle of Inclusions: Any rock that contains fragments of an adjacent
rock must be younger than the adjacent rock.
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What was the sequence of events?
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