Historical Geology Slides

8/13/2019 Historical Geology Slides

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Harry Williams, Historical Geology 1

HISTORICAL GEOLOGY

LECTURE 1. STRATIGRAPHY

Stratigraphy:

Many past geologic events have resulted in strata or layers ofsediment 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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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

Fault A

Conglomerate

Shale

Fault B

BatholithDyke B & sill

Dyke A

Older

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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Historical Geology Slides