Stratigraphy Stratigraphy is the branch of geology that deals with the arrangement of rocks in layers.
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Relative Dating The procedure by which scientists determine the geologic events which formed these rocks and the order in which these events occurred.
Law of Uniformitarianism The geologic processes which shape the earth today are the same as they were in the past. “The present is the key to the past”
Law of Superposition In an undisturbed sequence, each rock later is older than the layers above it. “Oldest on the bottom, youngest on the top.”
Law of Original Horizontality Sediments are always deposited in horizontal layers. If sedimentary rock layers are not horizontal, then an event like folding, faulting,intrusions, etc. took place to change the layers after they were deposited.
Law of Cross-Cut Relationships If a fault or an igneous intrusion cuts across or through rock layers, then the fault or intrusion is younger than the rock layers.
the Law of Crosscutting Relationships stated that if a fault or other body of rock cuts through another body of rock then it must be younger in age than the rock through which it cuts and displaces.
Principle of Unconformity Often the deposition of rock layers is stopped for one reason or another and the layers undergo erosion. This causes a break in the geologic record. This break or missing time sequence is called an unconformity. There are 3 types of unconformities.
A. Disconformity The sedimentary rock area is eroded, then undergoes deposition without any change in the angle of the rock layers. The old erosional surface is parallel to the beds above and below it.
DISCONFORMITIES Disconformities are characterized by an irregular erosional surface which truncates flat-lying sedimentary rocks. The layers of sedimentary rocks above and below the unconformity are parallel with one another.
B. Angular Unconformity When sedimentary rock layers are tilted, exposed to the new surface and then eroded, and covered by new deposits, an angular unconformity results. The new layers ARE NOT parallel to the older layers.
ANGULAR UNCONFORMITIES Angular unconformities are characterized by an erosional surface which truncates folded or dipping (tilted) strata. Overlying strata are deposited basically parallel with the erosion surface. The rocks above and below the unconformity are at an angle to one another.
C. Nonconformity This occurs when igneous or metamorphic rocks are eroded and younger sedimentary rocks are deposited on the erosional surface.
NONCONFORMITIES Nonconformities are characterized by an erosional surface which truncates igneous or metamorphic rocks. At a nonconformity, sedimentary rocks unconformably overlie igneous or metamorphic rocks.
DisconformityNonconformity Angular Unconformity
Cambro-Ordovician sea floor life.
Relative time scale Faunal succession. Selected members of the long-extinct group, trilobites, indicate changing styles with time
Evolution of life Correlating rocks, etc. Correlation by fossils. Certain index fossils are keys to matching sedimentary strata in widely separated outcrops
Fly reserved in amber for over 3 million years.
Carbonization conversion to coal carbon film impressions
PRACTICE
1.Shale 2.Sandstone 3.Shale 4.Conglomerate 5.Shale 6.Limestone 7.Tilting 8.Angular unconformity 9.Igneous (granite) 10.Shale 11.Sandstone
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