ES 105 I II 12 January 2006 Chapter 11 Correlation of rock layers a Develop geologic time scale for entire Earth b Match up rocks in different regions c Example i Bryce Canyon ii Grand Canyon iii Zion Canyon d Correlation i can be done by tracing from one area to another if they are close and continuous ii from one area to another gives us a more complete view of geologic history iii Usually relies upon fossils e Correlation across southwestern United States Fossils evidence of past life a Types of fossilization Petrified Formed by replacement Mold Cast Carbonization Impression Preservation in amber Indirect b With enough time most remains will be modified by geologic processes c Examples i petrified show oreodont from John Day Fmtn ii formed by replacement show wood on doc cam microscopic details may be preserved iii mold show bivalve 1 perfected when original material dissolves 2 only shows shape and surface markings iv cast space in mold filled with mineral material v carbonization 1 insect in book is also a compression fossil 2 show graptolite carbon film also has impression vi Impression when carbon is lost 1 of fishes from book 2 show leaf vii Preservation in amber show picture in rocks and fossils d Indirect evidence includes i Tracks footprints in soft sediment ii Burrows worms mammals some clams III iii Coprolites dung stomach contents tell us what animal ate iv Gastroliths gizzard stones v Track of dinosaur at Tuba City e Preservation of evidence of past life actually not common i Rapid burial of the organism or its trace ii Possession of hard parts soft parts very uncommon in fossil record Fossils and correlation a Principle of faunal succession i fossils in beds of differing ages have distinctly different fossils and the order of occurrence is definite and determinable 1 William Smith observed this in excavations for canals in England in the late 1700s 2 Beds in widely separated areas could be predicted by noting the fossils in beds above compared to the same fossils in another area ii Fossils organisms succeed one another in a definite and determinable order and therefore any time period can be recognized by this fossil content 1 Age of trilobites age of fishes age of reptiles age of mammals 2 Recognition of fossils as time indicators became a very useful means of correlation of rock units iii Index fossils 1 index fossils are particular organisms that define an age quite specifically 2 exist for a short range of geologic time 3 are widespread geographically 4 however some formations do not contain index fossils a Groups of fossils with overlapping ages can be utilized in cases where index fossils are absent b Assemblage of fossils is useful here c Notice the bivalve fern starfish and brachipod span long lengths of time d However the trilobite vertebrate and maple have much shorter times e Noting the maple and the vertebrate in rock unit A defines its time of formation quite precisely f Similarly the trilobite and fern define rock unit B to be older than unit A 5 Methods such as these have allowed precise correlation of rock units in the Colorado Plateau area IV Geologic Time Scale a Subdivides 4 5 billion years 4500 million years b Eons based on large changes in the fossil record i Hadean hidden life ii Archean primitive life cyanobacteria is a plant like singlecelled organism iii Proterozoic early multi cellular organisms without hard parts iv Phanerozoic life that can be seen beginning of hard parts c The first three Eons are collectively known as the preCambrian Cambrian is the first subdivision of the first Phanerozoic Era d Eras divisions of the Phanerozoic i Paleozoic Ancient Life nearly all major groups of life phyla evolve in this era 1 Arthropods dominant early 2 fishes significant in the middle 3 land plants 4 insects 5 amphibians abundant later 6 reptiles 7 mammals only minor presence 8 Paleozoic ended with the greatest mass extinction in geologic record a 90 of marine species die out 70 of terrestrial vertibrates b Fungus dominant life form on land ii Mesozoic Middle Life retiles rise to prominence 1 Dinosaurs rule Earth 2 Birds evolve from dinosaurs 3 Flowering plants appear 4 Ends with another great extinction 50 of all genera including non avian dinosaurs iii Cenozoic Era Recent Life rise of mammals iv Each Era is divided into Periods distinguished by lesspronounced changes in life 1 Periods divided into Epochs each of which may be several million years long 2 Periods are tens to hundreds of millions of years long 3 Eras may be several hundred million years 4 Eons hundreds to thousands of millions of years e Actual numeric dates of the subdivisions of the geologic time scale determined by radiometric age dating of igneous rocks i Ash layers within sedimentary sequence ii Crosscutting relationships of intrusive rocks iii Thousands of instances give consistent results of age
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