ES 105 11 January 2007 Chapter 11 I. Unconformities a. •An unconformity is a break in the rock record b. •Types of unconformities i. •Angular unconformity – 1. tilted rocks are overlain by flat-lying rocks 2. tilting event, erosion, deposition ii. •Nonconformity –look for inclusions!! 1. •Metamorphic or igneous rocks below 2. •Younger sedimentary rocks above iii. •Disconformity – hardest to recognize 1. strata on either side are parallel 2. lack of sedimentation, or actually erosion II. Correlation of rock layers a. Match up rocks in different regions b. 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 c. Correlation across southwestern United States i. Bryce Canyon ii. Grand Canyon iii. Zion Canyon d. Develop geologic time scale for entire Earth III. Fossils—evidence of past life a. Types of fossilization i. Petrified ii. Formed by replacement iii. Mold iv. Cast v. Carbonization vi. Impression vii. Preservation in amber viii. Trace evidence b. With enough time, most remains will be modified by geologic processes c. Examples i. petrified—oreodont from John Day Formation. ii. formed by replacement—microscopic details may be preserved iii. mold— 1. perfected when original material dissolves 2. only shows shape and surface markings iv. cast—space in mold filled with mineral material v. carbonization—vi. Impression when carbon is lost vii. Preservation in amber— d. Trace evidence includes i. Tracks—footprints in soft sediment: of dinosaur at Tuba City ii. Burrows—worms, mammals, some clams iii. Coprolites—dung, stomach contents tell us what animal ate iv. Gastroliths—gizzard or stomach stones 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 fossils IV. 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: 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 V. 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 single-celled 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 Period of the first Era of the Phanerozoic Eon) 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. reptiles7. 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 vertebrates b. Fungus dominant life form on land ii. Mesozoic Era—Middle Life: reptiles 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 less-pronounced 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 VI. Index fossils—particular organisms that define an age quite specifically a. exist for a short range of geologic time b. are widespread geographically c. however, some formations do not contain index fossils i. Groups of fossils with overlapping ages can be utilized in cases where index fossils are absent ii. Assemblage of fossils is useful here iii. Notice the bivalve, fern, starfish and brachipod span long lengths of time iv. However, the trilobite, vertebrate and maple have much shorter times v. Noting the maple and the vertebrate in rock unit A defines its time of formation quite precisely vi. Similarly, the trilobite and fern define rock unit B to be older than unit A d. Methods such as these have allowed precise correlation of rock units in the Colorado Plateau
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