Ch 5 rocks, fossils, and time

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Ch Rocks, Fossils, and Time ESCI 102 Geologic Record • The fact that Earth has changed through time is apparent from evidence in the geologic record • The geologic record is the record of events preserved in rocks • Although all rocks are useful in deciphering the geologic record, sedimentary rocks are especially useful • We will learn to interpret the geologic record using uniformitarianism Geologic Record • Fossils in these rocks provide a record of climate change and biological events • The rocks themselves help reconstruct the environment John Day Fossil Beds National Monument, Stratigraphy • Stratigraphy deals with the study of any layered (stratified) rock, but primarily with sedimentary rocks and their • • • • composition origin age relationships geographic extent • Sedimentary rocks are almost all stratified • Many igneous rocks and metamorphic rocks are also stratified Stratified Igneous Rocks • Stratification in a succession of lava flows in Oregon Stratified Metamorphic Rocks • Stratification in Siamo Slate, in Michigan Stratified Sedimentary Rocks • Stratification in sedimentary rocks consisting of alternating layers of sandstone and shale, in California Vertical Stratigraphic Relationships • Surfaces known as bedding planes – separate individual strata from one another • Rocks above and below a bedding plane differ – in composition, texture, color – or a combination of these features • The bedding plane signifies – a rapid change in sedimentation – or perhaps a period of nondeposition Superposition • Nicolas Steno realized that he could determine the relative ages of horizontal (undeformed) strata by their position in a sequence • In deformed strata, the task is more difficult – sedimentary structures, such as cross-bedding, and fossils – allow geologists to resolve these kinds of problems • more later in term Principle of Inclusions • According to the principle of inclusions – inclusions or fragments in a rock are older than the rock itself • Light-colored granite showing basalt inclusions (dark) • Which rock is older? – basalt, because the granite includes it northern Wisconsin Fossils from Different Areas • Compare the ages of rocks from different localities Principle of Fossil Succession • Using superposition, Smith was able to predict the order in which fossils would appear in rocks not previously visited – lead to the principle of fossil succession Principle of Fossil Succession • Principle of fossil succession – holds that fossil assemblages (groups of fossils) succeed one another through time in a regular and determinable order • Why not simply match up similar rocks types? – because the same kind of rock has formed repeatedly through time • Fossils also formed through time, but because different organisms existed at different times, fossil assemblages are unique Matching Rocks Using Fossils youngest oldest • The youngest rocks are in column B • Whereas the oldest are in column C Relative Geologic Time Scale • Investigations of rocks by naturalists between 1830 and 1842 based on superposition and fossil succession – resulted in the recognition of rock bodies called systems – and the construction of a composite geologic column that is the basis for the relative geologic time scale Geologic Column and the Relative Geologic Time Scale Absolute ages (the numbers ) were added much later Correlation • Correlation is the process of matching up rocks in different areas • There are two types of correlation: – lithostratigraphic correlation • simply matches up the same rock units over a larger area with no regard for time – time-stratigraphic correlation • demonstrates time-equivalence of events Lithostratigraphic Correlation • Correlation of lithostratigraphic units such as formations – traces rocks laterally across gaps Time Equivalence • Because most rock units of regional extent are time transgressive we cannot rely on lithostratigraphic correlation to demonstrate time equivalence – for example: sandstone in Arizona is correctly correlated with similar rocks in Colorado and South Dakota • but the age of these rocks varies from Early Cambrian in the west to middle Cambrian farther east (THAT'S MILLIONS OF YEARS!) Time Equivalence • For all organisms now extinct, their existence marks two points in time – their time of origin – their time of extinction • One type of biozone, the range zone, – is defined by the geologic range • total time of existence – of a particular fossil group, a species, or a group of related species called a genus • Most useful are fossils that are – easily identified – geographically widespread – had a rather short geologic range Guide Fossils • The brachiopod Lingula is not useful because, although it is easily identified and has a wide geographic extent, – it has too large a geologic range • The brachiopod Atrypa and trilobite Paradoxides are well suited for time-stratigraphic correlation – because of their short ranges • They are guide fossils Short Duration Physical Events • Some physical events of short duration are also used to demonstrate time equivalence: – distinctive lava flow • would have formed over a short period of time – ash falls • take place in a matter of hours or days • may cover large areas • are not restricted to a specific environment • Absolute ages may be obtained for igneous events using radiometric dating Absolute Dates and the Relative Geologic Time Scale • Ordovician rocks – are younger than those of the Cambrian – and older than Silurian rocks • But how old are they? – When did the Ordovician begin and end? • Since radiometric dating techniques work on igneous and some metamorphic rocks, but not generally on sedimentary rocks, this is not so easy to determine Indirect Dating • Absolute ages of sedimentary rocks are most often found by determining radiometric ages of associated igneous or metamorphic rocks Indirect Dating • Combining thousands of absolute ages associated with sedimentary rocks of known relative age gives the numbers on the geologic time scale [...]... abrupt • at the edge of a depositional basin, and • where eroded • where truncated by faults Gradual Terminations – or they may be gradual • where a rock unit becomes progressively thinner until it pinches out • or where it splits into thinner units each of which pinches out, called intertonging • where a rock unit changes by lateral gradation as its composition and/ or texture becomes increasingly different... heat the rocks above and below – sill might also have inclusions of the rocks above and below – but neither of these rocks will have inclusions of the sill Unconformities • So far we have discussed vertical relationships among conformable strata • sequences of rocks in which deposition was more or less continuous • Unconformities in sequences of strata represent times of nondeposition and/ or erosion that... Both intertonging and lateral gradation indicate simultaneous deposition in adjacent environments • A sedimentary facies is a body of sediment – with distinctive physical, chemical and biological attributes deposited side-by-side with other sediments in different environments Sedimentary Facies • On a continental shelf, sand may accumulate in the high-energy nearshore environment • Mud and carbonate deposition... • Mud and carbonate deposition takes place at the same time in offshore low-energy environments ∴ Different Facies Marine Transgressions • A marine transgression occurs when sea level rises with respect to the land • During a marine transgression – the shoreline migrates landward – the environments paralleling the shoreline migrate landward • Each laterally adjacent depositional environment produces... environments Marine Transgression • Rocks of each facies become younger in a landward direction during a marine transgression • One body of rock with the same attributes (a facies) was deposited gradually at different times in different places so it is time transgressive – ages vary from place to place older shale younger shale A Marine Transgression in the Grand Canyon • Three formations deposited in... such as bones, teeth and shells – rarely we might find entire animals preserved by freezing or mummification Trace Fossils • Indications of organic activity including tracks, trails, burrows, and nests are called trace fossils • A coprolite is a type of trace fossil consisting of fossilized feces that may provide information about the size and diet of the animal that produced it Trace Fossils • A land-dwelling... transgressions and regressions adapted from Van Wagoner et al., 1990; ~strata/sequence/pa rasequences.html Extent and Rates of Transgressions and Regressions • Since the Late Precambrian, 6 major marine transgressions followed by regressions have occurred in North America • These produce rock sequence, bounded by unconformities, that provide the structure for U.S Paleozoic and Mesozoic... both of which are parallel to one another (implies sed rx) – nonconformity • cuts into metamorphic or intrusive rocks • is covered by sedimentary rocks – angular unconformity • tilted or folded strata • over which younger rocks were deposited Types of Unconformities • Unconformities of regional extent may change from one type to another • They may not represent the same amount of geologic time everywhere... specimen measures about 5 cm long and contains small fragments of bones Body Fossil Formation • The most favorable conditions for preservation of body fossils occurs when the organism – possesses a durable skeleton of some kind – and lives in an area where burial is likely • Body fossils may be preserved as – unaltered remains, meaning they retain their original composition and structure,by freezing,... unconformities, that provide the structure for U.S Paleozoic and Mesozoic geologic history • Shoreline movements are a few centimeters per year • Transgression or regressions with small reversals produce intertonging Causes of Transgressions and Regressions Causes of Transgressions and Regressions • Uplift of continents causes local regression • Subsidence causes local transgression • Widespread glaciation
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