Chapter 5 Rocks, Fossils and Time—Making Sense of the Geologic Record
Stratigraphy • Stratigraphy deals with the study of any layered (stratified) rock, but primarily with sedimentary rocks and their • composition • origin • age relationships • geographic extent • Many igneous rocks • such as a succession of lava flows or ash beds are stratified and obey the principles of stratigraphy • Many metamorphic rocks are stratified
Stratified Igneous Rocks • Stratification in a succession of lava flows in Oregon.
Stratified Sedimentary Rocks • Stratification in sedimentary rocks consisting of alternating layers of sandstone and shale, in California.
Stratified Metamorphic Rocks • Stratification in Siamo Slate, in Michigan
Vertical Stratigraphic Relationships • Surfaces known as bedding planes separate individual strata from one another • or the strata grade vertically from one rock type to 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
Age of Lava Flows, Sills • Determining the relative ages of lava flows, sills and associated sedimentary rocks uses alteration by heat and inclusions • How can you determine whether a layer of basalt within a sequence of sedimentary rocks is a buried lava flow or a sill? • A lava flow forms in sequence with the sedimentary layers. • Rocks below the lava will have signs of heating but not the rocks above. • The rocks above may have lava inclusions.
Sill • A sill will heat the rocks above and below. • The 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 conformablestrata, which are sequences of rocks in which deposition was more or less continuous • Unconformities in sequences of strata represent times of nondeposition and/or erosion that encompass long periods of geologic time, perhaps millions or tens of millions of years • The rock record is incomplete. • The interval of time not represented by strata is a hiatus.
The origin of an unconformity • For 1 million years erosion occurred and removed 2 MY of rocks • and giving rise to a 3 million year hiatus • The process of forming an unconformity • deposition began 12 million years ago (MYA), • continues until 4 MYA • The last column • is the actual stratigraphic record • with an unconformity
Types of Unconformities • Three types of surfaces can be unconformities: • A disconformity is a surface separating younger from older rocks, both of which are parallel to one another • A nonconformity is an erosional surface cut into metamorphic or intrusive rocks and covered by sedimentary rocks • An angular unconformity is an erosional surface on 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
A Disconformity • A disconformity between sedimentary rocks in California, with conglomerate deposited upon an erosion surface in the underlying rocks
An Angular Unconformity • An angular unconformity, Santa Rosa
A Nonconformity • A nonconformity in South Dakota separating Precambrian metamorphic rocks from the overlying Cambrian-aged Deadwood Formation
Lateral Relationships • In 1669, Nicolas Steno proposed his principle of lateral continuity, meaning that layers of sediment extend outward in all directions until they terminate • Terminations may be • Abrupt at the edge of a depositional basin where eroded • where truncated by faults
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
Sedimentary Facies • 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 • while mud and carbonate deposition takes place at the same time in offshore low-energy environments
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 as the sea progressively covers more and more of a continent
Marine Transgressions • Each laterally adjacent depositional environment produces a sedimentary facies • During a transgression, the facies forming offshore become superposed upon facies deposited in nearshore environments
Marine Transgression • The 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 • meaning the ages vary from place to place
A Marine Transgression in the Grand Canyon • Three formations deposited in a widespread marine transgression exposed in the walls of the Grand Canyon, Arizona
Marine Regression • During a marine regression, sea level falls with respect to the continent • the environments paralleling the shoreline migrate seaward
Marine Regression • A marine regression • is the opposite of a marine transgression • It yields a vertical sequence with nearshore facies overlying offshore facie sand rock units become younger in the seaward direction
Walther’s Law • Johannes Walther (1860-1937) noticed that the same facies he found laterally were also present in a vertical sequence, now called Walther’s Law • holds that • the facies seen in a conformable vertical sequence will also replace one another laterally • Walther’s law applies to marine transgressions and regressions
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 sequences, bounded by 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 • Uplift of continents causes regression • Subsidence causes transgression • Widespread glaciation causes regression • due to the amount of water frozen in glaciers • Rapid seafloor spreading, • expands the mid-ocean ridge system, • displacing seawater onto the continents • Diminishing seafloor-spreading rates • increases the volume of the ocean basins • and causes regression
Fossils • Fossils are the remains or traces of prehistoric organisms • They are most common in sedimentary rocks and in some accumulations of pyroclastic materials, especially ash • They are extremely useful for determining relative ages of strata but geologists also use them to ascertain environments of deposition • Fossils provide some of the evidence for organic evolution and many fossils are of organisms now extinct
How do Fossils Form? • Remains of organisms are called body fossils. and consist mostly of durable skeletal elements such as bones, teeth and shells • rarely we might find entire animals preserved by freezing or mummification
Body Fossil • Skeleton of a 2.3-m-long marine reptile in the museum at Glacier Garden in Lucerne, Switzerland
Body Fossils • Shells of Mesozoic invertebrate animals known as ammonoids and nautiloids on a rock slab in the Cornstock Rock Shop in Virginia City Nevada
Trace Fossils • Trace fossils are indications of organic activity including • tracks, • trails, • burrows, • nests • A coprolite is a type of trace fossil consisting of fossilized feces which may provide information about the size and diet of the animal that produced it
Trace Fossils • Paleontologists think that a land-dwelling beaver called Paleocastormade this spiral burrow in Nebraska
Trace Fossils • Fossilized feces (coprolite) of a carnivorous mammal • 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, mummification, in amber, in tar • altered remains, with some change in composition • permineralized • recrystallized • replaced • carbonized
Unaltered Remains • Insects in amber • Preservation in tar
Unaltered Remains • 40,000-year-old frozen baby mammoth found in Siberia in 1971. It is 1.15 m long and 1.0 m tall and it had a hairy coat. • Hair around the feet is still visible
Altered Remains • Petrified tree stump in Florissant Fossil Beds National Monument, Colorado • Volcanic mudflows 3 to 6 m deep covered the lower parts of many trees at this site
Altered Remains • Carbon film of a palm frond • Carbon film of an insect
Molds and Casts • Molds form when buried remains leave a cavity • Casts form if material fills in the cavity
Mold and Cast Step a: burial of a shell Step b: dissolution leaving a cavity, a mold Step c: the mold is filled by sediment forming a cast
Cast of a Turtle • Fossil turtle showing some of the original shell material • body fossil • and a cast
Fossil Record • The fossil record is the record of ancient life preserved as fossils in rocks • Just as the geologic record must be analyzed and interpreted, so too must the fossil record • The fossil record is a repository of prehistoric organisms that provides our only knowledge of such extinct animals as trilobites and dinosaurs
WHY is the fossil record incomplete??? Why are there large gaps of time and biological strata?
Fossil Record • The fossil record is very incomplete because of destruction to organic remains • bacterial decay • physical processes • scavenging • metamorphism • In spite of this, fossils are quite common
Fossils and Telling Time • William Smith • 1769-1839, an English civil engineer independently discovered Steno’s principle of superposition • Realized that fossils in rocks followed the same principle • He discovered that sequences of fossils, especially groups of fossils, are consistent from area to area • Thereby discovering a method of relatively dating sedimentary rocks at different locations
Fossils from Different Areas • To compare the ages of rocks from two different localities • Smith used fossils
Principle of Fossil Succession • Using superposition, Smith was able to predict the order in which fossils would appear in rocks not previously visited • Alexander Brongniart in France also recognized this relationship • Their observations lead to the principle of fossil succession