Chapter 5 Rocks Fossils and Time Making Sense of the Geologic Record 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 The geologic record is complex and requires interpretation which we will try to do Uniformitarianism is useful for this activity Geologic Record for nearly 14 million years of Earth history preserved at Sheep Rock in John Day Fossil Beds National Monument Oregon Fossils in these rocks provide a record of climate change and biological events 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 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 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 but some sedimentary structures such as cross bedding and some fossils allow geologists to resolve these kinds of problems we will discuss the use of sedimentary structures more fully later in the term Principle of Inclusions According to the principle of inclusions which also helps to determine relative ages inclusions or fragments in a rock are older than the rock itself Light colored granite in northern Wisconsin showing basalt inclusions dark Which rock is older Basalt because the granite includes it 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 conformable strata 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 In the process of forming an unconformity deposition began 12 million years ago MYA continuing until 4 MYA For 1 million years erosion occurred removing 2 MY of rocks and giving rise to a 3 million year hiatus 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 in Colorado between steeply dipping Pennsylvanian rocks and overlying Cenozoic aged conglomerate 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 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 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