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Structural Geology- Introduction Basic Principles for Determining Sequence of Events Law of superposition: In a sequence of sedimentary rocks that have not been disturbed, the oldest rocks will be on the bottom while the oungest will be on top Law of crosscutting relationships: That which is cut is older than that which cuts . Law of inclusions: An inclusion is older than the rock that contains the inclusion Law of initial horizontality Sedimentary rocks are deposited in basins. The lower surface of any sequence of rock will conform to the shape of the basin, while the upper surface will tend toward horizontality Law of initial continuity: Any deposit of rocks will initially be continuous, without any sharp breaks or changes in the rocks Law of faunal succession: Ancient life forms are unique for the period of that they exist. Life forms do not replicate themselves through time. Therefore, an individual fossil* or fossil group will be unique to a certain time. These fossils or fossil groups can be used to determine relative time Unconformities An unconformity is a break in the rock record representing a loss of rock record or a time hiatus; erosion or non-deposition. The rocks above an unconformity surface are ounger than the rocks below the unconformity surface Nonconformity: Sediments deposited on crystalline rocks. The unconformity surfac preserves a history of intrusion or metamorphism, uplift and erosion followed by subsidence and deposition sedimentary rocks /unconformity surface fragments of crystalline rocks Last revised August 27, 1999 Fall 1999 ! " # $ " %& & ' $ & ' & ' ( & ! ' $ ! & ' ( & ' ( $ & ) $ '"# ( % * + '