2 Introduction Stratigraphy crucial to Historical Helps us understand vertical and lateral changes in rocks/environments with timeHelps us understand paleogeograpy e.g. where the land was, where the ocean was, where the mountains were, etc
3 What is Stratigraphy?A branch of geology concerned with composition, origin, age relationships, and geographic extent of layered or stratified rocksValley of the Gods, Utah
4 Stratigraphy Stratigraphic section Tracing stratigraphic sections from outcrops leads to correlation of rock unitsSequence of sedimentary rocks on a bed-by-bed basisDemonstrates equivalency across an areaTo draw a stratigraphic section, you must have data from a sequence of rocks. You will need to have data on the thickness of each bed, and all of the physical, chemical, and biological characteristics of that bed, as well as the character of its contacts. Before you start, you need to examine your data to determine the total thickness of the section you plan to draw. Then determine a proper scale so that the entire section will fit on your paper (such as, 1" = 100'). Draw a vertical column in which you will plot your data, and then mark off the thickness of each bed or formation using the scale you established. Draw in the contacts between units; if the contacts are erosional, you should use a wavy line. Once you have drawn in contacts, draw in the lithologic symbols for each unit. Information on fossils and sedimentary structures, etc. may be placed within the unit, or beside it using a special symbol or small sketch. Color may be illustrated with a special symbol, or by coloring your section. There are standard symbols which have been established by oil companies and core logging companies. You may use theirs (see a reference book), or create your own. Your instructor may give you further instruction on this.Once you have drawn several stratigraphic sections for an area, you may begin to correlate them
5 Sedimentary FaciesBody of sediment/rock characterized by physical, chemical, and biological attributesSediment/rock makes it recognizable and different from laterally adjacent rockCould be texturesCould be fossilsCould be compositionTransgression – Sea Level rises, environments march landwardRegression– Sea Level drops, environments march seaward
6 Lateral Changes in Stratigraphy Beds may change laterally in thickness or lithologyDifferences due to changes in sedimentation rate or depositional environment
13 Transgressive and Regressive Sequence V-shaped pattern produced by migrating facies during a transgression followed by a regressionThree stratigraphic sections illustrate facies generated in vertical succession at three different locations
14 Causes of Transgressions and Regressions Cold and warm climatesCold climate glaciationWarm climate ice meltUplift and SubsidenceUplift regressionSubsidence transgressionSeafloor spreadingFast spreading transgressionSlow spreading regression
15 Fossils and Correlation William SmithDetermined relative ages of rocks using fossils and assemblages of fossilsEstablished Principle of Faunal Succession
16 Principle of Fossil Succession Fossils succeed one another through time in a regular and determinable orderBecause of this, any time period can be recognized by its fossil content
17 Fossil AssemblagesSpecies have a time of origination and a time of extinctionBecause of this, different times have different fauna and floraIf sedimentary rocks from different places contain the same fossils, they were deposited at the same timeIf fossil A range = 40 – 20 myIf fossil B range = 25 – presentWhat is age of rocks containing last appearance of A & first appearance of B.
18 Establishing Correlation of Units How do we establish correlation or equivalency of units?