Stratigraphy Rocks, Fossils, and Time

Introduction Stratigraphy crucial to Historical Helps us understand vertical and lateral changes in rocks/environments with time Helps us understand paleogeograpy e.g. where the land was, where the ocean was, where the mountains were, etc

What is Stratigraphy? A branch of geology concerned with composition, origin, age relationships, and geographic extent of layered or stratified rocks Valley of the Gods, Utah

Stratigraphy Stratigraphic section Tracing stratigraphic sections from outcrops leads to correlation of rock units Sequence of sedimentary rocks on a bed-by-bed basis Demonstrates equivalency across an area To 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

Sedimentary Facies Body of sediment/rock characterized by physical, chemical, and biological attributes Sediment/rock makes it recognizable and different from laterally adjacent rock Could be textures Could be fossils Could be composition Transgression – Sea Level rises, environments march landward Regression– Sea Level drops, environments march seaward

Lateral Changes in Stratigraphy Beds may change laterally in thickness or lithology Differences due to changes in sedimentation rate or depositional environment

Lateral Changes Pinchout Intertonguing Lateral Gradation

Walther’s Law Those facies found in a conformable (no faults, unconformities, folds) lateral sequence will replace one another vertically

Conformable Vertical Successions Transgression Regression Sea level ↑; shoreline moves inalnd Sea level ↓; shoreline moves seaward Facies are time transgressive Ages of facies vary from place to place

Did sea level rise or fall?

Transgressive Sequence Note time lines cut across units

Regressive Sequence

Transgressive and Regressive Sequence V-shaped pattern produced by migrating facies during a transgression followed by a regression Three stratigraphic sections illustrate facies generated in vertical succession at three different locations

Causes of Transgressions and Regressions Cold and warm climates Cold climate  glaciation Warm climate  ice melt Uplift and Subsidence Uplift  regression Subsidence  transgression Seafloor spreading Fast spreading  transgression Slow spreading  regression

Fossils and Correlation William Smith 1769-1839 Determined relative ages of rocks using fossils and assemblages of fossils Established Principle of Faunal Succession

Principle of Fossil Succession Fossils succeed one another through time in a regular and determinable order Because of this, any time period can be recognized by its fossil content

Fossil Assemblages Species have a time of origination and a time of extinction Because of this, different times have different fauna and flora If sedimentary rocks from different places contain the same fossils, they were deposited at the same time If fossil A range = 40 – 20 my If fossil B range = 25 – present What is age of rocks containing last appearance of A & first appearance of B.

Establishing Correlation of Units How do we establish correlation or equivalency of units?