Lecture 6: Dating Rocks and Fossil Succession PDF

Lecture 6: Dating Rocks and Fossil Student Accessibility Succession Services ©JoAnna Wendel Reminder Quiz 2 due tonight at 11:59pm! Objectives for Today Last class: plate boundaries, geologic principles, unconformities...

Lecture 6: Dating Rocks and Fossil Student Accessibility Succession Services ©JoAnna Wendel Reminder Quiz 2 due tonight at 11:59pm! Objectives for Today Last class: plate boundaries, geologic principles, unconformities Today: Understand the differences between relative and absolute dating Learn the principle of fossil succession Combine all principles to reconstruct geologic history and age beds  Time Deep time: Earth is very old, and the geologic time scale is vast two typesrods of dating Relative dating: Order of deposition of a body of rock based on position → uses lithostratigraphy, biostratigraphy, magnetostratigraphy, chemostratigraphy Answers, “what age are the rocks?” L which ones < relative other torocks older first what came Absolute dating: A number representing the time a body of rock was deposited → radiometric dating Answers, “how old are the rocks?” Image: Stanley (2009)  Lithostratigraphic correlation Correlation based on rock types youngest and sequences & , - pitches, oldest 2nd Rock units are correlated Fig 5.18 oldest without regard to their age -Sequence  Lithostratigraphic correlation Fig 5.9 Principle of Fossil Succession extinct Vertical ordering of fossils in geological record Fossil assemblages (groups of fossils) vary through time Relative ages of fossil assemblages determined using superposition Occurrence of fossils can be independent of rock type ~ vague Divided into biozones that do not necessarily correspond to those of lithostratigraphic units Biostratigraphy Index fossil (Guide fossil): Any easily identified fossil with a evolved representsrange possible wide geographic distribution and short geologic range; - useful for determining relative ages of strata in different -specific areas. better Fig 5.19 Fig 5.15  Time Deep time: Earth is very old, and the geologic time scale is vast Relative dating: Order of deposition of a body of rock based on position → uses lithostratigraphy, biostratigraphy, magnetostratigraphy, chemostratigraphy Answers, “what age are the rocks?” Absolute dating: A number representing the time a body of rock was deposited → radiometric dating Answers, “how old are the rocks?” Image: Stanley (2009)  Absolute age unstable decaye will get a Radioactive decay , will decad into Radioactive molecules - (parent isotopes) of certain elements are locked into igneous rocks during cooling After crystallization, parent isotope begins to decay (break down) This decay produces the daughter isotope (stable form of element) half-life Fig 4.9 Amount of decay indicates time since rock was formed Image: Chernicoff and Whitney (2002)  Absolute age Isotopic dating: Radioactive isotopes decay at constant exponential rate Eventually, half of the parent present will survive and half will decay to daughter Half-life: interval of time for half of parent to decay Fig. 4.8  Absolute age Radiocarbon dating: 14 Half life = 5730 years Carbon Maximum age for dating: ~70,000 yearsamount the Lafter this Bone, teeth, wood , left of parent isotope two is Mammuthus columbi smalltoea Fig 4.12 Images: commons.wikimedia.org; Absolute age  Chronostratigraphy Combines relative dating (lithostratigraphy and biostratigraphy) with radiometric dating methods Integration of multiple methods generates the most robust signals, to put dates on rock strata (layers) Sequences of rocks are broken up into blocks of time based on the fossils they contain. Volcanic layers (geologically ‘instantaneous’), given the absolute context. used ask deposits Commonly - to exact get arges  Absolute age Some physical events of short duration can demonstrate time equivalence between two or more widely separated rock bodies Examples: lava flows and volcanic ash falls Fig 5.21  Numerical Ages for Fossil Zones Provided by Ash Fall picture (relative) of dates big /range dates ↳ group exact Ash falls, plutons, lava flows, and metamorphic rocks associated with fossil-bearing sedimentary rocks Have provided numerical ages for the geologic time scale Igneous and metamorphic rocks associated with fossil-bearing rocks May provide age ranges for the fossils Fig 5.23 Let’s put it all together (crosscut BFAC DE Ages of Sedimentary Rocks youngest Fig 5.22 Ages of Sedimentary Rocks Fig 5.22  1) deposition of Trekker Formation 2) folding of Trekker Formation 3) Igneous intrusion A B 4) erosion/unconformity between Trekker Fm and Old Red Sandstone (Unconformity or angular unconformity or disconformity between A and Old Red Sandstone) 5) deposition of Old Red A Sandstone (can mention size sorting, large clasts and small clasts) Trekker Formation 6) Igneous intrusion B Fish is between 170-150 million years old  -last · -First