Unit 4: Stratigraphy, Fossils, and Earth's History

Concept of Stratigraphy

Definition: Stratigraphy is the branch of geology concerned with the study of rock layers (strata) and their sequence in time (layering).

It focuses on the order, correlation, and interpretation of layered rocks (mainly sedimentary rocks) to understand Earth's history.

The Principles of Stratigraphy

These are the fundamental "rules" geologists use to read the rock record. They were largely developed by Nicolaus Steno in the 17th century.

1. Principle of Superposition: In an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom, and the youngest layers are at the top.
2. Principle of Original Horizontality: Sediments are almost always deposited in horizontal layers. If layers are tilted, folded, or faulted, it means they were deformed by tectonic activity *after* they were deposited.
3. Principle of Lateral Continuity: Sedimentary layers extend horizontally in all directions until they thin out, grade into another rock type, or are cut off by an erosional edge. This allows usto correlate rock layers across valleys.
4. Principle of Cross-Cutting Relationships: Any geologic feature (like a fault or an igneous intrusion) that cuts across another feature must be younger than the feature it cuts.
5. Principle of Faunal Succession: Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. (This is the basis for using index fossils).

Understanding the past from stratigraphic records

The stratigraphic record is like a history book with most of its pages missing and the rest out of order. Geologists reassemble this history by:

• Correlation: Matching up rock layers (strata) of the same age from different locations. This can be done by:
  • Lithostratigraphy: Matching based on rock type.
  • Biostratigraphy: Matching based on the fossils they contain (the most reliable method).
• Interpreting Past Environments: The type of rock tells a story. For example:
  • Sandstone with ripple marks might indicate a beach or river bed.
  • Shale (made of mud) indicates a calm, deep-water environment like a lake bottom or deep ocean.
  • Coal indicates a swamp environment.

Nature of stratigraphic records

The rock record is not continuous. There are large gaps in time where either no rocks were deposited or rocks were eroded away. These gaps are called Unconformities.

Unconformities

Definition: An unconformity is a buried surface of erosion or non-deposition that separates two rock masses of different ages. It represents missing time in the geological record.

There are three main types:

1. Disconformity: An unconformity between parallel sedimentary layers. This represents a period of erosion or non-deposition, but with no tilting or folding.
2. Angular Unconformity: An unconformity where older, tilted or folded sedimentary layers are overlain by younger, horizontal layers. This shows a sequence of: 1. Deposition, 2. Tilting/Folding, 3. Erosion, 4. New Deposition.
3. Nonconformity: An unconformity where younger sedimentary rocks lie on top of older, eroded igneous or metamorphic rocks.

Introduction to age dating of rocks

Geologists use two main methods to date rocks and events in Earth's history.

Relative Dating

• What it is: Determining the chronological order of events (i.e., this rock is older or younger than that rock). It does *not* provide a specific age in years.
• How it's done: By applying the Principles of Stratigraphy (Superposition, Cross-Cutting, etc.) and using fossils.
• Example: In the Grand Canyon, we know the lowest layer (Vishnu Schist) is the oldest, and the top layer (Kaibab Limestone) is the youngest.

Absolute Dating (Radiometric Dating)

• What it is: Determining the actual age of a rock or fossil in years (e.g., 66 million years old).
• How it's done: By measuring the decay of radioactive isotopes in minerals.
  • Radioactive isotopes (the "parent") are unstable and decay at a fixed, predictable rate into a stable "daughter" isotope.
  • The time it takes for half of the parent atoms to decay into daughter atoms is called the half-life.
  • By measuring the ratio of parent-to-daughter atoms in a mineral, scientists can calculate how many half-lives have passed since the mineral formed, thus giving its age.
• Common Examples:
  • Uranium-Lead (U-Pb) Dating: Used for very old rocks (billions of years).
  • Potassium-Argon (K-Ar) Dating: Used for volcanic rocks.
  • Radiocarbon (Carbon-14) Dating: Used for young (less than ~50,000 years), organic materials (like wood, bones, fossils). It cannot be used on rocks.

Fossil and its types

Definition: A fossil is the preserved remains, trace, or imprint of a once-living organism from a past geological age.

Types of Fossils (Modes of Preservation)

1. Body Fossils (Preserved Remains):
   • Unaltered Remains: Rare. The original hard parts (or even soft parts) are preserved. Examples: Insects in amber, mammoths frozen in ice, original shells.
   • Altered Remains: The original material is changed.
     • Petrifaction (or Petrification): "Turned to stone." This happens in two ways:
       • Permineralization: Minerals (like silica) dissolved in groundwater fill the empty pore spaces (e.g., petrified wood).
       • Replacement: The original material (like a shell) dissolves away and is simultaneously replaced by new minerals.
     • Carbonization: Only a thin black film of carbon is left, common for plants and fish.
2. Molds and Casts:
   • Mold: A hollow impression of an organism (like a seashell print in wet sand that hardens). An external mold is the outside, an internal mold is the filled-in inside.
   • Cast: A 3D copy of the organism, formed when a mold is later filled in with sediment or minerals.
3. Trace Fossils (Ichnofossils):
   • Evidence of an organism's activity, not its body.
   • Examples: Footprints, tracks, burrows, coprolites (fossilized feces).

Evolution of life through geological time

The fossil record, read using the Geological Time Scale, shows a clear history of the evolution of life on Earth.

Era / Eon	Major Life Forms & Events
Precambrian (Archean/Proterozoic)	- Origin of life (first simple cells). - Photosynthesis begins (bacteria), adding oxygen to the atmosphere. - First complex cells (eukaryotes). - First multicellular organisms (e.g., Ediacaran fauna - soft-bodied animals).
Paleozoic Era ("Ancient Life")	- Cambrian Explosion: A rapid diversification of life, most major animal groups appear. - Age of Invertebrates (e.g., Trilobites). - First fish appear. - First land plants and insects. - First amphibians. - Ends with the Permian-Triassic Mass Extinction ("The Great Dying"), the largest extinction event ever.
Mesozoic Era ("Middle Life")	- Age of Reptiles. - Dinosaurs, marine reptiles, and flying reptiles evolve and dominate. - First mammals (small, rodent-like). - First birds. - First flowering plants. - Ends with the Cretaceous-Paleogene (K-Pg) Extinction, which killed the dinosaurs (likely by an asteroid impact).
Cenozoic Era ("Recent Life")	- Age of Mammals. - Mammals diversify rapidly to fill the niches left by the dinosaurs. - Evolution of grasses. - Evolution of primates, leading to the first humans.