Stratigraphy What is Stratigraphy The branch of geology

Stratigraphy

What is Stratigraphy? • The branch of geology that studies rock layers (strata) and layering (stratification). – Primarily used in the study of sedimentary and layered volcanic (igneous) rocks.

Steno’s Law • Nicholas Steno (16381686) – Danish-born pioneer of geology, “The Father of Stratigraphy”. – Observed rock layers and suggested that geology is not totally chaotic but instead preserves a chronological record of Earth’s history and past life.

Principles of Stratigraphy 1. Law of Original Horizontality – Sediments are deposited in water in horizontal layers due to gravitational settling. 2. Law of Superposition – In undisturbed rock layers, the oldest layer lies at the bottom and the youngest layer at the top.

Principles of Stratigraphy 3. Law of Cross-Cutting – An igneous intrusion that cuts through existing rock is always younger than that disturbed rock.

Principles of Stratigraphy 4. Law of Unconformity – A surface that has a significant gap, layers missing, from its rock record from erosion or nondeposition. – 3 Types: • Disconformity • Angular Unconformity • Nonconformity

Unconformities 1. Disconformity – The eroded surface is between sedimentary layers that are parallel to each other. – All sedimentary rocks! Disconformity

Unconformities 2. Angular Unconformity – Younger strata overlie an erosional surface on tilted or folded rock layers.

Unconformities 3. Nonconformity – When igneous or metamorphic layers are eroded and covered by younger sedimentary or volcanic rock.

Deformation Processes • Strata can become deformed when changes to its shape or volume occur. – Changes in shape and volume occur when stress and strain causes rock to fold and/or fracture. • most visible in rocks that contain layering • 3 Types of Folds: – Monocline – Anticline – Syncline

Fold Types 1. Monocline - The simplest type of fold, involves a slight bend in otherwise parallel layers of rock.

Fold Types 2. Anticline - fold in rock that resembles an arch like structure with the rock beds

Fold Types 3. Syncline - rock layers are warped downward

Deformation Processes • Faults form in rocks when the stresses (friction) overcome the internal strength of the rock resulting in a fracture. – faults are named according to the type of stress that acts on the rock and by the nature of the movement of the rock blocks either side of the fault plane.

Characteristics of a Fault • Fault Plane- A break in Earth’s crust where slabs of rock slip past each other. • Hanging Wall - is the block positioned over the fault • Foot Wall - is the block positioned under the fault Foot wall Hanging wall

Fault Types 1. Normal Fault - occur when tensional forces act in opposite directions and cause one slab of the rock to be displaced up and the other slab down Hanging wall Foot wall

Fault Types 2. Reverse Fault - when compressional forces cause one block to be pushed up and over the other block. Hanging wall Foot wall

Fault Types 3. Graben Fault - produced when tensional stresses result in the sinking of a block of rock. – On a large scale these features are known as Rift Valleys Foot wall Hanging wall