Time and Stratigraphy Basics Stratigraphic Laws and Geohistory

Objectives of the 6 Laws of Stratigraphy Module You will learn: The 6 basic laws and principles of Stratigraphy (i.e., the study of Strata or Layers of the Earth) Original Horizontality Lateral Continuity Superposition Cross-cutting Relationships Law of Inclusions Law of Faunal Succession

Given the three kinds of rocks and their possible deformation, let’s put them into a framework of time and consider different aspects of the layering (stratigraphy) of sedimentary rocks

Stratigraphic Laws Stratigraphic Laws are basic principles that all geologists use in deciphering the spatial and temporal relationships of rock layers. These laws were developed in the 17th to 19th centuries based upon the work of Niels Steno, James Hutton and William Smith, among others. The 6 Stratigraphic Laws will follow.

The 6 Laws of Stratigraphy Original Horizontality Lateral Continuity Superposition Cross-cutting Relationships Inclusions Faunal Succession The 6 Laws of Stratigraphy: Original Horizontality Lateral Continuity Superposition Cross-cutting Relationships Law of Inclusions Law of Faunal Succession

Law of Original Horizontality Assumes that the sedimentary rocks are deposited in horizontal layers. Therefore, if the rocks are tilted or folded we can infer that they have been subjected to a deformation event.

Law of Lateral Continuity Assumes that sedimentary rocks are laterally continuous over large areas. “Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way.” Steno, 1669

Law of Superposition Assumes that most sedimentary rocks were originally deposited in horizontal layers (law of horizontality) and therefore the oldest rocks will be on the bottom of the sequence and the youngest on the top.

Superposition in the Stratigraphic Column

Law of Cross-Cutting Relationships Igneous intrusions and geological structures such as folds, faults, and unconformities must be younger than the rocks they affect or cut across.

Law of Cross-Cutting Relationships Principle of Cross-Cutting Relationships If a geological structure or feature cuts across another geological structure or feature, it must be younger than the structure or feature it is cutting. Here: Folding  Intrusion  Faulting 11

Law of Inclusions Rock fragments (clasts) from another rock must be older than the rock containing the fragments.

Law of Faunal Succession Fossils can be used to correlate sedimentary rocks. Certain fossil groups are preceded or succeeded by other fossil groups. This allows for the development of a fossil stratigraphy that can be used around the world. Fossils used for age dating and correlation of strata from one region to another are called Index Fossils.

Using Fossils for Correlation

Using Fossils for Marine / Non-Marine Biostratigraphy Relative time scale

Objectives of the 6 Laws of Stratigraphy Module You have learned: The 6 basic laws and principles of Stratigraphy (i.e., the study of Strata or Layers of the Earth) Original Horizontality Lateral Continuity Superposition Cross-cutting Relationships Law of Inclusions Law of Faunal Succession

Geohistory Block Diagram Exercise Geohistory Block Diagram Geohistory Block Diagram Exercises 2, 3, and 4