Stratigraphic Layer
A stratigraphic layer (stratum, plural strata) is a single bed or layer of sedimentary rock, volcanic ash, or other deposited material that was laid down during a specific interval of geological time, distinguished from adjacent layers by differences in composition, texture, or fossil content. The principle of superposition, established by Nicolas Steno in 1669, states that in an undisturbed sequence, lower layers are older than upper ones, making stratigraphy the primary method for establishing relative geological time. Correlation of strata across regions using index fossils and radiometric dating allows geologists to reconstruct the geological history of continents and oceans.
Key Formula
S = Δd / Δt
LaTeX: S = \frac{\Delta d}{\Delta t}
| Symbol | Meaning | Unit |
|---|---|---|
| S | Sedimentation rate | m/yr |
| Δd | Thickness of stratum deposited | m |
| Δt | Time interval of deposition | yr |
Worked Example
Problem
A limestone stratum is 45 m thick and was deposited over approximately 9 million years. Calculate the average sedimentation rate in mm per 1,000 years.
Solution
Step 1: Convert units — 45 m = 45,000 mm; 9,000,000 yr = 9,000 thousand years. Step 2: Apply the formula — S = Δd / Δt = 45,000 mm ÷ 9,000 = 5 mm per 1,000 years. Step 3: Interpret — This is a slow carbonate platform sedimentation rate, typical of deep marine limestone environments.
Answer
Average sedimentation rate = 5 mm per 1,000 years (5 m/Ma)
Stratigraphic Principles and Their Applications
| Principle | Statement | Established By | Application |
|---|---|---|---|
| Superposition | Lower strata are older in undisturbed sequences | Nicolas Steno, 1669 | Relative age dating |
| Original horizontality | Strata are deposited horizontally | Nicolas Steno, 1669 | Identifying tectonic deformation |
| Lateral continuity | Strata extend laterally in all directions | Nicolas Steno, 1669 | Correlating distant outcrops |
| Fossil succession | Fossil assemblages change predictably upward | William Smith, 1815 | Biostratigraphic correlation |
| Cross-cutting relationships | Intrusions are younger than surrounding rock | Charles Lyell, 1830 | Dating igneous events |
Interactive Tools
Wolfram Alpha — Geological Time Calculator
Calculate sedimentation rates, layer thicknesses, and geological time intervals.
Open ToolKhan Academy — Relative vs. Absolute Dating
Explains stratigraphic principles and how they are used to determine the relative ages of rock units.
Open ToolMacrostrat — Stratigraphic Column Database
Digital database of rock unit descriptions, ages, and lithologies for North America and globally.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Fossil
A fossil is the preserved remains, trace, or impression of a once-living organism, typically found in sedimentary rock and dating to more than 10,000 years ago. Fossilization requires rapid burial by sediment, which prevents decay and allows mineral replacement of organic tissues (permineralization), leaving a durable record of past life. Fossils are the primary evidence for reconstructing the history of life on Earth, documenting evolutionary relationships, ancient environments, and the timing of major biological events through the fossil record.
Geologic Time Scale
The Geologic Time Scale (GTS) is the internationally standardized chronological framework that divides Earth's 4.54-billion-year history into hierarchical time units—eons, eras, periods, epochs, and ages—based on stratigraphic evidence, fossil assemblages, and radiometric dating. The boundaries between units correspond to major geological or biological events such as mass extinctions, sea-level changes, or tectonic reorganizations. The GTS is maintained by the International Commission on Stratigraphy (ICS) and is essential for communicating geological ages, correlating rock formations globally, and understanding Earth's evolutionary and environmental history.
Radiocarbon Dating
Radiocarbon dating (¹⁴C dating) is a radiometric technique used to determine the age of organic materials up to approximately 50,000 years old by measuring the decay of the radioactive carbon isotope ¹⁴C. Living organisms continuously incorporate ¹⁴C from the atmosphere through photosynthesis and respiration; at death, ¹⁴C incorporation ceases and the remaining ¹⁴C decays at a known rate with a half-life of 5,730 years. Developed by Willard Libby in 1949, for which he received the 1960 Nobel Prize in Chemistry, radiocarbon dating revolutionized archaeology, geology, and environmental science by providing absolute ages for recent geological and archaeological materials.
From Latin "stratum" (something spread or laid down), past participle of "sternere" (to spread, to lay flat). "Stratigraphy" as a discipline was named and systematized by William Smith, whose 1815 geological map of Britain demonstrated that strata could be identified and correlated by their fossil content.