Relative Dating Techniques: Stratigraphy, Fossils, Structures

Stratigraphic Methods

Stratigraphic Discontinuities

Breaks in sedimentation are called diastems. A time lapse without sedimentation is called a hiatus. If erosion occurs during this time, it results in a stratigraphic unconformity. Types of unconformities include disconformity, angular unconformity, and nonconformity.

Principle of Superposition of Strata

This principle states that in an undisturbed sequence of strata (layers), the order from oldest to most recent ranges from bottom to top. Related principles include the Principle of Original Horizontality and the Principle of Lateral Continuity of strata.

Exceptions:

• The Principle of Original Horizontality may not be valid at the edges of sedimentary basins.
• The Principle of Lateral Continuity does not account for changes in lithology (facies changes) that can occur within a layer during sedimentation.

Criteria for Layer Polarity

To differentiate the top (techo) from the bottom (muro) of a layer, several criteria are used, such as:

• The shape of strata
• Internal sedimentary structures
• Surface features of stratification (e.g., ripple marks, mud cracks)

Glacial Varve Analysis

Glacial varves are pairs of strata produced annually due to seasonal changes. They typically consist of a coarser, lighter-colored sandy layer (summer deposit) and a finer, darker-colored clay or silt layer (winter deposit).

Biological Methods

Fossils

Fossils provide crucial information for:

• Paleogeography: Indicating the relative positions of lithospheric plates in the past, and the spatial and temporal distribution and migrations of various species.
• Paleoecology: Providing information on the sedimentary environment where fossilization occurred.
• Paleoclimatology: Helping specify the paleoclimatic and paleoecological conditions of the sedimentary environment.
• Biostratigraphy (Biochronostratigraphy): Acting as indicators of geological time in sedimentary rocks, allowing the establishment of relative chronologies between rocks from different locations or within the same stratum.

Index Fossils

Index fossils (or guide fossils) are used to establish accurate relative dating. They define a very narrow margin of geological time and possess the following characteristics:

• Wide geographical distribution
• Rapid evolutionary process (short species lifespan)
• High ecological tolerance (found in various environments)
• Easy to recognize and interpret

A stratum containing a specific index fossil represents a particular time horizon.

Dendrochronology

This method involves studying the annual growth rings of trees. Each ring is typically composed of a lighter part (earlywood, spring/summer growth) and a darker part (latewood, late summer/fall growth). The number of rings allows us to determine the tree's age. The width of annual rings varies depending on climatic conditions: narrow rings during drought, wider rings with ample water. Similar growth rings, reflecting intermittent growth, can also be observed in corals.

Molecular Clocks

This method utilizes mutation rates in DNA and proteins (DNA clocks and protein clocks) to estimate evolutionary divergence times. A greater degree of similarity between the DNA and proteins of two species indicates a closer degree of kinship on the evolutionary scale.

Structural Methods

These methods are based on relationships of relative chronology, utilizing principles like the superposition of strata, lateral continuity, and concepts from tectonics and structural geology. Key principles include:

Principle of Cross-Cutting Relationships

If a geological feature (e.g., a fault or an igneous intrusion) cuts across existing rocks, that feature must be younger than the rocks it cuts through.

Principle of Inclusions

This principle allows relative dating based on inclusions. If a rock body contains fragments (inclusions) of another rock, the fragments must be older than the rock body containing them.

Historical Context

The principles of cross-cutting relationships and inclusions were initially recognized by James Hutton in the 18th century, further defined by John Playfair in 1802, and later popularized by Charles Lyell in his influential book Principles of Geology (first published in the 1830s).