Lithostratigraphy: An Introduction to Earth’s Sedimentary Records

Lithostratigraphy

Definition of Lithostratigraphy

Lithostratigraphy is a branch of stratigraphy focused on the recognition, subdivision, and correlation of sedimentary rocks based on their lithology.

Definition of Lithology

Lithology involves the study and description of the physical characteristics of rocks, including:

• Rock type
• Color
• Mineral composition
• Grain size

Lithostratigraphic Units

Lithostratigraphic units are bodies of sedimentary, extrusive igneous, metasedimentary, or metavolcanic rock, defined by their lithologic characteristics. These units are commonly stratified and tabular, recognized, and defined based on observable rock characteristics. Boundaries between different units may be placed at clearly identifiable or distinguished contacts or may be drawn arbitrarily within a zone of gradation. Lithostratigraphic units are defined strictly based on lithic criteria as determined by descriptions of actual rock materials. They carry no connotation of age. They cannot be defined based on paleontologic criteria, and they are independent of time concepts.

Terminology

• Lithosomes: The term lithosome refers to masses of rock of essentially uniform character and having intertonguing relationships with adjacent masses of different lithology. The shape of a lithosome is largely controlled by its relationship to adjacent lithosomes.

Classification of Lithostratigraphic Units (Formal)

There is a hierarchical framework of terms used for lithostratigraphic units, and from largest to smallest, these are: ‘Supergroup’, ‘Group’, ‘Formation’, ‘Member’, and ‘Bed’.

Formation

The basic unit of the lithostratigraphic division of rocks is the formation, which is a body of material that is mappable and can be identified by its lithological characteristics and its stratigraphic position. A formation should have some degree of lithological homogeneity and its defining characteristics may include mineralogical composition, texture, primary sedimentary structures, and fossil content in addition to the lithological composition. It may encompass a single lithosome, or part of an intertonguing lithosome, and thus consist of a single lithology. Alternatively, a formation can be composed of two or more lithotomies and thus may include rocks of different lithology. All other lithostratigraphic units are defined as either assemblages or subdivisions of formations. A formation is not defined in terms of its age either by isotopic dating or in terms of biostratigraphy. A formation may be, and often is, a diachronous unit, that is, a deposit with the same lithological properties that was formed at different times in different places.

Group / Supergroup

The major division of rock formations is called groups, each group includes a thick succession of rock formations extending over a large period. Large unconformities separate one group from another. A Supergroup is formed when two or more groups are joined together.

Member

Members are smaller stratigraphic units within the formation.

Bed

The smallest lithological unit is a single set of sedimentary rock units that has a distinct set of mineralogical or fossil characteristics.

Hierarchy of Lithostratigraphic Units

• Supergroup: A formal assemblage of related or superposed groups or of groups and formations.
• Group: Consists of assemblages of formations, but groups need not be composed entirely of named formations.
• Formation: A body of rock, identified by lithic characteristics and stratigraphic position, that is prevailingly but not necessarily tabular and is mappable at Earth’s surface and traceable in the subsurface. It must be of sufficient areal extent to be mappable at the scale of mapping commonly used in the region where it occurs. The fundamental lithostratigraphic unit – formations are grouped to form higher-rank lithostratigraphic units and are divided to form lower-rank units.
• Member: The formal lithostratigraphic unit next in rank below a formation and always part of some formation. A formation need not be divided entirely into members. A member may extend laterally from one formation to another.
• Lens (or lentil): A geographically restricted member that terminates on all sides within a formation.
• Tongue: A wedge-shaped member that extends beyond the main boundary of a formation or that wedges or pinches out within another formation.
• Bed: Distinctive subdivisions of a member; the smallest formal lithostratigraphic unit of sedimentary rock. Members commonly are not divided entirely into beds.
• Flow: The smallest formal lithostratigraphic unit of volcanic rock.

Major Types of Stratigraphic Units (North American Stratigraphic Code/Nomenclature, 1983)

Material Categories Based on Content or Physical Limits

• Lithostratigraphic Units: Conform to the law of superposition and are distinguished based on lithic characteristics and lithostratigraphic position.
• Lithodemic Units: Consist of predominantly intrusive, highly metamorphosed, or intensely deformed rock that generally does not conform to the law of superposition.
• Magnetopolarity Units: Bodies of rock identified by remnant magnetic polarity.
• Biostratigraphic Units: Bodies of rock defined and characterized by their fossil content.
• Pedostratigraphic Units: Consist of one or more pedologic (soil) horizons developed in one or more lithic units now buried by a formally defined lithostratigraphic or allostratigraphic unit and are identifiable based on bounding discontinuities.
• Allostratigraphic Units: Mappable stratiform (in the form of a layer) bodies defined and identified based on bounding discontinuities.

Categories Expressing or Related to Geologic Age

Material Categories to Define Temporal Spans

• Chronostratigraphic Units: Bodies of rock established to serve as the material reference for all rocks formed during the same periods.
• Polarity-Chronostratigraphic Units: Divisions of geologic time distinguished based on the record of magnetopolarity as embodied in polarity-chronostratigraphic units.

Temporal (Nonmaterial) Categories

• Geochronologic Units: Divisions of time distinguished based on the rock record as expressed by chronostratigraphic units.
• Polarity-Chronologic Units: Divisions of geologic time distinguished based on the record of magnetopolarity as embodied in polarity-chronostratigraphic units.
• Diachronic Units: Comprise the unequal periods represented by one or more specific diachronous rock bodies, which are bodies with one or two bounding surfaces that are not time-synchronous and thus “transgress” time.
• Geochronometric Units: Isochronous units (units having equal time duration) that are direct divisions of geologic time expressed in years.

Categories and Ranks of Stratigraphic Units as Defined in North American Commission on Stratigraphic Nomenclature Note 63

Important Questions???

1. What is the concept of Lithostratigraphy?

Lithostratigraphy is the study and classification of rock layers (strata) based on their lithological characteristics, such as composition, grain size, and texture, without considering their age.

2. What is the law of Lithostratigraphy?

A lithostratigraphic unit conforms to the law of superposition, which states that in any undisturbed succession of strata, younger rocks lie above older rocks.

3. What type of rock is mostly used in chronostratigraphy?

Igneous rocks are mostly used in chronostratigraphy because they occur at specific intervals in time, are geologically instantaneous, and contain minerals that can be accurately dated using isotopic methods.

4. What are the 7 laws of stratigraphy?

The principles of stratigraphy include:

• Law of Superposition
• Original Horizontality
• Lateral Continuity
• Cross-Cutting Relationships
• Inclusions
• Unconformities
• Fossil Succession

Additionally, uniformitarianism and catastrophism play roles in stratigraphic studies.

5. Why do sedimentary rocks form in layers?

Sedimentary rocks form in layers as particles settle out of water or air, accumulating in horizontal beds. Some form by precipitation of minerals from water, and their distinct layers reflect changes in environmental conditions over time.

6. What are the three types of unconformities?

The three types of unconformities are:

• Angular Unconformity – Tilted or folded rocks overlain by younger, horizontal layers.
• Disconformity – A gap in deposition between parallel sedimentary layers.
• Nonconformity – Sedimentary rock layers deposited on top of older igneous or metamorphic rocks.

7. What is lithostratigraphy in geology?

Lithostratigraphy is a branch of stratigraphy that classifies rock layers based on their lithological characteristics, such as composition, grain size, and texture.

8. What is the most common metamorphic rock on Earth?

The most common metamorphic rock is gneiss, which forms under high-grade metamorphic conditions from a variety of parent rocks except limestone.

9. How many eons are there?

There are four eons in Earth’s history:

• Hadean Eon
• Archean Eon
• Proterozoic Eon
• Phanerozoic Eon

10. What is the concept of lithology?

Lithology refers to the physical characteristics of a rock, including its color, texture, grain size, and mineral composition.

11. What is the simple definition of stratigraphy?

Stratigraphy is the branch of geology that studies rock layers (strata) and their formation, primarily in sedimentary and volcanic rocks.

12. What is the basic unit of Lithostratigraphy?

The formation is the fundamental unit of lithostratigraphy. It is a distinct mappable rock unit that has specific lithological characteristics.

13. What is the concept of petrology?

Petrology is the scientific study of rocks, including their mineralogy, texture, composition, and formation conditions. It has three subdivisions:

• Igneous Petrology
• Metamorphic Petrology
• Sedimentary Petrology

14. What is the difference between chronostratigraphy and lithostratigraphy?

• Chronostratigraphy classifies rocks based on their age and position in the geological time scale.
• Lithostratigraphy classifies rocks based on their physical characteristics, without considering time.

15. What are three examples of stratigraphy?

1. Lithostratigraphy – Classification based on rock type.
2. Biostratigraphy – Based on fossil content.
3. Chronostratigraphy – Based on absolute dating techniques.

16. What does lithology mean in geology?

Lithology refers to the general physical characteristics of rocks, including color, texture, and composition.

17. What is lithography in geology?

Lithography in geology refers to the detailed description of the physical characteristics of rocks.

18. What is the difference between a formation and a bed?

A formation is a distinct, mappable unit of rock, whereas a bed is a smaller lithologically distinct layer within a formation.

19. Who is the father of stratigraphy?

Nicolaus Steno is considered the father of stratigraphy.

20. What is the law of superposition?

The law of superposition states that in an undisturbed sequence of rock layers, the oldest layer is at the bottom, and each successive layer is younger than the one beneath it.

21. What things does a sedimentologist learn about sediments?

A sedimentologist studies the processes of:

• Weathering
• Erosion
• Transportation
• Deposition
• Diagenesis

These help reconstruct past environments and geological history.

22. Which rock type erodes faster?

• Hard rocks (e.g., granite) erode slowly.
• Soft rocks (e.g., limestone, chalk) erode quickly.

23. How to read lithology?

Lithology is interpreted using neutron logs, density logs, and other geological methods to determine porosity and rock composition.

24. What is the difference between lithology and petrology?

• Lithology focuses on physical rock characteristics.
• Petrology studies rock composition, formation, and mineralogy.

25. How do sedimentary rocks record Earth’s history?

Sedimentary rocks preserve information about past environments, climate changes, and biological activity through their composition and fossil content.

26. Which layer of rock is most likely to be the oldest?

According to the law of superposition, the lowest layer in an undisturbed sequence is the oldest.

27. Why is the law of superposition important?

It helps geologists determine the relative ages of rock layers, contributing to the understanding of Earth’s geological history.

28. How can you tell how old a sedimentary rock is?

Sedimentary rock age is determined using radiometric dating (e.g., uranium-lead dating) and relative dating techniques.

29. Is Earth the only planet with sedimentary rocks?

No, Mars also has vast deposits of sedimentary rocks that record its geological history.

30. What is the difference between lithification and diagenesis?

• Lithification is the process of turning sediment into solid rock.
• Diagenesis includes all physical, chemical, and biological changes after sediment deposition.

31. What law says that something found inside a rock must be the same age as the rock it is in?

The Law of Inclusions states that a rock containing an inclusion must be younger than the inclusion itself.

32. What are the lines in rocks called?

The lines in rocks are called bedding planes, which represent changes in sediment deposition.

33. Why do scientists use the law of superposition on sedimentary rocks?

It helps determine the relative age of rock layers, essential for reconstructing geological history.

34. How is the law of superposition used in everyday life?

An example is the double-slit experiment, where light behaves as both a wave and a particle.

35. What is the usefulness of Lithostratigraphy in rock correlation?

Lithostratigraphy helps geologists correlate rock units by identifying similar lithological characteristics across different locations.

36. What is the smallest lithostratigraphic rock unit?

A bed is the smallest recognisable stratigraphic unit.

37. How do geologists correlate rock layers?

They use index fossils, rock type, key beds, and sequences to correlate rock units across different regions.

38. What is the smallest rock in the world called?

The smallest rock particles are called clay, followed by silt, then sand.

39. Which rock unit is the oldest?

The Precambrian basement rocks are the oldest, forming the Earth’s foundation.