Stratigraphic Classification and Correlation Methods
Categories of Stratigraphic Classification
Lithostratigraphic units – Units based on the lithologic properties of the rock bodies. Unconformity-bounded units – Bodies of rock bounded above and below by significant discontinuities in the stratigraphic succession. Biostratigraphic units – Units based on the fossil content of the rock bodies. Magnetostratigraphic polarity units – Units based on changes in the orientation of the remanent magnetization of the rock bodies. Chronostratigraphic units – Units based on the time of formation of the rock bodies.
Hierarchy of Lithostratigraphic Units
- Supergroup: A collection of related groups or formations.
- Group: Assemblages of formations, not necessarily named.
- Formation: A distinct rock body identified by lithic characteristics, mappable at Earth’s surface.
- Member: A unit below a formation, always part of it, and can extend across formations.
- Lens: A small, geographically restricted member that terminates within a formation.
- Tongue: A wedge-shaped member that extends beyond or pinches within another formation.
- Bed: The smallest subdivision of a member, a distinctive layer of sedimentary rock.
- Flow: The smallest unit of volcanic rock.
Biostratigraphy
Biostratigraphy uses the appearance of fossils to define the relative ages of rock layers. Key terms include:
- FAD (First Appearance Datum): The first appearance of a species in the fossil record.
- LAD (Last Appearance Datum): The last appearance of a species in the fossil record.
In subsurface stratigraphy, FAD is often more reliable than LAD due to factors like contamination and bioturbation.
Biozones
Biozones are defined based on fossil content:
- Interval zone: Defined by the presence of a specific species between two datums.
- Assemblage zone: Defined by the presence of a group of species.
- Abundance zone: Defined by the abundance of a particular species.
Different Zones: Taxon range, concurrent, lineage, interval (high and low), abundance.
The Earth’s polarity has shifted in the past. A period in which polarity remains stable is called a magnetic chron.
Correlation
Correlation is the process of matching rock units across different areas. Correlation involves determining the relative ages of local units and establishing their ages relative to the geologic time scale, with index fossils being useful for accurate and reliable correlation. Lithostratigraphic correlation: Matching the same rock units without considering time. Time-stratigraphic correlation: Demonstrating time-equivalence of events.
Correlation by Instrument Logs
Correlation by instrument logs involves using tools like the electric (resistivity) log, which records resistivity of rock units in a borehole. Resistivity reflects lithology and pore fluid content; for example, marine shale has lower resistivity than porous sandstone or limestone. Geologists use well logs, combined with lithologic data from cores and cuttings, to interpret the rock types and formations, helping to correlate geological units.
Time Equivalence
Time equivalence is best demonstrated through time-stratigraphic correlation using biozones, as most regional rock units are time-transgressive. Lithostratigraphic correlation alone cannot confirm time equivalence. For example, a sandstone in Arizona can be correlated with similar rocks in Colorado and South Dakota based on their time-equivalent fossil content. A biozone marks the time span between the origin and extinction of extinct organisms. A range zone is a type of biozone defined by the total geologic time a fossil group, species, or genus existed. The most useful fossils for biozones are those that are: Easily identifiable; Geographically widespread; Had a short geologic range.
Index Fossils
The brachiopod Lingula is not useful because, although it is easily identified and has a wide geographic extent, it has too large a geologic range. The brachiopod Atrypa and trilobite Paradoxides are well suited for time-stratigraphic correlation because of their short ranges. They are index fossils.
A concurrent range zone is established by plotting the overlapping ranges of two or more fossils with different geologic ranges.
Short-Duration Physical Events
Short-duration physical events, like distinctive lava flows and ash falls, can demonstrate time equivalence. Lava flows form over a brief period, while ash falls occur within hours or days and can cover large areas. These events are not restricted to a specific environment. Absolute ages for such igneous events can be determined using radiometric dating.
Event Stratigraphy: High-Resolution Correlation
This method relies on identifying many short-term cycles (up to 100,000 years) and events that occur close together in the rock record. These events are preserved as nearly identical surfaces and thin layers. This allows for a high level of dating accuracy, with chronological resolution of 20,000 to 50,000 years or even less.
Units Representing Physical and Chemical Events
Units representing physical events: Paleomagnetic excursions, impact-related beds, volcanic ash beds, short-term depositional events, varves, disconformities. Units representing chemical events: Elemental chemostratigraphy, light stable isotope, organic and carbonate carbon.
Absolute Dating
Absolute dates for sedimentary rocks are usually determined indirectly by dating nearby igneous and metamorphic rocks. According to the principle of cross-cutting relationships, a dike is younger than the rock it cuts. Therefore, the absolute age of the dike provides a minimum age for the host rock and a maximum age for any sedimentary rocks deposited after the dike was eroded.
Cyclostratigraphy
Cyclostratigraphy is the subdiscipline of stratigraphy that deals with the identification, characterization, correlation, and interpretation of cyclic variations in the stratigraphic record and, in particular, with their application in geochronology by improving the accuracy and resolution of time-stratigraphic frameworks.