Stratigraphic concepts and lithostratigraphy UNIT - 6

GEOLOGICAL TIME The passage of time since the formation of the Earth is divided into geochronological units. These are divisions of time that may be referred to in terms of years or by name. The abbreviations used for dates are ‘Ma’ for millions of years before present and ‘ka’ for thousands of years before present. The time billlion of years before present is abbreviated to ‘Ga’ (Gigayears).

Geological time units The geoogical time units have been divided into 5 groups. They include: Eons: These are the longest periods of time within the history of the Earth, which are now commonly divided into three eons: the Archaean Eon up to 2.5 Ga, the Proterozoic Eon from 2.5 Ga to 542Ma, and the Phanerozoic Eon from 542Ma up to the present.

Geological time units Eras are the three time divisions of the Phanerozoic: The Palaeozoic Era up to 251 Ma, The Mesozoic Era from then until 65.5Ma and The Cenozoic Era up to the present. Precambrian eras have also been defined, for example dividing the Proterozoic into the Palaeoproterozoic, the Mesoproterozoic and the Neoproterozoic.

Geological time units Periods are the basic unit of geological time and are the most commonly used terms when referring to Earth history. The Mesozoic Era, for example, is divided into three periods, the Triassic Period, the Jurassic Period and the Cretaceous Period. The term system is used for the rocks deposited in this time, e.g. the Jurassic System.

Geological time units Epochs are the major divisions of periods Ages/Stages: The smallest commonly used divisions of geological time are ages. They are typically a few million years in duration. Chrons are short periods of time that are sometimes determined from palaeomagnetic information, but these units do not have widespread usage outside of magnetostratigraphy.

The time is given in millions of years. THE GEOLOGICAL COLUMN AND TIME SCALE The time is given in millions of years. Eg: 505 stands for 505 million years.

STRATIGRAPHIC UNITS The International Stratigraphic Chart and the Geologic Time Scale that it shows provides an overall framework within which we can place all the rocks on Earth and the events that have taken place since the planet formed. It is, however, of only limited relevance in determining the stratigraphic relationships between rocks in the field. Strata do not have labels on them which immediately tell us that they were deposited in a particular epoch or period, but they do contain information that allows us to establish an order of formation of units. There are a number of different approaches that can be used, each based on different aspects of the rocks, and each of which is of some value individually, but are most profitably used in combinations.

Sub-division of rock record Lithostratigraphy Study of the physical relationship among rock units; No time connotation other than superposition Physical properties and stratigraphic position relative to other lithostratigraphic units Chronostratigraphy Integrated approach to establishing the time relationships among geologic units

Sub-division of rock record Chronostratigraphy: Integrated Approach to Establishing the Time Relationships Among Geologic Units Biostratigraphy Study of the fossil record with emphasis on faunal succession to establish relative time relationships The correlation web Magnetostratigraphy Study of the magnetic properties of rock units for the purpose of correlation using magnetic polarity reversals Allostratigraphy Study of rock units defined by unconformities and other features generated by base level change Geochronology Various techniques, especially isotope geochemistry, to establish the absolute age of rock units

Methods of Stratigraphic sub-divison

Stratigraphic Correlation

LITHOSTRATIGRAPHY In lithostratigraphy rock units are considered in terms of the lithological characteristics of the strata and their relative stratigraphic positions. The relative stratigraphic positions of rock units can be determined by considering geometric and physical relationships that indicate which beds are older and which ones are younger. The units can be classified into a hierarchical system of members, formations and groups that provide a basis for categorising and describing rocks in lithostratigraphic terms.

Principles (Laws) of Stratigraphy Principle of… Original Horizontality Superposition Lateral Continuity Cross Cutting Relationships Inclusions Faunal Succession

Stratigraphic relationships The principle of original horizontality It is based on the fact that sediment usually accumulates in horizontal layers. If sedimentary rocks lie at an angle, we can infer that tectonic forces tilted them after they formed

Stratigraphic relationships The principle of superposition It states that sedimentary rocks become younger from bottom to top (as long as tectonic forces have not turned them upside down). This is because younger layers of sediment always accumulate on top of older layers. In the figure below the sedimentary layers become progressively younger in the order E, D, C, B, and A.

Principle of Lateral Continuity It states that if layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance.  Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.

Stratigraphic relationships The principle of crosscutting relationships It states that a rock must first exist before anything can happen to it. The figure below shows sedimentary rocks intruded by three granite dikes. Dike B cuts dike C, and dike A cuts dike B, so dike C is older than B, and dike A is the youngest. The sedimentary rocks must be older than all of the dikes.

Stratigraphic relationships Principle of Inclusions The Law of Inclusions states that if a rock body (Rock B) contained fragments of another rock body (Rock A), it must be younger than the fragments of rock it contained. The intruding rock (Rock A) must have been there first to provide the fragments.

Stratigraphic relationships Principle of Faunal Succession Law of Faunal Succession staes that fossils occur in a definite, invariable sequence in the geologic record. As you can see in the image below the fossil remains of living things are present in the rock layers at definite intervals, and exist within a discrete period of time. In this instance, using the Law of Superposition, would the age Rock Unit A be older or younger than the age of Rock Unit B?

Stratigraphic relationships UNCONFORMITIES Layers of sedimentary rocks are conformable if they were deposited without interruption. An unconformity represents an interruption in deposition, usually of long duration. During the interval when no sediment was deposited, some rock layers may have been eroded Thus, an unconformity represents a long time interval for which no geologic record exists in that place. The lost record may involve hundreds of millions of years There are several types of unconformities

Stratigraphic relationships

Stratigraphic relationships Nonconformity In this case sedimentary rocks lie on igneous or metamorphic rocks

Stratigraphic relationships Angular unconformity In this case tectonic activity tilted older sedimentary rock layers before younger sediment accumulated

Stratigraphic relationships Disconformity In this case the sedimentary layers above and below the unconformity are parallel Geologists identify disconformities by determining the ages of rocks using methods based on fossils and absolute dating

Way-up indicators in sedimentary rocks The folding and faulting of strata during mountain building can rotate whole successions of beds (formed as horizontal or nearly horizontal layers) through any angle, resulting in beds that may be vertical or completely overturned. In any analysis of deformed strata, it is essential to know the direction of younging, that is, the direction through the layers towards younger rocks. The direction of younging can be determined by small-scale features that indicate the way-up of the beds or by using other stratigraphic techniques to determine the order of formation.

Lithostratigraphic units There is a hierarchical framework of terms used for lithostratigraphic units, and from largest to smallest these are: ‘Supergroup’, ‘Group’, ‘Formation’, ‘Member’and ‘Bed’. The basic unit of lithostratigraphic division of rocks is the formation, which is a body of material that can be identified by it lithological characteristics and by 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.

Description of lithostratigraphic units The formation is the fundamental lithostratigraphic unit and it is usual to follow a certain procedure in geological literature when describing a formation Members and groups are usually described in a similar way.

Description of lithostratigraphic units Lithology and characteristics The field characteristics of the rock, for example, an oolitic grainstone, interbedded coarse siltstone and claystone, a basaltic lithic tuff, and so on form the first part of the description. Although a formation will normally consist mainly of one lithology, combinations of two or more lithologies will often constitute a formation as interbedded or interfingering units. Sedimentary structures (ripple cross-laminations, normal grading, etc.), petrography (often determined from thin-section analysis) and fossil content (both body and trace fossils) should also be noted.

Description of lithostratigraphic units Definition of top and base These are the criteria that are used to distinguish beds of this unit from those of underlying and overlying units; This is most commonly a change in lithology Where the boundary is not a sharp change from one formation to another, but is gradational, an arbitrary boundary must be placed within the transition.

Description of lithostratigraphic units Type section A type section is the location where the lithological characteristics are clear and, if possible, where the lower and upper boundaries of the formation can be seen. The type section will normally be presented as a graphic sedimentary log and this will form the stratotype. It must be precisely located (grid reference and/or GPS location) to make it possible for any other geologist to visit the type section and see the boundaries and the lithological characteristics described.

Description of lithostratigraphic units Thickness and extent The thickness is measured in the type section, but variations in the thickness seen at other localities are also noted. The limits of the geographical area over which the unit is recognized should also be determined. The variability of rock types within an area will be the main constraint on the number and thickness of lithostratigraphic units that can be described and defined. Quality and quantity of exposure also play a role, as finer subdivision is possible in areas of good exposure.