Unconformity Recognition of Unconformity Unconformities may be recognized

Unconformity Recognition of Unconformity Unconformities may be recognized in various ways, of which observation in a single outcrop is the most satisfactory. Angular Unconformity lack of parallelism lowest beds above the unconformity may consist of conglomerate with pebbles derived from the underlying formations faults and dykes may be truncated at the contact Department of Geology, University of Dhaka Elementary Structural Geology Slide 1 of 16 ©SHA

Unconformity Recognition of Unconformity Unconformities may be recognized in various ways, of which observation in a single outcrop is the most satisfactory. Disconformity sharp contrast in colour between the rocks upper and below. the surface of disconformity may be wavy. conglomerate bed above the unconformity. Department of Geology, University of Dhaka Elementary Structural Geology Slide 2 of 16 ©SHA

Unconformity Disconformity Recognition Basal Conglomerate Department of Geology, University of Dhaka Elementary Structural Geology Slide 3 of 16 ©SHA

Unconformity Disconformity Recognition Channel Scouring Department of Geology, University of Dhaka Elementary Structural Geology Slide 4 of 16 ©SHA

Unconformity Disconformity Recognition Paleosoil formation Department of Geology, University of Dhaka Elementary Structural Geology Slide 5 of 16 ©SHA

Unconformity Disconformity Recognition Department of Geology, University of Dhaka Elementary Structural Geology Slide 6 of 16 ©SHA

Unconformity Recognition of Unconformity Unconformities may be recognized in various ways, of which observation in a single outcrop is the most satisfactory. Local Unconformity Department of Geology, University of Dhaka same criteria but regional relationship must be considered to distinguished from disconformity. Elementary Structural Geology Slide 7 of 16 ©SHA

Unconformity Recognition of Unconformity Unconformities may be recognized in various ways, of which observation in a single outcrop is the most satisfactory. Nonconformity Department of Geology, University of Dhaka conglomerate bed above the nonconformity and the pebbles derived from the igneous or metamorphic rock. arkose above the nonconformity. Elementary Structural Geology Slide 8 of 16 ©SHA

Unconformity Nonconformity Recognition Department of Geology, University of Dhaka Elementary Structural Geology Slide 9 of 16 ©SHA

Unconformity A surface of erosion may covered by thick residual soil that grades into the underlying bed rock. Younger sediments deposited above this erosion surface may incorporate some of the residual soils, and a sharp contrast may be lacking. Such a contact is called blended unconformity. Paleosoil formation Department of Geology, University of Dhaka Elementary Structural Geology Slide 10 of 16 ©SHA

Unconformity Many unconformities are not exposed in an outcrop. This may be due to poor exposures, igneous intrusions or faulting. In such cases other methods must be employed to detect the unconformity. Aerial mapping map or aerial photographs marked changed in the strike of beds angular relationship. truncation of beds. pebbles of truncated beds. Sharp contrast in the degree of induration. Unconsolidated sands and clays when associated with well consolidated / cemented sandstone and compact shale. Distinct difference in the grade of regional metamorphism. Significant difference in the intensity of folding. Highly / strongly folded and gently inclined or horizontal less deformed rocks are probably unconformable above the more deformed rocks. Department of Geology, University of Dhaka Elementary Structural Geology Slide 11 of 16 ©SHA

Unconformity Relation of plutonic rocks may be important. Two sedimentary formations 1 and 3 may be separated from each other by granite (2) and may no where come into contact with each other. Formation 1 is intruded by granite but formation 3 is resting on the granite unconformably, and it contains pebbles of granite. It is apparent, therefore, that formation 3 is above unconformity and that formation 1 is beneath the unconformity. 3 2 1 Department of Geology, University of Dhaka Elementary Structural Geology Slide 12 of 16 ©SHA

Unconformity Paleontology may indicate an unconformity. break through Jurassic Department of Geology, University of Dhaka Elementary Structural Geology c c Cretaceous fossil Triassic fossil Slide 13 of 16 ©SHA

Unconformity Distinguishing Faults from Unconformities In some cases unconformities are confused with the faults and are difficult to differentiate. Dip, diagonal and transverse faults offer no difficulties. But if the bedding on one or both sides of the contact has the same strike as the contact, as in the figure below, either an unconformity or a fault may exits. The map may be interpreted as unconformity or as fault. Department of Geology, University of Dhaka Elementary Structural Geology Slide 14 of 16 ©SHA

Unconformity Distinguishing Faults from Unconformities x c’ y 6 5 7 20 20 6 3 2 1 40 x y 40 x 5 y 2 5 4 3 2 1 4 1 c Department of Geology, University of Dhaka Elementary Structural Geology Slide 15 of 16 ©SHA

Unconformity In a region of sufficient relief, it may be possible to ascertain the attitude of the contact cc’ from its relation to topography. Under the simplest conditions, the dip of the unconformity would be essentially parallel to the dip of the beds in formation 5. The greater the divergence of the dip of the contact from the dip of the formation 5, the greater the probability that the contact is a fault. The presence of pebbles of formation 1, 2, 3 and 4 in formation 5 would indicate an unconformity between the two formations. In the final analysis, every effort should be made in the field to observe the actual contact. If it is an unconformity, small ridges of the older rock may project into the younger rocks, and a conglomerate or sandstone, with fragments of the older rocks, may lies above the contact. Silikensides, gouge and breccia would be absent from an unconformity but, would likely be present along a fault. Department of Geology, University of Dhaka Elementary Structural Geology Slide 16 of 16 ©SHA