Principles of Stratigraphy MD. ABDULLAH SALMAN LECTURER DEPARTMENT OF GEOLOGY AND MINING UNIVERSITY OF BARISHAL BARISHAL-8200 Depositional environment Depositional environment is part of earth surface that has certain chemical, biology, and physics characteristics where sediments are laid on. There are 3 kinds of depositional environments, they are continental, marginal marine, and marine environments.[1] Each environments have certain characteristic which make each of them different than others. And different depositional environment, will have different structure and texture of sediments. Transgression and Regression A Marine Transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water (or decreasing in capacity). Transgressions and regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load. In either case, sea water rises farther up onto land than it did before. Transgression In this case we will have deeper sea sediments (shales and limestones) being deposited on top of continentally-derived beach sediments (sand). This forms a sequence (from bottom to top) of: sand ► shale ►limestone. A maximum transgression occurs where the finest sediments reach the farthest landward. Regression Marine Regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land. Regression In this case, continental sediments are being deposited farther out to sea than they once were. Therefore, we see a sequence (from bottom to top) of: limestone ► shale ► sandstone. A maximum regression occurs where the coarsest sediments reach the farthest seaward. Parasequence Sets and Stacking Patterns Parasequences are defined as a relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces. In addition to these defining characteristics, most parasequences are asymmetical shallowing-upward sedimentary cycles. Sets of successive parasequences may display consistent trends in thickness and facies composition and these sets may be progradational, aggradational, or retrogradational. Progradation & Retrogradation Lateral outbuilding, or progradation, of strata in a sea-ward direction. progradation can occur as a result of a sea-level rise accompanied by a high sediment flux (causing a regression). A progradational stacking pattern of parasequences refers to the pattern in which facies at the top of each parasequence becomes progressively more proximal (Posamantier, 1999, Wilgus, 1988, Emery, 1986). The movement of coastline land-ward in response to a transgression. This can occur during a sealevel rise with low sediment flux. retrogradational stacking patterns of parasequences refer to patterns in which facies become progressively more distal when traced upward vertically (Posamantier, 1988; Wilgus, 1988; Emery, and Meyers, 1996). Aggradation Usually occurs when there is a relative rise in sea level produced by subsidence and/or eustatic sea-level rise, and the rate of sediment influx is sufficient to maintain the depositional surface at or near sea level Occurs when sediment flux = rate of sea-level rise. Produces aggradational stacking patterns in parasequences when the patterns of facies at the top of each parasequence are essentially the same (Posamentier, 1999; Wilgus et al.; 1988, Emery, 1996). Coarse grained sediments are deposited in most of the upper part compare to the fine grained Coarsening upward means coarse grains which are deposited in the upper part of the sedimentary sequences. Fine grained sediments are deposited in most of the upper part compare to the coarse grained Fining upward means fine grains which are deposited in the upper part of the sedimentary sequences.
