EDIMENTARY BASINS What is a Sedimentary Basin? by Prof. Dr. Abbas Mansour
WHAT IS A SEDIMENTARY BASIN? A low area on the Earth s surface relative to surroundings e.g. deep ocean basin (5-10 km deep) e.g. intramontane basin (2-3 km a.s.l.) May be of tectonic (or erosional origin) A receptacle for sedimentation; erosion may also be important Sedimentation may be interrupted - unconformities Basins may be small (kms2) or large (106+ km2)
WHAT IS A SEDIMENTARY BASIN? Basins may be simple or composite (sub-basins) Basins may change in size & shape due to: erosion sedimentation tectonic activity eustatic sea-level changes Basins may overlap each other in time
WHAT IS A SEDIMENTARY BASIN? The evolution of sedimentary basins may include: tectonic activity (initiation, termination) magmatic activity metamorphism as well as sedimentation all may be contemporaneous
WHAT IS A SEDIMENTARY BASIN? Basins may develop on oceanic crust, island arc crust, or continental crust Basins may be sedimentary basins sedimentary fill is relatively undeformed basin margin facies are preserved OR, structural (remnants of) basins sedimentary fill is deformed dips > original depositional slopes basin margin facies are eroded
Distribution of Sedimentary Rocks Sedimentary rocks cover a large part of the earth's surface, including some 75% of the land areas. Yet sedimentary rocks make up only 5% of the lithosphere (data from Pettijohn, 1957, p. 7). From this it follows that sedimentary rocks cover the earth only as a thin and superficial veneer. This cover is not evenly distributed. Thick sedimentary deposits were laid down in localized areas; loosely termed sedimentary basins. Large areas of the continents lack a thick sedimentary cover; Pre-Cambrian igneous and metamorphic rocks occur at or near the surface. These stable continental cores are termed cratons.
Sedimentary basins Why should sedimentation occur in one place at a particular time? What is the spatial organization of large volumes of sediment and what are the factors which control their facies? Thick sedimentary sequences are essentially attributable to tectonic subsidence for which there are three causes (Stoneley, 1969). They may occur where subcrustal displacement of the mantle leads to down-dragging and compressional warping of the crust. This occurs principally at what are called zones of subduction, linear features which are the site of geosynclinal sedimentation.
Sedimentary basins Sedimentation may also occur on a large scale where changes in the mantle cause foundering and subsidence of the crust. This process is responsible for intracratonic basins. Conversely these changes can cause the crust to dome. Thick volcanic and sedimentary sequences may form in crustal rift basins.
Sedimentary basins Thick sedimentary sequences may form where the weight of the sediment itself causes isostatic depression of the crust. This process obviously requires an outside mechanism to create an initial crustal void, since it poses the old problem of which came first, the hen or the egg?
Sedimentary basins The most likely place for such a process is the continental margin, where a whole ocean basin waits to be infilled. Sedimentation at the foot of the continental slope may cause isostatic depression of the crust. Some geophysical data support this (e.g. Drake et al., 1968, Fig. 2).
Sedimentary basins A hole in the ground is a necessary prerequisite: sedimentation needs subsidence. Considerable attention has been paid to the actual mechanics of basin subsidence, a problem requiring geophysics and structural analysis for its elucidation (e.g. Bott and Johnson, 1967).
Sedimentary basins Sedimentary basins are, in a very broad sense, all those areas in which sediments can accumulate to considerable thickness and be preserved for long geological time periods. A sedimentary basin is an area of the earth s crust that is underlain by a thick sequence of sedimentary rocks.
Sedimentary basins Hydrocarbons commonly occur in sedimentary basins and are absent from intervening areas of igneous and metamorphic rocks (North, 1971). This fundamental truth is one of the cornerstones of the sedimentary-organic theory for the origin of hydrocarbons.
Sedimentary basins and their Economic potential Sedimentary basins are depressions on the Earth's surface that are filled by sediment and organic material deposited by wind, river, and ocean processes. Sedimentary basins are sensitive recorders of regional tectonics, paleoclimate, and the history of life; they contain many of the freshwater aquifers and energy resources necessary for our society; and they are often used as repositories for community and industrial waste.
Sedimentary basins Tectonic subsidence and uplift are prerequisites for basin formation and terrigenous sediment supply, but sedimentary processes in a basin are governed by other factors, including water circulation and recycling of nutrients, sediment transport, and redistribution. The sedimentary facies of a basin are largely controlled by the interrelationship between subsidence, sedimentation rate and relative sea level change.
Sedimentary basins Directly relevant to sedimentation studies is the structural level of the faults which permit an area to subside. Two types of basement: sediment interface can be defined. In one variety deposition occurs on platforms or gently subsiding basins which are undefaced by syndepositional surface faults. Faulting may be taking place, however, at deeper crustal levels to accommodate basin subsidence.
Sedimentary basins The second type of sediment: basement interface is actually disrupted by faults which have penetrated to the surface. These may be horst and graben fractures within the basement. The Sirte basin of Libya, for example, shows this type of basin floor. Alternatively, faults may be generated within the sediment cover, independent of the tectonic style of the basement. Growth faults, as these are termed, are due to sediment compaction. They are characteristic of areas of rapid deposition and are, therefore, especially common in deltas such as the Mississippi and the Niger (e.g. Shelton, 1968; Carver, 1968; Weber, 1971).
Sedimentary basins This distinction between smooth basement: sediment interfaces and faulted ones is important. Not only does shallow faulting cause rapid lateral facies changes, but its presence may actually be responsible for the development of additional facies. Having analysed what may be termed the tectonic base level, it is relevant to consider other base levels which control the nature and distribution of sedimentary facies. These include the depositional surface, the water level and wave base.
Sedimentary basins The depositional surface is the boundary which separates sediment from the overlying fluid, air or water. This surface may be horizontal or inclined at angles of up to about 30. This angle is the depositional dip. The water level is a horizontal surface. Mean sea level is obviously particularly critical in differentiating sedimentary facies.
Sedimentary basins Wave base level is a rather more difficult parameter to define. Wave base is generally understood to mean the water depth which separates high-energy traction deposits from low-energy suspension deposits (including turbidites). This boundary depends not just on waves, however, but also on the power of tidal currents and on the amount of suspended sediment load. Effective wave base does not, therefore, parallel water level at a constant depth. Wave base is one of the most important parameters of environmental (and therefore of facies) control
Sedimentary basins Three main types of sedimentary pattern can be defined according to whether the sediment: basement interface is faulted, whether it has a smooth platform, or a basin shape. These three types are further subdivided into a total of nine assemblages, depending on whether the depositional surface is above, at, or below water level
Sedimentary basins, in plan view Sedimentary basins can have numerous different shapes; they may be approximately circular or, more frequently, elongate depressions, troughs, or embayments, but often they may have quite irregular boundaries. The size of sedimentary basins is highly variable, though they are usually at least 100km long and tens of km wide (Einsele, 1992). Sedimentary basins are of many types, ranging from small alluvial intermontane valleys to vast mountain ranges of contorted sediment, kilometres thick.
Sedimentation, Tectonics, Paleogeography The regional deposition of sediments, nondeposition, or denudation of older rocks are controlled mainly by tectonic movements. In addition to the tectonic movements in the basinal area itself, sedimentary processes and facies are controlled by the paleogeography of the regions around the basin (pre-basin morphology and climate, rock types and tectonic activity in the source area), the depositional environment, the evolution of sediment producing organisms, etc. (Einsele, 1992).
MECHANISM OF BASIN FORMATION Sedimentary basins form part of the earth s crust, or lithosphere; they are generally distinguishable from granitic continental and basaltic oceanic crust by their lower densities and slower seismic velocities. Beneath these crustal elements is the more continuous subcrustal lithosphere. The crust is thin, dense, and topographically low across the ocean basins, but thick, of lower density, and, consequently, of higher elevation over the continents (Fig. 8.4). The lithosphere is made up of a series of rigid plates, which overline the denser, yet viscous, asthenosphere.
MECHANISM OF BASIN FORMATION Basins can form in four main ways (Fischer, 1975). Three of these processes are summarized in Fig. 8.7. One major group of basins, the rift basins, form as a direct result of crustal tension at the zones of sea floor spreading. A second major group of basins occurs as a result of crustal compression at convergent plate boundaries. A third type of basin can form in response not to lateral forces but to vertical crustal movements. A fourth mechanism of basin formation is simple crustal loading due to sedimentation.
MECHANISM OF BASIN FORMATION Basins formed as a result of crustal thinning and rifting are of particular interest to the petroleum industry because they are an important habitat for petroleum. Many theories have been advanced to explain their formation. A useful review of these can be found in Allen and Allen (1990). Of the many models proposed, three are particularly significant: 1. 1. Salveson (1976, 1979) proposed a model of passive crustal separation, in which the continental crust was deemed to deform by brittle failure, while the subcrustal lithosphere is thinned by ductile necking.
MECHANISM OF BASIN FORMATION 2. 2. McKenzie (1978) proposed a model that assumed that both the crust and subcrustal lithosphere deformed by brittle failure. 3. 3. Wernicke (1981, 1985) proposed a model for crustal thinning by means of simple shear, in which a low angle fault extends from the surface right through the lithospere. These three models are illustrated in Fig. 8.8 at left The McKenzie model has received particular interest in the oil industry because it offers a means of predicting the history of heat flow in a sedimentary basin.