Sedimentary Basins: An Integrated Study

BASIN ANALYSIS Basin analysis (Bogges, 2006) is an integrated study involves application of sedimentologic, stratigraphic, and tectonic principles to develop a full understanding of the rocks that fill sedimentary basins to interpreting their geologic history and evaluating their economic importance. Sedimentary basin analysis (Nichols,2009) is the aspect of geology dealing with all the controls on the accumulation of a succession of sedimentary rocks to develop a model for the evolution of the sedimentary basin as a whole. The spatial distribution of depositional facies and variations in the environment of deposition through time will depend upon the tectonic setting, so a comprehensive analysis of the sedimentology and stratigraphy of an area must take place in the context of the basin setting. 1

Basin analysis studies aimed to understanding and predicting basin formation within the framework of plate tectonics and mantle convection hydrocarbon generation and migration during basin evolution present and historic ground-water flow and chemical transport changes in basin fill and thermal evolution with tectonic environment spatial and temporal variations of subsurface porosity and permeability The record of tectonics, climate, and sea-level change preserved in sedimentary basins. 2

SEDIMENTARY BASIN CONCEPT Sedimentary basins : depressions on Earth's surface over geologic time are filled with sediments and organic materials that have been transported by wind, rivers, and ocean currents, they are come in many shapes and sizes, pervasive on Earth and form in response to complex geologic processes.They reach to thousands of kilometers in horizontal dimensions and contain more than 1015 m3 of buried materials. A sedimentary basin (Bogges,2006) is a depression of some kind capable of trapping sediment. Sedimentary basins (Nichols,2009) are regions where sediment accumulates into huge successions thickness over giant areas. 3

The basin-filled materials is important in two respects. First, it preserves unique information regarding the history of tectonic, biologic, oceanographic, and atmospheric events during Earth's evolution. Second, basin fill contains most of the fuel and water, and many of the mineral resources, that are critical for society and industrial civilization. Some Basins are filled with strata deposited entirely in terrestrial environments, others with strata deposited below sea level in marine environments; many basins include both kinds of sediment. The formation of sedimentary basins is ultimately controlled by three elements: topography that defines the surface depressions that receive the sediments, the elevated regions that provide sediment sources, and the topographic and bathymetric gradients that transport sediments from source to basin. 4

MECHANISMS OF BASIN FORMATION (SUBSIDENCE) Isostatic compensation: This concept assumes that: local compensation of the crust occurs as if Earth consists of a series of free-floating blocks. Adjacent blocks of crust of different thickness and / or density structure will have different relative relief. Thus, adding a load to the crust (e.g., filling a basin with sediment) causes subsidence; removing a load (e.g., erosion of the crust) causes uplift. There are three physical principles effects of loading operate as a basin originally filled with water will be deepened gradually by load of the accumulated sediment . flexing of the crust also occurs, to various degrees depending upon the rigidity of the underlying lithosphere, because of tectonic forces: over thrusting, underpulling, underthrusting of dense lithosphere. thermal effects (e.g., cooling of lithosphere, increase in crustal density caused by changing temperature/pressure conditions) may also be important in basin formation. 5

Table 1: GEO-MECHANISMS OF BASIN FORMATION (SUBSIDENCE) Crustal thinning: Extensional stretching, erosion during uplift, and magmatic withdrawal Mantle-lithospheric thickening: stretching or heating due to adiabatic melting or rise of asthenospheric melts Sedimentary and volcanic loading: lithospheric flexure, dependent on flexural rigidity of lithosphere, during sedimentation and volcanism Tectonic loading: Local isostatic compensation of crust and regional lithospheric flexure, dependent on flexural rigidity of underlying lithosphere, forces(overthrusting and/ or underpulling) Subcrustal loading: Lithospheric flexure during underthrusting of dense lithosphere Asthenospheric flow: Dynamic effects of asthenospheric flow, commonly due to descent or delamination of subducted lithosphere Crustal densification: Increased of crust density due to changing pressure/ temperature conditions and/ or emplacement of higher- density melts into lower-density crust. Cooling of lithosphere following either cessation of Local isostatic compensation of crust and regional during tectonic 6

Basin plains Essential critical interfaces that control sedimentary patterns and facies and the distribution of organisms,they are: (1) The sea water level (horizontal plain identify general level of sea surface). (2) lower and the upper boundaries of the tides (control the distribution of organisms), (3) The base of the photic zone (controls the distribution of light- dependent phototrophic organisms), (4) The effective wave base level (the plain where wave effect becoming zero, above where bottom currents and wave action may lead to erosion and cementation or, the plain, which is separate high- energy traction deposits from low energy suspended deposits) (5) The storms wave base level (base of storms action on the sea bottom) 7

(6) The O2 minimum zone (strongly limiting life on and in the sea bottom), (7) The thermocline (the layer of water that is too cold for most carbonate-producing organisms) (8) The pycnocline (the layer of water where salinity is too high for most organisms). (9) Aragonite compensation depth (ACD): Level in the oceans where aragonite is dissolved, about 3 Km. (10) Calcite compensation depth (CCD): The level in the deep oceans where the rate of dissolution of calcium carbonate (calcite) balances the rate of deposition and below which carbonate-free sediments accumulate. The level is characterized by a transition from carbonate ooze to deep-marine clay or siliceous ooze, about 4-5 Km. The CCD varies between ocean basins. The position of the ACD and the CCD depends on the fertility of the surface waters and the degree of undersaturation of deep water. 8

(11) Silica compensation depth (SCD): Level in the oceans where silica (SiO2) is dissolved, about 6 Km. (12) Depositional base level (the interface between the sediments and liquids (water, air), it may be horizontal or inclined at 30 then called depositional dip) (13) Tectonic base level (the interface between basement of basin and sediment or it s a structural plain of faults that forming basin).it is the main plain affecting directly on sedimentation processes, there are two types: a- Level without faults or smooth basements as platforms or low subsiding areas. The sediments here are undeformed with clear structures and features. b- Level with faults interface that exist on deeply faulted basement as Horst and Graben ,the sediments here are deformed by faults without tectonic effect, these faults called growth faults(non-tectonic causes, compaction cause, recognized in quick sedimentation basin as deltas ex: Mississippi ,Niger) the good example is Sirit basin in libya. The discrimination between two types are important in basin analysis. 9