Sedimentary Facies Analysis - Essay Example

Sedimentology Insert Insert Sedimentary facies is forms of residue visibly different from neighboring residue deposited in dissimilar depositional environment (Amanz Gressly, 1838). Physical, chemical and biological aspects of the sedimentary bed and geological age differentiate facies. Physical aspects include temperature, wind speed and direction, velocity and water depth. Chemical aspects include mineralogy of the residue source and water salinity. Biological aspects include grass and forest cover on land, type of organisms at sea, their activities and the skeleton composition. Some other aspects include earth movements, volcanic activities, and tectonic activities may affect the development of facies in a certain location. Sedimentary facies can be formed only where residues are deposited and remain undisturbed for long time in their sedimentary basins where they compress and are concreted into layers or hard beds. The size of a sedimentary facies is determined by the sedimentary basin the residue deposited themselves. There are different types of sedimentary facies; biogenic facies that occurs through buildup of whole or split shells and other solid portions of organisms or fossils, chemical facies develop when there is mineral precipitation of materials from solvents. Terrigenous facies forms up when eroded elements from older rocks are accumulated. Each facies has a three-dimension shape, but this may change with time. Facies Analysis This is the description and interpretation of the physical process controlling formation of sedimentary facies through weathering of sediments, the deposition process, lithification and even erosion. To describe facies analysis, a number of facies will be taken into considerations; 1. Coal facies Coal facies is made up of leaves, stems and bark of plants. At upper parts, ash, carbonaceous shells may be seen. Post dispositional features may include calcareous mineralization in clear spaces and modular pyritization. Formation of coal facies is through buildup, burial and coalification of its contents. This is manifested by the presence of organic content, and plant remains in leaves, stems, brachiopod shells, etc. despite underclay facies being at its bottom they are never genetically related (McCabe, 1984). Sandstone facies and conglomerate are coal that overly non-rooted facies that were washed away and transported from neighboring peat-forming setting are genetically related to original residues (McCabe, 1984) they come from. Coal with high sulfur content and black shale facies developed in a place with oceanic conditions. Coal with high ash developed in non-marine siliciclastic environment (McCabe, 1984). Coals of Desmoinesian developed in marginal marine mire environment raised, floating or low-lying (McCabe, 1987; 1991). 2. Black Shale Facies They are in three variations; phosphatic black shale, fossiliferous black silistone and pyritic black shale (Lange, 2003). It consists of black, dark gray siltstone, silty-shale. Phosphatic and fossiliferous black shale is about 1 foot (0.3metres); pyritic black shale is almost 10 feet (3metres). Alluvial structures include very tiny unfossiliferous flat laminate. Upper and contacts are sharp. The underlying facies on black shale includes coal, underclay, and heterolithic siltstone. Overlying facies includes gray shale and mudstone-wackestone. Horizontal, vertical and spiral traces of fossils from undermined species. The black color in the black shale facies indicates deposition by suspension residue in low energy, offshore maritime environment. Absent of plant residue, on black shale facies, show that the maritime condition was not favorable for preservation. Traces of Phosphate in black shale facies indicate deep water (Heckle, 1977; Schultz, 1991). 3. Underclay Facies It resembles “Blocky Mudstone” facies (Lange, 2003). It is approximately 1 to 20 feet; it is light to dark gray in color. Post depositional structures include platy, angular or sub angular-blocky mudstone sometimes columnar and prismatic shaped mudstone. Lower contacts are gradational with post-depositional structures while upper contacts are typically sharp. Underclay facies include stems, roots, and leaves. Paleosolea from underclay facies include root traces, soil horizons and soil structure (Retallack, 1988; 2001). Features of paleosols vary depending on paleoclimate, paleoenvironment and overlying conditions from marine flooding and mineralogy of parent material (Gustavson, 1991; Retallack, 2001). Root traces on the underclay facies shows the water table and soil drainability. Presence of carbonaceous root traces on the facie describes poorly drained, watery soils. Vertically spread roots on the facies shows more drained soils but literally, spread roots traces shows poorly drained waterlogged soils especially in swampy, low-lying areas. Siderite-lined or rhizoconcentration root traces shows the drier, well-drained, alkaline soils with fresh water and has nutrients favorable for root development.in stagnant water, Drab-holed root traces are seen (Retallack, 2001). Structures of soil show soil maturity and soil drainability. Padogenic structures or Platy soil clods indicate soil immaturity and possible disruption of bedding. Mature soils are showed by angular and sub-angular blocky peds form. High salt content and a high shrink-swell capability soil show columnar and prismatic peds. Peds are padogenic structures. Padogenic slickensides are found on underclay facies and are clay shrink-swell features in more developed soils found in environments with wet and dry seasons (Retallack, 2001). Over time, soils may separate into visible horizons depending on grain size, chemical attributes, padogenic structures, alluvial features and earlier alluvial features. Desmoinesian paleosols are been either non-oxidized; gray colored soil, semi-vertical, very weakly developed or even weakly developed. It may also be interpreted as moderately developed vertisols in other cases (Retallack, 1988; 2001). 4. Gray Shale Facies This facie is a collection of medium-gray to dark gray siltstone, or shale (Lange, 2003). Mainly found in incised/notched valleys. Gray shale facie is 2 to 20 feet thick. It contains thin, repetitive, upward laminae in its parts. Upper contacts are gradational and have heterolithic siltstone, lower contacts are sharp and underlying facies include black shale, coal or underclay facies. Body focills are absent except minute shells since the facie does not allow preservation of the focills due to salty condition. Presence of plant fragments indicates sedimentation occurred near a marginal marine environment. Due to sediment fallout during dispositional process parallel, horizontal laminae are observed. Presence of lenticular laminae show disposition from low-energy interrupted sedimentation (Potsma, 1982; Buatois et al.). Gray shale facies is created when suspended residues fallout under low-energy currents in an offshore slightly restricted sedimentation basin. 5. Heterolithic Siltstone Facies It resembles interlaminated sandstone and siltstone facies (Lange, 2003). The heterolithic siltstone is composed of bimodal, well sorted, and medium to dark gray siltstone and silty very fine-grained sandstone. It is 1 to 10 feet thick. Alluvial structures include very thin, thin to thick horizontal silt laminae coated with thin to thick lenticular silty-sand laminae and slight starved unidirectional ripple lamination sets. It has reactive surfaces. Overlying facies includes heterolithic sandstone underlying facies include coal, gray shale, black shale and heterolithic sandstone facies. Body focills are rarely seen in the facies, but horizontal burrows and structureless bioturbated zones are observed. Leaves, stems, and barks are common. Heterolithic siltstone facies form from irregular flood-ebb tidal currents and overriding the slack water suspension fallout in inferior tidal flat or subtidal coastline settings (Nio and Yang, 1992). Tidal processes can be high-class criterion as mud couplets, lamination bundles, reactive surfaces and diurnal cyclic bundle thickness variations; all these are observed in facies. Tide-related criteria of tidal rhythmites include, coarse and fine-grained lamination couplets, vertical couplets thickness variation observed in facies. Presence of trace fossils (Plano lites) show brackish-water environment with restrained sedimentation rates that allow burrowers actively maintain burrows (Pemberton et al., 1992; Buatois et al., 1999). Presence of Siderite shows fresh water infiltration; plant remains show formation near continental and marginal marine environments (Potsma, 1982). 6. Mudstone-Wackestone Most mudstone-wackestones are light gray, light tarnish gray and medium brownish gray, but some are black in color. It is 2 to 10 feet thick. Sedimentary structures include thick laminated bed, but some are thin with insignificant fenestal fabrics. Its matrix consist of mud or colloidal mud. Upper and lower contacts are sharp. Underlying and overlying facies depend on the position of mudstone-wackestone facie itself within the cyclothem model (Hackel, 1977). If mudstone-wackestone facie is located at the upper part, the overlay is packstone-grain store facie or black shale facies if located at the core, the underlay is underclay, packstone-grain store or black shale facies. If located in the middle it can overly packstone-grainstone, underclay or coal facies and underlay could be black or gray shale facies (Lange, 2003). The formation of this facie mainly occurs in low-energy, open aquatic and limited shallow aquatic slope environments that are the reason why mudstone-wackestone facie has root traces and low faunal abundance (Heckle, 1977). All facies are different in their formation, content and size. This is due to different environmental conditions that are present during formation. With time, facies may change due to change in present conditions. Changes in sea level for example rise in sea level will allow finer sediments to be received since water moves inland. Facies analysis is very important especially in industries that exploit earth resources as fossil fuels since it may lead to forecast about coal, petroleum, and natural gas. Apart from the examination of rock specimens on fossil fuels, facie analysis may also rely upon geographical properties of the rocks as density and electrical, magnetic and radioactive properties. Facie analysis may give information on recognition and correlation of important resources obtained from boreholes and other possible sites (Lange, 2003). List of References Boer, P., Gelder, A. and Nio, S. (1988). Tide-influenced sedimentary environments and facies. Dordrecht, Holland: D. Reidel Pub. Co. McCabe, P. (1975). The sedimentology and stratigraphy of the Kinder scout grit group (Namurian, Rsub(1)) between Wharfedale and Longdendale. University of Keele. Plint, A. and Reading, H. (1995). Sedimentary facies analysis. Oxford, England: Blackwell Science. Reading, H. (1986). Sedimentary environments and facies. Oxford: Blackwell Scientific. Reading, H. (2002). Sedimentary environments. Oxford: Blackwell Science. Read More