Glossary of terms for rock cores and the Ferm code
Bed. A distinctly recognizable unit or layer of sedimentation in a sedimentary rock, distinguishable from overlying and underlying rock layers, generally with thickness in excess of 1 centimeter to several meters. Beds in sedimentary rocks represent deposition of sediment under similar conditions, and in some cases, by one event. Bedding in sedimentary rocks is commonly described by its thickness. These subdivisions are not included in the Ferm coding system for core description, but can be included as notes.
Bedding. The physical geometry of constituent layers, beds, or laminae making up a sedimentary rock mass. Bedding is commonly described by its grain size, shape or dip of bounding contacts, continuity or discontinuity of internal structures or bounding contacts, and sedimentary structures.
Biogenic structure. A structure or feature caused by bioturbation. See Bioturbation.
Bioturbation. A rock fabric resulting from the action of plants or animals when the rock was unlithified sediment. Fabrics range from general particle disorientation and churning to discrete, infilled tubes and structures.
Bone or bone coal. Dull, coaly material that has a moderately high density.
Brecciated structure. A rock composed of angular pieces separated by finer matrix material or natural cement, in which the pieces have rotated from their original undisturbed positions. Differs from mosaic structure in the Ferm classification, in which the pieces are separated but remain in relative original position to each other.
Brittle deformation. Deformed sedimentary structure that resulted from stresses applied after the rock was lithified. Involves clear cracking or breaking rather than compacting or molding.
Cannel coal. A dull coaly material having a massive fabric, waxy luster, and conchoidal fracture.
Carbonaceous. Containing an appreciable amount of organic material of either plant or animal origin. Carbonaceous rocks generally have a dark gray to black color because of the abundance of organic material they contain.
Carbonate mineral. A mineral containing calcium carbonate (CaCO3). Calcium carbonates react (effervesce) with hydrochloric acid.
Carbonate rock. A sedimentary rock composed primarily of calcium carbonate (CaCO3), such as limestone and dolomite. Limestones are composed of more than 50 percent calcium carbonate. Dolostones (dolomites) are composed of more than 50 percent calcium-magnesium carbonates (CaMg(CO3)2). Limestones are much more common in coalfield strata than dolostones. Calcium carbonates react (effervesce) with hydrochloric acid. See Limestone.
Clastic. A sedimentary rock composed of eroded particles, originally transported from somewhere else (Bates and Jackson, 1980). Includes the rock types claystone, siltstone, and sandstone. Does not include sedimentary rocks formed from precipitation of their constituent grains or matrix, such as carbonate rocks like limestone. See also Siliciclastic rocks.
Claystone. A sedimentary rock composed primarily of clay-size particles. Clay-size particles have diameters less than 0.00016 inch (0.0039 mm, 8 phi units) (Bates and Jackson, 1980). Claystones are part of the mudstone (fine-grained) classification in the Ferm code. See Grain size.
Conchoidal fracture. A rock fracture characterized by a smooth, curved surface.
Conglomerate. A coarse-grained sedimentary rock that contains particles greater than 0.079 inches (2 mm) in diameter, such as cobbles and pebbles (Bates and Jackson, 1980).
Core barrel. Hollow tubes used in drilling for core samples. The outer barrel is attached to the drill bit, and rotates with the bit. The inner barrel collects the cut rock core. Standard core barrels are 10 to 15 feet long.
Core bit. A hollow drill bit for cutting cylindrical sections of core. The rim of the bit is coated with tungsten carbide or diamond chips, so that it can cut the rock with the aid of a lubricant, usually water. As the outer edge of the bit cuts rock, the core passes upward through the center of the bit into the core barrel. Different sizes of bits and tubing are used to cut different diameters of core. It takes more power to cut larger diameters, so sometimes the diameter must be reduced as the hole depth increases. Standard core diameters are designated as “Q” sizes: AQ = 1.0 inch, BQ = 1.4 inch, NQ = 1.87 inch, HQ = 2.5 inches, and PQ = 3.35 inches.
Core loss. The expected thickness of core minus the amount of core retrieved. In some cases, core may be lost during drilling and recovery (for example, an inch of clay shale may be lost in a 10-foot core run). It is important to note lost core in each core run (generally as a percentage of the total if the specific interval is not known) so that an accurate record of true depth and rock layer thickness is maintained.
Core runs. An individual interval of coring, usually the length of a core barrel. If the amount of core that needs to be retrieved is longer than the core barrel, multiple core “runs” are made to collect increments of cores until the total length of core is obtained. The length of a core run is determined by the standard core barrel used, usually 10 or 15 feet.
Coring rig. A specialized drilling rig designed for coring rock rather than rotary drilling through rock. Like a rotary rig, it has a derrick or tower above a platform with motors and a drill string. Threaded steel tubes the length of the core barrel are lowered down the drill string into the hole for each core run. With conventional core rigs, the entire length of drill rods must be extracted from the hole in order to retrieve a completed core barrel. Wireline core rigs have an inner barrel that permits extraction of the core barrel without the drilling rods having to be removed, a significant savings in time and expense.
Crossbedding. Inclined layering in a sedimentary rock bed resulting from deposition by moving water or wind. Crossbeds are greater than 1 centimeter in thickness. Crossbeds are restricted to sandstones (or limestones) with grain sizes greater than 1 millimeter (0.039 inches), which is very fine sand or coarser. They can be more than 15 feet thick, but generally are 1 to 3 feet thick.
Dewatering structure. A deformed structure in sedimentary rocks resulting from expulsion of water when the rock was still sediment. Most common in grain sizes of silt to very fine-grained sand.
Dewatering pipe. A relatively large-scale dewatering structure that separates flow rolls.
Dilute hydrochloric acid. A 10 percent solution of full-strength hydrochloric acid used to test for the presence of carbonate minerals.
Drill pipe. Hollow steel pipes used in drilling. Drill pipes (1) connect the drill bit to the drill rig and transfer the motion of the motor to the drill bit and (2) allow transfer of drilling mud from the surface to the drill bit.
Drill string. The connected downhole string of drill bit, core barrel if used, and drill pipes to the surface drill rig.
Drilling mud. A mixture of water and clays (bentonite) used during drilling. Chemicals may also be added for lubrication and viscosity control. Drilling mud helps keep the drill bit cool, keeps pressure in the wellbore to prevent formation fluids from entering the wellbore, and are used to circulate cuttings from the bit to the surface.
Drilling pad. A level gravel or cement surface constructed to put the drilling rig on so that it is stable and stays level during drilling. Mostly used for larger drilling rigs.
Ductile. Deformation without breaking. Nonbrittle. See also Soft-sediment deformation.
Durability. See Rock durability.
Effervescence. The bubbling reaction seen when hydrochloric acid is applied to carbonate (CO3) minerals. Calcium carbonates (CaCO3) such as limestone effervesce readily, whereas many other carbonate minerals such as dolomite (CaMg(CO3)2) and siderite (FeCO3) effervesce weakly.
Fabric. See Rock fabric.
Fault. Offset of bedding along a fracture plane in rock across which movement has occurred.
Fault gouge. Pulverized rock and sedimentary material along a fault, formed by the friction between two rock masses that have moved along a fault plane. Rocks are commonly brecciated within fault gouges.
Fine-grained. In sedimentary rock and sedimentary rock core description, grain sizes of silt or smaller (less than 0.0025 inch., 0.0625 mm, 1/16 mm, 62 microns, 4 phi units), which includes siltstones, shales, mudstones, claystones, flint clays, and fire clays. In general, fine-grained applies to grain sizes in which individual grains are smaller than can be detected by the eye (Bates and Jackson, 1980).
Fire clay (also spelled Fireclay). A hard claystone composed mostly of silica. The formal definition of a fire clay is a silica-rich clay that can be used for refractory ceramics, but in the coal fields the term has sometimes been used for underclays (Bates and Jackson, 1980).
Flaser bedding. Ripple bedding in which mudstone drapes or streaks between coarser grained ripples are discontinuous or incompletely preserved (Reinick and Wunderlick, 1968). Compare with Wavy bedding and Lenticular bedding. This subdivision of ripple bedding is not specifically designated in the Ferm code, but is commonly included in geologic notes and descriptions of cores.
Flat bedding. Flat-lying and parallel sedimentary beds.
Flint clay (also spelled Flintclay). A claystone composed mainly of the mineral kaolinite that has a characteristic milky luster and conchoidal fracture (Bates and Jackson, 1980).
Flow roll. Convoluted bedding in sedimentary rock resulting from ductile, soft-sediment deformation processes, with the general appearance of bedding that has been folded into a semicircular or oval shape. Horizons of flow rolls are commonly separated by dewatering structures (also called dewatering pipes).
Foreset. Inclined laminae within ripple bedding and crossbedding, representing the avalanche front of a migrating bedform.
Fracture. A break in a rock or rock bedding resulting from mechanical failure or stress. Includes cracks, joints, and faults (Bates and Jackson, 1980).
Framework grains. The dominant particles in a rock that support each other in a sedimentary rock. Cements, matrix, and pores occur between the grains.
Graded bedding. Flat bedding in which the grain size of individual layers shows an internal, gradual transition. In normal grading, grain size diminishes from the bottom to the top of the bed. In inverse grading, grain size increases from the bottom to the top of the bed.
Grain size. The size of particles in sediment or sedimentary rock. Grain sizes are used to classify different types of sedimentary rocks (e.g., siltstone, sandstone, etc.). Grain size is typically measured in millimeters, microns, and phi units.
HCl. Hydrocloric acid.
Homogeneous fabric. A sedimentary rock having a uniform appearance with no obvious lines, streaks, or layers. See also Massive.
Ironstone. A rock composed primarily of iron-bearing minerals. Siderite (FeCO3) is the most common iron carbonate mineral in ironstones of eastern and midwestern United States coal fields.
Laminae (lamination). The thinnest recognizable unit of sedimentation in a sedimentary rock, less than 1 centimeter (0.39 in) in thickness (Bates and Jackson, 1980). Many laminae may constitute a bed.
Lenticular bedding. Ripple bedding in interlaminated or interbedded mudstones, siltstones, and sandstones in which the intervening finer-grained (mudstone, siltstone) layers between ripples are continuous and equal to or greater than the thickness of the coarser-grained (sandstone, siltstone) ripple laminae (Reineck and Wunderlich, 1968). Grades from continuous ripple laminae with overlying and underlying mudstone laminae to isolated or disconnected ripples surrounded by mudstone. Compare with Flaser bedding and Wavy bedding. This subdivision of ripple bedding is not specifically designated in the Ferm code, but is commonly included in geologic notes and descriptions of cores.
Limestone. A sedimentary carbonate rock composed primarily (> 50 percent) of calcium carbonate (CaCO3). See Carbonate rocks.
Lithology. A description of a rock based on its aggregated properties.
Massive (fabric or bedding). Bedding or laminae that lack internal structures. Homogenous beds.
Matrix. The finer-grained material in sedimentary rocks occurring in the pore spaces between framework grains.
Megaripple. A large ripple bed having a wavelength greater than 1 meter (39.37 in) or a ripple height greater than 10 centimeters (3.94 in) (Bates and Jackson, 1980). Generally used for large ripples, but is smaller than features described as crossbeds.
Mineral. A naturally occurring inorganic element or compound having an orderly structure and characteristic chemical composition, crystal form, and chemical properties (Bates and Jackson, 1980). The building blocks of rocks.
Mineral composition. The types of minerals that make up a rock.
Moisture susceptibility. The degree to which a sedimentary rock will disaggregate or begin to fall apart when exposed to moisture.
Mosaic structure (or fabric). A sedimentary rock composed of angular fragments separated by thin cracks filled with matrix material, in which the fragments are in the original, undisturbed positions relative to adjacent pieces. Differs from a breccia or brecciated fabric in the Ferm classification, in which the pieces are rotated or out of position relative to each other.
Mud flow. In the Ferm classification, a small-scale deformation structure, generally caused by ductile, soft-sediment deformation. The term is more commonly used in geomorphology for larger-scale mass movements on slopes involving a flowing mass of fine-grained sediment and fluids.
Mudstone. In siliciclastic rocks, a dark sedimentary rock containing a mixture of clay- and silt-size particles (see claystone, siltstone). In some uses, refers to all fine-grained rock types, whether laminated or unlaminated. In other uses, the term mudstone more specifically refers to nonlaminated fine-grained rocks, in contrast to shales, which are laminated fine-grained rocks. See Claystone and Siltstone. In the Dunham classification of carbonate rocks, refers to a limestone with less than 10 percent discernable particles.
Nodular structure (or fabric). A sedimentary rock containing rounded or lumpy masses of material set in a matrix of typically finer-grained material.
Oolitic (pisolitic). A sedimentary rock composed mainly of highly rounded particles. The particles formed from accretionary processes. Oolites are rocks, usually limestones, composed of ooliths. Ooliths are small (0.25 to 2 millimeters), circular to ellipsoid grains composed of concentric layers or radiating fibrous structures. Pisolites are rocks, usually limestones, composed of pisoliths. Pisoliths are pea-size or larger oolites, usually showing concentric internal layering (Bates and Jackson, 1980).
Paleosol. An ancient soil horizon. Paleosols are generally clay-rich and rooted. They may exhibit leaching and color mottling and contain mineral nodules such as siderite or limestone. Paleosols are often gradational with underlying rock layers.
Piezoelectricity. A mineral property in which electric potential develops in a preferred crystallographic direction when a stress is applied.
Pyrite. A common iron sulfide mineral (FeS2). Typically has a green-gold color and metallic luster. Informally called “fool’s gold.”
Quartzose. Describes a sandstone variety in which more than 90 percent of the grains are quartz.
Ripple bedding (or lamination). Ripples are small-scale sedimentary structures, typically 1 centimeter or less in thickness, formed by migrating ripple bedforms in sediment. When ripples or stacked ripple units are less than 1 centimeter thick, they are termed ripple laminae or ripple laminations. When ripple units are stacked into units more than 1 centimeter thick, they are termed ripple beds. Ripple beds and lamination may be further subdivided or classified as flaser, wavy, and lenticular beds or laminae, depending on the continuity of mudstone streaks or layers between individual ripples.
Rock durability. As used in core properties, the ability of a rock to resist weakening and disintegration when exposed to cycles of wetting and drying.
Rock fabric. The orientation or lack of orientation of the constituent particles, cements, matrix, etc., in a rock (Bates and Jackson, 1980).
Rock strength. The ability of a rock to resist failure when strain is applied either parallel or orthogonal to bedding direction.
Rock texture. A description of the size, shape, and relationships of grains making up a sedimentary rock.
Rotary drilling rig. A common type of shallow drilling rig that has a motor that imparts a rotating motion to a series of drill pipes attached in a drill string, which cause the drill bit at the end of the drill string to rotate so that it cuts a cylindrical hole into the earth. Rotary rigs use water and drilling mud or pressurized air for circulation and lubrication of the drill bit and drill string downhole.
Sandstone. A sedimentary rock composed primarily of sand-size particles. Sand-size particles have diameters less than granules and more than silt-size particles. Sand-size particles have diameters ranging from 0.0025 in. to 0.08 inches. Sandstones are subdivided into very fine-grained, fine-grained), medium-grained, coarse-grained and very coarse-grained, based on grain size (Wentworth, 1922). See Grain size. These subdivisions are not included in the Ferm coding system for core description, but can be included as notes.
Sandy shale. A sedimentary rock composed primarily of silt-size particles or mixtures of clay, silt, and sand particles or layers.
Seat earth. See Underclay.
Sedimentary bedding. See Bedding.
Sedimentary structure. A structure in a sedimentary rock formed at the time of deposition or shortly after deposition by moving sediment. Includes ripples, crossbeds, etc.
Sedimentary texture. See Rock texture.
Shale. Laminated fine-grained rock. Differs from mudstones, which generally are unlaminated fine-grained rock, although in some usages may include laminated and unlaminated fine-grained rock. See Mudstone.
Siderite. A common iron carbonate mineral (FeCO3).
Siliciclastic. A sedimentary rock composed primarily of silicate minerals. See Clastic.
Silicate mineral. Minerals containing both silica and oxygen in the SiO4 tetrahedra (Bates and Jackson, 1980). The most common minerals on earth. Silicates include quartz, feldspars, clay minerals, micas, garnets, and zircons.
Siltstone. A sedimentary rock composed primarily of silt-size particles. Silt-size particles are smaller than sand-size and larger than clay-size particles. Silt-sized particles range from 0.00016 to 0.0025 inches (0.0039 to 0.0625 mm, 8 to 4 phi units) (Wentworth, 1922). Siltstones are part of the mudstone (fine-grained) classification in the Ferm code. See Grain size.
Slickensided. A polished, smooth to striated, often glassy surface formed by friction. Slickensides are most common in fine-grained rocks (shales, mudstones, claystones). They are common in underclays and paleosols, in shales directly beneath thick sandstones, along fault planes, and in clay dikes (clay veins). Sometimes called “slips” by miners.
Slump. A deformed sedimentary mass that resulted from rotational movement along a glide plane, such as commonly occurs in stream-channel bank collapses. Slumps are characterized by inclined and deformed beds with a polished lower bounding surface representing the glide plane. In most slumps, the angle of bedding increases down the glide plane from the head of the slump to the toe of the slump. At the top of the glide plane, bedding may be horizontal, and relatively undisturbed. In contrast, toes of slumps commonly contain contorted bedding, bedding offset by secondary and tertiary offset faults, and steeply inclined bedding.
Soft-sediment deformation structures (or features). Structures resulting from stresses applied to sediments during burial or shortly after burial that result in contortion, convolution, or folding of sedimentary layers, flow rolls, and a variety of water-escape structures. Most soft-sediment deformation is formed by at least partial liquefaction or fluidization of the sediment. Stresses resulting in soft-sediment deformation include rapid loading of burial sediments; density contrasts between layered, water-saturated sediments; sediment movement on inclined slopes; and fluid flow over a sediment surface.
Splint coal. A dull coal variety having a steely gray luster, low density, and common association with very thick bands of shiny coal.
Texture. See Rock texture.
Underclay. A layer or bed of clay beneath a coal bed, representing the ancient soil or surface on which the coal-forming peat originally accumulated. Underclays often contain slickensides and fossil roots.
Wavy bedding. Ripple bedding in interlaminated or interbedded mudstones, siltstones, and sandstones in which the finer-grained (mudstone, siltstone) laminae between ripples are continuous across the top of the ripples and fill ripple troughs, and have thicknesses less than or equal to the coarser grained ripple laminae or beds (Reineck and Wunderlich, 1968). Compare with flaser and lenticular bedding. This subdivision of ripple bedding is not specifically designated in the Ferm code, but is commonly included in geologic notes and descriptions of cores.
- Bates, R.L., and Jackson, J.A., 1980, Glossary of geology [2d ed.]: Falls Church, Va., American Geological Institute, 751 p.
- Ingram, R.L., 1954, Terminology for the thickness of stratification and parting units in sedimentary rocks: Geological Society of America Bulletin, v. 66, p. 937–938.
- Reineck, H.E. and Wunderlich, F., 1968, Classification and origin of flaser and lenticular bedding: Sedimentology, v. 11, p. 99–104.
- Wentworth, C.K., 1922, A scale of grade and class terms for clastic sediments: Journal of Geology, v. 30, no. 5, p. 377–392.