Kluth & Associates


                        GLOSSARY OF STRUCTURAL GEOLOGY
A-DIRECTION: One of the coordinate directions set up to describe the geometry of features formed during folding and of the fold itself. In a similar fold, the a-direction is defined as the direction of tectonic transport of material, parallel to the trace of the cleavage planes in cross section. In cross sections of parallel or concentric folds, the transport direction is parallel to bedding because the main movement is bedding plane slip. In this case the fold is assumed to have formed as a cleavage fold and the same coordinate directions are used. That is, in an upright parallel fold, the a-direction is vertical. See also B-DIRECTION.
ALLOCHTHONOUS: The condition of having moved during deformation, thus not being in an original position. Rocks that have moved from their original position during an orogenic episode are referred to as allochthonous. This is true regardless of whether these rocks have been deformed or not. For instance, undeformed rocks riding merrily along on the hanging wall of a thrust sheet are referred to as ALLOCHTHONOUS. Compare with AUTOCHTHONOUS.
ANOMALY: A term used in structural mapping to denote a deviation from the regional structure and a feature that could be capable of trapping hydrocarbons but needs further evaluation.
ANTICLINAL COMPLEX: A term used in the Gulf Coast for a large anticlinal structure that may be due to salt or shale flowage at depth or a large rollover anticline associated with a series of listric normal faults which may localize a number of hydrocarbon traps near the crest of the structure.
ANTICLINE: A fold in the rocks that is convex upward.
ANTI-LISTRIC FAULT: A normal fault that has a concave downward geometry - becomes steeper with depth. Compare with LISTRIC FAULT.

ANTITHETIC FAULT: A subsidiary fault that dips in the opposite direction of the master fault in a fault system. The displacement on the fault decreases toward the intersection with the master fault. In the Gulf Coast, these faults are referred to as compensating faults. See also SYNTHETIC FAULT.
ANTITHETIC SHEAR: Shear planes dipping in the opposite direction of a normal fault.
ARCUATE FAULT: A highly curved, listric, normal fault that is concave toward the local axis of deposition.
AREA BALANCE: The amount of rock in an area is the same after deformation as before deformation, or the lost material can be accounted for
APPARENT DIP: The dip of a rock layer measured not at a right angle to its strike: The apparent dip is always less than the true dip.
AUTOCHTHONOUS: The condition of being in an original position and not having been moved during deformation. Compare with ALLOCHTHONOUS.

AXIAL PLANE: A plane that includes all of the lines of maximum curvature (fold hinges) of surfaces (beds) involved in a fold. This is the plane of maximum curvature in a cross section of a fold, and in certain cases may bisect the angle between the fold limbs (interlimb angle). This plane is often curved, in which case it is referred to as the Axial Surface. The fold axis of any fold must lie within the axial plane of that fold. The trace of the intersection of the axial plane and the ground surface on a map is the Axial Trace. Active Axial Surfaces are those which rock material moves through as a fold develops. Inactive Axial Surfaces are those that rocks do not move through as the fold develops. Axial surfaces can change from active to inactive and back again as the fold develops.
"BACKBONE" FAULT: A growth fault located at or near the crest of a rollover anticline and usually exhibiting normal drag bed geometry in the hanging wall.
BACKLIMB: The more gently dipping panel of dip in an asymmetric fold.
BACK THRUST: A thrust fault that carries rocks in its hanging wall in the opposite direction of the master thrust.
BACKWARD BREAKING: The sequence of imbrications of a thrust fault (usually) in which the imbricates get younger in the opposite direction to that of tectonic transport. This has nothing to do with moving your rock collection. That's backbreaking. See also FORWARD BREAKING.

BALANCED CROSS SECTION: A cross section constructed using structural styles that can be seen to be present in the region around the section (admissible structures) and which can be restored to its original, pre-deformation configuration (it is viable).
B-DIRECTION: In the coordinate system set up to describe folds and fold related features, the b-direction is the same as the axis of the fold. It is the long dimension of the fold. See also A-DIRECTION.
BEDDING PLANE FAULT: A fault that is parallel to bedding.
BEDDING PLANE SLIP: Relative motion between adjacent beds. This motion is usually the upper bed moving away from (out of) the syncline and toward the anticline. An easy model to use when trying to visualize this type of slip is to fold a phone book or a dock of cards. The upper pages or cards slip outward past the lower ones.
BETA: The ratio of the thickness of stretched to unstretched lithosphere. A measure of how much stretching has taken place (during deformation. Thus,
BETA-FACTOR: Extension or stretching factor. εβ+=1 where ε is the ratio of change in crustal profile length to the original length. So
or in terms of bed lengths,
BRITTLE: The term used to describe a material that undergoes little permanent deformation before fracturing. Quartzites and dolomites are examples of rocks that usually behave brittly. Often, this type of rock is described as COMPETENT.
BUOYANCY: In terms of salt tectonics, buoyancy is the resultant of upward forces exerted by the surrounding sediments due to the density differential between the salt and surrounding sediments. The force acting on the salt may be positive (upward), negative (downward), or neutral.
BY-PASS: The en echelon arrangement of anticlines along a trend of folds. The zone of overlap in adjacent anticlines is called, within Chevron, the By-Pass Zone. The syncline between the anticlines plunging in opposite directions is referred to as the by-pass syncline.
C-DIRECTION: In the fold coordinate system set up to describe features of folds, the C-direction is defined as being perpendicular to both the A coordinate axis and the C coordinate axis. See also A-DIRECTION and B-DIRECTION.
CLEAVAGE: Closely spaced planes within the rock, usually created by the alignment of planar mineral grains, such as mica.
CLOSURE: Dip in all directions away from a structural high.

COLLAPSED SALT RIDGE: An elongated synclinal area exhibiting structural thinning toward the axis of the syncline. This type of Gulf Coast structure is thought to be due to later salt removal from an early linear salt anticline with consequent subsidence of the thinned overburden to the pre-salt surface.
COMBINATION TRAP: A hydrocarbon accumulation where the trap geometry has a component of stratigraphic trap as well as a structural component.
COMPACTION: The decrease in the volume of sediment (thickness of a rock unit) resulting from continued deposition and burial to deeper depths; the decrease in thickness of a rock unit due to reduction of pore space produced by the weight of the overburden.
COMPARTMENTAL FAULT: A fault that is transverse to the main structural trend and that separates areas of different structural geometries. The amount of regional extension or shortening is the same on both sides of the fault even though geometries on either side may be quite different. See also TRANSFER ZONE.
COMPETENCY: The measure of a rock's ability to withstand stress before deforming significantly. For our purposes, this is essentially related to the strength of the rock (stronger rocks are more competent).
CONCENTRIC FOLD: Originally defined as all layers involved in a fold having the same center of curvature. Now defined as a fold in which the layers involved do not change thickness around the fold. Also referred to as a Parallel Fold, or sometimes as
CONCORDANT: A term used to describe a salt diapir in which the salt-sediment contact is parallel to the bedding of the overburden.

CONICAL FOLD: A fold that is shaped like a section of a cone. This shape requires an apex toward which the radius of curvature of the folded layers decreases. The apex can be up-plunge (a normal conical fold) or downplunge (a reverse conical fold).
1. All generating lines pass through a point known as the vertex.
2. Vertex may be up-plung or down-plunge.
3. If vertex is up-plung, anticlines get smaller and sharper with depth.
4. Vertical dips are not parallel to the axis.
COUNTER-REGIONAL FAULT: A normal fault that is downthrown in the opposite direction of the regional dip toward the basin axis; in the Gulf Coast this usually means down to the coast or down toward the continent.
CREST: The part of the fold where the beds attain their highest elevation. For any particular unit, the highest point on the fold. The crests for all of the layers can be joined together by a Coastal Plane. Note that on most geologic maps, the place where the dip reverses is usually mapped as the axial trace, but unless the fold is upright what is really mapped is the trace of the crestal plane.
CRESTAL COLLAPSE GRABEN: An elongated downdropped block formed by several opposite dipping normal faults on or near the crest of a rollover anticline. Outside of the Gulf Coast, this type of structure is called a KEYSTONE GRABEN.

CRITICAL DIP: Sometimes used to denote the dip in the direction of poorest control.
CRITICALLY TAPERED WEDGE: An orogenic wedge (in a thrust belt) of rocks in which the angle between the upper (topographic) surface and the basal (usually basement) surface is correct to allow internal deformation (thrust faulting) to take place. Below the critical angle, internal deformation (such as out of sequence thrusts, etc.) tend to thicken the wedge and increase the angle between the surface and basement. Above the critical, angle the deformed wedge moves forward without internal deformation.
CYLINDRICAL FOLD: A fold that is shaped like a section of a cylinder. It is the surface that is generated by moving a line (fold axis) parallel to itself in space.
DATUM: Any numerical or geometrical quantity which may serve as a reference plane for other quantities. A level reference elevation used in structural mapping and cross sections - usually sea level.
DECOLLEMENT: The independent deformation by folding or faulting of sedimentary beds by detaching and sliding over the underlying rocks.
DECOMPACTION: The act of restoring a rock unit to true depositional thickness by accounting for the reduction in pore space produced by the weight of the overburden.
DEEP PIERCEMENT: In the Gulf Coast, this term refers to an intrusive salt body that has pierced the, overlying strata and where the crest of the salt is greater than 10,000' below sea level.
DEEP SEATED STRUCTURE: An anticlinal structure in the Gulf Coast area that has resulted from structural deformation at great depth - usually thought to be due to salt or shale flowage into a deep anticline, but also may be a turtle structure or deep rollover.
DEFORMATION: A general term for the process of changing the shape or position of rocks as a result of various earth forces.
DEPOCENTER: An area or site of maximum deposition; the thickest pan of any specified stratigraphic unit in a depositional basin.
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DEPTH TO DETACHMENT: The depth at which folding terminates; for a normal fault,
The depth at which a fault parallels regional.
DETACHED: A term applied to diapiric salt bodies that have been disconnected (pinched off) from the Louann Salt or source layer.
DETACHMENT: A fault that separates panels of rocks that deform with different structural geometries. In most of the uses at Chevron, different structural geometries at different stratigraphic levels indicates that a detachment is present, even if it is difficult to see because of poor exposure. The shortening of units above and below the detachment must be the same even though the geometries are different. If local balance is not evident the shortening will be balanced somewhere in the region. This term is sometimes used to refer to a low angle normal fault (detachment fault).
DIAPIR: Body of material (rock of one sort or another) that rises because it is less dense than the surrounding rock. The displacement of the surrounding rock can take place by several mechanisms (for instance stoping of the roof, assimilation of the country rocks, or forceful intrusion to name just a few). Salt and shale diapirs in deltaic settings like the Gulf Coast and West Africa are the most important examples of these features to our business. These diapirs can take on may different shapes, such as Domes, Walls, Detached Masses, etc. In the Gulf Coast, 1500' of salt may be necessary to form diapirs.

DIP: The angular deviation of a bed from a horizontal plane.
DIP SLIP: The component of movement on a fault that is oriented parallel to the dip of the fault plane.
DISCORDANT: A contact (structural or stratigraphic) between rock units whose orientations are not the same.
DISPLACEMENT: The distance a rock moves (is displaced) by movement on a fault. This movement is measured parallel to the fault surface and in the direction of net slip, unless otherwise noted. See also HEAVE, THROW.
DISSECTED SALT TONGUE: An allochthonous body of salt that has been remobilized by differential loading or tectonic processes and flows into second cycle diapirs or has a large volume of salt lost by dissolution.
DOMINO FAULT SYSTEM: A linked system of straight planar normal faults where all the faults are active at the same time and the fault bounded blocks rotate during extension. The pattern of deformation is like a stack of dominos being rotated.
DOWNBUILDING: A theory of salt dome growth where the crest of the salt core is regarded as stationary or static and the base is regarded as moving downward.
DOWN-PLUNGE VIEWING OF GEOLOGIC MAPS: A technique of viewing the geologic map of a plunging structure parallel to the plunge of the structure in order to see a right section (or profile view) of the structure. This technique utilizes the fact that a map view of a plunging structure is essentially a distorted cross section. It allows the worker to get a picture of the structural geometry very quickly.

DOWN TO THE BASIN FAULT: A normal fault in the Gulf Coast whose fault plane is dipping toward the basin axis.
DRAG: Bending of beds near a fault plane due to frictional resistance to movement along the fault plane. The bending is upward on the hanging wall of a normal fault and downward on the hanging wall of a reverse fault.
DUAL FAULT ZONE: A type of fault zone commonly present at the edges of basement cored blocks in the Wyoming foreland. In this zone two moderately dipping (commonly 350 ± 100) reverse faults form the edge of the uplifted block. The two faults are separated by an overturned and deformed sliver of rocks. The overhang on the uplifted block overlies the fault zone and the upright basin footwall is below the two faults and the overturned sliver
DUCK HEAD: An asymmetric salt mass whose shape (perhaps controlled by the presence of an earlier fault) includes a large bulbous top part and a thinner lateral or downslope(?) intrusion (sill). The shape of the mass in cross section resembles that of a duck's head. Also used to refer to an unpopular supervisor, but in these cases it is often mispronounced.
DUCTILE: A term used to describe a material that undergoes much internal and permanent deformation before failure. Shales and salt are often ductile during deformation. Usually, this term corresponds to Incompetent.
DUPLEX: A series of imbricates that link two faults and serve to transfer motion from the first fault to the second. These linking imbricate systems were first described in thrust systems where they (generally) transfer displacement from a lower "floor" fault to an upper "roof" thrust. These types of geometries are now recognized in strike-slip and normal faults as well.
DYNAMICS: A branch of mechanics that deals with forces.
EFFECTIVE STRESS: The net stress on a plane. This includes the weight of overburden and differential loading, minus the value of pore fluid pressure. If the pore pressure is high, it decreases the effective strength of the rock, making it easier to deform. Effective stress = Total stress - Pore pressure.
EMBAYMENT FAULT: An arcuate concave-basinward, listric normal fault that has movement contemporaneous with deposition and exhibits a much thicker time-rock unit in the hanging wall.

EN ECHELON: The pattern of features in which the end of one overlaps and is slightly offset from the end of the next by some short distance. For instance, the north plunge end of one en echelon anticline can overlap and be offset from the south plunge end of its neighbor. The two oppositely plunging ends are separated by a syncline.
ENJOY: Rocks do not suffer deformation, they enjoy it!
ESCARPMENT: A steep slope. In the Gulf Coast, a steep descent in the sea floor such as the West Florida escarpment or Sigsbee escarpment where there is a pronounced drop-off into deeper water.
EXPANSION INDEX: The ratio of the thickness of a unit on the downthrown side of a fault to the thickness of that same unit on the upthrown side. This index provides valuable insight into the movement history of the fault. upthrownthicknessdownthrownthicknessIE=..
EXTENSION: A strain term signifying increase in length.
FACING DIRECTION: The direction to the top of a bed. The direction that was originally up.
FAULT: A fracture in the rock along which there has been some amount of movement.
FAULT-BEND FOLD: A fold resulting from the movement of the hanging wall across a change in dip (ramp) in the fault plane. The geometry of the fold is directly related to the geometry of the fault. 4/25/91
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FAULT CUT: The vertical thickness of the stratigraphic section missing or repeated in a wellbore as a direct result of a normal or reverse fault cutting through the section.
FAULT GAP: The horizontal distance between the upthrown and downthrown fault traces measured perpendicular to the fault trace as depicted on a structure contour map.
FAULT LINE FEATURE (STRUCTURE): Commonly a small structure where the critical closure on the feature is provided by faulting bifurcation of the fault, or arcuate strike of the fault and may be due in part to normal drag on the fault.
FAULT-PROPAGATION FOLD: A fold created when the displacement on a fault plane is consumed by folding. As the fault loses displacement, the shortening is taken up by the fold. The geometry of the fold is directly related to that of the geometry and displacement pattern along the fault.
FAULT-RELATED FOLD: A fold that is related to the change in dip angle of a fault. See FAULT-BEND FOLD or FAULT-PROPAGATION FOLD. Work shows that in many places these folds have kink geometries. This allows the rigorous analysis of these folds in terms of a balanced cross section.
FAULT TRACES: The two lines on a structure map representing the intersection of a fault with the structure contoured surface in the upthrown and downthrown blocks. Not the same as Fault Strike.
FAULT TRAP: A trap whose closure results from or is dependent on the presence of one or more faults.
FLANK: The periphery of a geologic structure, especially Gulf Coast Salt Domes.
FLAT: A part of the fault that is subparallel, to bedding. See RAMP.
FLEXURAL-SLIP: A mechanism of folding that includes flexing of beds as well as slip between the beds. The slip pattern is the upper bed slipping toward the anticlinal axis (such as that observed when folding a phone book or a deck of cards).
FLEXURE: A term used in the Texas Gulf Coast to denote an apparent sudden increase in basinward dip associated with a growth fault system such as the "Vicksburg Flexure" or "Chick Flexure."

FLOWER STRUCTURE: Folds and faults related to slight convergence or divergence along a strike-slip fault zone. The faults converge downward into the main strike-slip zone and may have oblique slip movement. If there is convergence across the zone (positive flower structure), the upward-diverging splays exhibit reverse or thrust movement as material is squeezed out of the strike-slip zone. The folds can be quite complex. If there is divergence across the main strike-slip zone (negative flower structure), the upward-diverging splays will have normal fault displacement and complex grabens will develop. The name "flower structure" is derived from the diverging upward pattern of fault splays and the folds in cross sections across these zones. These features are also referred to as Palm Tree Structures by some workers.
FOLD: A bend or change in orientation in the rock surface. Folds are often described by the "degree" of folding, based on the interlimb angle (α): Isoclinal - both limbs are parallel (α = 0°), Tight - α < 30° between the limbs, Close - 30°< α < 70 α, Open - 70°<α <120°, Gentle - 120° α <180°, Un – α =180°.
FOLD AXIS: An imaginary line that includes all of the points of maximum curvature in a fold of an individual surface (bed). The trend and plunge of this line uniquely defines the orientation of the fold in space. Sometimes referred to as the B-AXIS.
FOLD ZONE - FOLD SYSTEM: A group of folds showing common characteristics and trends and presumably of common origin. The term is used in the Gulf of Mexico to denote compressional folding at the down-dip margin of the structural system such as the "Perdido Fold Zone."
FOOT WALL: The lower plate below' a dipping fault. In a tunnel along the fault plane, you would be standing on the foot wall. See also HANGING WALL.

FORCE: A vector quantity that changes or tends to produce a change in the motion of a body.
FORELIMB: The steep panel of dip in an asymmetric fold. Sometimes referred to as the front limb.
FORWARD BREAKING: The sequence of imbrication of a thrust fault in which the imbricates get younger in the direction of tectonic transport. See also BACKWARD BREAKING.
FOUR-WAY CLOSURE: A term used to describe an anticlinal feature that has structural dip in all directions from the crest of the structure-a closed structure.
FRACTURE: A break or crack in rock. A fault is a fracture along which there has been movement. A joint is a fracture along which there has been no movement.
FRICTION: The resistance to shear on any plane.
GLIDE PLANE: A term used to describe the low angle portion of a listric normal fault where the fault plane becomes parallel to bedding.
GOUGE: In the Gulf Coast, a term more or less synonymous with shale sheath that pertains to out of place, deepwater shale section associated with the flanks of salt domes.
GRABEN: In extensional terranes, a low block that is bounded by normal faults.
GRAVITY CREEP: A type of sediment movement which is caused by the downslope component of gravity. This term is unrelated to your little brother when he was a kid, although he was a creep too.
GRAVITY FAULT: A fault along which the hanging wall has moved down relative to the footwall. (See also NORMAL FAULT.)
GRAVITY SPREADING: A type of mass sediment or salt movement toward the basin axis or downslope region caused by the downslope component of gravity on a dipping surface. This is thought to occur in the Gulf Coast in overpressured environments with slopes of 1°.
GROWTH: In the context of structural geology, the continued sedimentation while a structure develops. That is, the stratigraphic section continues to "grow" during structural development.

GROWTH CURVE: A method used to describe the age of fault movement by comparing the upthrown and downthrown thickness of a time-rock unit.
Id is the thickness on the upthrown side.
GROWTH FAULT: A fault that moves contemporaneously with sedimentation, so that the thickness of the synorogenic units change across the growing structure. In normal fault terranes, the growth section is thicker on the downthrown (hanging wall) block. In shortened terranes, the growth section is thinner on the hanging wall (upthrown) block. Analysis of the thickness and age of the synorogenic (growth) section yields valuable information about the geometry and history of the structure. In the Gulf Coast, the thickening of the section is termed the Expansion of the section. See also EXPANSION INDEX.
GROWTH HISTORY: Pertaining to the age of structural development.
HALF GRABEN: A tilted basin that is bound on one side by a normal fault. The deepest part of the basin is adjacent to the bounding fault.
HALOKINESIS: Isostatic or gravity driven salt flow.
HALOTECTONIC: Salt movement driven by tectonic forces, usually restricted to lateral compression.
HANGING WALL: The upper plate or higher block above a dipping fault. In a tunnel along the fault plane, you would bump your head on the hanging wall. See also FOOT WALL.
HANGING WALL SEQUENCE DIAGRAM: A series of cross sections oriented parallel to strike of the thrust system that illustrate the position of faults before and after movement on successive faults.
HANGING WALL SYNCLINE: A synclinal feature in die hanging wall of a normal fault that has a ramp and flat fault plane geometry. The synch overlies the ramp that allows downfolding of the overlying rocks.
HEAVE: The horizontal component of displacement on a fault.
HEAVED OUT: A description of a listric normal fault that has a large heave component to denote that a stratigraphic unit is not present near the fault in the hanging wall. As used here, it has nothing to do with being hung over.
HINGELINE FAULT: A term used in the Texas Gulf Coast for a growth fault -- usually near the paleo-shelf edge of the growth interval associated with the fault.

HORIZON: The surface separating two beds and hence having no thickness.
HORSE: The name given, for whatever reason, to the slivers of rock, no matter how thick, bounded by two imbricates of a duplex system or by strands of an anastomosing fault pattern. That is, a body of rock that is completely surrounded by faults.
HORSETAIL: A normal fault that has a number of upward directed splays. The upward directed splay faults from a normal fault.
HORST: In extended terranes a high block bounded by normal faults. See also GRABEN and HALF GRABEN.
IMBRICATE: A subsidiary fault that branches off of the main fault.
INCOMPETENT BED: A bed that is relatively weak and thus cannot transmit pressure for any distance-may deform by flowage.
INFLECTION PLANE: A plane containing all points of change in curvature from anticlinal curvature to synclinal curvature.
INTERDOMAL: An anticlinal structure located between shallower salt features -- usually a low relief deep structure between shallow diapirs.
INTERMEDIATE BLOCK: A fault block within a fault zone that has an intermediate elevation between the highest uplifted block and the lowest downdropped block.
INTERMEDIATE PIERCEMENT: A salt body that has pierced shallower overburden and where the crest of the salt is 5,000'- 10,000' below sea level.
INTER-SALT BASIN: A local or sub-regional depositional thick lying between shallow diapiric salt structures.
ISOCHORE MAP: A map on which the drilled (apparent) thickness of a stratigraphic unit is contoured.
ISOCHRON: A line on the surface of the earth connecting points at which a characteristic time or interval has the same value.
ISOPACH MAP: A contour map indicating the varying thickness of a stratigraphic unit measured perpendicular to its bedding.
ISOSTACY: The equilibrium of units of differing thicknesses and densities above the aesthenosphere. Lighter, thicker lithosphere is in equilibrium at higher elevation than thinner, denser lithosphere.
ISOTIME MAP: A map on which the seismic vertical travel time between two events is contoured.
KINEMATICS: The movement pattern of material within a structure during deformation.
KINK FOLD: Folds that have straight limbs and sharply curved hinges.

LATERAL RAMP: A ramp that is oriented at high angles to the structural strike and subparallel to the direction of tectonic transport.
LINKED SYSTEM: Faults whose displacement is linked so that the displacement on the system as a whole is constant even though displacement on any fault may change. As displacement on one of the faults dies out along strike, displacement on one of its mates increases by the same amount.
LISTRIC FAN: A linked system of listric normal faults where a series of synthetic faults are associated with the rollover anticline.
LISTRIC FAULT: A fault that is concave upward in shape. This term is used synonymously with the less common term "spoon shaped." A fault that is concave downward is often referred to as "Anti-listric."
MOHR CIRCLE: A graphical construction showing the relationship between the maximum and minimum normal stresses and the shear stresses in a(n infinitesimal) body. It is used primarily to graphically show the stress and failure conditions in a rock.
MOTHER SALT: A term that refers to the source layer of a salt dome province where the salt for the shallower salt bodies has been derived. In the Gulf Coast, the "Mother Salt" is the Middle Jurassic Louann Formation.
MUD LUMP: A near surface, possibly diapiric, low density clay or shale body that may form a small island 5 to 10 feet above sea level at the mouth of the Mississippi Delta.
MUSHROOM DIAPIR: A broad salt bulb fringed by one or more peripheral pendant lobes (skirts).

NAMAKIER: An extrusive body of salt that way be flowing downslope, especially during periods of rain.
NEUTRAL LEVEL OF BUOYANCY: An elevation at which a low density body such as salt is in equilibrium with the surrounding and/or overlying higher density cover rocks. Diapirs can rise until they reach their neutral level of buoyancy, generally above the level where the cover rock density is equal to that of salt.
NON-DIAPIRIC: A salt body in conformable contact with its overburden.
NORMAL DRAG: Folding due to frictional drag during movement of a fault plane.
NORMAL FAULT: A fault in which the hanging wall moves downward relative to the foot wall.
NOSE: A partially developed anticlinal structure that lacks closure and is open in one direction.
OBLIQUE SLIP FAULT: A fault on which the movement has not been purely dip slip or purely strike slip but rather a combination of the two.
OROGENY: An episode of regional deformation, such as the Alleghenian Orogeny that produced the Valley and Ridge Province in the Appalachians or the Hercynian Orogeny, Laramide Orogeny, etc.
OUT-OF-THE-SYCLINE CROWD FEATURE: A subsidiary structure (fault or fold) that is formed by the movement of rocks out of the adjacent syncline and toward the anticline. As the syncline tightens, the volume is too small to fit all of the rock material that was originally in the unit now in the core of the fold. Material is moved by flowage or on a detachment out of the syncline.

OUT-OF-SEQUENCE THRUSTING: Faults within a thrust system that do not move in the usual "younger in the direction of tectonic transport" sequence. In these cases, movement on thrusts may be simultaneous, or it may be younger in the opposite direction to that of tectonic transport.
OVERHANG: A flank or margin of a salt body (Gulf Coast) or basement (Rocky Mountains) that is underlain by sedimentary rocks in a vertical sense.
OVERPRESSURED: A sedimentary section that has a pressure gradient greater than 0.465 psi/foot.
PENDANT: A peripheral lobe on the outer margin of a diapiric salt body.
PERIPHERAL SALT ANTICLINE: A low relief anticline cored by salt near the updip margin of the Louann Salt and usually associated with the rim graben system in the Gulf Coast.
PLANAR FAULT: A fault whose fault plane has no curvature.
PLANE STRAIN: Strain that does not include movement of material in or out of the plane of the cross section. This is a common assumption that allows the three dimensional deformation to be reduced to two dimensions and analyzed with balanced cross sections.
PLASTIC: The time dependent, non-elastic, non-recoverable, stress dependent deformation under a uniform sustained load.
PLUNGE: The inclination of any linear structural element (fold axis, etc.) from the horizontal.
POLES: An imaginary line at right angles to a plane (bedding, foliations, fault planes). The density of lines plotted on equal-area stereonets are used for analysis of patterns of orientation. Poles are used because large data sets are easiest to analyze and manipulate if they can be depicted as points rather than lines, especially curved lines.
PORE PRESSURE: The value of the pressure of interstitial fluids in a rock. See also EFFECTIVE STRESS.
POTHOLE: A rounded, steep sided depression resulting from downward surface solution of salt. Small scale examples are common to New Orleans streets.
PRESSURE CHAMBER: A unit of sedimentary rocks of widely varying areal extent that is overpressured relative to rocks of the same sedimentary unit adjacent to the area of overpressure.
PRIMARY PERIPHERAL SINK: An early syncline containing over-thickened section that forms between two developing salt pillows or salt anticlines. A local sedimentary thick that decreases in thickness toward the salt pillow.
PURE SHEAR: Deformation by flattening or stretching. This deformation causes elongation in at least one direction and shortening in at least one direction. The deformation can be visualized by lightly stepping down on a balloon. See also SIMPLE SHEAR.

RABBIT EAR: A subsidiary fold generated by volume problems in the syncline in front of the steep forelimb of a concentric fold. Material is forced out of the syncline on a detachment and up the steep forelimb of the anticline. This extra material forms a smaller anticline at the top of the steep limb of the larger main fold. The name rabbit ear comes from the fact that the tight subsidiary fold form a map outcrop pattern that has the shape (sort of) like the outline of a rabbit's ear. Now that the subsidiary fold can generate its own volume problems in front of its steep limb, resulting in another ear on top of the first one. Also note that the term "rabbit ear" is not common outside of Chevron.
RAFT TECTONICS: An extensional structural system formed above a detachment that includes listric normal faults which have rafted or heaved out strata above the detachment.
RAMP: A part of the fault that cuts obliquely across the stratigraphic section. See also FLAT and LATERAL RAMP.
REGIONAL DIP: The general inclination of strata, over a large area in which they dip in one direction with or without interruption.
REGIONAL ELEVATION: The elevation of a horizon (bed, formation, etc.) throughout the region if there had been no deformation. This plane provides a reference horizon for measuring the amount of uplift or downdropping of structures. Sometimes referred to as just "Regional."
RELICT STRUCTURE: An older anticlinal structure usually associated with the flank of a piercement salt dome that has bad its closure tilted out by later growth of the salt dome, but exhibits structural growth or thinning of the stratigraphic section onto the crest of the structure and may retain fault closure on older growth faults.
RESIDUAL STRUCTURE: An anticlinal structure between shallow piercement salt domes that is salt cored and where the salt is conformable with the overburden. The structure is caused by salt that is left behind as salt flowed into the adjacent domes.
RESTORED SECTION: A structural cross section that has been restored to the time of deposition of a stratigraphic unit. The restoration may include decompaction and movement of strata back up any fault planes present in the section. Also, the replacement of a stratigraphic section that has been faulted out of a well by correlations with nearby wells.

REVERSE DRAG: Rotation of beds on the hanging wall of a normal fault downward into the fault plane. This term is a misnomer because drag usually means bending caused by frictional resistance. In the case of reverse drag, bending is related to the geometry of the fault surface. The beds that are located against the fault rotate as movement along the fault causes them to be displaced along the curving plane. The term is descriptive however, in that the rotation is the opposite to that expected by true drag. Commonly referred to as ROLLOVER in the Gulf Coast. See also DRAG.
REVERSE FAULT: A fault in which the hanging wall moved upward with respect to the footwall. See also NORMAL FAULT.
RHEIDITY: The time required for the viscous flow component of strain to be 1000 times the other components of strain. For example, salt and ice have much lower rheidity than granite.
RHEOLOGY: The general type of deformational response of a rock to stress. The manner in which a rock deforms is a reflection of its theology.
RIFT SYSTEM: A structural system of normal faults that downdrops a block or blocks of the earth's crust and extends the overburden.
RIGHT SECTION: A cross section that is at a right angle to the axis of the structure. This section provides an undistorted cross sectional view. (See also DOWN-PLUNGE VIEWING.)
RIM GRABEN SYSTEM: A normal fault system at or near the updip limit of the Louann Salt in the Gulf Coast that is presumably due to gravity creep of the salt downslope toward the basin axis.
RIM SYNCLINE: A depositional thick or synclinal area adjacent to a piercement salt dome that tends to develop around the periphery of salt domes due to salt flowage into the dome.
ROLLING PIN EFFECT: The southward or basinward movement of a large volume of the Louann Salt in the Gulf Coast area due to differential loading by the progradation of the Mississippi Delta in the Tertiary.
ROLLOVER-ROLLOVER ANTICLINE: An anticlinal structure developed in the hanging wall of a listric normal al fault, generally due to the concave geometry of the fault plane. Strata in the upward hanging wall adjacent to the fault dip towards the fault. See REVERSE DRAG.
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ROOTED: A term used in Gulf Coast salt tectonics to denote a salt structure that is connected to the Iouann Salt source by a salt stock or stem-sometimes called a feeder root.
SALT ANTICLINE: A moderate to high relief anticline cored by salt with a conformable contact between the salt and overlying strata. Intermediate to high relief salt anticlines may exhibit a complex graben faulted crest.
SALT BUD: A salt diapir that grows or develops from a laterally extensive sheet-like salt body, often at the downdip margin of the salt tongue.
SALT BULB: A diapiric salt body having a shape similar to a light bulb overhung on all sides.
SALT CANOPY: A shallow sheet-like salt body composed of multiple mushroom-shaped diapiric salt bodies or salt tongues that have coalesced and which have a number of salt stems, stocks, or feeder roots extending downward to the sat source layer.
SALT CHUTE: An inter-salt area or sub-basin elongated in a basinward direction where coarse clastic deposition may be focused into the deeper part of the basin during the time interval when the shallow salt may have sea floor relief.
SALT DOME: A structure resulting from the relative upward movement of a salt mass where the salt has pierced the overburden and where there is a discordant relationship between the salt and the overburden.
SALT GLACIER: An extrusive body of salt.
SALT INVERSION: A process whereby a salt body rises to a shallower level as the salt is buried by higher density rocks.
SALT MASSIF: A large salt complex. A deep-seated uplift of salt with outward dipping flanks and irregular plan shape, with a crest consisting of smaller anticlines or salt stocks.
SALT NAPPE: An intrusive sheet-like salt mass that moves tons of miles laterally by climbing up sections on thrust ramps.
SALT OVERTHRUST: An extensive sheet-like body of salt that cuts up the stratigraphic section at a low angle over the underlying strata.
SALT PILLOW: An anticlinal structure produced when a small area of a once continuous salt layer is preserved after the rest of the layer is removed. The rocks above the area where the salt has been removed subside to take up the space left by the removal of the salt. The rocks above the area where salt remains cannot subside because the salt is still there, and an anticline results.
SALT PLUG: The salt core of a salt dome.
SALT REDUCTION: A process whereby a salt body decreases in volume due to burial by denser rocks or tectonic processes that causes the salt to move laterally or vertically to another area and may include dissolution of the salt.

SALT REMOBILIZATION: A process pertaining to the mobilization of a previously emplaced salt body that results in the formation of 2nd cycle salt structures.
SALT RIDGE: An elongated non-intrusive, low to moderate amplitude salt anticline Salt ridges may evolve into intrusive salt walls, high relief salt anticlines, and thence into columnar stocks.
SALT ROLLER: A low relief salt anticline associated with a listric normal fault detaching in or at the base of the salt Salt roller structures are common in the Offshore Alabama Norphlet Trend.
SALT SILL: An allochthonous tabular body of salt that is concordant with the overburden and underlying strata.
SALT STOCK: A columnar body of salt, circular or oval in plain view and cylindrical in nature, that may extend vertically to a great depth or to the salt source layer.
SALT SUTURE: A line or zone of contact between two or more salt tongues or mushroom diapirs that have coalesced.
SALT TECTONICS: A branch of Tectonics or Geology that deals with all types of salt movement.
SALT TOE: The downdip compressional structure associated with an extensional fault system and have a core of salt that may be either diapiric or non-diapiric.
SALT TONGUE: A sheet-like salt body of large areal extent that is presumably emplaced near the surface and spreads downslope by gravity flow. A salt tongue is distinguished from a salt sill by being discordant with the underlying or overlying strata and usually cutting up the stratigraphic section toward the downslope area. The salt may be locally conformable with the underlying section.
SALT TRAIL: A disconformity caused by displacement of an allochthonous salt body and subsidence of the overburden down to the strata that previously was present below the salt. The strata are first separated by the salt and later welded together as the salt is displaced. A disconformity may not be apparent in a local arm.
SALT WALL: An elongated, high amplitude, intrusive salt anticline. Salt walls in Northern Germany average 4-5 km in width and may reach a length of 120 km.
SALT WINDOW: An area or inter-salt basin where no salt is present between the surface or sea floor and the salt source layer which is defined as the in-place Lou Salt in the Gulf of Mexico.
SALT WITHDRAWAL: The process where salt is displaced horizontally or vertically, thus, creating space for deposition of sediment.
SCAT (Statistical Curvature Analysis Technique): The analysis of orientation data (usually dipmeter data but this analysis can be applied to field dam also) by recognition of patterns of orientations and changes in orientations with respect to their position on die structure.

SCISSORS FAULT: A fault on which there is increasing offset or separation along the strike from an initial point of no offset, with reverse offset in the opposite direction. Often a combination of two normal faults having opposite displacements in the Gulf Coast. The sense of displacement along the strike of the fault may be changed by differential salt withdrawal at depth.
SECONDARY PERIPHERAL SINK: A depositional thick adjacent to a salt diapir due to subsidence above a shrinking of collapsing flank of a salt pillow. The sedimentary thick may increase in thickness toward the salt deeper. See also RIM SYNCIINE.
SEPARATION: The apparent displacement on a fault. This description of movement direction or amount should be used when the actual displacement is not known but is being estimated from a map view, logs, or some other data. For example, it is possible for a normal fault to produce a map pattern that could be interpreted as being formed by reverse faulting. In interpreting the map, it is then more accurate to describe the map as showing reverse separation to indicate the ambiguous nature of the data.
SHALE DIAPIR, SHALE DOME: An intrusive body composed of low density shale.
SHALE SHEATH: A body of shale originally deposited in deepwater (upper slope-abyssal) that is out of its normal stratigraphic position due to transport by a salt diapir - usually a relatively thin, overpressured, low resistivity shale unit overlying part of the salt body. In the case of an allochthonous salt body emplaced near the sea floor in deepwater, the shale sheath would be in its normal stratigraphic position overlying the salt body.
SHALLOW PIERCEMENT: A diapiric salt body where the crest of the salt is shallower than 5000' below sea level.
SHEAR STRAIN: Angular displacement by internal deformation.
SHEAR STRESS: The force exerted tangential to a plane of specified area. Shear stress is often noted by τ.
SHELF EDGE: The point at which there is a marked increase in slope at the outer margin of the continental shelf. In the Gulf of Mexico, this commonly occurs at 600 feet of water on the present day shelf but may have been different for paleo-shelf edges.
SIMILAR FOLD: A fold in which the thickness of the beds involved in the folding is constant when measured parallel to the axial plane of the fold. Thicknesses when measured at right angles to the bedding planes change around the fold. Sometimes referred to as a Cleavage Fold because these folds are thought to form by movement on cleavage planes that are parallel to the axial plane, like the transposition of a deck of cards along card surfaces.
SIMPLE SHEAR: Deformation by rotation and translation, causing most lines drawn at right angles to each other within the plant of deformation to change to some other angle. This style of deformation results in elongation in at least one direction and shortening in at least one direction. This deformation can be visualized by the displacement of cards in a deck with the upper cards displaced more than the next lower card. See also PURE SHEAR.

SKIRT: A downward facing anticlinal lobe on the margin of a bulb-shaped or mushroom-shaped diapir.
SLICKENSIDE: Striations along a fault plane caused as one block moves past and grinds against the adjacent block. The striae are oriented in the direction of movement of at least one period of movement on the fault.
SLIP: The actual movement on a fault. That is, how did a point at A get to B. This movement can be analyzed in terms of the components of movement, i.e., amount of strike-slip and dip slip, amount of horizontal movement and vertical movement, etc. Compare with SEPARATION.
SNAIL TRAIL: A disconformity or previous detachment zone related to the removal or dissection of a sheet-like body of allochthonous salt. See SALT TRAIL.
SOURCE LAYER: A buoyant layer of salt.
SOLE: A term referring to the point or level at which a listric normal fault becomes approximately parallel to bedding.
SPLAY: A subsidiary fault that branches off of a larger main fault.
SPOON FAULT: In the Gulf Coast, the term is usually applied to a highly arcuate local or subregional listric normal fault.
STEM: The salt body that extends downward to the salt source layer from a shallow mushroom or bulb-shaped diapir.
STICK-SLIP: A pattern of intermittent motion on a fault. There is no motion when the fault sticks and stress builds up. There is relative motion when the fault slips.
STRAIN: The ratio of the deformed dimension compared to the original dimension (new length compared to original length, for instance). This can also be measured as an angular distortion (that is, the change in angular relationships).
STRAIN RATE: Proportional change in length per second of a deforming body.
STRATIGRAPHIC SEPARATION: The offset of a fault measured in terms of the thickness of stratigraphic units (given in formation names or ages). Also referred to as Stratigraphic Throw.
STRENGTH: For our purposes, the ability of a rock to resist significant deformation (stronger rocks do not deform significantly until higher stress levels). Specifically, the value of differential stress required to cause significant deformation.
STRESS: The force per unit area tending to deform a body. A stress may be resolved into its three mutually perpendicular components.
STRESS: Force exerted over a specified area (kilograms per square meter, psi). The normal stresses within a body can be resolved (by making some assumptions) to three perpendicular, principal axes denoted by σ. The usual convention for identifying these axes is that the largest compressive stress is called σ1, the smallest compressive stress is σ3, and the intermediate stress is called σ2.

STRIKE: The azimuth of the intersection of a (dipping) planar geologic feature (bedding) and a horizontal plane.
STRIKE-SLIP FAULT: A fault along which the movement has been horizontal, with one block moving horizontally past another block. The movement direction is parallel to the strike of the fault plane.
STRUCTURAL TRAP: A hydrocarbon entrapment resulting from salt tectonics, folding, faulting, or a combination thereof.
SUB-BASIN: A depositional thick of limited areal extent that is part of a larger basin. In the Gulf Coast, these depositional thicks are often due to growth faulting, localized salt withdrawal or a combination of the two.
SUB-SALT: A reference to strata, faults, hydrocarbon accumulation or other quantity that lies under an allochthonous salt body or severely overhung salt dome.
SUBSIDENCE: A lowering of elevation of a part of the earth's crust. This may be due to a rift event and decay of the thermal anomaly, isostatic compensation for a sediment load, salt withdrawal or low angle normal faults.
SUBSIDIARY FOLD: Folds that are smaller, but kinematically and geometrically related to a larger fold. These smaller folds (how small and how much smaller depends on the size of the "main" fold and the lithologies of the rocks involved, as well as other things) are produced by interbed shear, volume problems in the cores of the synclines, or other factors associated with the development of the larger fold. The asymmetry of these folds is useful in areas of incomplete exposures because they are asymmetric towards the anticlinal crest. The asymmetry of the "S" or the "Z" indicates the direction to the anticlinal crest. Also referred to as Drag Folds or Parasitic Folds.
SUPRADOMAL: A reference to strata, faults, hydrocarbon accumulation or other quantity that overlies a salt dome.
SYNCLINE: A fold in the rocks in which the beds are concave upward.
SYNOROGENIC: At the same time as deformation (as the orogeny). This term is often applied to rocks deposited during deformation.

SYNTHETIC FAULT: A subsidiary fault that dips in the same direction as the master fault in a fault system. The term has nothing to do with the way that the fault was made. Sometimes referred to as a Sympathetic Fault, a term that likewise has nothing to do with how the fault feels. Compare with ANTITHETIC FAULT.
TEARDROP: A bulb-shaped piercement salt dome where the root or stem of the diapir has been pinched off from the salt source layer leaving a "teardrop" body of salt suspended in the sedimentary section.
TEAR FAULT: A fault that strikes subparallel to the transport direction and allows the rocks on one side to be transported farther than those on the other side. Generally, this term is used in shortenedterranes, especially thrust belts. Tear faults are commonly second-order strike-slip features that dip steeply, and are only present in the hanging wall. The geometry of shortening may be different on opposite sides of the fault.
TECTONICS: A branch of Geology dealing with the broader structural features of the earth and their causes. It is closely related to Structural Geology but Tectonics usually deals with large features and speculations on their causes.
TECTONIC TRANSPORT DIRECTION: The direction in which the rocks move during deformation.
TEEPEE STRUCTURE: A sharp anticlinal fold similar in geometry to a Chevron fold but of limited area extent and often observed in association with normal faults that may have oblique or strike slip movement.
THROW: The vertical displacement component of fault movement. Sometimes referred to as True Throw.
TOE STRUCTURE: In the Gulf Coast, a compressional type structure at the downdip margin of an extensional fault system exhibiting an amount of shortening that may be comparable to the amount of extension on the linked fault system and having a common detachment with the master fault - usually a salt or shale cored anticline which may or may not have associated normal and reverse faults.
TOROIDAL FLOW: A viscous circulation of salt in a rising diapir whose stream surfaces are centered on the core of the salt and are concentric about a horizontal circular axis. The crest of the diapir widens to form a bulb and may exhibit internal folds.

THRUST FAULT: A low angle reverse fault (<45°). These faults, like other reverse faults, shorten the crustal section and cause duplication of the section, at least locally. Thrusts often dip parallel to bedding in less competent units (called a flat) and then cut obliquely across bedding in more competent units (called a ramp). This geometry is referred to as "ramp-flat" geometry.
TRANSFER ZONE: A compartmental fault or fault zone in extensional terranes that separates areas with different structural geometries. See also COMPARTMENTAL FAULT.
TRANSPRESSION: Horizontal convergence combined with strike-slip offset.
TRANSTENSION: Horizontal divergence combined with strike-slip offset.
TRAP DOOR STRUCTURE: An anticlinal structure or salt dome in which one flank or limb of the structure has been uplifted with considerable rotation of the strata with a fault or hingeline acting as the fulcrum and commonly exhibiting an angular unconformity at the top of the rotated section often due to upward relative salt movement in the footwall of a growth fault.
TREND: The compass direction that defines the orientation of a linear structural feature (fold axis, etc.). Used with the amount of plunge, these two pieces of data uniquely define the orientation of a fold in space.
TRIANGLE ZONE: An area at the frontal part of a thrust in which the thrust fault wedges itself into the foreland stratigraphic section, the upper part. The name comes from the triangular-shaped cross section of the zone of deformation. Deformation within the triangle zone is usually quite complex. Also referred to as a Delta Zone or Structural Wedge by some people.
TROUGH: An elongated structural depression or depositional thick which may be bounded by normal faults dipping toward the axis of the trough.
TURTLE STRUCTURE: An anticlinal structure caused by the two phase withdrawal of a salt layer during continuous sedimentation. These structures are formed when salt is removed (dissolved) in a small area during the first phase of salt removal. Sediment fills the void left by the salt, thus rocks deposited during this first phase are thicker in that area. During a later phase, the rest of the salt layer is removed and the rocks above the salt subside to replace the salt. The earlier thick will not permit the overlying rocks to subside as much and an anticlinal structure is produced. Also referred to as a Sombrero Structure, especially in Canada.

UPTHRUSTING-UPBUILDING: The base of a salt body is regarded as stationary and the crest is regarded as uplifted. During pure upbuilding, the midpoint of the salt rises and the base is static.
UP TO THE BASIN FAULT: In the Gulf Coast, a normal fault that dips toward the continent.
VERGENCE: The direction of asymmetry in folds within a fold belt or system. This direction is usually interpreted to be, the direction in which material was transported during deformation to produce the folds.
VERTICAL EXAGGERATION: The ratio between the horizontal scale and the vertical scale of a cross section - equal to the horizontal scale divided by the vertical scale. The tangent of the exaggerated fault angle is equal to the tangent of the true fault angle times the vertical exaggeration.
VERTICAL SEPARATION: The separation between two parts of a displaced bed measured in a vertical direction - equal to the fault throw only when the upthrown and downthrown beds are flat. The missing section faulted out of a well by a normal fault is equal to the vertical separation.
VERTICAL SHEAR: Shear planes or surfaces dipping at an angle of 90°.
VOLUME PROBLEM: The loss of volume in the core of a concentric fold leaving insufficient room for all of the original rocks. Rocks are therefore deformed in some manner to accommodate this loss of space. Generally parts of the rock mass are detached, deformed and moved out of the core zone. Also referred to as the Space Problem or Room Problem.
VORTEX STRUCTURE: A structure that is formed by more than one overturn of diapiric material and possibly of cover rocks in second order folds. The first overturn produces downward facing folds (skirt and infold) which may enter the drag zone of the rising core and be entrained into a second overturn.
WRENCH FAULT: A nearly vertical strike slip fault. See STRIKE-SLIP FAULT.