Secondary Structures, Unconformity and Sedimentary Structures PDF

SECONDARY STRUCTURES, UNCONFORMITY AND SEDIMENTARY STRUCTURES Chapter Objectives After reading this chapter the students should be able to: 1- Define Geometric elements of faults and folds. 2- Compare between different types of faults and folds. 3- Describe the...

SECONDARY STRUCTURES, UNCONFORMITY AND SEDIMENTARY STRUCTURES Chapter Objectives After reading this chapter the students should be able to: 1- Define Geometric elements of faults and folds. 2- Compare between different types of faults and folds. 3- Describe the different types of unconformity surfaces. 4- Describe the different types of primary sedimentary structures. I- Tectonic or Secondary Structures Tectonic or secondary structures are those structures which created on the rocks after their original formation due to tectonic forces (such as compression or extension). They are divided into the following: - Tilting of strata - Joints or fractures (brittle deformation) - Faulting of strata (brittle deformation) - Folding of strata (ductile deformation) Faults Faults are fractures planes in the Earth’s crust along which the opposite walls of a rock column have moved relative to each other. 1- Geometric elements of faults. Dip Hade Slip: Slip of the fault is the total movement (separation) of geological Slip features present on either side of a fault Throw plane. Hanging wall Foot wall 1 Heave Hanging wall: Hanging wall is the block above the fault plane. Footwall: It is the block below the fault plane. Throw: The throw of the fault is the vertical component of the dip-slip separation. Heave: The heave of the fault is the horizontal component of the dip-slip separation. Hade: It is the angle between the fault plane and the vertical. Dip: The dip of the fault is the angle between the fault plane and the horizontal. 2- Classification of faults. There are different concepts according which the faults are classified into various types: (1) On the basis of the apparent movement of the fault blocks According to the apparent movement of the fault blocks there are four basic types of faults are recognized: - Normal fault: This type of faulting occurs in response to extension stress where the fault plane is more or less vertical and the hanging wall moves down relative to the footwall. - Reverse fault: This type of faulting is common in areas of compression stress where the hanging wall moves upward relative to the footwall. Thrust fault, is special type of reverse fault and characterized by small fault angle, less than 45 degrees. 2 - Strike-slip fault: It is yielded in response to shearing along the adjacent fault blocks. The movement along this fault is approximately parallel to the strike of the fault plane they may be left or right-lateral. - Oblique-slip fault: It is created due to both extension and shear stresses. The movement along the fault plane is inclined to both the strike and the dip of the fault surface. (2) Based on the orientation of the slip: This classification concerns with the movement direction of the hanging wall block: - Dip-slip faults: Faults which have the movement along the direction of the dip plane are dip-slip faults and described as either normal or reverse. - Strike-slip faults: They are faults in which the movement is approximately parallel to the strike of the fault plane. They are classified as either right-lateral or left-lateral. - Oblique-slip Faults: Faults which show both dip-slip and strike-slip motions are known as oblique-slip faults. Horst, graben and step faults. A horst: A horst represents a block pushed upward relative to the blocks on either side. It is a result of its bounding faults dipping away from each other A graben: It is a block that has been lowered or down dropped relative to the blocks on either side due to the 3 faulting. It is a result of its bounding faults dipping towards each other. Step Faults: A series of parallel faults that, all inclined in the same direction. II- Folds Folds are wavelike undulations in layered rocks that develop through ductile deformation in rocks undergoing compressional stress. Geometry of Folds. Folds are described by the following: The hinge line: It is the line where the limbs Axial plane of the fold meet. It is also the line of Fold axis maximum curvature. Limb Fold axis: It is the line formed by the intersection of the axial plane with any bed. Core The plunge: It is the inclination of the fold axis from the horizontal line. The axial surface (or axial plane): is the plane or surface that passes through the hinge lines and most nearly divides the fold into two equal parts. Crest: The crest is the highest point of an anticline. Trough: The Trough is the lowest point of a syncline. Limb: It represents the sides of the fold. 4 Classification of Folds. 1- Classification based on the appearance of folds in cross sections: Anticline: In anticline folds rock layers convex upward with the oldest rocks in the center. The two limbs of anticline folds dip away from each other. Oldest rocks Syncline: It is a fold that is concave upward with the Youngest rocks youngest beds in the center. The two limbs of syncline folds dip toward each other. Dome: Is a fold with approximately circular, closed outcrop patterns. It is convex upwards where the beds dip uniformly in all directions away from the center which contains the oldest rocks. Basin: Is a fold with approximately circular or slightly elongate, closed outcrop patterns. It is concave upwards where the beds dip uniformly in all directions toward the center which occupied by the youngest rocks. Youngest Oldest Youngest rocks rocks rocks Oldest rocks Dome Basin 5 2- Based on the orientation or the attitude of the axial plane. Based on the inclination of the axial surface folds are classified into: Symmetrical fold: It is one in which the axial plane is vertical. In other words, the two limbs have the same angle of dip. Asymmetrical fold: It is one in which the axial plane is inclined. The two limbs dip in opposite direction but at different angle. Overturned folds: An overturned fold has inclined axial pane but the two limbs have been tilted in the same direction, usually at different angles. Recumbent fold: A recumbent fold has an essentially horizontal axial plane. Recumbent fold Overturned fold Asymmetrical fold Symmetrical fold 3- Based on the orientation of fold axis. Non-plunging: It is the fold in which the fold axis horizontal. Plunging fold: It is the fold in which fold axis is inclined. Non-plunging fold Plunging fold 6  III- Unconformity An unconformity is a surface of erosion or non-deposition that separates younger strata from older rocks. Types of unconformities 1- Angular Unconformity: An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rocks are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity. 2- Disconformity: It is an unconformity surface separating essentially parallel strata. The contact between younger and older beds is marked by a visible, irregular or uneven erosional surface is a disconformity. 3- Nonconformity: a surface separating the igneous or metamorphic rocks from the overlying sedimentary strata. They usually indicate that a long period of erosion occurred prior to deposition of the sediments. 4- Paraconformity: A paraconformity is a type of unconformity in which strata above and below the unconformity contact are parallel and in which no erosional surface or other physical evidence of unconformity is discernable. The contact surface resembles a simple bedding plane. Paraconformity is not easily recognized and must be identified on the basis of missing strata as determined from paleontological evidence such as absence of faunal zones or abrupt faunal changes. 7  IV- Primary Sedimentary structures Primary Sedimentary structures are those which develop at the time of the deposition of the rocks. It is divided into the following types: 1- Bedding (or Stratification): It is defined as the deposition of sediments or fragments in the series of beds that differ from each other in mineralogy, rock type, color or other characteristics. The upper and lower surfaces of these layers are called bedding planes. Limestone Bedding planes Shale Sandstone 2- Ripple Marks: They are wave like structures produced in the granular sediments such as sands by unidirectional currents of wind or water or by oscillating waves. It is divided into: i- Current (Asymmetrical) Ripple Marks: They are asymmetrical in profile, with a gentle up-current slope and a steeper down-current slope. They are produced by unidirectional currents of wind or water. ii- Oscillatory (Symmetrical) Ripple Marks: They are symmetrical in cross sections. They have generally sharp crests peaks and rounded trough. They are formed by waves.. 3- Cross-stratification: It also known as cross-bedding, false-bedding is layering within a stratum and deposited at an angle to the main bedding plane. Cross-bedding can be subdivided according to the overall geometry of the sets and the nature of the 8 bounding surfaces of the cross-bedded units. The most commonly described types are tabular cross-bedding and trough cross-bedding. i- Tabular (planar) cross-beds: Tabular (planar) cross- Planar forest beds consist of cross-bedded units that are large in horizontal Curved foreset tangential extent relative to set thickness to the basal surface and that have essentially planar bounding surfaces. The foreset laminae of tabular cross-beds are commonly planar or curved so as to become tangential to the basal surface. Tabular cross-bedding is formed mainly by migration of large scale straight-crested ripples and sand waves. ii- Trough cross-beds: Trough cross-beds have lower surfaces which are curved or scoop shaped and truncates the underlying beds. The foreset beds are also curved and merge tangentially with the lower surface. They are associated with sand dune migration. 4- Graded bedding: The term "grading," applied to individual bed or laminae that show a progressive change in grain size from the coarse grain at the base to fine grain at the the top. When sediments settle down in the basin or lake, heavier particles settled down first then the lighter ones according to their specific gravity. 5- Mud Cracks: An irregular fracture or crack formed by shrinkage of clay, silt, or mud under the drying effects of atmospheric conditions at the surface, and preserved when the beds are changed to rock. 9 6- Rain Imprints: Rain drop impressions are a geological feature characterized by small hole like pits with slightly raised edges that are the result of the impact of rain on soft sediment surfaces. 10