Primary Structures in Sedimentary Rocks Engr. Sultan A. Khoso
Sedimentary rocks Sedimentary rocks are those rocks which are formed by the weathered sediments of pre existing rocks (igneous or metamorphic rocks). The geological processes that involved in the formation of sedimentary rocks are as under: 1. Weathering, 2. Erosion, 3. Deposition, 4. Compaction 5. cementation
Sedimentary rocks
Sedimentary rocks
Primary Sedimentary Structures Major Primary Sedimentary Structures are: 1. Bedding (Stratification) 2. Graded Bedding 3. Cross Bedding 4. Ripple Marks 5. Mud Cracks 6. Rain Drop Imprints
1. Bedding or Stratification Bedding (or Stratification) is defined as the deposition of sediments or fragments in the series of beds that differ from each other by mineralogy, rock type, color or other characteristics. The upper and lower surfaces of these layers are called bedding planes.
1. Bedding or Stratification Layer Thickness Names > 300 cm Massive Bed 100-300 cm Very thick bed 30-100 cm Thick bed 10-30 cm Medium bed 3-10 cm Thin bed 1-3 cm Very thin bed 0.3-1 cm Thick lamination <0.3 cm Thinly lamination
1. Bedding or Stratification
2. Graded Bedding These types of structures are classified on the basis of particle size: coarser size and Finer size. In graded bending structures, beds with particle of finer size occurred mostly at top and the beds with particles of coarser size occurred at base. This may be due to the different in density, weight of coarser to finer particles. When sediments settle down in the basin or lake, heavier particles settled down first then the lighter ones according to their specific gravity. Thus a sequence formed is called Graded bedding
2. Graded Bedding
3. Cross Bedding or Cross-Stratification Cross-bedding or cross-stratification is an arrangement of beds or laminations in which one set of layers or beds is inclined relative to the others. The layering is inclined at an angle to the horizontal, dipping downward in the down current direction. Hence, cross-beds may be used as indicators of ancient current directions. Cross-beds usually curve at the bottom edge, becoming tangent to the lower bed surface. The upper edge of individual inclined cross-beds is usually at a steep angle to the overlying bedding plane.
4. Ripple Marks Ripple Marks are wavelike (undulating) structures produced in the granular sediments such as sand by unidirectional of wind or water currents or by oscillating wave currents. Ripple marks are characteristic of shallow water deposition. Types: Oscillatory Ripple Marks/Symmetric Ripple Marks Current Ripple Marks/Asymmetric Ripple Marks
A. Symmetric ripple marks or wave ripples. B. Asymmetric ripple marks or current ripples.
a. Oscillatory Ripple Marks Oscillatory Ripple Marks (also called wave-formed or bidirectional) are produced by waves or oscillating water (back and forth motion of water). These types of ripples are generally formed in the bodies of standing water. Like lake and pond. They have almost sinusoidal profile. Means a smooth symmetric cross sections and long relatively straight crests. That why known as symmetric ripple marks. They have generally sharp crests peaks and rounded trough. These ripple marks indicate an environment with weak currents where water motion is dominated by wave oscillations (sand grains are moving in both directions, forward and backward).
Sharp crest and Rounded trough.
b. Current Ripple Marks Current Ripple Marks (also called unidirectional) form in unidirectional currents (such as in streams or rivers). Unidirectional flow of water or wind are the main cause of these ripple marks. Current Ripple Marks are asymmetrical in profile, means having asymmetric cross sections. That is why called as asymmetric ripple marks., Asymmetric ripples have a steep slope on the downstream side, and a gentle slope on the upstream side. They have generally rounded crests peaks and rounded trough. Because of this unique geometry, asymmetrical ripples in the rock record may be used to determine ancient current direction.
Asymmetrical ripple marks with current from left to right. Rounded crest as well as Rounded trough
Differentiate
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. Also known as desiccation crack; sun crack.
6. Rain Imprints Formed due to the impact of rain drops over the soft (e.g. clay) beds. 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.