Sedimentary Structures UNIT - 4

TRANSPORT MEDIA Gravity is the simplest mechanism of sediment transport. It includes the movement of particles under gravity down a slope. Rock falls generate piles of sediment at the base of slopes, typically consisting mainly of coarse debris. These accumulations are seen as scree along the sides of valleys in mountainous areas.

TRANSPORT MEDIA Water : Transport of material in water is by far the most significant of all transport mechanisms. Water flows on the land surface in channels and as overland flow. Currents in seas are driven by wind, tides and oceanic circulation. These flows may be strong enough to carry coarse material along the base of the flow and finer material in suspension. Material may be carried in water hundreds or thousands of km before being deposited.

TRANSPORT MEDIA Air :Wind blowing over the land can pick up dust and sand and carry it large distances. The capacity of the wind to transport material is limited by the low density of air. Ice: Water and air are clearly fluid media but we can also consider ice as a fluid because over long time periods it moves across the land surface. Ice is capable of transporting large amounts of clastic debris. Movement of detritus by ice is significant in and around polar ice caps and in mountainous areas with glaciers

Transport of particles in a fluid Particles of any size may be moved in a fluid by one of three mechanisms Rolling: the clasts move by rolling along at the bottom of the air or water flow without losing contact with the bed surface. Saltation: the particles move in a series of jumps,periodically leaving the bed surface, and carried short distances within the body of the fluid before returning to the bed again. Suspension: turbulence within the flow produces sufficient upward motion to keep particles in the moving fluid more-or-less continually. Particles being carried by rolling and saltation are referred to as bedload, and the material in suspension is called the suspended load. At low current velocities in water only fine particles (fine silt and clay) and low density particles are kept in suspension Sand-size particles move by rolling and some saltation. At higher flow rates all silt and some sand may be kept in suspension with granules and fine pebbles saltating and coarser material rolling. These processes are essentially the same in air and water but in air higher velocities are required to move particles of a given size because of the lower density and viscosity of air compared with water.

Introduction to Primary sedimentary structures The term “structure” can be used in two different senses: Features, on the scale of hand specimens to large outcrops, produced within a depositional environment, during or (usually) not long after deposition. These are usually called sedimentary structures. Features, on the scale of hand specimens to whole regions, produced by deformation associated with regional rather than local deforming forces, (e.g. folding and faulting) These are called tectonic structures.

Importance of Sedimentary Structures Study of sedimentary structures is important because they are far and away the most valuable features for interpreting depositional environment. How most structures are formed is known, so finding them in the rocks can tell a lot about the conditions of deposition. They are much more useful than textural things like grain-size distribution and grain shape.

Classification of Sedimentary Structures Two ways of classifying sedimentary structures is on the basis of: kind of mechanism that produces them (physical sedimentary structures, chemical sedimentary structures, and biogenic sedimentary structures) and time of development relative to time of deposition (primary sedimentary structures and secondary sedimentary structures).

Classification of Sedimentary Structures Physical Chemical Biological Primary Stratification Bioturbation (Tracks, Trails) Sole Marks Secondary Deformation Nodules (Burrows) Intrusion Stylolites Desiccation

Classification of Sedimentary Structures Physical primary sedimentary structures are certainly the most common and widespread and striking, in general are the most useful in interpreting the depositional environment Most are related to transportation and deposition of sediment particles at a fluid/sediment interface.

Stratification Stratification is by far the most important sedimentary structure. Stratification can be defined simply as layering brought about by deposition. Stratification comes about by changes in depositional conditions with time.

Stratification Stratification is usually obvious, especially on the scale of large outcrops, In looking for the stratification, always think in terms of changes in composition, texture, and/or structure from bed to bed. Here's a list of things that tend to make stratification apparent to the eye: differences in grain size differences in composition color/shade differences caused by slight differences in composition differential weathering caused by differences in composition/texture zones of larger or smaller concentration of individual components, like pebbles or fossils in otherwise homogeneous sediment;

Stratification Stratification is officially subdivided into bedding and lamination, depending upon the thickness of the strata, and bedding and lamination are in turn subdivided according to thickness. Strata between 1 cm to 100 cm are known as beds Strata less than 1 cm thick are known as laminations Lamination

Origin of Stratification Quiet-fluid deposition of particles by settling ocean bottom (plus lakes) mainly; low-velocity currents carrying a supply of suspended sediment usually fine-grained but not always usually thin lamination, because deposition rate is slow usually nearly or perfectly even and planar, unless later deformed Deposition of particles by tractional currents deposition onto a well defined fluid-sediment interface during transport by moderate to strong currents; stratification thick to thin depending on nature of variations in sediment supply, currents, and deposition rate; even stratification and cross stratification can be formed Mass deposition of coarse and fine sediment by sediment gravity flows (high-concentration sediment-water mixtures flowing as a single fluid) coming to rest without differentiation or particle-by-particle deposition; usually thick-bedded, with little or no internal stratification.

Cross Stratification Cross stratification is stratification that is locally at some angle to the overall stratification as a consequence of changes in the geometry of the depositional surface during deposition. Usually one or more beds in some part of a section show cross stratification. The vertical scale of cross stratification varies from millimeters to several meters, and the geometry is infinitely varied. Cross stratification varies enormously in geometry. Cross stratification is probably the single most useful tool in interpreting the physical aspects of loose-sediment depositional environments. Cross stratification can be classified as either cross bedding or cross lamination.

Clast-size variations: graded bedding The grain size in a bed is usually variable and may show a pattern of an overall decrease in grain size from base to top, known as normal grading, or a pattern of increase in average size from base to top, called reverse grading Normal grading is more common and can result from the settling of particles out of suspension or as a consequence of a decrease in flow strength through time.

Graded Bedding In graded bedding, the largest grains collect at the bottom of a layer and the grain size decreases toward the top.

Ripples and Sand Dunes A bedform is a morphological feature formed by the interaction between a flow and sediment on a bed. Ripples in sand in a flowing stream and sand dunes in deserts are both examples of bedforms. Ripples result from flow in water. Sand dunes are formed due to airflow. The patterns of ripples and dunes are products of the action of the flow. The formation of bedforms creates distinctive layering and structures within the sediment that can be preserved in strata. Recognition of sedimentary structures generated by bedforms provides information about the strength of the current, the flow depth and the direction of sediment transport.

Current Ripples Ripple marks are small waves of sand that develop on the surface of a sediment layer by the action of moving water. The ridges form at right angles to the direction of motion. If the ripple marks were formed by water moving in essentially one direction, their form will be asymmetrical. These current ripple markswill have steeper sides in the downcurrent direction and more gradual slopes on the upcurrent side. Ripple marks produced by a stream flowing across a sandy channel is an example of current ripples. When viewed from above current ripples show a variety of forms . They may have relatively continuous straight to sinuous crests (straight ripples or sinuous ripples) or form a pattern of unconnected arcuate forms called linguoid ripples.

Wave Ripples Other ripple marks have a symmetrical form. These features, called oscillation ripple marks, result from the back-and-forth movement of surface waves in a shallow near shore environment. The oscillatory motion of the top surface of a water body produced by waves generates a circular pathway for water molecules in the top layer . In shallow water, the base of the water body interacts with the waves. Friction causes the circular motion at the surface to become transformed into an elliptical pathway, which is flattened at the base into a horizontal oscillation. This horizontal oscillation may generate wave ripples in sediment.

Ripple marks

Distinguishing wave and current ripples Distinguishing between wave and current ripples can be critical to the interpretation of palaeo-environments. Wave ripples are formed only in relatively shallow water in the absence of strong currents, whereas current ripples may form as a result of water flow in any depth in any subaqueous environment. These distinctions allow deposits from a shallow lake or lagoon to be distinguished from offshore or deep marine environments, for example. The two different ripple types can be distinguished in the field on the basis of their shapes and geometries. In plan view wave ripples have long, straight to sinuous crests which may bifurcate (divide) whereas Current ripples are commonly very sinuous and broken up into short, curved crests. When viewed from the side wave ripples are symmetrical with cross-laminae dipping in both directions either side of the crests. In contrast, current ripples are asymmetrical with cross-laminae dipping only in one direction,

Trough Cross-beds Trough cross-beds have lower surfaces which are curved or scoop shaped and truncate the underlying beds. If the bedform is a ripple the resulting structure is referred to as cross-lamination. Ripples are limited in crest height to about 30mm so cross-laminated beds do not exceed this thickness. Migration of dune bedforms produces cross-bedding. A single unit of cross-laminated, cross-bedded or cross-stratified sediment is referred to as a bed-set. Where a bed contains more than one set of the same type of structure, the stack of sets is called a co-set.

Mud cracks Mud cracks are polygonal cracks that form when mud shrinks as it dries. They indicate that the mud accumulated in shallow water that periodically dried up. The spacing of desiccation cracks depends upon the thickness of the layer of wet mud, with a broader spacing occurring in thicker deposits.

Fossils Fossils are another feature of the sedimentary rocks that are formed during the time of deposition of these rocks. Fossils are the remains or impressions of plants or animals that were persevered in the crust of the earth due to natural causes.

Erosional Sedimentary Structures Small-scale erosional features on a bed surface are referred to as sole marks. They are preserved in the rock record when another layer of sediment is deposited on top leaving the feature on the bedding plane. Sole marks may be divided into those that form as a result of turbulence in the water causing erosion (scour marks) and impressions formed by objects carried in the water flow (tool marks).