Sedimentary structures has been generated on the recognition and interpretation of primary physical sedimentary structures in terms of sedimentary process and associated depositional product. As a result, the sedimentologist is now provided with the tools to better interpret the sedimentary rock record in terms of the sedimentary processes that were acting at the time of deposition.Note particularly that in addition to flow velocity, variables such as sediment grain size and flow depth are extremely important in controlling the primary physical sedimentary structure product.
1. Laminae and Beds
Laminae and bed define stratification. Bed is thicker than 1 cm whereas lamination is thinner than 1 cm.
- Laminae
Are produced by less severe or shorter lived, fluctuations in sedimentation conditions than the fluctuations that generate beds. The laminae result from changing depositional conditions that cause variations in (1) grain size, (2) content of clay and organic material, (3) mineral composition, or (4) microfossil content of sediments. Laminae produced by alternating layers of finer and coarser grained sediment are probably the most common kind. Lamination may be produced by such differences in mineral composition as those that occur in alternating mica-enriched and mica-poor laminae; as in some beach deposits; and alternating laminae of anhydrite and dolomite in evaporite deposits.
- Beds
Are produced under essentially constant physical, chemical or biological conditions. Many beds have been produced very rapidly by a single event such as a flood that lasted only a few hours or days. Thus, an individual bed may be produced rapidly by an event such as a single episode of deposition of fine sediment from suspension. Many beds are not preserved to become part of the geologic record bur are destroyed by succeeding erosional episodes.
2. Ripples, Dunes and Sand Waves
They are bed form developments chiefly in sand sized sediments. The migration of ripple, dunes and sand waves under condition of net sedimentation gives rise to various types of cross stratification, which is one of the most common internal depositional sedimentary structures. Both wind and water can move sediments to produce these structures.
- Ripples
Ripples are common and occur on bedding surfaces, but the larger scale dunes and sand waves are rarely preserved. Ripples are the smallest bedform, ranging in length from about 0.05 to 0.2 m and in height from about 0.005 to 0.03 m. They form in sediment ranging in size from silt to sand as coarse.
- Dunes
Dunes form at higher velocities and except for size is similar in appearance to ripple. They form in sediment coarser than about 0.1 mm. In the lower part of the dune stability field, ripples may be superimposed on the backs of dunes.
- Sand waves
Sand waves are characterized by straight, continuous crest, produced at lower energies than the megaripples. Sand waves is bedforms that have wave lengths ranging from 5 to 100+ (under natural conditions) and a ripple index of about 50. They form at higher flow velocities than ripple and in sediment coarser than about 0.25 mm.
3.3 Cross-Lamination and Cross-Beddings
Cross-lamination and cross-beddings are cross stratification forms either a single set or many sets within one bed. Cross lamination is where the set height is less than 6cm and the thickness of the cross laminae is only a few millimeters, and cross bedding is where the set height is generally greater than 6cm and the individual cross beds are millimeters to a centimeter or more in thickness.
4 Massive
Massive is used to describe beds that appear to be homogeneous and lacking in any form or structure (internal structures), e.q. massive beds are those without internal grading, and lacking sedimentary structures.
5 Tool marks
Tool marks is the impression made on the surface of a soft bed of sediment by the impact of an object (tool), or the dragging of an object over the sediment by a current. Tool marks include bounce, prod, skip, groove, and chevron marks, which develop by differing interaction of the tools with the sediment.
6 Load Cast
Load Cast are sole marking generally preserved on the lower side of the sand layer overlying the mud layer. They are often associated with turbidities with a thin layer of coarser sediment on the top.
Load Cast can also be defined as a bulbous depression formed on the base of a bed of sediment, developed by the differential singking of the sediment, while still soft, into less dense sediment below. Load casts are not strictly casts, as they do not infill an existing depression as in the case of flute casts.
Load casts as swelling ranging from slight bulges, deep or shallow rounded sacks, knobby excrescences, or highly irregular protoberances’ (Potter & Pettijohn, 1977).
7 Hummocky Cross Stratification (HCS)
This type is in fact more closely related to parallel bedding than cross bedding, as the laminations are generally parallel to the depositional surface. HCS is characterized by undulating sets of cross laminae that are both convex-up domes (hummocks) separated by concave-up troughs.
This structure is not very widely recognized and its occurrence and description to date are restricted to ancient sandstones. It occurs in interbedded sandstone/mudstone sequences, both within thicker units of sandstone and also within sharp –based sandstone beds. The structure consists of undulating sets of cross bedding that gently cut into one another with curved erosion surfaces. HCS is generally preserved in area of weak tidal activity that lies below fair weather wave base.
8 Swaley Cross Stratification (SCS)
The term swaley cross stratification (SCS) was created by Leckie and Walker (1982) for sandbodies by definition thicker than 2 m. The internal stratification is dominantly flattish to very gentle undulating, and the swales cut into this lamination. The swales are circular to elliptical in plan view, a meter or more in diameter and a few tens of centimeters deep. The infilling laminae conform to the shape of the swale, gradually flattening out upward. The swales rarely pass laterally into hummocks, although there are enough convex-upward laminae in a swaly sandbody to indicate a descriptive similarity to classical HCS.
9 Flame
Flame structures are wavy of flame-shaped tongues of mud that project upward into an overlying layer, which is commonly sandstone. The crests of some flames are bent over or overturned and tend to all point in the same direction. Flame structures are commonly associated with other structures caused by sediment loading. They are probably caused mainly by loading or water saturated mud layers less dense than overlying sands and are consequently squeezed upward into the sand layers.
10 Convolute
This structure occurs commonly in single beds of sand or silt in a wide range of environmental settings. They are most commonly recognized in vertical section, but are also seen o bedding surface and associated with water and sediment escape structures such as sand volcanoes. Convolute is a structure formed by complex folding or intricate crumpling of beds or laminations into irregular, generally small scale antidunes and syncline.
11 Flaser-wavy-lenticular bedding
- Flaser bedding is produced in environments in which conditions for deposition and preservation of sand are more favorable than or mud (Reineck-Sigh, 1980) and where cross lamination contains mud streak, usually in the ripple troughs It is a type of ripple bedding in which thin streaks of mud occur between sets of cross laminations. Mud is concentrated mainly in the ripple troughs but may also partly cover the crests. The flaser bedding suggests deposition under fluctuating hydraulic conditions. Period of current activity, when traction transport and deposition of rippled sand take place, alternate with periods of quiescence, when mud is deposited.
- Wavy bedding is where thin ripple cross laminated beds alternate with mud rock. In wavy bedding, mud and sand layer alternate and form continuous layers. The mud layers almost completely fill the ripple troughs and make a thin cover over the ripple crests, so that the surface of mud layer only slightly follows the concavity and convexity of the underlying rippled surface. The thicker of mud layer, the less the form of the underlying rippled surface is traceable on the upper surface of the mud.
- Lenticular bedding is where mud dominates and the cross lamination occurs in sand lenses. It is produced in environments in which condition favor deposition and observation of mud over sand. This structure formed by interbedded mud and ripple cross laminated sand in which the ripple or sand lenses are discontinuous and isolated in both vertical and horizontal directions.
12. Flute Cast
The casts occur singly or in swarms in which all of the flutes are oriented in the same general direction. On a given sole, the flutes tend to be about the oriented in the same size; however, flute casts on different beds can range in width from a centimeter or two to 20 cm or more, in height (relief) from a few centimeters to 10 cm or more.
Flute casts are formed by filling of a depressions scoured in cohesive sediment by current eddies created behind some obstacle or by chance eddy scour. Flute casts thus make excellent paleocurrent indicators because they show the unique direction of current flow. These particularly prevalent on the soles of turbidite sequences, but they occur also in sediments deposited in shallow-marine and non marine environments. They have been reported on the soles of limestone beds as well as sandstone beds.
13. Debris Flow
Debris flows reflect high-energy sedimentary processes relative to traction flows, although they are commonly associated with traction-flow deposits. In many cases, in the subaerial environment, traction flow reworks the debris flow top for long periods after deposition, substantially modifying the debris flow and producing cross-stratified and sorted conglomerate facies.
Debris flow deposits are differentiated from tractive deposits by a massive or graded fabric and poor sorting. Commonly they are matrix supported, but this is not a unique characteristic.
The highest energy sediment gravity flow deposits is the clast-rich debris flow. It typically forms on steep (>5 degrees) alluvial fan surfaces. It will not be present on fluvial- dominated terminal fans.
14. Bouma Sequences
Bouma sequence is an ideal turbidity sequence. It consists of five structural units that included the characteristics of both types of turbidites. These structural subdivisions presumably record the decay of flow strength of a turbidity current with time and the progressive development of different sedimentary structures and bedform in adjustment to different flow regimes.