Sedimentary Rocks Continued

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Sedimentary Rocks Continued sediment may suggest seasonal deposition with *forms on the upper surface!!*
varying rates of supply.
Grain size, Shape Sorting and Mineralogy: Dessication Cracks- in existing environments,
The maturity of sediment is a reflection of its Graded Bedding- graded bedding is usually mud cracks are familiar structures of tidal flats,
history. A Detrital rock can be described in found where, within a bed, there is a gradual dried up river courses, lake beds and flood plains.
terms of its textural maturity (grain size, shape diminution in the size of the grains from the lower They are most characteristics of subaerial
and sorting) and it mineralogy maturity (number to the upper part of the bed. Frequently, the dissication, but can occur with any situation
of minerals present). structure is repeated through beds and a textural where the drying out of water fill argillaceous or
Sediment is transported by the wind, river or sea rhythm is established. calcareous sediments may take place.
but not by ICE. The constant components Mainly caused by variation in seasonal supply The cracks are a result of shrinking, and he
become more rounded and rounded. Truly with finer material arriving as the carrying surface breaks up into a network of polygonal
spherical grains are rarely seen anywhere other power of the transport wanes, the progressive units. The width and depth of the cracks vary
than Aeolian or wind blown sands. settling of grain sizes from large to small in widely. Once the sediment has dried and attained
These tend toe red in colour due to the oxidation comparatively calm bodies of water, stirring some measure of cohesion, than any new
of the iron minerals such as haematite which is up bottom deposits in storms and sediment would be swept into the cracks to form
common in desert conditions. differentiation on settling. downward pointing fingers.
Mainly seen in Turbidites which are rocks laid *forms on the upper surface!!*
Sediments that are composed of a variety of don by turbidity currents or submarine
minerals are said to be mineralogically avalanches which tend to flow down submarine Sole Structures:
immature. Detrital sediment that is mainly made canyons to deposit material is a graded fashion They are called this because they are seen on
up from mainly one mineral is said to be on submarine fans as the current wanes in the bottom of sedimentary units, and often
mineralogically mature. strength on the gentler slop of the deep sea bed. represent the casts from moulds made on the
*coarse grained bottom, finer grained surface* underlying material by some physical process.
If sediment is made up of grains that show wide There are two important groups, Flute casts and
range of sizes then it is said to be texturally Cross Bedding- this is a common structure in Groove casts.
immature. If the sediment has a narrow range sandy deposits of both aqueous and Aeolian Flute casts appear to be produce by the turbulent
of grain sizes then it is textually mature. If it is origin. The structure appears as arcuate, sub- flow of water which is probably charged with
difficult to decide one way or the other, than the parallel plans inclined to the main bedding plane. sandy sediments. Hollows that are scooped out
mineralogy or texture can be said to be This type of sedimentary structure represents the by the flow on the sea floor are filled in by
submature. advance of a sloping front of sedimentation. sediment as the current wanes in strength. These
filled excavations then appear as casts on the
Structures seen in Sedimentary Rocks: Ripple Marks- these are the most familiar seen under surface of the overlying bed hence the
*Comes up every year!!* on the upper surface of a sedimentary rock. name sole structures. The casts are often a
They are most common on intertidal beaches and conical form. They are asymmetric in profile with
Bedding- the most distinctive structure of a are formed by the disturbance and discontinuous the steeper, deeper face being `up current' end
sedimentary rock is the bed or stratum. Each transport of unconsolidated material over short which merges gradually down current into the flat
bed is separated from the next (above and below) distances and finally accumulate in ridges. surface of the overlaying sand. The long axis of
by the bedding plane. Beds are usually of great Ripple marks vary in their characteristics, with the structure lies in the direction of the current.
lateral extent and are often of surprisingly uniform variations usually being related to the medium Groove casts are also formed by current action,
thickness. Thin accumulations within an individual responsible for their formation. but in this case, the current drags some solid
bed are called laminae. Coarser beds of rocks Wavelength, amplitude, symmetry or body though the surface of the muddy sediment
such as conglomerates and breccias are often asymmetric and roundness or sharpness of developing ridges and furrows roughly parallel to
devoid of any bedding. A bed usually represents the ridges are all variables. Ripples commonly the current.
the single accumulation and it is probable that the have straight crests and they develop at right
bedding plane represents a period of cessation of angles to the current, which can be wind or water.
deposition or a change in the nature of the supply As such ripple marks are useful as current or
of sediment. The occurrence of laminae within wind direction indicators in fluvial, Aeolian or
marine environments.


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