Grai

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A chart for estimating grain size in the field

The distinction between well-sorted and poorly-sorted refers to the distribution of grain sizes

within an individual bed

Eolian dune sands, many fluvial deposits shown by the well-sorted sandstone

Debris-flow conglomerates are typically very poorly sorted

Siltstone and mudstone can be distinguished in a hand specimen by the presence or absence of

a gritty texture.For conglomerates, maximum clast size is often a useful parameter to measure:

Taking the average of the 10 largest clasts visible within a specified region of an outcrop.

It is also important to note the degree of sorting, clast shape and roundness, matrix content, and fabric of conglomerate beds.

Do the clasts “float” in abundant matrix, a rock type termed matrix-supported conglomerate.do the clasts rest on each other with minor amounts of matrix filling the interstices clast-supported conglomerate?

Field geologists traditionally take a dropper bottle of 10 % hydrochloric acid with them to

test for carbonate content and to aid in distinguishing limestone from dolomite (on the

basis of “fizziness”).Dolomite commonly can be distinguished from limestone by its yellowish weathering color in

the field, but a better field test is to use alizarin red-S in weak acid solution.

Evaporites are difficult to study in surface outcrops.

They are soft and recessive and commonly poorly exposed, except in arid environments

Reduced sediments may contain organically derived carbon and Fe2+ compounds, such as sulfides, imparting green or drab gray colors. Oxidized sediments may be stained various

shades of red, yellow, or brown by the presence of Fe3+ compounds such as hematite and

limonite.

Thickness relates to rate of environmental change and to depositional energy.In some cases, bed thickness and maximum grain size are correlated, indicating that both are controlled by the capacity and competency of single depositional events.Bed-thickness changes may be an important indicator of cyclic changes in the environment, and sedimentologists frequently refer to thinning-upward and fining-upward or coarsening-and thickening-upward cycles.

Sediment carried in turbulent suspension by mass gravity-transport processes, such as

debris flows and turbidity currents, is subjected to internal sorting processes.

When the flow slows and ceases, the sorting may be preserved as a distinct texture termed

Graded Bedding. Grading commonly consists of an upward decrease in grain size, this is termed normal grading. However, certain sedimentary processes result in an upward increase in grain size, termed inverse

grading

Cohesive grains are those that are small enough that they tend to be bound by

electrostatic forces and thus resist erosion once deposited on a bed (clay minerals and fine silt particles). Larger clastic grains, including

siliciclastic, evaporite and carbonate fragments, of silt to cobble size, are noncohesive

a) W

ave

rippl

es in

shal

low-

wate

r dol

omite

.

b) R

ippl

es a

nd c

limbi

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rippl

es in

gla

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ou

twas

h.

c) Planar crossbedding in drill core.

d) tr

ough

cro

ssbe

ddin

g flo

w di

rect

ion

towa

rds t

he ri

ght.

e) low-angle crossbedding; flow direction towards the left.

Plan

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bedd

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in sa

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Plan

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bedd

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in m

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Herri

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bedd

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(cro

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180

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m se

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set)

view

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sh

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There are three main classes of bedforms and crossbedding found in ancient rocks:

Those formed from unidirectional water currents such as are found in rivers and deltas, and oceanic circulation currents in marine shelves and the deep sea.

Those formed by oscillatory water-currents, including both wave- and tide-generated features. Although the time scale of current- reversal is, of course, quite different, there are comparable features between the structures generated in these different ways.

Those formed by air currents. Such currents may be highly variable, and the structure of the resulting deposits will be correspondingly complex. However, examination of ancient wind-formed (eolian) rocks indicates some consistent and surprisingly simple patterns