Udden wentworth grain size scale

Lecture 5

Dr. Seyed Mehdi Alizadeh

General Aspects of Sedimentary Rocks

Composition of grains/clasts: tells about source.

Age of grains: tells about age of source and transportation

history (e.g., zircon).

Texture/Maturity: tells about transportation history.

Sedimentary structures: tell about depositional

environment.

Fossils: tell about depositional environment and age of

deposit.

Grade Scales

• A grade scale provides such a standard for verbally expressing

and quantitatively describing grain size. • Any good grade scale should: (i) Define ranges or classes of grain size (grade is the size of particles

between two points on a scale. e.g., “very fine sand”, is a grade between maximum and minimum size limits)

(ii) Proportion the grade limits so that they reflect the significance of the differences between grades.

• For example, the change in size from 1 mm to 2 mm diameter sand is an increase of 100%, however, the change in size from 10 mm to 11 mm is on the order of 10%. Therefore, a grade scale in which grade limits vary by 1 mm would not be useful.

• The most widely-used grade scale is the Udden-Wentworth Grade Scale (Table R-5). Note that most of the grade boundaries increase by a factor of 2, reflecting significant changes in grain size. Also, the scale defines limits for the verbal expression of grain size. "Very fine sand" is sand which ranges in size from 0.0625 mm to 0.125 mm

• Krumbein (1934) introduced a logarithmic transformation of

the scale which converts the boundaries between grades to whole numbers. This scale is known as the Phi Scale, it’s values being denoted by the Greek symbol (φ):

Udden-Wentworth Tale

φ = -log d(mm) 2

where d(mm) is just the grain size expressed in millimetres.

Maturity is a function of sediment transport

• Textural maturity refers to:

- The degree of roundness of the grains

- The amount of sorting of the grain sizes

•

Texturally mature sandstones have well-rounded

and well-sorted grains, immature if not • Mineralogical maturity refers to the percentage of

quartz grains

 Feldspars break down with transport

 Quartz grains more resistant • Mineralogically mature sandstones have mostly

quartz grains

• Arkose is mineralogically immature

Maturity

• Sorting is the degree of similarity in particle size in a sedimentary

rock. • For example, if all the grains in a sample of sandstone are about

the same size, the sand is considered well sorted. Conversely, if the rock contains mixed large and small particles, the sand is said to be poorly sorted (see figure 6.5)

• The shapes of sand grains can also help decipher the history of a

sandstone (Figure 6.5B). • When streams, winds, or waves move sand and other larger

sedimentary particles, the grains lose their sharp edges and corners and become more rounded as they collide with other particles during transport.

• Thus, rounded grains likely have been airborne or waterborne.

Sorting

Particle sorting – comparison chart

Increasing Roundness=increasing maturity

• Example: a poorly sediment containing

glassy angular volcanic fragments, olivine crystals and plagioclase is texturally immature because the fragments are angular.

• This indicates they have not been transported very far and the sediment is poorly sorted, indicating that little time has been involved in separating larger fragments from smaller fragments.

• A well sorted beach sand consisting mainly of well rounded quartz grains is texturally mature because the grains are rounded, indicating a long time in the transportation cycle.

• Such sediment is well sorted, also indicative of the long time required to separate the coarser grained material and finer grained material from the sand.

Clastic Sediment Textures

Roundness/Angularity:

Transport by wind or water – rounding occurs.

Transport by ice or gravity – angular.

Degree of Sorting:

Selection/separation of grains is on the basis

of size, shape, specific gravity.

Poorly sorted: fast deposition, high energy.

Well sorted: slow deposition, less chaotic.

Poorly Sorted Sample

Sedimentary Structures  Other than rock type (lithology), one of the

best ways to determine depositional environment is by observing sedimentary structures:

 bedding – flat layering  cross-bedding – sediment moved by wind or water

deposited at an incline  ripple marks – deposition by wave oscillating

 graded bedding – coarser sediment on bottom, finer on top (currents dropping load)

 mud cracks – sediment drying up and shrinking

Sedimentary Structures

• Give evidence of depositional environments.

• Sedimentary rocks are deposited originally

in horizontal beds.

• Later deformation causes the beds to be inclined.

• Which was the original way up?

Structures can give that information.

Bedding – horizontal sheets differing in composition.

Sedimentary structures can give idea of paleocurrents.

Bedding & Cross-Bedding

• In geology a bed is the smallest division of geologic formation or

stratigraphic rock series marked by well-defined divisional planes (bedding planes) separating it from layers above and below.

• The term is generally applied to sedimentary strata, but may also be

used for volcanic flows or ash layers. • An important feature of a bed is its internal structure, which is

characterized by conditions of deposition. • A bedding type can be recognized and named on the basis of a

single bed; or it is only possible when same type of bed is repeated a several times- a bedset; or it is then possible when two or more beds of different natures are repeated in certain sequences.

• Thus a bedding type can be made of the same type of beds (cf.

Reineck and Wunderlich 1969).

Examples - various bedding types

Cross Bedding (formation)

• Cross beds form as sand blows up the windward side of a dune and then accumulates on

the slip face. At a later time we see that the dunes migrate, and eventually bury the layers below. • Cross-bedded strata can be seen on this cliff face of sandstone in Zion National Park.

We are looking at the remnants of ancient sand dunes. Cross beds indicate the wind direction during deposition. • With time the dune crest moves

Development of Cross Bedding

Cross Bedding (Field Example)

Ripple Marks • Ripple marks are sedimentary structures (i.e. bedforms of the lower flow

regime) and indicate agitation by water (current or waves) or wind. (A) A current that always flows in the same direction (like in a stream)

produces asymmetric ripples. (B) A current that moves back and forth (like on a wave-washed beach)

produces symmetrical ripples.

(A) (B)

Ripple Marks – wave action, form on top of beds.

Ripple Marks

• A ripple is conventionally described in terms of its size and shape. Traditionally, ripple marks are represented and described in terms of vertical profile parallel to flow, at right angle to the ripple crest. A ripple is composed of a crest and a trough.

Ripple Mark Description

Ripple Marks

• A graded bed is one characterized by a systematic

change in grain or clast size from the base of the bed to the top. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones.

• In above example of a graded bed, pebbles lie at the bottom

of the bed and silt at the top

Graded Bedding

Graded Bedding

(turbidity current) – beds

grade from coarse grained

at the bottom to fine

grained at the top.

Graded Bedding

• Mudcracks (also known as desiccation cracks or mud cracks) are

sedimentary structures formed as muddy sediment dries and contracts.

• They are formed as a result of drying on top of bed