The Sedimentary Archives

CHAPTER 3CHAPTER 3

Controls on sedimentary rock features

• Tectonic setting • Physical, chemical, and biological processes in the

depositional environment • Method of sediment transport • Rocks in source area from which sediment is

derived • Climate (and its effect on weathering) • Post-depositional processes of lithification

(cementation, compaction)

Tectonic Setting

Tectonics:

• The forces controlling deformation or structural behavior of a large area of the Earth's crust over a long period of time.

• Tectonic Settings influence: Size of clastic particles Thickness of deposit Rate of erosion or subsidence

Continental Tectonic Regimes

• Craton - stable interior of a continent; undisturbed by mountain-building events since the Precambrian Shields- large areas of exposed crystalline rocks) Platforms- like shields but covered by flat-lying

or gently warped sedimentary rocks

• Orogenic belts - elongate regions bordering the craton which have been deformed by compression since the Precambrian

Figure 3-1 (p. 62)The craton and orogenic belts of North America.

Environments of Deposition • All of the physical, chemical, biologic and

geographic conditions under which sediments are deposited.

1. Sediments formed from the weathering of pre-existing rocks outside the basin, and transported to the environment of deposition

Or2. Sediments form inside the basin; includes

chemical precipitates, most carbonate rocks, and coal.

Marine Environments

1. Continental shelf (shallow)

2. Continental slope

3. Continental rise

4. Abyssal plain (deep)

Figure 3-4 (p. 65)Deep-sea fan built of land-derived sediment emerging from the lower part of a submarine canyon. Such fans occur in association with large rivers, such as the Amazon, Congo, Ganges, and Indus. (Vertical exaggeration 200:1.)

Transitional Environments

Transitional environments = shoreline deposits

Examples include:• Deltas• Beaches and Barrier Islands • Lagoons• Tidal flats

Deltas

Deltas • fan-shaped accumulations of sediment • river flows into a standing body of water, such as a lake or sea• sediments are dropped, forming this progradational feature.

Mississippi DeltaMississippi Delta Niger DeltaNiger Delta

Barrier Islands• Beaches and Barrier Islands are shoreline

deposits exposed to wave energy and dominated by sand with a marine fauna.

• Lagoons are bodies of water on the landward side of barrier islands.

• Tidal flats are low-lying plains near lagoons. Marshy

• Subenvironments:

Barrier Island System

The Outer Banks

Dauphin Island

Continental Environments• Continental environments are those environments

which are present on the continents.

Examples:• Fluvial (River)• Alluvial fans • Lakes (lacustrine)• Glacial • Eolian (wind)

Fluvial

Lacustrine

Alluvial Fan

Glacier

Eolian

Color of Sedimentary Rocks

• Clues about depositional environment: Black and dark gray coloration in sedimentary

rocks generally indicates the presence of organic

carbon and iron

Reddish coloration in sedimentary rocks indicates

the presence of oxidized iron

Green and gray coloration in sedimentary rocks

indicates the presence of reduced iron

Rock Colors

Dolomite

Red Siltstone

Gray Evaporite

Size and Sorting of Clasts

• Texture refers to the size, shape, sorting, and arrangement of grains in a sedimentary rock.

• Three textural components in clastic rocks: Clasts Matrix Cement

Clasts and matrix

Clasts

Matrix

Interpretation of Clastic Sedimentary Rocks

• The texture of a sedimentary rock can provide clues to the depositional environment.

1. Fine-grained= quiet water

2. Large grains= higher energy (velocity) deposition

Grain Size

• Sedimentary grains are categorized according to size using the Wentworth Scale.

• Wentworth Scale for sedimentary grain size:

GRAVEL (>2mm)SANDSILT

CLAY (<1/256mm)

Sorting• Sorting refers to the distribution of grain sizes in a

rock.

Sorting

• In general, windblown sediments are better sorted than wave-washed sediments.

• Well-sorted sands 1. Have higher porosity and permeability than poorly-

sorted sands (if not tightly cemented),

2. May be good reservoirs for petroleum and natural gas.

• Poor sorting is the result of rapid deposition of sediment without sorting by currents.

• Examples:1. alluvial fan deposits

2. glacial tillites.

Grain Shape

• Grain shape is described in terms of rounding of grain edges and sphericity (equal dimensions).

• Rounding results from abrasion and grain impact during transport.

Figure 3-13 (p. 72)

Shape of sediment particles. (A) An angular particle (all edges sharp). (B) A rounded grain that has little sphericity. (C) A well-rounded, highly spherical grain. Roundness refers to the smoothing of edges and corners, whereas sphericity measures the degree of approach of a particle to a sphere.

• Sedimentary structures are visible at the scale of an outcrop (LARGE!)

that formed at the time of deposition or shortly

thereafter (before lithification)

Evidence of processes operating

Sedimentary Structures

• Sedimentary rocks generally have bedding or stratification

Bedding

Individual layers less than 1 cm thick are laminations • common in

mudrocks Beds are thicker

than 1 cm• common in rocks

with coarser grains

• Some beds show an upward gradual decrease in grain size, known as graded bedding

Graded Bedding

• Graded bedding is common in turbidity current deposits

Graded Bedding

• Cross-bedding forms when layers come to rest at an angle to the surface

• Cross-beds result from transport by either water or wind

http://www.wwnorton.com/earth/egeo/flash/5_1.swf

Cross-Bedding

Cross Beds

• Small-scale alternating ridges and troughs are known as ripple marks and are common in sandstone

1. Current ripple marks form in response to water or wind currents flowing in one direction and have asymmetric profiles

2. Wave-formed ripple marks result from the oscillation of waves tend to be symmetrical

Ripple Marks

• Ripples with an asymmetrical shape

• Internally cross- bedded

• Flow upper right to lower left

Current Ripple Marks

• As the waves wash back and forth, symmetrical ripples form

• Produced by wave (shallow)

Wave-Formed Ripples

• When clay-rich sediments dry, they shrink and crack into polygonal patterns fractures called mud cracks

• Mud cracks require wetting and drying to form,

Mud Cracks

Ancient Mud Cracks

Geopetal Structures• Which way is up??• Sedimentary structures can be used to determine "up

direction". graded beds cross beds mudcracks Flute marks symmetrical (but not asymmetrical) ripples stromatolites burrows tracks,

Interpretating Sands in Clastic Rocks

• Quartz-rich (mature; quartz sand)

• Feldspar-rich (immature; arkosic sand)

• Rock Fragment-rich (immature; lithic sand)

• Clay-rich (immature; greywacke)

Figure 3-23 (p. 77)Four categories of sandstone as seen in thin section under the microscope. Diameter of field is about 4 mm.

Figure 3-24 (p. 78)Idealized geologic conditions under which quartz sandstone may be deposited. There is little tectonic movement in this environment. Water depth is shallow, and the basin subsides very slowly.

Figure 3-26 (p. 79)Geologic environment in which arkose may be deposited.

Figure 3-28 (p. 80)Tectonic setting in which graywacke is deposited. Frequently graywackes are transported by masses of water highly charged with suspended sediment. Because of the suspended matter, the mass is denser than surrounding water and moves along the sloping sea floor or down submarine canyons as a turbidity current. Graywacke sediment characteristically accumulates in deep-sea fans at the base of the continental slope.

Figure 3-29 (p. 80)Deltaic environment in which lithic sandstones may be deposited.

Interpretation of Carbonates

• Main Processes Chemical direct precipitates (carbonate mud)

Biochemical: organic contribution (shells, etc.)

Characteristics of most marine carbonate environments

• Warm water

• Tropical climate (30 ° N - 30 ° S of equator)

• Shallow water (less than 200 m deep)

• Clear water (low to no terrigenous input)

• Sunlight required for photosynthesis by algae

Some limestones may be the accumulation of shells

Microscopic Foraminifera (chalk)

Shell fragments (coquina)

Fossiliferous limestone

Dolomite• CaMg(CO3)2

• Rock and mineral

• Original (forming today) is rare

• Many older rocks have altered (dolomitized) over time

Interpretation of Shales

• Shale - very fine-grained rock composed of clay, mud, and silt.

Types:

• Quartz-rich shales (quartz sandstones)

• Feldspar-rich shales (arkoses)

• Chlorite-rich shales (greywackes)

• Mica-rich shales (greywackes)

• Unconformities in sequences of strata represent times of nondeposition and/or erosion that encompass long periods of geologic time,perhaps millions or tens of millions of years

• The rock record is incomplete!

Unconformities

For 1 million years erosion occurred

removing 2 MY of rocks

The origin of an unconformity

Deposition began 12 million years ago (MYA), continuing until 4 MYA

• The last column is the actual stratigraphic record with this unconformity

** Total of 3 million year hiatus**

• Three types of surfaces can be unconformities: A disconformity is a surface separating younger

from older rocks, both of which are parallel to one another

A nonconformity is an erosional surface cut into metamorphic or intrusive igneous rocks and covered by sedimentary rocks

An angular unconformity is an erosional surface on tilted or folded strataover which younger rocks were deposited

Types of Unconformities

Figure 3-48 (p. 92)Four types of erosional unconformities. (A) Angular unconformity. (B) Nonconformity. (C) Disconformity.