Sedimentary RocksSedimentary Rocks

John Day Fossil Beds Natl. Mon.,Oregon

Sedimentary rocks composed of sediment

(particles derived by weathering)

• Detrital sediments particles derived by physical weathering

• Non-detrital sediments minerals precipitated from solution by inorganic chem. processes or activities of organisms

Classification of Sedimentary ParticlesSize Sediment Name>2 mm gravel1/16-2 mm sand1/256-1/16 mm silt<1/256 mm clay

Why Should You Study Sedimentary Rocks?

• Sediments are the most common Earth materials.

• Rock Features record environmental conditions at the time sediment was deposited. (impt. in deciphering Earth history)

• Fossils knowledge of pre-existent life preserved in sedimentary rocks

• Some sed. rocks contain resources (i.e. water, petroleum, natural gas, etc.).

Transport & DepositionIce

-- carries particles of any size.Wind

-- transports sand & smaller particles. Water (most prolific agent)

Larger particles require more vigorous current for transport.

• Depositional environment geographic area where sediment accumulates

• Rounding -- abrasion during transport reduces particle size & smoothes sharp corners

• Sorting -- variety of particle sizes present in sediment or sed. rock (influenced by transport & depositional processes)

well-rounded, poor-sorted gravel angular, poor-sorted gravel

Processes (Physical, chemical, & biological) operating in the depositional environment impart distinctive charac. to accumulating sediment.

How Are Sediments Transformed into Sedimentary Rocks?

Compaction– due to weight of overlying sediment – particles pack more closely & pore space is reduced– deposit’s volume reduced

Cementation – binds one particle to another by chemical precipitation of minerals in

sediment pore space. – {Common cements include quartz, calcite, & hematite.}

Lithification – involves compaction & cementation – converts sediments to sedimentary rocks

Classification of Detrital Sed. RocksBased on:

Sediment Size

Composition

Detrital sedimentary rocks comprised of solid particles derived from

parent material by phys. weathering

Conglomerate = rounded gravel (>2 mm)

Breccia = angular gravel

B

A

Sandstone = sand-size (1/16 - 2 mm) particles • classified by mineralogy

Quartz sandstone most common Arkose sandstone 25% feldspar

Mudrock any combination of silt & clay

Siltstone silt (1/16 – 1 / 256 mm)

Claystone clay (< 1 / 256 mm)

• Shale is fissile clay. (splits along many

closely spaced planes)

• Mudrocks = most common sed. rock (quiet-water environ)

shale

Chemical sedimentary rocks ions taken into soln by weathering of parent material **crystalline texture (interlocking mineral grains)

Biochemical sedimentary rocks Organisms aid in the precipitation of minerals.

Classification of Chemical Sed. Rocks

Based on:

Mineral compositionTexture

Evaporites formed by precipitation of minerals from evaporating water

\

Rock salt {halite (NaCl)} (A) & Rock gypsum {gypsum (CaSO4 . 2H2O)} (B) are most common.

A

B

contains much pore space

Limestones composed of Calcite

(CaCO3)

Fossiliferous limestone

Coquina

skeletal fragments of marine invertebrates

-- pores filled w/ cement & mud --

Clastic texture shell fragments

Coalcrystalline texture

carbon

(compressed, altered plant remains that occupied

swamps & bogs)

B

Chert crystalline texture

silica

(layers of microscopic shells of marine organisms)

A

Sedimentary Facies-- sets of sed. rock w/ distinct attributes imparted by their depositional environment

Coastal areas facies accumulate simultaneously on various areas of seafloor (w/ unique charac.).

Strata (beds) layers differ in color, texture, & comp. from rock layers above & below.

Regression (seaward shift in the shoreline)

• Drop in sea level

Nearshore deposits overlie offshore deposits.

1

2

3

Transgression (landward

shift in shoreline)

• Rise in sea level

Nearshore seds overlie old land surfaces, & offshore seds are stacked on top.

Grand Canyon Muav Limestone (offshore)

Bright Angel Shale

Tapeats Sandstone (onshore)

(deposition record of laterally adjacent environ.)

Transgression Sed structures, fossils & ripple marks

Determining the Depositional Environment

• Cross bedding

• Graded bedding

• Oscillation ripple marks

• Current ripple marks

• Mudcracks

- formed by physical processes at the time of deposition

Sedimentary Structures

Cross bedding

inclined layers w/in a bed Formed by wind or water

slopes downward in flow direction

Graded bedding upward decrease in grain size w/in a bed

{deposits of turbidity currents}

Ripple marks -- small ridges separated by intervening troughs

Oscillation ripple marks (A) symmetric cross-sections generated by the back-&-forth motion of waves.

A

B

Current ripple marks (B) asymmetric cross-sections record flow in one direction (i.e., streams).

Mudcracks

shrinkage (polygonal patterns of intersecting fractures) • clay-rich sediment

• depositional environ. periodic drying (i.e., a river floodplain, lake shore, or tidal flat)

Morphology of organisms reveals info about environment the organisms lived in.

1. Heavy-shelled clams = shallow-water, turbulent

2. Thin, fragile-shelled clams = low-energy

3. Filter-feeding organisms (corals) = clear seawater (suspended sediment clogs their feeding organs.)

Fossils are NOT Sedimentary Structures. remains or traces of ancient organisms

Resources in Sedimentary Rocks?• Sand & gravel building & road construction• Limestone cement• Gypsum wallboard/plaster• Phosphate-bearing sedimentary rock fertilizer• Sand-sized quartz glass • Carnotite (uranium mineral) fuels nuclear reactors

[assoc w/ plant remains in sandstones formed in ancient stream channels]

• Hematite & magnetite (banded iron formations) iron ores