Pages of Earth’s Past: Sedimentary Rocks

Sedimentary Cover Earth is covered by a thin ‘veneer’ of sediment.

The veneer caps igneous and metamorphic “basement.”

Sediment cover varies in thickness from 0 to 20 km.

Thinner (or missing) where ig and meta rocks outcrop.

Thicker in sedimentary basins.

Sedimentary Rocks Sediments are the building blocks of sedimentary rocks.

Sediments are diverse, as are the rocks made from them.

Four classes:

Clastic – Made from weathered rock fragments (clasts).

Biochemical – Cemented shells of organisms.

Organic – The carbon-rich remains of plants.

Chemical – Minerals that crystallize directly from water.

ChemicalClastic OrganicBiochemical

Clastic Sedimentary Rocks Clastic sedimentary rocks reflect several processes.

Weathering – Generation of detritus via rock disintegration.

Erosion – Removal of sediment grains from rock.

Transportation – Dispersal by wind, water, and ice.

Deposition – Settling out of the transporting fluid.

Lithification – Transformation into solid rock.

Lithification – Transforms loose sediment into solid rock.

Burial – More sediment is added onto previous layers.

Compaction – Overburden weight reduces pore space.

Sand – 10 to 20%

Clay – 50 to 80%

Cementation – Minerals grow in pores, “gluing” sediments.

Clastic Sedimentary Rocks

Clastic Sedimentary Rocks Classified on the basis of texture and composition.

Clast (grain) size.

Clast composition.

Angularity and sphericity.

Sorting.

Character of cement.

These variables produce a diversity of clastic rocks.

Clastic Sedimentary Rocks Clast (grain) size – The average diameter of clasts.

Range from very coarse to very fine.

Boulder, cobble, pebble, sand, silt, and clay.

With increasing transport, average grain size decreases.

Clastic Sedimentary Rocks

Clast composition – The mineral makeup of sediments.

May be individual minerals or rock fragments.

Mineral identities provide clues about…

The source of the sediment.

The environment of deposition.

Clastic Sedimentary Rocks Angularity and sphericity – Indicate degree of transport.

Fresh detritus is usually angular and non-spherical.

Grain roundness and sphericity increases with transport.

Well-rounded – Long transport distances.

Angular – Negligible transport.

Clastic Sedimentary Rocks Sorting – The uniformity of grain size.

Well-sorted – Uniform grain sizes.

Poorly sorted – Wide variety of grain sizes.

Sorting becomes better with distance from source.

Clastic Sedimentary Rocks Cement – Minerals that fill sediment pores.

Fluids with dissolved solids flush through pore system.

Dissolved ions slowly crystallize and fill pores.

Cementation varies from weak to strong.

Common cements:

Quartz

Calcite

Hematite

Clay minerals

Clastic Sedimentary Rocks Coarse clastics – Composed of gravel-sized clasts.

Breccia – Comprised of angular fragments.

Angularity indicates a lack of transport processing.

Deposited relatively close to source.

Clastic Sedimentary Rocks

Coarse clastics – Composed of gravel-sized clasts.

Conglomerate –

Indicates water transport.

Collisons round angular corners and edges of clasts.

Conglomerates are deposited at a distance from the source.

Clastic Sedimentary Rocks Sandstone – Clastic rock made of sand-sized particles.

Forms in many depositional settings.

Quartz is, by far, the dominant mineral in sandstones.

Sandstone varieties.

Arkose – Contains abundant feldspar.

Quartz sandstone – Almost pure quartz.

Clastic Sedimentary Rocks Fine clastics - Composed of silt

and clay.

Silt-sized sediments are lithified to form siltstone.

Clay-sized particles form shale.

Fine clastics are deposited in quieter waters.

Floodplains, lagoons, mudflats, deltas, deep-water basins.

Organic-rich shales are the source of petroleum.

These are sediments derived from living organisms.

Biochemical – Hard mineral skeletons.

Organic – Cells of plants, algae, bacteria and plankton.

Biochemical and Organic Rocks

Biochemical Rocks Biochemical limestone – CaCO3 skeletal (shell) remains.

Warm, tropical, shallow, clear, O2-rich, marine water.

Diverse organisms (plankton, corals, clams, snails, etc.).

Many textural varieties.

Reefs.

Shell debris.

Lime mud (micrite).

Chert – Rock made of cryptocrystalline quartz.

Formed from opalline silica (SiO2) skeletons.

Diatoms.

Radiolarians.

Opalline silica added to bottom sediments dissolves.

Silica pore fluids solidify to form chert nodules or beds.

Biochemical Rocks

Made from organic carbon.

Coal – Altered remains of fossil vegetation.

Accumulates in lush tropical wetland settings.

Requires deposition in the absence of oxygen.

Oil shale – Shale with heat altered organic matter.

Organic Rocks

Comprised of minerals precipitated from water solution.

Evaporites – Created from evaporated seawater.

Evaporation triggers deposition of chemical precipitates.

Examples include halite (rock salt) and gypsum.

Chemical Sedimentary Rocks

Chemical Sedimentary Rocks

Travertine – Calcium carbonate (CaCO3) precipitated from groundwater where it reaches the surface.

Dissolved calcium (Ca2+) reacts with bicarbonate (HCO3

-).

CO2 expelled into the air causes CaCO3 to precipitate.

Thermal (hot) springs.

Caves.

Chemical Sedimentary Rocks Dolostone – Limestone

altered by Mg-rich fluids.

CaCO3 altered to dolomite CaMg(CO3)2 by Mg2+-rich water.

Dolostone looks like limestone, except…

It has a sugary texture and a pervasive porosity.

It weathers to a buff, tan color.

Chemical Sedimentary Rocks

Replacement chert – Nonbiogenic in origin.

Many varieties.

Flint – Black or gray from organic matter.

Jasper – Red or yellow from Fe-oxides.

Petrified wood – Wood grain preserved by silica.

Agate – Concentrically layered rings.

Sedimentary Structures

Features imparted to sediments at or near deposition.

Layering.

Surface features on layers.

Arrangement of grains.

Help decipher conditions at or near time of deposition.

Sedimentary Structures

Sedimentary rocks are usually layered or “stratified.”

Arranged in planar, close-to-horizontal “beds.”

Bedding is often laterally continuous for long distances.

Beds are often similar in composition, color and texture.

Sedimentary Structures Bedding reflects changing conditions during deposition.

These can be changes in…

Energy conditions, and hence, grain size.

Disturbance by organisms.

Bedding may also reflect non-deposition or erosion.

Sedimentary Structures

A series of beds are referred to as strata.

Formation: Strata recognized on a regional scale.

Geologic maps display the distribution of formations.

i.e. Coconino Formation

Sedimentary Structures Water flowing over loose sediment creates bedforms.

Bedforms are linked to flow velocity and sediment size.

Ripples, cm-scale ridges, and troughs, indicate flow.

Asymmetric ripples – Unidirectional flow.

Symmetric ripples – Wave oscillation.

Ripples are commonly preserved in sedimentary rocks.

Bedforms

Cross beds – Created by ripple and dune migration.

Sediment moves up the gentle side of a ripple or dune.

Sediment piles up, then slips down the steep face.

The slip face continually moves downstream.

Added sediment forms sloping “cross-bedded” layers.

Bedforms

Dunes – Similar to ripples except much larger.

Form from wind-blown sand in desert or beach regions.

Often preserve large internal cross-laminations.

Bedforms Turbidity currents.

Sediment moves on a slope as a pulse of turbid water.

As pulse wanes, water loses velocity and grains settle.

Coarsest material settles first, medium next, then fines.

This process forms graded beds in turbidite deposits.

Bed-Surface Markings Occur after deposition while sediment is still soft.

Mudcracks – Polygonal desiccation features in wet mud.

Indicate alternating wet and dry conditions.

Necessitate deposition in a terrestrial setting.

Scour marks – Troughs eroded in soft mud by current flow.

Fossils – Evidence of past life.

Footprints.

Shell impressions.

Locations where sediment accumulates. They differ in…

Energy regime.

Sediment delivery, transport, and depositional conditions.

Chemical, physical and biological characteristics.

Environments range from terrestrial to marine.

Depositional Environments

Depositional Environments Terrestrial environments – Deposited above sea level.

Glacial – Due to movement of ice.

Ice carries and dumps every grain size.

Creates glacial till; poorly sorted gravel, sand, silt, and clay.

Depositional Environments Terrestrial environments –

Deposited above sea level.

Mountain streams.

Water carries large clasts during floods.

During low flow, these cobbles and boulders are immobile.

Course conglomerate is characteristic of this setting.

Depositional Environments

Terrestrial environments – Deposited above sea level.

Alluvial fan - Sediments that pile up at a mountain front.

Rapid drop in stream velocity creates a cone-shaped wedge.

Sediments are immature conglomerates and arkoses.

Depositional Environments Terrestrial Environments– Deposited above sea level.

Sand dunes – Wind-blown piles of well-sorted sand.

Dunes move according to the prevailing winds.

Result in uniform sandstones with gigantic cross beds.

Depositional Environments Terrestrial environments– Deposited above sea level.

Rivers – Channelized flow transports sediment.

Sand and gravel fill concave-upward channels.

Fine sand, silt, and clay are deposited on nearby floodplains.

Depositional Environments Terrestrial environments– Deposited above sea level.

Lake – Large ponded bodies of water.

Gravels and sands trapped near shore.

Well-sorted muds deposited in deeper water.

Often capped with wetland muds.

Depositional Environments Marine environments – Deposited at or below sea level.

Deltas – Sediments dropped where a river enters the sea.

Sediment carried by the river is dumped when velocity drops.

Deltas grow over time, building out into the basin.

Often develop a topset – foreset – bottomset geometry.

Depositional Environments

Marine environments – Deposited at or below sea level.

Coastal beaches – Surf zone.

Sediments are constantly being processed by wave attack.

A common result? Well-sorted, well-rounded medium sand.

Beach sandstones may preserve oscillation ripples.

Depositional Environments

Marine environments – Deposited at or below sea level.

Shallow marine – Finer version of beach sediment.

Fine silts and muds turn into siltstones and mudstones.

Usually support an active biotic community.

Depositional Environments

Marine environments – Deposited at or below sea level.

Shallow water carbonates – Tropical.

Skeletons of marine invertebrates.

Born in the carbonate factory.

Warm, clear, shallow, normal salinity, marine water.

Depositional Environments

Marine environments – Deposited at or below sea level.

Deep marine – Fines predominate far from land sources.

Skeletons of planktonic organisms make chalk or chert.

Fine silts and clays turn to shale.

Sedimentary Basins Sediments vary in thickness across Earth’s surface.

Thin to zero edge where non-sedimentary rocks outcrop.

Thicken to 10 to 20+ km in sedimentary basins.

Subsidence – Sinking of the land during sedimentation.

Due to crustal flexure and faulting.

Compounded by the weight of added sediments.

Basins are important locations for natural resources.

Coal.

Petroleum.

Natural gas.

Uranium.

Sedimentary Basins Basins form where tectonic activity creates space.

Rift basins – Divergent (pull-apart) plate boundaries.

Crust thins by stretching and rotational normal faulting.

Thinned crust subsides.

Sediment fills the down-dropped basin.

Sedimentary Basins Basins form where tectonic activity creates space.

Passive margin basins – Non-plate-boundary continental edge.

Underlain by crust thinned by previous rifting.

Thinned crust

subsides as it cools.

Sedimentary Basins Basins form where tectonic activity creates space.

Intracontinental basins – Interiors far from margins.

Result from differential thermal subsidence.

May be linked to failed crustal rifts.

Sedimentary Basins Basins form where tectonic activity creates space.

Foreland basins – Craton side of collisional mountain belt.

Flexure of the crust from loading creates a downwarp.

Fills with debris eroded off of the mountains.