Pages of Earth’s Past: Sedimentary Rocks Sedimentary Cover  Earth is covered by a thin ‘veneer’ of .  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  are the building blocks of sedimentary rocks.  Sediments are diverse, as are the rocks made from them.

 Four classes:

 Clastic – Made from weathered fragments (clasts).

 Biochemical – Cemented shells of organisms.

 Organic – The carbon-rich remains of plants.

 Chemical – that crystallize directly from water.

Clastic Biochemical Organic Chemical Clastic Sedimentary Rocks

 Clastic sedimentary rocks reflect several processes.  – Generation of detritus via rock disintegration.  – 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. Clastic Sedimentary Rocks

 Lithification – Transforms loose sediment into solid rock.  Burial – More sediment is added onto previous layers.  Compaction – Overburden weight reduces pore space.  – 10 to 20%  – 50 to 80%  Cementation – Minerals grow in pores, “gluing” sediments. 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.  , , , sand, , and clay.  With increasing transport, average decreases. Clastic Sedimentary Rocks

 Clast composition – The 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 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:

 Calcite

 Hematite  Clay minerals Clastic Sedimentary Rocks

 Coarse clastics – Composed of -sized clasts.  – 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.  – Indicates water transport. Collisons round angular corners and edges of clasts. Conglomerates are deposited at a distance from the source. Clastic Sedimentary Rocks

– Clastic rock made of sand-sized particles.  Forms in many depositional settings.  Quartz is, by far, the dominant mineral in .

 Sandstone varieties.  – Contains abundant .  Quartz sandstone – Almost pure quartz. Clastic Sedimentary Rocks

 Fine clastics - Composed of silt and clay.  Silt-sized sediments are lithified to form .  Clay-sized particles form .

 Fine clastics are deposited in quieter waters.  Floodplains, lagoons, mudflats, deltas, deep-water basins.  Organic-rich are the source of petroleum. Biochemical and Organic Rocks

 These are sediments derived from living organisms.  Biochemical – Hard mineral skeletons.  Organic – Cells of plants, algae, bacteria and plankton. 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). Biochemical Rocks

– 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. Organic 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. Chemical Sedimentary Rocks

 Comprised of minerals precipitated from water solution.  – Created from evaporated seawater.  Evaporation triggers deposition of chemical precipitates.  Examples include halite (rock salt) and gypsum. 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.  – Red or yellow from Fe-oxides.  Petrified wood – Wood grain preserved by silica.  Agate – Concentrically layered rings.

 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. Depositional Environments

 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

 Terrestrial environments – Deposited above sea level.  Glacial – Due to movement of ice. Ice carries and dumps every grain size. Creates glacial ; 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 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 . 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.  and 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 and muds turn into and . 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 .

 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.

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.