Chapter 6: Sedimentary Rock
These notes include Interlude B
Interlude B
A Surface Veneer: Sediments and Soils
Sediment - loose fragments of rocks or minerals broken off bedrock, mineral crystals that precipitate directly out of water, and shells or shell fragments.
These sediments come from weathering (physics and chemical breakdown) of pre-existing rock.
Sediments form a veneer (cover) over bedrock.
Thickness range is from 0 km to 20 km.
Sediments can turn into soil.
Regolith - any loose debris (sediment or soil) that covers bedrock.
Weathering: Forming Sediment
Weathering is the processes that break up and corrode solid rock and turning it into loose debris.
Ions dissolve in surface water and groundwater.
Dissolved ions can eventually form a solid rock again.
Weathered rocks may look discolored or rough.
Physical Weathering (aka mechanical weathering)
Breaks solid rock into unconnected grains or chunks. ◦
Clast - a fragment or grain produced by the physical or chemical weathering of a pre-existing rock. Grains.
Jointing
Naturally formed cracks in rock.
Joints may form parallel to the mountain face.
Breaks bedrock into many separate blocks.
When exposed to a slope, they may tumble down and break into smaller pieces.
Talus - the rock rubble at the base of a slope.
Rock can be carried away by rivers as the base of a cliff.
Erosion - the breaking off and removal of rock or sediment).
Frost Wedging
Freezing in joints forces joints to open.
It can break blocks free from intact bedrock.
Salt wedging
Dissolved salt in groundwater precipitates in open pore spaces in rocks, crystallizing and pushing apart surrounding grains.
Weakens rock so when exposed to wind and rain, the rock disintegrates into separate grains.
Root wedging
Roots of trees pushing through joints.
Thermal expansion
Heat of intense forest fires bakes a rock, the outer layer of the rock expands.
On cooling, the layer contracts, generating a force that is sufficient to break off the outer part of the rock.
Animal attack
Burrows created by creatures can move rock fragments.
Chemical Weathering
Chemical weathering is the many chemical reactions that alter or destroy minerals when rock comes in contact with water solutions or air.
Dissolution - water solvent flows over or through rock and dissolves minerals (affects salts and carbonate minerals).
Hydrolysis - water reacts chemically with minerals and breaks them down to form other minerals. Example: Feldspar turning into clay.
Oxidation - rocks transform iron-bearing minerals into a rusty brown mixture of various iron-oxide and iron-hydroxide minerals.
Hydration - absorption of water into the crystal structure of minerals causing certain minerals to swell, weakening the rock.
Quartz is resistant to chemical weathering.
Physical and Chemical Weathering Working Together
Physical weathering speeds up chemical weathering.
Smaller chunks of rock or larger surface areas makes chemical weathering work faster.
Soil
Soil consists of rock or sediment that has been modified by physical and chemical interaction with organic material, rainwater, and organisms at or just below the earth's surface over time.
Soil is our planet's most valuable resources.
How soil forms:
Physical and chemical weathering
Downward-percolating water, mostly from rain that has seeped into the ground, redistributes ions and fine clay flakes.
Close to the ground surface, in the zone of of leaching, the water extracts ions, picks up clay, and carries this material downward.
Deeper down, in the zone of accumulation, new mineral crystals precipitate out of the percolating water, and because the rate of water movement slows, the water leaves behind its load of fine clay.
Detritus interacts with organisms just below the ground surface.
Soil horizons - distinct zones within soils defined by certain characteristics.
Highest horizon: O-horizon, made almost entirely of humus (plant debris)
A-horizon - humus decays, mixes with clay, silt, and sand.
O and A horizon make up topsoil.
E-horizon - soil level that has undergone substantial leaching but has not yet mixed with organic material.
A, O, and E make up the the zone of leaching.
B-horizon - subsoil, where ions and clay accumulate (zone of accumulation)
Final horizon: C-horizon - consists of material derived from the substrate that has been chemically weathered and broken apart, but has not yet undergone leaching or accumulation. Goes into bedrock or unweathered sediment.
Soil profiles - vertical sequences of distinct zones.
What determines the nature of soil development:
Climate
Substance composition
Slope steepness
Wetness
Time
Local ecosystem
There are 12 soil orders based on physical characteristics and environment of soil formation.
Chapter 6: Sedimentary Rocks
6.1: Introduction
Sedimentary rock - Rock that forms, either by cementing together of fragments, broken off, pre-existing rock, or by the precipitation of mineral crystals out of water solutions at or near the earth surface.
6.2: Classes of Sedimentary Rocks
Four major classes based on mode of origin:
1.) Clastic sedimentary rocks forms from cemented together clasts, solid fragments and grains broken off of pre-existing rocks.
2.) Biochemical sedimentary rock consists of shells
3.) Organic sedimentary rock consists of carbon-rich relicts of plants or other organisms.
4.) Chemical sedimentary rock is made up of minerals that precipitated directly from water solutions.
Clastic sedimentary rocks
Sandstone - coarse grain sedimentary rock consisting almost entirely of quartz.
Clasts - grains of individual minerals or chunks of rock.
Five steps:
Weathering
Erosion
Transportation
Deposition - sediment settles out of the transporting medium
Lithification - the transformation of loose sediment into solid rock through compaction and cementation.
Compaction - when the weight of overburdened squeezes air or water out from between grains, so the grains can fit together more tightly.
Cementation - when minerals, commonly quarts or calcite, precipitate from ground water, and fill the remaining spaces between clasts, to form a cement that binds grains together.
Classification
Clast size - diameter of fragments or grains.
Clast composition - the makeup of clasts in the rock (ex: rock fragments or individual mineral grains).
Angularity and sphericity - clasts having smooth vs sharp corners.
Sorting - the proportion of clasts in a rock that are the same size (well sorted, poorly sorted, etc).
Character of cement - not all kind of cement is the same (examples of cement: quartz, calcite).
These characteristics depends on the transporting medium (water, ice, wind).
Breccia - Course sedimentary rock consisting of angular fragments, or rock, broken into angular fragments by faulting.
Conglomerate - Very coarse grained, sedimentary rock consisting of rounded clasts.
Arkose - A coarse sandstone containing at least 25% feldspar grains.
Siltstone - Fine grained sedimentary rock, generally composed of very small quartz grains.
Shale - Very fine grained sedimentary rock that breaks into thin sheets.
Mudstone - Very fine grained sedimentary rock that will not easily split into sheets.
Biochemical Sedimentary Rocks
When organisms die the solid material in their shell survives. When lithified, this material constitutes biochemical sedimentary rock.
Biochemical limestone (fossil shells and shell fragments)
Limestone is a type of carbonate rock (contains calcite and/or dolomite.
Fossiliferous limestone - consisting of visible fossil shells or shell fragments.
Micrite - consisting of very fine carbonate mud.
Chalk - consisting of plankton shells.
Biochemical chert is made from cryptocrystalline quartz.
Organic Sedimentary Rocks
Organic-rich sediment.
Two most common types: coal and shale.
Coal - black, combustible rock containing 40% and 90% carbon. Consists of clay and quartz.
Oil shale - contains clay and between %15 and %75 organic material in a form called
kerogen.
Chemical Sedimentary Rocks
Rocks consisting of rock formed primarily by the precipitation of minerals from water
solutions.
Typically have crystalline texture either formed during their original precipitation and partly when, at a later time, new crystals grow at the expense of old ones through a process called recrystallization.
Evaporites - salt deposits that form as a consequence of precipitation from saline water.
Salt precipitation occurs where saltwater becomes supersaturated, meaning that it can’t keep all the dissolved ions that it contains in solution.
Happens in desert lakes and along the margins of restricted seas.
The specific types of salt minerals that make up an evaporite depend on the amount of evaporation.
When 80% of the water evaporates, gypsum forms; and when 90% of the water evaporates, halite precipitates.
Travertine (Chemical Limestone) - a rock composed of crystalline calcium carbonate that precipitates directly from ground water that has seeped out at the ground surface, either in hot or cold water springs, or on the walls of caves (speleothems).
Dolostone - Carbonate rock, contains the mineral dolomite which contains equal amounts of calcium and magnesium.
Chemically precipitated chert and replacement chert.
Chert examples: petrified wood and agate.
6.3: Sedimentary Structures
Sedimentary structure describes the layering of sedimentary rocks, surface features on layers formed during deposition, and the arrangement of grains within layers.
Bedding and Stratification
A single layer of sediment or sedimentary rock with a recognizable top and bottom is called a bed.
The boundary between two beds is a bedding plane.
Several beds together constitute strata and the overall arrangement of sediment into a sequence of beds is bedding or stratification.
A sequence of strata that is distinctive enough to be traced as a unit across a fairly large region is called a stratigraphic formation, or simply a formation.
Bed-forms - sedimentary structures that develop at the interface between the sediment and fluid that carried it. There are two kinds:
Ripple marks are relatively small (generally no more than a few centimeters high), elongate ridges that form on a bed surface at right angles to the direction of current flow.
Dune - a pile of sand generally formed by deposition from the wind.
Cross beds - Internal laminations in a bed, inclined at an angle to the main bedding, they are a relict of slip face ripples or dunes.
Turbidity currents and graded beds - moving submarine sediment suspension as a turbidity current.
Graded bed - a layer of sediment in which grain size varies from coarse at the bottom to fine at the top.
Turbidite - a deposit from a turbidity current.
Many overlapping turbidities built into a broad apron called a submarine fan.
Bed Surface Markings
Mud cracks - mud layers dry after deposition and leave behind hexagonal plates that have curled up edges. Opening between plates.
Scour marks - Currents flow over a sediment surface and erode small troughs.
Fossils - relicts of past life.
Sedimentary structures provide important clues that help geologists understand the depositional environment in which sediments accumulated.
6.4: Recognizing Depositional Environments
Geologists refer to the conditions in which sediment was deposited as a depositional environment.
Examples: beach, glacial, and river environments.
Geologists examine grain size, composition, sorting, sedimentary structures, and fossils to identify a depositional environment.
Geologic clues can tell us if the sediment was deposited by ice, strong currents, waves, or quiet water, and in some cases can provide insight into the climate at the time of deposition.
Terrestrial (non-marine) depositional environments:
Develop inland, far enough from the shoreline that they are not affected by ocean tides and waves.
Terrestrial sediments/non-marine sediments accumulate on dry land or under and adjacent to freshwater.
Red beds - sediment that was a result from oxygen in surface water reacting with iron and rusting.
Glacial environments
Ice can move sediment of any size.
Glacial till - unsorted and unstratified, contains clasts ranging from clay size to boulder size.
Mountain stream environments
Streams rushed downslope in steep sided valleys, fast moving water carried large clasts.
Slopes decrease and water flow slows is where larger clasts settle out to form gravel and boulder beds, while stream carries finer sediments like sand and mud farther. downstream.
Alluvial fan environments
Mountain front, fast moving streams empties onto a plain.
Desert environments
River environments
Lake environments
Delta
Coastal and marine depositional environments
Marine environments start at the high-tide line and extend offshore to the deep seafloor.
The type of sediment deposited at a marine location depends on the climate, water depth, and whether or not clastic grains are available.
Marine delta deposits
Coastal beach sands
Shallow marine clastic deposits
Shallow marine carbonate environments
Deep marine deposits
6.5: Sedimentary Basins
Geologists use the term subsidence to refer to the process by which the surface of the lithosphere sinks, and the term sedimentary basin for the sediment-filled depression.
Categories of basins in the context of plate tectonics theory
Rift basins: Form in continental rifts, regions where the lithosphere is stretching horizontally. Low areas, narrow basins bordered by elongated mountain ridges. Fill with terrestrial sediment.
Passive-margin basins: Form along edges of continents that are not plate boundaries. Underlain by stretched lithosphere. form because subsidence of stretched lithosphere continues long after rifting ceases.
Intracontinental basins: Develop in the interiors of continents, intially because of subsidence over a rift.
Foreland basins: Form on the continental side of a mountain belt because of the forces produced during convergence or collision push large slides of rock up faults and onto the surface of the continent.
Transgression - the inward migration of shoreline resulting from a rise in sea level.
Regression - the seaward migration of a shoreline caused by lowering of sea level
Diagenesis - all the physical, chemical, and biological processes that transform sediment into sedimentary rock and that alter characteristics of sedimentary rock after the rock has formed.