Topic 6: Sedimentary Rocks

Sedimentary Rocks

Types of rock that are formed by the accumulation or deposition of sediments (weathering, transportation, deposition, and lithification) at Earth's surface, followed by cementation

Petrology

The study of rocks by studying the rock occurrence, composition, texture, and other characteristics

Petrography

Basically petrology but using a petrographic microscope to do it

Sedimentology

The processes by which sediments are transported and deposited

Stratigraphy

The study of all aspects of the sedimentary rock from the perspective of its age, regional relationships, and correlations of sedimentary rocks across regions

Paleontology

The study of the history of life on Earth based on fossils

Petroleum Geology

Involves the exploration of the Earth for oil and gas deposits and the production of these resources

Weathering

The breaking down or dissolving of rocks and minerals on Earth’s surface

Mechanical Weathering

Involves the breaking apart of rocks along fractures, such as joints generated by tectonism or cooling of igneous rocks, or exhumation during erosion

Chemical Weathering

Weathering that results in the alteration of the molecular structures of rocks and soil with reactions that can take place over long periods

Frost Wedging

A type of physical weathering where water seeps into cracks in rocks and expands upon freezing, causing the cracks to widen and eventually break the rock apart

Abrasion

The process where rocks and sediments grind against each other, causing the surfaces to wear down through friction

Sandblasting

The natural process of wind-blown sand eroding rock surfaces

Salt Wedging

A type of physical weathering where salt crystals grow within rock cracks and pores, causing the rock to break apart

Exfoliation

A type of physical weathering where outer layers of rock peel off in thin sheets or slabs

Bioturbation

The biological reworking of sediments and soils by organisms, particularly through burrowing and other activities

Hydrolysis

The breaking apart of molecules by water

Hydration

When water is added to the chemical structure of a mineral

Oxidation

When iron atoms lose electrons, their properties change; in the presence of water, oxygen happily accepts these electrons

Mine Drainage

Simple weathering of of iron oxides through oxidation

Carbonation

Reaction of carbon dioxide and water forms carbonic acid (weak acid) that can dissolve rock over time

Acidification

When compounds such as sulfur dioxide and nitrogen gases react with water molecules in the atmosphere where clouds form, this results in acid rain

Reactions With Living Organisms

Living organisms perform chemical reactions to obtain minerals from soil and rocks, resulting in chemical weathering

Sediments

Solid particles, like rock fragments, soil, or organic matter, that are moved and deposited in a new location

Sedimentation

The process by which sediments are transported

Grain Size

The size of the sediments in a sedimentary rock, can be measured visually or using sieves, and relates to the energy of its environment

Clastic Sediment

Are composed of fragments of rocks and minerals that have been transported and deposited

Detritus Sediment

Particles of rock derived from pre-existing rock through weathering and erosion

Udden-Wentworth Scale

A grade scale for classifying the diameters of sediments in millimeters and ranges from cobbles to silt/clay

Cobble

Grain size above 64 mm

Pebble

Grain size from 4 to 64 mm

Granule

Grain size from 2 to 4 mm

Very Coarse Sand

Grain size from 1 to 2 mm

Coarse Sand

Grain size 0.5 to 1 mm

Medium Sand

Grain size 0.25 to 0.5 mm

Fine Sand

Grain size 0.125 to 0.25 mm

Very Fine Sand

Grain size 0.0623 to 0.125 mm

Silt

Grain size 0.0039 to 0.0623 mm

Clay

Grain size less than 0.0039 mm

High Energy Environments

Glaciers, mountain, alluvial fans, rivers, and streams; sediments tend to be fairly large

Medium Energy Environments

Deltas, channels, flood plains, beaches, lagoons; sediments tend to be sand-sized

Low Energy Environments

Lakes, ocean basins, protected bays, estuaries, abyssals, continental shelves; sediments tend to be fine

Sorting

How different grain sediments are distributed within a sedimentary rock, a process which is determined by transportation

Well Sorted

Sediments tend to be even and have an uniform size

Moderately Sorted

Some sediments may have a similar size but sediments of varying sizes are present

Poorly Sorted

Most sediments have wide variation in grain sizes, giving it an uneven appearance

Angularity

Refers to the sharpness or smoothness of the edges and corners of sediment particles, like grains of sand or pebbles

Angular

Mineral fragments with sharp, rough edges and corners

Subangular

Grains that have somewhat smooth surfaces but retain some degree of sharpness and edges

Subrounded

Grains that have some degree of rounding, meaning they have lost some of their sharp edges and corners, but are not fully smooth

Rounded

Particles that have smooth, rounded shapes due to abrasion from being transported by water or other forces

Sediment Maturity

The degree of alteration and sorting of sediment grains due to weathering, transportation, and diagenesis

Immature Sediment

Consists of angular, poorly-sorted grains in which a considerable amount of mud-sized particles still remain mixed with sand sized particles (not exposed to significant transport)

Submature Sediment

The mud-sized fraction has been removed, but the grains remain angular and poorly sorted

Mature Sediment

Well-sorted and subangular grains

Supermature Sediment

Well-sorted and rounded grains

Glacial Geomorphology

Sediments form different landforms

Organically Produced Sediment

Sediment composed of or derived from the remains of living organisms, such as plants and animals, eg. limestone

Oil

Isn't considered a sediment, rather, it is a direct product of the burial of animal and plant remains that forms via breakdown of kerogen from organic material at slightly-elevated temperatures

Natural Gas

Flammable gas, consisting largely of methane and other hydrocarbons, occurring naturally underground (often in association with petroleum) and used as fuel

Fracking

A process where you use high pressure water and gas to artificially break the rock to retrieve natural gas; it leaves a lot of groundwater and is bad for the environment

Chemically Produced Sediment

Sediments formed through chemical processes, primarily precipitation, where dissolved minerals in water are deposited as solid sediments

Banded Iron Formation (BIF)

Distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert

Laminar Flow

Flow of fluids as thin sheets or laminae that slide over each other

Turbulent Flow

Irregular, highly distorted flow that occurs when the viscosity of the fluid is low, and the velocity is high

Rolling

When fluid passes over a particle at rest, the shear force applied by the fluid tends to roll the particle

Saltation

As fluid flows over a stationary particle, flow lines are deflected and forced closer together, causing the fluid to momentarily move faster over the top of the grain

Suspension

If flow is turbulent (wind and most moving water), turbulent eddy currents can lift particles and carry them in suspension, as long as the particles are not too large or dense

Stokes Law

A mathematical equation that expresses the drag force resisting the fall of small spherical particles through a fluid medium, F=6πrηv

Bedload

Sediment that moves along the bottom of a river or stream, typically during periods of high flow

Ripple

Form at the lowest velocities, with wavelengths of centimeters to meters

Sand Wave

A lower regime sedimentary structure that forms across from tidal currents

Dune

A mound of sand formed by the wind, usually along the beach or in a desert

Plain

A large area of flat land, typically with little variation in elevation and few trees

Antidune

Super rare, develop the greatest velocities (wave shape of the bedform is in phase with the surface of the water)

Unidirectional Flow

Produces asymmetric ripples with pointed crests and rounded troughs, eg. stream, aeolian dune

Bidirectional Flow

Produces symmetrical ripples, eg. beach

Aeolian Dune

Mounds or ridges of loose sand shaped and deposited by wind

Cross Bedding

A linear depression in the Earth's surface through which water and sediment flow

Channel

A linear depression through which water and sediment flow, typically within a stream or river

Floodplain

A naturally flat area next to a river or stream that gets flooded when the water level rises

Turbidity Current

Powerful, sediment-laden flows of water that move downslope under the influence of gravity

Turbidite

The geologic deposit of a turbidity current

Fissility

The splitting apart of a bed into thin sheets

Sedimentary Bed

A layer that is distinguishable from layers above and below it on the basis of: rock type, grain size, fissility, etc.

Nicholas Steno (Niels Steensen)

Danish scientist and pioneer in anatomy and geology, questioned the idea that fossils grew in the ground and early explanations of how rocks formed, and developed the four of the defining principles of stratigraphy

The Law of Superposition

A sequence of rocks in their original orientation will have the oldest rocks on the bottom and the youngest rocks on top

The Principle of Original Horizontality

At the time when one of the upper strata was being formed, the lower stratum had already gained the consistency of a solid ( i.e., sediment tends to be deposited horizontally under the action of gravity)

Angle of Repose

The steepest angle at which a sloping surface formed of a particular loose material is stable

The Principle of Lateral Continuity

Layers of sediment initially extend laterally in all directions, and as a result, rocks that are otherwise similar but are now separated by an erosional feature can be assumed to be originally continuous

Facies

A body of rock with specified characteristics (chemical, physical, biological) that distinguishes it from an adjacent rock, and can be grouped spatially by associating genetically-related lithologies that reflect linked environments in the stratigraphic record

The Principle of Cross-Cutting Relationships

If a body or discontinuity cuts across a stratum, it must have formed after that stratum, a technique for determining the relative ages of features in geology

Structural Relationships

The way rocks and geological features are arranged and how they interact with each other like folding or fracture

Intrusional Relationships

Occur when an igneous pluton or dike is intruded into pre-existing rocks

Stratigraphic Relationships

May be an erosional surface (or unconformity) that cuts across older rock layers, geologic structures, or other geologic features

Sedimentological Relationships

Occur where currents have eroded or scoured older sediment in a local area to produce, for example, a channel filled with sand

Paleontological Relationships

Occur where animal activity or plant growth produces truncation (e.g., animal burrows penetrate pre-existing sedimentary layers)

Geomorphological Relationships

Occur where a surficial feature, such as a river, flows through a gap in a ridge of rock (or where an impact crater excavates into a subsurface layer of rock)

Walter's Law

States that any vertical progression of facies is the result of a succession of depositional environments that are laterally juxtaposed to each other

Marine Transgression

A geologic process during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding