Sedimentary Architecture, Facies, and Ichnofacies
Sedimentary Architecture and Facies
Sedimentary architecture is the division of the stratigraphic succession into a hierarchy of architectural units. Facies are bodies of rock with specified characteristics that may be identified from adjacent bodies of rock by means of color, bedding, composition, texture, fossils, and sedimentary structures.
Types of Facies
- Biofacies: Defined by biological content.
- Lithofacies: Defined on the basis of the physical and chemical characteristics of the rock.
- Microfacies: Definition depends on features seen in thin sections. Usually, it is used to describe limestones.
An architectural element (Miall, 2006) is a component of a depositional system equivalent in size to, or smaller than, a channel fill, and larger than an individual facies unit, characterized by a distinctive facies assemblage, internal geometry, and external form.
Nine Ichnofacies
- Scoyenia
- Skolithos
- Psilonichnus
- Cruziana
- Zoophycus
- Nereites
- Glossifungites
- Trypanites
- Teredolites
Ichnology
Ichnology is the study of trace fossils (also called ichnofossils) produced by organisms (animals, plants, and microbes) on or within a substrate.
Trace Fossils Record
- The behavior of animals that lived where their traces are found.
- Physical and chemical conditions.
- Ecology.
- Sedimentological events (depositional or erosional events).
Controls on the Nature of Trace Fossils
- Sediment grain size and consistency
- Water depth
- Temperature
- Salinity
- Degree of oxygenation of the water
- Food supply
- Life habits and trace-making methods of the organism
- Sedimentation rate
Sediment Gravity Flows
Sediment gravity flows are flows composed of mixtures of sediment and water, which move downslope as mass flows driven by gravity. Sediment motion is initiated when the downslope component of the gravity force on a sediment mass exceeds the forces resisting motion.
Effective stress (σ′) is the normal stress (σ) minus stress due to the excess of pore-water pressure over normal hydrostatic pressure (u).
Grain-Support Mechanisms and Resulting Flows
Sediment gravity flow can occur only when grains become separated and dispersed to the point that internal friction and cohesiveness are sufficiently reduced to initiate movement.
Debris Flow
Flow mechanism: Laminar flow. Significant content of clay, which imparts cohesive strength. Plastic behavior with yield strength. The flow stops when the gradient decreases, by gradual dewatering or a decrease in flow thickness. The flow will then stop abruptly, “freezing” the internal fabric of the flow.
Debris flow deposits: The base of the deposits can be sharp and/or erosional.
Grain Flow
Flow mechanisms: Laminar flows (Re≈20) of cohesionless sediments; that is, muddy matrix is mostly absent. Particles are supported by intergranular collisions, which create a so-called dispersive pressure.
Sediment sorting and orientation in grain flows: Preferential movement of the larger grains away from the flow boundary due to kinetic sieving.
Grain flow deposits: Sharp, possibly erosional base. Normal, normal-reverse, reverse grading, or massive. Grain-flow deposits are lenticular.
Liquefied Sediment Flows
These flows occur when a sediment mass loses strength due to excess pore-water pressure (i.e., it becomes liquefied) and relatively low permeability inhibits the upward escape of the pore fluid. Distinctive features are water-escape structures such as pipes, sheets, and dish structures.
Turbidity Currents
Flow mechanisms: Turbidity currents are turbulent density currents in which the excess density that drives their downslope motion is caused by suspended sediment. Hyperpycnal flows display different sedimentological features than “turbidity currents.”
- Head: A region of erosion: flutes marks, gutter casts, and tool marks.
- Body: Dominated by deposition.