Sed Strat Week 5

sedimentary structures

Bedform

Form of the sediment bed (how it looks)

Bedforms of unidirectional flow (river)

ripples → dunes

Froude number

Flow velocity / (square root of gravitational constant * water depth).

Represents velocity of a ripple (gravity wave) on the surface of the water flow

Subcritical flow

Froude number < 1. Slow, deep, laminar flow, ripples propagate downstream and upstream. Flow velocity is less than celerity (gD)

Supercritical flow

Froude number > 1. Fast, shallow, often turbulent flow, ripples propagate entirely downstream. Flow velocity is more than celerity (gD)

Factors influencing grain flow

Grain size, flow depth, flow velocity

Hjulstrom diagram

Grains in suspension will join the bed as a static grain based on their size in response to the flow velocity.

High velocity flow is required to move heavier particles and fine, sticky sediment off the bed

Cohesive and non-cohesive sediment

Slight negative and positive surface charges on clay-sized grains cause them to stick together. There is also large surface/volume ratio due to clay being a flat sheet silicate, causing further cohesion. As grain size increases, cohesion decreases due to lower surface area and weaker charge (sand, gravel)

Bedding planes

Surface of a layer of sediment called the bed. Used to determine order/timing of accumulation of sediments that form beds. Characteristics of these (erosion, cementation, boring, etc) are used to aid in sedimentary rock interpretation

Individual beds are produced under constant physical, biological, or chemical conditions

Bed

Building block of stratigraphy. Tabular or lenticular layers (> 1cm) of sedimentary rock that have lithologic or structural differences that distinguish them from the layers above and below. Lithologic composition, geometry, and stacking pattern is used to interpret depositional setting

Laminae

Layers < 1cm in width. Alterations of thin layers of different lithologies are interbedded and are considered as one unit

Cross stratification

Any layering in sedimentary rock that is oriented at an angle to the horizontal deposition. Often caused by dune migration; ripples produce cross-lamination

Groups of cross beds

Sets are one unit of planar, cross bedded, or cross stratified sediment. A bed with more than one set of the same structure is a co-set

Composite bed

group of beds differing in composition, texture, or internal structures but associated genetically, representing a common type of deposited sequence (turbidite)

Graded beds

units characterized by distinct vertical gradation in grain-size (upward fining or coarsening)

Current ripples

Migrate as sand is added to the crest, accreting on lee slope. Sediment moves up the stoss side and collects in the trough, forming laminae and moving crest/attachment point downstream

referred to as cross laminae; may be straight, sinuous or linguoid

Flow effect on ripple shape

Higher velocity flow causes straight ripples to turn into linguoid ripples

Planar cross laminae (2D ripples)

Cross laminae that all dip in the same direction and lay in the same plane, caused by straight ripples

Trough cross laminae (3D ripples)

Curved cross laminae that all dip in different directions are formed mainly in the trough-shaped low areas between adjacent ripple forms; caused by linguoid ripples

Climbing ripple

High sand addition causes no net removal of sand from stoss side, causing ripples to migrate on top of each other.

When rate of sediment addition exceeds the forward movement of the ripple, deposition occurs on the stoss and lee side

Current ripples do not give a way to estimate ____.

Water depth. They only form in sands with grain size <0.6 mm and are independent of depth

Dune

Beds of sand in rivers, estuaries, and beaches that are much larger than ripples, leaving cross beds. They are different in the sense that dune crests (wavelengths) are larger in spacing and higher than ripples. However, sand migration works in the same way

May have 2D and 3D dune cross beds

Dune cross bedding types

Planar (low velocity flow), tangential (high velocity flow), tangential with ripples climbing up lee side (highest velocity flow)

Upper plane bed

Only forms at high velocities in fine sand. Ripple and dune bedforms become washed out with increased flow speed. Often have primary current (parting) lineation that shows flow direction where grains have dragged along the surface

Antidunes

Form when flow is in phase with surface waves. Migrate upstream