L5: Sedimentary Structures 1 Flashcards
Original Horizontality of beds
Layers of sediment are deposited on horizontal or low angle slopes; The depositional bed was originally horizontal. Does not apply to cross stratification.
Original continuity of beds
Beds used to be continuous before they were eroded so outcrops and exposures can be used to extrapolate
Superpositions of beds
For a bed to be deposed on top the bottom bed must already have been there therefore the bottom beds are older
Way up rule
Tops and bottoms that have undergone movement can still be determined using sedimentary structures to determine which way was up.
Cross-cutting relationships
If a feature cuts aross or into a bedding plane then the feature must be younger than the bed.
Truncation of beds
indicates erosion must have occured
termination of strata or seismic reflections interpreted as strata along an unconformity surface due to post-depositional or structural effects.
Erosion surfaces indicate
geological record is incomplete
Law of faunal succession
The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances. A fossilized Neanderthal bone will never be found in the same stratum as a fossilized Megalosaurus, for example, because neanderthals and megalosaurs lived during different geological periods, separated by many millions of years. This allows for strata to be identified and dated by the fossils found within.
Sedimentary structures
Structures formed at the time of deposition
Bedforms
Depositional features that occur by the movement of wind and water over loose unconsolidated sand
Bedforms
Depositional features that occur by the movement of wind and water over loose unconsolidated sand
Explain how bedforms work
The bedform moves downstream and sand is deposited on the lee side, on a dipping surface inclined in the downstream direction these record the movement of bedforms and form cross-stratification/lamination
2 techniques used to understand sedimentary structures
Modern analogues
Process sedimentology - knowldge of physical processes to interpret
Entrainment
entrainment is the process by which surface sediment is incorporated into a fluid flow (such as air, water or even ice) as part of the operation of erosion.
Bernoulli’s effect
Bernoulli’s principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure. This happens above the grain and generates a lift force causing entrainment.
Sole
Base of a sedimentary bed
Lamination
less than 1cm layers
stratification layer thickness
Greater than 1cm
Thin 1-10cm
Medium 10-30
Thick 30-100cm
Gutters
(Sole structure) small scale localised elongated erosional scours cut into cohesive muddy sediment by strong stormy currents
Comet marks
Scour around an obstacle where the scour is deepest upstream and shallower downstream
Scour
Large scale sole structure where a bed is eroded around an object or obstacle deeper
Flutes
Created by fluid flow and can be formed without an obstacle; a type of sole structure
Current ripples
Sedimentary structure
Wavelength of less than 50cm and amplitude of less than 3cm. Formed in currents with 30cm/s to 60cm/s
Only in sand less than 0.6mm
Asymetrycal
Show paleocurrents as they move downstream with a steep lee slope and a low stoss slope
Climbing ripple lamination
The rate of sediment deposition from suspension is sufficient to raise the bed at the same time as the ripple is migrating downstream.
When are dunes and ripples preserved
When horizontal translation is in equilibrium with vertical growth
Mutually erosive bedforms
No sediment supply or accretion so the bedforms will migrate downstream with no preservation
Subcritical climbing of ripples and dunes
if the angle of climb is less than stoss slope angle therefore the ripples will be accreted p[reserving the lower part of the lee slope (toesets) but eroding the tops
Critical angle of climbing in dunes and ripples
The lee slope is preserved as the angle of climb equals the stoss angle
Supercritical climbing of dunes and ripples
The angle of climb is greater than the stoss slope angle leading to the preservation of the whole bedform morphology with the stoss slope
Wave ripples
cause by the Oscillatory flow of water moving back and forth in shallow waters
symmetrical
straight crestlines
perpendicular to wave direction
Dunes
Larger than current ripples
wavelength greater than 1m and amplitude greater than 10cm
Dune form is rarely preserved but cross-stratification is commonly formed by the migration of dunes
2 types of dune stratification sets and causes
1) Planar set - straight Crestline formed by 2-dimensional dunes
2) Trough set - curved crestlines formed by 3-dimensional dunes
Bedform phase diagrams
Used to study which bedforms will be created under which conditions
Plane bed lamination
formed in high-velocity, rapid transport environments
Turbulence is suppressed by high sediment load resulting in a flat planar bed
On bedding planes, exposure lineations can be seen in the direction of paleocurrent called primary current lineation
Also found in lower slop regimes with coarser grained sand and gravel
Antidunes
High current velocities and turbulent flows
low angle sinusoidal bedforms
low preservation due to erosions
Way up methods
Sole structures
Graded bedding (usually fines upwards)
Cross stratification - down lap at bottom
Bioturbation (burrows etc.)
Mudcracks and dewatering
Fossils in life position (corals etc.)
Geopetal infill with mud at bottom and cement at the top
