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.
