7: Sedimentary Structures

what does sedimentary structures include? what are these called?

structures formed just before, during, or just after deposition, primary

what does sedimentary structures not include? what are these called?

structures formed during or after lithification, secondary

what are the types of classification sedi structures?

depositional, erosional, deformational, biogenic

what are deformational structures?

soft-sediment deformation after deposition

what are biogenic structures? examples?

modifications by live organisms, trace fossils, coral

what are beds?

tabular or lenticular layers of sedi rock that have lithologic, textural, or structural similarities, distinguished from strata above and below

what are bedding planes?

upper and lower surfaces of beds

what are laminae?

layers less than 1 cm thick

how to dinstinguish between wavy parallel and wavy nonparallel?

nonparallel will have intersecting lines

what does tabular mean?

even thickness

what does lenticular mean?

shaped like a lens, thicker in the middle and tapered on the edge

how are parallel laminae formed?

settling of very fine grained particles out of suspension or traction in water or air. OR, by very fast flow allowing no ripples

how is cross bedding formed?

sediment deposited on the lee side of a dune, layer by layer creating an angled bed

if the bottom of the dune structure is the head of an arrow, which way do crossbeds point?

downstream

what is the difference between tilted rocks and tilted bedding?

tilted rocks are macroscopic, the current will push them over, so the top points downstream, tilted bedding is microscopic, the sediment is deposited on a dune, so the bottom points downstream

what are bedforms? examples?

distinct topographic forms that develop on a surface of sedi due to specific conds of fluid velocity and sedi transport depositions, ripples and dunes

what is the difference between ripples and dunes?

ripples are less than 5-6cm in height, dunes are larger and are spaced 1-100m apart

what are ripples controlled by?

viscous sublayer

what is the lee side?

downwind, the side where sediment gets deposited

what is the stoss side?

upwind, the side where sediment is pushed uphill

what is angle of climb?

the angle that the base of the ripple/dune is building on

what is important about angle of climb?

if it’s zero, none of the ripples will be preserved

what are sinuous/undulatory ripples/dunes? what will it cause? what type of foreset will it cause?

the “line” visible from the top is wavy, not straight, curved preservations in the cross section, asymmetrical

what are linguoid ripples/dunes? what will it cause? what type of foreset will it cause?

the lines from the top look like channels, curved preservation sin the cross section, tangential and asymptotic

what will 3D transverse ripples/dunes look like? what will the cross section look like?

sinoidal shape, out of phase, in direction of flow cross section will look lenticular intersecting, direction parallel to flow will look like regular 2D with angle of climb

what will a low flow velocity over a ripple/dune cause?

sharp angular lee side

what will medium flow velocity over a ripple/dune cause?

a curved asymptotic lee side

what will high flow velocity over a ripple/dune cause?

counter-current ripples on the lee side

what will occur in low flow regimes? what froude number?

Fr<1, ripples and dunes form

what will occur in high flow regimes? what froude number?

Fr>1, flat plane bed, then antidunes, then chutes and pools

what are antidunes? where are they formed?

fairly symmetric dunes migrating upstream with low angle cross beds dipping upstream (opposite of expected), in high flow regimes

briefly explain the controls on bedform stability graph?

coarse grains don’t produce ripples, the transition between no motion to lower plane then dunes then upper plane is gradual, the transition between ripples to dunes or ripples to upper plane is abrupt. there is a diagonal trend between small grains at low speed = ripples and large grains at higher speed = dunes

what is required for significantly high climbing ripples? why? why wouldn’t another way work?

low velocity with a lot of sediment, lots so they will be deposited and not just wash away, and if it was high flow velocity they would crash into each other with force and destroy the ripples

what is important about wave ripples?

the surface will be symmetrical but the cross section can be intertwining

what can symmetrical ripples have than asymmetrical may not?

laminations or cross bedding angled in both directions

what is hummocky cross stratification

dune scale bedforms with cross laminae dipping in all directions away from a high convex crestal area

what is information about HCS? (wavelength, angle, grain size, formed by?)

1-5m wavelengths, thin laminae draping over hummocks and dip at low angles (12*), with low angle intersections, only fine-grained sands, formed in combined flow of strong oscillatory currents (storms) and unidirectional flow

how to distinguish HCS?

look at big picture, small areas may appear planar, look for intersections at low angles

what is herringbone cross stratification? what is it caused by? how is it formed?

looks like cross bedding, in tidal areas, flow going in one direction, then ebbing and letting fine grained particles deposit, and direction changing, causing laminae to be angled the opposite direction

what is slack water? what does it allow?

when the water velocity goes to zero as the flow direction is changing, mud deposition (much finer grains)

what is heterolithic bedding? where does it occur?

interbedded mud and sand laminae, variations in currents and sediment supplly

what are the types of heterolithic bedding?

lenticular, wavy, flaser

what is lenticular bedding?

more mud than sand

what is wavy bedding?

same mud and sand

what is flaser bedding?

more sand than mud

describe this photo

climbing ripples, flow direction to the right, ripples are asymmetric

what are flute casts? what are they usually carving into? how to know upstream?

sediment fills a depression that was scoured in cohesive sediment by turbulent eddies, mud, deeper and steeper upstream

what are tool marks? how are they formed?

elongated, irregularly shaped ridges that result from infilling of erosional relief, sticks or shells dragging or bouncing along the bottom

what are channels?

elongate continuous depressions that cut older bedding

how to identify channels?

look for a border where the angle of bedding is different above vs below

what are the forms of soft sediment deformation?

convolute bedding, flame structures, ball and pillow

what is convolute bedding? what is it caused by?

complex folding and intricate crumpling of beds and laminae, from loading/dewatering/slumping

what are flame structures? what are they caused by?

wavy or flame shapes of sediment squeezed upward into the overlying layer, from loading a denser material (sand/gravel) on top of less dense, saturated sediment (mud)

what are ball and pillow structures? what are they caused by?

hemispherical masses of sand, partially separated from the base of a unit with internal or deformed lamination, from breaking up semi-consolidated sand beds due to liquefaction of underlying mud

what can you infer from ball and pillow structures?

ball is fully separated, pillow is not, can be caused by earthquake, used to date seismic activity if many are at the same level

what are dessication/mud cracks? what are they caused by?

downward tapering V shaped fractures, show a polygonal pattern in plan view, area between cracks becomes concave, from drying of an exposed surface

what are synaeresis cracks? what are they caused by?

more variable and discontinuous, spindle shaped, from fluctuations in saline or swelling clays, start straight but often become compacted and deformed