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292 Cards in this Set
- Front
- Back
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what is the average width of a continental shelf?
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50-100km
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As sediment progrades...
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the shoreline regresses
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What is the shelf slope break dependent on?
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sediment supply, water level (base level), and tectonics/subsidence
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What is the shelf slope break also known as?
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the nick point
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what is the chokepoint?
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the transition from steep to less steep. sediment slows and is deposited
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why is it important to understand where the nickpoint and chokepoint are?
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it determines where there is erosion and deposition
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what is the primary control on where sediment is eroded or deposited?
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base level
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When is shear stress its highest? what does this mean?
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when we have a steep slope. this means the flow can do more work
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Where is deposition favored?
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decreasing slopes/decreasing water speeds/increase in water depth
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Where is erosion favored?
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increasing slopes/decreasing water depth/increasing veolocity of water
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What are the different types of base level?
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stratigraphic and geomorphic
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What is stratigraphic base level?
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imaginary surface that separates erosion from deposition. systems tend toward 'equilibrium' profile (ie graded river profiles, graded marine shelf, angle of repose)
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What is geomorphic base level?
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the lowest point to which the land's surface tends. (approimately at sea level, implies at flat surface)
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how can base level change?
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through actual changes in sea level or changes in the land's surfaces (through tectonics)
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What is eustasy?
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it refers to all oceanic waters. It's an absolute change in sea level
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What is relative sea level?
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it's a relative change in sea level compared to a datum
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What are some examples of equilibrium profiles?
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graded river profiles, graded marine shelf, and angle of repose
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what is the angle of repose? what is its value?
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it's the critical angle where material is on the verge of instability/sliding ~32 degrees
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what is the steepest lateral accretion slope?
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15 degrees
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what is a typical continental slope?
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3-4 degrees
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how steep is a typical continental shelf?
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<0.1 degrees
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How are carbonates different in their slope building than siliciclatics?
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carbonates can build to be completely vertical, whereas siliciclastics cannot
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What happens generally when a nickpoint is exposed?
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the rivers can run faster due to an increase in slope, leading to more erosion, and sediment being deposited at chokepoint deepwater fans
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what occurs further inland with nickpoint exposure?
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there is headward erosion that may occur. may form gullied erosion that later can connect with a stream or river forming an incised valley (know diagram)
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What happens to the gullies during a transgression?
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they fill with mud
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what controls the size of a submarine fan?
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the area being excavated (ie big rivers feed bigger fans)
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Why are sand reservoirs from channels hard to find?
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because most rivers carry mud
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Is sequence stratigraphy scale dependent?
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no. it is similar no matter what the scale is
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where does gully erosion occur?
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in the break in slope
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Where are the steepest areas in a continental system?
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the incised valleys
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steeper slopes exhibit what feature?
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they are more channelized
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When do attributive/distributive properties occur?
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attributive before the nickpoint, distributive after the nickpoint
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where does alluvial deposition occur?
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at the chokepoint
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when do nickpoint incise?
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when there is a drop in sea level
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what kind of feature is the shelf edge? and why?
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it's a lowstand feature because sea level is low enough to expose the shelf edge
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what kind of feature is a nickpoint? and why?
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it's a highstand feature because sea level is high enough to cover the shelf edge
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what is a buttress?
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it's a downstream point in which a river cannot extend any further (due to a damn or something that blocks the river)
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What is a buffer?
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a profile along the flow. at any point in time, the river has a specific buffer profile
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How can discharge change (Q) (Qs=sediment) (Qw=water)?
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it can change due to fluctuations in water level due to flooding.
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What happens with an increase is Qw?
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the river wants to pick up more sediment. the base of the river erodes and sediment gets deposited. There's incision and filling that can depend on milankovitch cycles.
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What does a lower buffer profile mean?
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the river is at its lowest levels with lowest uplift rate and sediment wants to erode if there's more water (known buffer profile diagrams)
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What are the upstream controls on base level and its resultant stacking patterns?
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climate, source area uplift, and subsidence
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What are the downstream controls on base level and its resultant stacking patterns?
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eustasy (know diagram)
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What happens to rivers with a fall of base level? what is needed?
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rivers incise if there's a nickpoint.
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What happens to rivers when the fall of baselevel ceases, then rises?
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the buttress is seaward. when sea level rises, the rivers begin to aggrade in confined systems. when the valley fills, they are free to avulse
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What happens to incised valleys during a highstand?
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the incised valleys are filled, and there's a muddier fluvial succession due to floodplains. when sea level drops, it leaves t he floodplain high and dry-> drier paleosols
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What do deltas and buttresses do when they get bigger?
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they move seaward
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what happens if a buttress falls?
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the river cuts down into the lower levels and that decrease in fall isn't felt as much further up the river's profile
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What is accommodation? is it stable?
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the space available for sediment infilling. the shape of the space can change. the rate at which space is created or destroyed can change.
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What are to tops and bottoms of accommodation spaces determined by?
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top=base level
bottom= compaction |
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can all accommodation spaces be filled?
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no. some spaces will never fill (ie ocean basins)
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what is accumulation? what is it controlled by?
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the sediment that fills the space. it's largely controlled by sediment supply
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What do basin fill patterns reflect?
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the sediment quantity (Q), now known as S, and the receptor value (R) now known as A. A/S ratio
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What happens with high S and low A?
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the accommodation can't contain sediment. therefore it's transferred somewhere else. this feature is similar to a river which are areas of net sediment transfer
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What happens with low S and large A?
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most of the space is filled with water (like a submarine fan)
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What happens with moderate S and moderate A?
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a delta forms
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What is the sedimentary stratigraphic record dependent on?
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the size, shape, and rate at which accommodation increases or decreases and the availability of sediment to fill that space
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What does physiography control?
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accommodation, shape of land's surface, and the slope
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What is sediment availability controlled by?
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relief and climate
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What is the A/S ratio sensitive to?
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the types of depositional systems present
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What do basin fill patterns ( facies successions) reflect?
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the accommodation vs. accumulation rate sed influx, and sea level rise/fall
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What characterizes a transgressive system?
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loss of sediment. sediment supply is low. subsidence is fast and/or sea level is rising.
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what characterizes a retrogradational system?
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back-stepping of sediment, sediment supply is low, subsidence is fast and/or sea level is rising
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what characterizes aggradational systems?
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it's pure aggradation, meaning it is a balanced system which renders it unstable. balanced=unstable. sed influx is moderate,subsidence is moderate and/or sea level rising is moderate
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what characterizes aggradation to progradation?
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nickpoint trajectory is negative (so it's a highstand). sed supply is high, subsidence is low/none, and sea level is static
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what characterizes progradation?
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with pure progradation, the trajectory is positive, so it's a lowstand, as it progrades to sea with no aggradation there is no excess accommodation. sed influx is high, subsidence is none, sea level is static
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what happens in a forces regression?
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shoreline moves seaward due to fall in relative sea level (tectonics?)
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how is a lowstand normal regression characterized?
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a positive shoreline trajectory. accelerated base level rise. the rates of progradation decrease with time, and the rates of aggradation increase with time. PA
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how is a highstand normal regression characterized?
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a negative shoreline trajectory. decelerating base level rise, with accelerated rate of accommodation being created. the rates of progradation increase with time, the rates of aggradation decrease with time. the degree of nickpoint migration decreases with time
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What did Neal and Abreu want to accomplish?
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wanted a geometric analysis of successions, not assumptions of sed supply. They call sequence sets, accommodation succession sets because this doesn't infer a scale.
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Does parasquence have a scale connotation?
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yes
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What do the facies successions show in highstands?
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AP
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what do the facies succession show in lowstands?
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PA
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What is baselap?
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it's where the upper clinoform terminates against a lower, towards the sea
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what is lapout?
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sequence stratigraphy! layers that are getting thinner and aren't seismically resolvable. there might be a thin toes, but it's not visible.
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what is sequence stratigraphy?
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lapout. the termination, analysis, and correlation of one stratum on top of another.
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What do posamentier and allen do in 1999?
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talk about the trajectory and stacking of facies. changes in accommodation (relative sea-level) are the main controls on the resulting stratigraphy (diagram)
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What do posamentier and allen assume in their 1999 diagram?
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assume a constant sediment supply!
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What is seismic resolution with 30 Hz data?
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approximately 50m. no tvery big. hard to see smaller sand bodies
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What is an attached forced regression?
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a quasi-continuous sand body. the trajectory goes below sea level with a negative trajectory (know picture)
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what is a detached forced regression?
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detached sand bodies resulting from drop in sea levl. the ultimate lowstand is detached from highstand. then following a transgression these sand bodies can be eroded
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Why should we care about detached forced regression?
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the sand bodies might not be seismically resolvable, but we might assume they exist, but they are hard to find. they make great reservoir because they are sand encased in mud (mud seal)
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What is the sedimentological expression of a forced regression?
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sharp-based shorefaces seen as indicative of non-genetic relationship between underlying 'shelf' mudstones and overlying shallow-marine sandstone
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What happens to the shoreface with a forced regression?
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it's is more proximal with its infill. the shoreface deposits migrate to be above the shelf. mudstone on the upper-shoreface most likely shows waves eroding transition area (which is not preserved)
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what is the shoreface and how does it form?
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it's the steeply dipping slope that separates the subaerial from the subaqueous plane. forms in response to shoaling waves (fairweather processes). Fairweather wave base ~5-15m. mud is typically unable to be deposited. shorefaces can be erosional (wave-cut terraces) or depositional
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What is a modern ravinement?
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a diacronous surface which may erode up to 40m. it scales to water depth. it's where the soft sed is eroded away until it reaches the firmer mud. these holes are filled when water deepens (transgression). trace fossils are common in these
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What is wave base?
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the depth at which sed can be moved by waves. there's a fairweather and storm wave base. (depths vary depending on wave energy and sediment types {ie sand is shallower than mud})
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what occurs during high base level?
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the shelf is 'flooded' and sedimentation is confined to the shelf. the slope and deepwater environments are starved. (assumes constant sed supply) rivers with highstand deltas, slope is underwater, mud and siltis on the slope and basin
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What happens when there's a fall in base level?
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the entire shelf is exposed, the nickpoint is exposed. deltas can't build because the water is too deep. incised valleys spew into the shelf. (when sea level rises again, these incised valleys are covered with mud, creating a trap) Rivers primarily erode, forming widespread erosional surfaces, forming a sequence boundary. sediment is deposited in deepwater environments and assumed to bypass the exposed shelf
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linked depositional systems are called?
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systems tracts- area (depositional systems area) describing the surface which defines the depositional system
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What did Krynine say about stratigraphy?
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Stratigraphy is the complete triumph of terminology over facts and common sense
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what is stratigraphy the basis for?
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mapping
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What is stratigraphy?
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the study of large-scale vertical and lateral relationships between units of sed rocks that are defined on the basis of lithological or physical properties, paleontological characteristics, geophysical characteristics, geophysical properties, age relationships, and geographic position and distribution
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What is stratigraphy based on?
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steno's law of superposition (young rocks overlie older rocks), original horizontality, and original continuity
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How did Steno come up with his laws?
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he found shark's teeth and theorized that rocks were once fluidized.
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Who are the fathers of modern sequence stratigraphy?
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Grabau, Blackwelder, and Barrell
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Who revolutionized sequence stratigraphy using data? (data revolution)?
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Wheeler, Sloss, and Vail
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What is correlation?
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determining the time-equivalency of rock units within a region, continent, between continents, by similar fossil assemblages or rock types
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What are the types of correlation?
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physical continuity and similarity of rock types
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What does physical continuity of correlation entail?
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physically tracing a continuous exposure of a rock unit. (easily done in the grand canyon)
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what does similarity of rock types correlation entail?
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assumes similar sequences of rocks formed at same time. this can be inaccurate if there are very common rock types involved
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What are the forms of formal stratigraphy?
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lithostratigraphy, allostratigraphy, biostratigraphy, chronostratigraphy, magnetostratigraphy, and pedostratigraphy
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What are the forms of informal stratigraphy?
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sequence stratigraphy (genetric sequence strat and depositional sequence strat), cyclostratigraphy, event stratigraphy, and morphostratigraphy
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What is lithostratigraphy?
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defined on the basis of lithologic properties, strata form bodies (sedimentary) groups, formation, members, beds.
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What are formations?
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the base unit for mapping. based on laws of superposition
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What is biostratigraphy?
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a formal stratigraphic scheme defined on the basis of fossils and faunal successions (biozones).
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How to we use biostratigraphy to obtain age?
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we use occurrence charts. can use the last appearance of an organism and the first appearance of their presence to determine a time period. Canbe based on a rangeof a single fossil, overlap between different fossils, and abundance of fossils/their overlap. the more fossil overlaps, the more definite the time period.
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how are biozones defined?
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as a unit of time of the first and last appearance of a species but can be different in different locales
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What is the problem with biostratigraphy?
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it can be a blunt tool
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Has any species on earth lived less than 1,000,000 years?
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no
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What is chronostratigraphy?
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defined as a unit of rocks that characterize a given time interval linked with geochronologic units measured in absolute time. Eon/eonathem, era/erathem, period/system, epoch/series, ages/stages. (mesozoic era=time, mesozoic erathem=rock; cretaceous period=time, cretaceous systme=rocks)
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What is the problem with rocks in time? what does this mean?
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not all geologic time is represented by existing rocks. they are diachronous
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What are some common chronologic units?
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eons, eras, periods, epochs, and ages.
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What is allostratigraphy?
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lithologically heterogeneous units defined on the basis of bounding discontinuities. basically any types of stratigraphic contact, diastem, and conformity is an allostratigraphic boundary
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What is magnetostratigraphy?
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mapping periods of normal vs reversed magnetic polarity
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What does informal stratigraphy do?
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reflects the increasing importance of subsurface data and cross sectional views of sed basins, rather than map views.
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Why is sequence stratigraphy helpful in oil and gas industry? is it useful for other things?
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it' helps to map reservoirs, seals, and traps. it's also useful in climate studies
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What is the highest level to subdivide geologic successions?
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sequence stratigraphy
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What is seismic stratigraphy?
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large scale stratal packages identified on the basis of stratal geometry, seismic character (facies) and lapout relationships(unconformities)
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What is sequence stratigraphy?
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the study of unconformity bounded sequences in the context of accommodation and accumulation concepts.
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What is event stratigraphy?
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the mapping and correlation of specific beds or events
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what is cyclostratigraphy?
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study of cyclic sed packages driven by apparently cyclic phenomena (milankovitch cycles, climate cycles)
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Whatis morphostratigraphy?
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study of morphologic elements bounded by surfaces of non-deposition or erosion, similar to facies architectural concepts
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What are PACs?
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punctuated aggradational cycles. stacked carbonate successions are referred to as PACs
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What is a lithosome?
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a rock body originally used to designate rocks of uniform character interfingering with different lithologies
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What are GIS?
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genetic increments of strata, one of the first sequence strat papers
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What were some problems with sequence stratigraphy in 60s?
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no coherency of names (too many caused confusion), no correlation of units, didn't understand how to classify interfingering and used vertical cutoffs instead.
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Who produced the first geologic map? and where?
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William 'strata' Smith, a canal builder produced map of england
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What did Grabau write about?
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the principles of sed overlap. first to write and use onlap, offlap relationships in paleozoic strata of North America.
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What did Barrell do?
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defined concepts of base level and hiatuses, reasoned that deltas might have been more important during other base levels. showed evolution of deltaic facies as a function of changing subsidence, aggradation, progradation, ect. Delta evolution as a function of tectonics
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What did Blackwelder recognize?
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the importance of unconformity-bounded sequences in North America. used grabau's work and documented a method to evaluate the time associated with an unconformity. Came up with chart through time and rocks that represented those times in North America (know diagram)
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What does base level control?
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the preservation of sedimentation
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how did we first measure the age of the earth?
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through the sedimentary record, which was obviously flawed because not all time is represented by rocks
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what did we use to first estimate the age of the earth?
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meteorites
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What did Harry Wheeler do? and how? and when?
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Wheeler, 1958 formalized the concept of time stratigraphy. He collected and correlated ideas of the previous stratigraphers. He recognized that the hiatuses were as important as the rocks. Depicted 'exploded' stratigraphic cross sections where time is on the vertical axis.
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What is a lacuna?
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a break in the rock record that is either a period of non-deposition or erosion
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What are unconformites divided into?
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rocks that were there, and periods of no deposition
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How did Wheeler and Mallory address interfingering units?
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they used arbitrary vertical cut offs
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What is a lithosome?
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describes a continuous body of rock
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What was Wheeler's definition of a sequence?
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a group of rocks bounded by an unconformity, (folded?) practical preservation with patches that represent rocks that were once there that are gone now
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Who is Larry Sloss and what did he do? (hint 2)
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He's the father of american stratigraphy. he applied blackwelder's and wheeler's work to north america. Named 6 major sequences in north america that represented major tectono-eustatic events. He also tried to model sequences using their A/S ratio. Showed that transgression and regression can be controlled either by changing subsidence or sed supply
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What are the Sloss sequences?
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Sauk, Tippecanoe, Kaskaskia, Absacoka, Zuni, and Tejas
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What did Sloss notes about the sequences?
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that the continent wide unconformities were larger at the center of the continent and more continuous near the edges
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Who studied under Sloss?
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Pete Vail
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What did pete vail do?
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used seismic data, and concepts developed by wheeler and others to use seismic stratigraphy. Had data from around the world since he worked for Exxon
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What are seismic reflections?
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assumed to image bedding surfaces. Reflection character and geometry is related to lithology and facies architecture
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What are depositional sequences bounded by?
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their unconformities and their correlative conformities
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What is the main problem with seismic data?
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it's resolution
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what is the scale of a geophysical thin bed?
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30m
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What did Pete Vail say about seismic stratigraphy in 1977?
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primary seismic reflections are generated by stratal surfaces which are chronostratigraphic rather than by boundaries of arbitrarily defined lithostratigraphic units
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what does chronostratigraphic mean?
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that rocks has significance in time
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is lithostratigraphy defined chronostratigraphically?
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no because all of time is not represented by rocks
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Why was sequence stratigraphy largely ignored in the 1970s?
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because there was endless debate about what to name units, arbitrary vertical cutoffs
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Why was Wheeler's science ignored?
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because he did not except plate tectonics
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Why didn't Dickinson agree with seismic stratigraphy?
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he didn't think seismic picks up more than lithologic boundaries
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What is an example of a coherent reflection?
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off of a bedding surface. These constructively interfere
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What is a inchoherent reflection?
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something that destructively interferes with reflections, like lithofacies shazams.
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What does velocity stacking do?
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greatly enhances the signal to noise ratio, and takes out the time asymmetry. allows for imaging of bedding contacts even where the changes in acoustic properties are minimal
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Do lithofacies picks from logs conform to seismic?
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no, seismic does not pick 'base sand' or whatever.
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What are seismic reflections interpreted to show?
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bed, bedset, or parasequence boundaries. Sharply defined surfaces across which there's a sharp lithologic change, even if impedance contrast is low
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What is toplap?
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the termination of upper surfaces onto a lower one.
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Is truncation a type on onlap?
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yes
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What do onlap and downlap tell us?
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give us evidence of regressions and transgressions.
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lapout geometries are useful for when paired with what?
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paired with seismic facies, they are useful to interpret depositional systems and facies.
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how will facies interpretations be enhanced?
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if there is a well log or core sample
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What happens to carbonates in lowstands and highstands?
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during lowstand, sea-level cuts off the factory (shuts it off). transgressive and highstand is the greatest carbonate sedimentation times
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3D seismic uses what kinds of data collections?
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inlines: collected parallel to direction of data acquisition and crosslines: collected perpendicular to data acquisition
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What is a reflection?
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the signal
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what is a relector?
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the surface. interpret reflector from reflection
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What did exxon theorize about sea level?
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that it had synchronous changes across the planet
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when were reservoirs/source rocks deposited according to exxon?
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reservoirs deposited during falls in sea level (lowstand). Ice ages has rivers and deltas spewing more sand over the shelf edge. Source rock deposited during rises. Greenhouse is more muddy because sand can't reach shelf.
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What does chronostratigraphically significant mean in Wheeler space?
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can separate everything below and above (lacuna) in time
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What are wilson cycles?
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super continent cycles that are 1st order changes in sea level
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what are second order changes in sea level?
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sloss sequences
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What was Vail famous for? who helped?
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establishing the first global sea level charts (heavily criticized) and Haq helped
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what do does a steep slope indicate about grain size on seismic?
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that it's most likely sand, because clay won't ever be that steep
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How did seismic stratigraphy help sequence stratigraphy?
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it led to a more theoretical understanding of how depositional systems change and are linked as a consequence of forcing parameters (accommodation/accumulation) Jervy and Posamentier 1988
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What does sequence stratigraphy equal?
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rock (facies) + time (wheeler diagrams)
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what is the formal definition of sequence stratigraphy?
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the study of rock relationships within a chronostratigraphic framework wherein the succession of rocks is cyclic and is composed of genetically related stratal units (include deltas with their delta fronts). The repacking of the rocks into their rock units bounded by unconformities and their correlative conformities (correlation of conformities and unconformitites)
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What do sequences consist of?
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depositional systems tracts and are interpreted in terms of accommodation/accumulations (includes all previous exercises we have done)
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What is posamentier and wagoner's definition of sequence strat?
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the study of rock relationship within a time-stratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition or their correlative conformities.
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What is Galloway's definition of sequence strat?
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the analysis of repetitive genetically related depositional units bounded in part by surfaces of non-deposition or erosion (genetic vs. depositional by using flooding surfaces, not erosional surfaces)
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What was posamentier and allen's definition of sequence strat?
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the analysis of cyclic sedimentation patterns that are present in stratigraphic successions as they develop in response to variations in sed supply and space available for sediment to accumulate
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What is catineau's definition of sequence strat?
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the analysis of the sedimentary response to changes in base level and the depositional trends (accommodations successions) that emerge from the interplay of accommodation (space available for sediments to fill) and sedimentation
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Why do we need sequence stratigraphy in junction with lithostratigraphy?
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lithostratigraphy only gives us the rocks, not the processes. Sequence stratigraphy is the surfaces
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Why does the relative sea level curve/ relative coastal onlap curve have a sawtooth pattern?
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because it reflects subsidence and relative sea level change
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What is mitchum's definition of a sequence?
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defined by unconformities and their correlative conformtities
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what is Galloway's definition of a sequence?
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sequences are bounded by max flooding surfaces
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What is Embry's definition of a sequence?
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sequences are separated into regressive and transgressive facies (quick and dirty method because he worked in Alaska with little sun, little time and bears)
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What is the most important surface according to exxon?
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the sequence boundary (subaerially exposed, eroded, unconformity, boundary)
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What is the forced regressive wedge systems tract also known as? what's the problem with it?
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the falling stage, early lowstand systems tract. it's not large enough to resolve seismically
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What is an unconformity according to Mitchum?
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a surface of erosion of non deposition that separates younger strata from older rocks and represents a significant hiatus. periods of erosion and non deposition occur at each global fall of sea level, producing interregional unconformities
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What is Van Wagoner's definition of unconformities?
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surface separating younger from older strata along which there is evidence of subaerial erosional truncation or subaerial exposure or correlative submarine erosion in some areas indicating significant hiatus. Forms in response to a relative fall in sea level. This is a much more restrictive definition of unconformity there is commonly used or used in earlier works on sequence strat. (sufficient, but not good definition)
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What is the length of the average stage?
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a few million years
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what is the smallest geochronologic unit?
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a biozone
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What does correlative conformity mean?
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that there's no hiatus or stop in deposition (or at least too short to notice)
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What are parasequences?
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the building blocks of sequences. they are genetically related strata bounded by flooding surfaces of their correlative surfaces. commonly characterized by shoaling upward facies succession
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what happens as a system progrades?
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the shoreline regresses
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When there's a transgression, what does it look like in a measured section?
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it shows an anomalous increase in sea level. there's an anomalous juxtaposition that violates Walther's law. It goes from distal to gradually proximal, but then gets deep suddenly again.
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what is order of events that occurs as a parasequence evolves?
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the delta occupies and creates a delta front sand. it abandons the area, attached barriers form from erosion of the headlands. subsidence occurs and barriers detach and have lagoons behind them. with further subsidence, the barrier islands become shallow marine shoals.
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What do progradational parasequence stacking patterns show? what is their gamma response?
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show downstepping, forestepping, seaward stepping, offlapping within each paraseuence. the gamma becomes sandier upward due to thickening sand units.
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What do the size of packages do in progradational sequences?
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they thin to thicken (distal to proximal)
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what do parasequence stacking patterns show when they are aggradational? what is their gamma response?
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vertically stacked paraseuences. show building, shale, building, then shale.
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What do retrogradational parasequences show? what is their gamma response?
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show backstepping, there's a gradual increase in marine vs. non marine. downstepping, forestepping, sea ward stepping and offlapping. their gamma response shows sand thinning as it's further up in the log. this is due to it becoming more shaley up-section
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what is the reason for learning the parasequence signatures in log profiles?
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we can predict their lateral variation from their log profile
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What might occur in all parasequence sets? What is wrong with this?
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regressive facies. it's debated
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are parasequence sets boundary surfaces more extensive than parasequences?
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yes
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parasequence set boundaries separate what?
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systems tracts
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How are systems tracts defined?
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physical location and lapout
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What drives the systems tracts relationships?
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baselevel, eustasy, and the A/S ratio
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What bounds the base of the LST?
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the sequence boundary
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when does the LST form?
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during rapid sea level fall
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What happens during the LST?
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there's subaerial exposure and fluvial degradation on the newly exposed shelf. Erosion is very complex.
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What marks the sequence boundary basinward?
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LST fans and deltas
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What happens near the end of the LST?
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the fall slows, and sea level stabilizes. depositional systems tracts deposit. rivers stabilize, shelf edge deltas and lobe fans form. valleys fill
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What is he HST bounded by?
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the MFS and SB.
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What happens at the beginning of the HST?
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baselevel rises rapidly, shelf edge deltas step back. the baselevel rise slows and stabilizes. the sediment starts to prograde onto the shelf.
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What happens at the end of the HST?
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Sea level starts to drop again, and there is degradation. We go into the FSST and then the LST. The HST is eroded (sometimes not preserved).
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What is the LST bounded by?
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the SB and TS
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What is the FSST?
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the falling stage systems tract.
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When and where is the FSST deposited?
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it's deposited during the time of rapid sea level fall.it's commonly deposited on the shelf and ramp style margins. no shelf-slope-break
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What is the BSFR? What is the problem with picking the BSFR?
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it's the basal surface of forced regression.there's a lack of subaerial accommodation and the SB becomes difficult to pick. sometimes referred to as the late highstand systems tract. (know diagram)
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What is the TST bounded by?
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the TS and MFS
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what do parasequences show in the TST? why?
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change from backstepping to progradational because the rate of accumulation is being exceeded by the rate of accommodation generation. shoreline transgresses
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What is debated about valley fill?
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many valley fills show transgressive facies fill, but the fill of incised valleys is interpreted as LST. only when the valleys fill, and the shelf floods is the TST defined
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What are condensed sections?
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coeval facies equivalents seaward of all systems tracts, so beyond the influence of terrigenous sedimenation. it's a thin unit that represents a lot of time
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what are some examples of condensed sections?
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chalk, chert, oozes, ect
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What is the gamma log response for a condensed section?
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very 'hot', faunal peaks
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what are condensed sections sometimes associated with?
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marine corrosion
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Where do the condensed sections occur?
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typically on the downlap facies. within the LST,and between the TST and HST
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What is a paraconformity?
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time is missing. can see this by missing biozones (need a small sampling interval for this, which can be expensive)
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what is a type 2 sequence?
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shelf margin systems tract. the unconformity is not exposed. There's a shelf slope break because the slope is never exposed. Rivers do not incise so there are no nickpoints
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What is missing in a type 1 sequence?
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a shelf-slope break
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What is most of the seismic stratigraphy work done on?
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passive margins
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Where is most sequence strat done and why?
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Foreland basins because they are typically subaerially exposed. there is no shelf-slope-break because the shoreface is the steepest point.
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What is the first step in sequence stratigraphic methodology?
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Evaluate previous work, regional seismic, and well log data
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What are the steps included in evaluating previous work?
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establish basin-scale, paleogeography, age, structure. orient x-sections along strike and dip, establish proximal and distal, and integrate paleontological data and chrono-stratigraphic data
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What is step 2 in the sequence strat methodology?
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establish the depositional facies (cores and well logs) to determine correlation styles
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why is it important to establish depositional facies?
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outcrops provide key insights into how facies are organized. Also seismic data shows regional scale
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What is the third step in the seq. strat. method?
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identify facies breaks which mark bounding discontinuities.
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what does a sharp sandstones top indicate?
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a marine flooding service
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what are some bounding discontinuities? what are these helpful for?
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erosional lags (bases of channels/channel belts/mudchips), truncation (bases of channel belts or incised valleys), exposure surfaces (paleosols, mudcracks), flooding surfaces (coals), and condensed sections (widespread lacustrine mudstones). helpful for reservoir modeling
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what is the 4th step in seq strat method? what is needed?
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establish facies correlation lengths from closely spaced data. use outcrop data and statistics
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what is the 5th step in seq strat method?
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expand correlations to larger scale. correlate anything you can, loop tie, and interpret lapout and stratal truncations, correlate key markers within mudstones
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What is the 6th step in the seq strat method?
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correlate muds
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what is the 7th step in the seq strat method?
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correlate the sandstones and explore for a reservoir
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What is the 8th, but optional step?
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make a time stratigraphic chart (wheeler diagrams). this helps find anomalous correlations
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what is the problem with using a datum?
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beds may be smooth, but it can distort the stratigraphic relationships.
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what is the best method when using a datum?
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use multiple datums. use sequence strat datum as opposed to a stratigraphic or structural datum.
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what are allostratigraphic units based on?
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bounding continuities and surfaces, not lithology
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what are lithostratigraphic units based on? what is the problem with this?
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defined lithologically heterogeneous units, but it ignores gradation b/w facies
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is the flooding surface a discontinuity?
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yes
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what is a safe assumptions if fluvial incisions are very large?
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that they are probably not channels, but more likely incised valleys filled with multi-story channels
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what does the HST show overall?
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fining up, low bioturbation (due to high sedimentation rates), then coarsens up due to deltas, then back into mudstones. lots of soft sed deformation
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what does the top of the HST show?
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roots due to subaerially exposure and bay/lagoonal facies due to flooding
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What is the bounding surface when there's a sandstone cliff with shale on top?
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typically the MFS
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what coarsens up then fines up?
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a flooding surface
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what happens in grain size at the sequence boundary?
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there's an anomalous increase in grain size
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Is the LST muddy?
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no it's typically very sandy
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What does attached vs detached forced regression relate to?
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the ability of shoreline sediments to keep pace with decreasing accommodation during fall
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What happens to the transition zone during a forced regression?
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it's a coarsening up transition that is eroded due to waves. so sand is deposited over mud.
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what is a sharp based shoreface indicative of?
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non-genetic relationship between underlying 'shelf' mudstones and overlying shallow marine sandstone
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What happens if the shoreline trajectory is less negative than the basin trajectory?
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a gradational base is created
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what are the types of valleys?
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river (incised valleys), subaerial rift valleys (structurally controlled), and subaqueous valleys (seascapes)
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are all valleys incised valleys?
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no
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How can we tell if a valley is created due to tectonics?
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there are typically angular unconformities. there's tilting of strata, complex truncation, and onlap. tectonics require deformation of some kind
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what in the typical EOD when there's a lack of bioturbation?
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fluvial influences
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how is the shoreface interpreted?
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gradationally based, top truncated, coarsening up, with a thin pebble bed in lower shoreface
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When is the ravinement surface exhumed? when is it overlain?
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it's exhumed during the early transgression and overlain by the transgression lag
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do shallow fluvial channels have a high preservation potential? why?
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no. because waves can erode 5-15m of stratigraphy, forming detached shelfs
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are ravinement surfaces flat? why?
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yes, because it has been planned off due to erosion. not cut by rivers
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why do deltas show low bioturbation?
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high sedimentation rates
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why do shelf mudstones show high bioturbation?
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low sedimentation rates
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what do deltaic depositional systems show?
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lobate to elongate geometry, radiating paleocurrents, basinward dipping internal clinoform beds, and upward coarsening faices successions
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are all forced regression sharp or erosional based?
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no
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what were the early fluvial stratigraphy models in the 70s, 80s, and early 90s based on? what was wrong with this?
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how much sand vs. mud there was. determined if the system was braided, meandering, ect. it created model driven interpretations
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what happens in strongly meandering systems?
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very isolated, disconnected sand bodies (reservoirs)
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what are the controls on fluvial architecture?
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channel migration, channel aggradation, and avulsion rate
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what are some examples of equilibrium profiles?
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graded river profiles, graded marine shelfs, and angle of repose
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If a buttress rises and falls due to sea level, what happens?
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the backwater length will aggrade or incise
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what is the backwater length dependent on?
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the slope
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are rises and falls felt past the backwater length?
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typically no
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when do the highest sedimentation rates occur?
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after a dry period
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what happens if a river doesn't have enough sediment?
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it wants to pick up more through erosion
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what is LGM?
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last glacial maximum
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what do rivers show in higher vs. lower slopes?
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lower slopes=meandering; higher slopes=braided
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In fluvial seq strat. what happens during the HST?
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there's no confinement, rivers are free to avulse and move anywhere. it is a very muddy system
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in fluvial seq strat, what happens during the TST?
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rate of transgression dependent, we see tidal influences in the valley fill (even though it's interpreted as LST; debated)
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in fluvial seq strat, what happens during the LST?
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valleys fill with river deposits. there's subsidence. since rivers are confined, there's a high net to gross (sand) channel deposit
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in fluvial seq strat what happens during the FSST?
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there's a bypass of sediment into shallow marine and deepwater systems. valleys form
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what happens during max flooding?
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the water table rises and there might be isolated limestones in the basin
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what do the upper valley fills show?
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more mud with tidal influence and bioturbation. This can cycle multiple times with transfer into the next incision in the FSST
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