1 - 2 Sequence Stratgraphy Flashcards
What do parasequences tell us ? (4.1, 4.2)
Parasequences record the creation and subsequent infilling of accommodation space, caused by low-amplitude relatively short term (Blackhawk 80ka, Viosca Knoll 30k) oscillations in relative sea-level.
Coarsen upwards (regression), capped by flooding surface (transgression). Asymmetry.
Parasequences are chronostratigraphical packages. They can be traced e.g. at Book Cliffs through many depositional environments.
What does parasequence stacking tell us? (4.2.1)
Stacking: longer term regression or transgression; balance of sediment supply and rate of creation of accommodation space.
– Progradational stacking: regression / progradation. Sediment supply > accommodation space. Can include forced regression.
– Aggradational: sediment supply and accommodation space balanced.
– Retrogradational: transgression. Sediment supply
What are the components of a sequence, and how may we recognise them? (4.3)
HST: aggradational - progradational.
Sequence boundary: erosion, bypass, proximal unconformity to distal correlative conformity. Base of incised valleys (BH, Nam).
FSST: downstepping, forced regression. Castlegate Sandstones.
LST: progradational - aggradational. Grouped with LSST in older literature.
Transgressive surface: onset of transgression. Can be minor unconformity due to changing wave energy intensity.
TST: retrogradational stacking. Infill of valleys (BH, Nam) coal formation due to high water table (BH), Goniatite mudstones (Nam).
Maximum flooding surface: most proximal extent of marine sediments; starvation, condensed fossiliferous beds and authigenesis distal.
Explain the use of seismic stratigraphy. (4.5)
Seismic reflection is a geophysical technique used to study sedimentary deposits on a basin-wide scale.
A seismic section comprises a series of seismic traces taken usually 25m apart along a line of survey, to provide a cross-section. The horizontal scale is two-way time, rather than depth.Seismic sections show clinoforms, formed along the boundaries of parasequences, as acoustic waves are reflected by the acoustic impedance contrast.
We can identify certain clinoforms as sequence surfaces, based on the termination of surrounding clinoforms.
Which terminations indicate (a) sequence boundary, (b) transgressive surface, and (c) maximum flooding surface? (4.5)
Sequence boundary: proximal erosional truncation giving way to top lap below; onlap above (these are FSST deposits).
Transgressive surface: onlap above.
Maximum flooding surface: downlap above (overlying HST deposits).
How may climatic processes affect eustatic sea-level? (5.1)
Icehouse: glacio-eustasy (60-80m if present ice melted). Growth of Quaternary sheets caused fall of 120m at rate of 1cm per year.
Greenhouse: thermal expansion (10m height for 10 degrees warmth).
How may climatic and tectonic processes affect sediment supply? (5.1.3, 5.2.5)
Increased precipitation and temperature increases rates of weathering and erosion. Vegetation cover: binds soils but chemical weathering.
Uplift also increases rates of weathering and erosion. Sediment supply from Himalayan oregony in 87Sr.
How may tectonic processes affect sea-level? (5.2)
Mid-ocean ridges buoyant. Break up of Pangaea in Cretaceous (250m+). Hays and Pitman (1973) got similar results to hypsometric.
Continental collision decreases continental area, so increases ocean extent.
Inhomogeneities in mantle: beneath southern Africa hot and buoyant so uplift 300m last 20Ma. Australia subsidence because more dense.
Local scale: uplift and subsidence.
Active faulting local and episodic.
Explain the importance of sediment compaction. (5.3)
Mudstones (dewatering) 80%.
Sandstones: 30%.
Delta lobe switching (e.g. modern Mississippi delta).
How can we recognise underlying controls from sediment architecture? (5.4)
Thick sedimentary packages and dominance of progradation: sediment supply.
Angular unconformities, abrupt deepening or uplift: tectonic.
Widespread interplate correlation and demonstrable synchroneity of sequences and sequence boundaries: eustatic.
Regular cyclicity: Milankovitch.
How do low-order cycles affect the development and preservation of higher order cycles? (5.5)
Tracts are preferentially developed and preserved in their equivalent sequence set (e.g. FSST in FSSS)
For example, HST parasequences are most common in Blackhawk Formation (an HSS).
This concept was used to construct hypothetical sequence curves in the Pennine Namurian, on the basis that some sequences were dominated by HST, some by TST and some by incised valleys.
How is sediment stratigraphy used to interpret the Pennine Namurian deposits south of Nottingham? (3.5, 4.4, 5.6)
Cycles of progradation and transgression, incision and fill of valleys. Connected with HST (progradation), SB (incision surface) and TST (valley infill, transgression, Goniatite mudstone horizons).Cycles dominated by HST interpreted as occurring in HSS; with incision: FSSS; and with thick TST: TSS. From this a low-order cycle is hypothesised.
How does Viosa Knoll aid our understanding of sequence stratigraphy? (6)
10km off shelf, past delta of Mississippi. 250m core, past 500ka. 15 parasequences.
Links tracts with wider context: climate and water depth.
- HST: interglacials (low d18O), warm water (fossils) 150m+ depth (fossils).
- FSST and LST: glacial (high d18O), cold water (fossils) as little as 20m depth (fossils).
- TST: shift to interglacial, deepening water.
How may we recognise HST deposits in the geological record?*
Falling rate of creation of accommodation space reflected in:
- siliclastics: aggradational to progradational stacking
- carbonates: upwards thinning
In Viosca Knoll, HST deposits associated with deep, warm waters and interglacial conditions. The thin deposits represent the toes of a prograding delta.
In the Pennine Namurian HST is represented by upwards thinning facies.
In Book Cliffs, HST are the most common, and associated with prograding wave-dominated delta systems.
How may we recognise sequence boundaries in the geological record?*
Unconformities, which may grade into correlative conformities distally. In carbonates: dissolution leads to karstic landscape or calcite cement.
Sequence stratigraphy.
E.g.
How may we recognise FSST in the geological record?*
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