Geology 326 lecture notes. (to find a specific lecture, use the Find command in your eb browser and search for the lecture code (e.g. lecture 1 = S06.326.L1).
S06.326.L1
Post-lecture notes for students: Some notes at the end of this first lecture pertain to the use of the North American Stratigraphic Code. The NASC, which is Appendix B in Boggs, is something that you should be familiar with. However, much of it is something that you can set aside as a reference. I would like you to read Section I (pages 654-657 in Boggs 3rd edition) for the midterm. This background will give you some basic understanding of the different types of stratigraphic units (see Appendix B Table 1).
Lecture 1 notes.
Course announcements:
Go over the course syllabus in class
Introduction: The development of stratigraphy as a science
Stratigraphy- the study of layered rocks as the record of Earth history
1) Time
1a) Relative (this all we had until start of 20th Century)
1b) Absolute
Radiometric dating of various kinds
Leads to the absolute calibration of other chronologic indicators
Geologic Time Scale
2) Environment- many environmental parameters can be read
Geography
Climate
Ocean conditions
Atmospheric conditions
Views of Earth by scientists
Note: 500 years ago if you wanted to be a scientist you’d be n the clergy.
Catastrophism – -Bishop James Ussher (1581-1656). Sudden, violent short-lived events sculpted the Earth into its present configuration. Calculated from bible that earth formed on Sunday October 23, 4004 B.C.
Idea of how Earth evolved- recognized that sedimentary layers found above sea level
Neptunism- Abraham Werner (1749-1817) the notion that the rocks all precipitated out of a primordial ocean attributed to the biblical flood.
Formation of rocks rich in SiO2 were said to be "acidic” because they were formed from silicic acid (H4SiO4). So terms “basic” and “acidic” igneous rocks are artifacts of neptunism.
Large caves under ground occasionally collapsed causing sea level to drop, and earthquakes to occur
Nicholas Steno (1638-1686)- Accredited with being the world’s first stratigrapher. Anatomist- compared living fossils with ancient fossils and realized that they had to be the same.
Recognized that hard pieces (e.g. fossils) found within other hard pieces (e.g. sedimentary rocks) meant that the rocks had to solidify at some date after the deposition of the fossils. This concept applied not only to fossils, but to mineral grains and layers or rocks (strata).
1669: Preliminary discourse to a dissertation on a solid body naturally contained within a solid
Steno recognized that strata were formed from sediments falling out of suspension in water, and so the layers were original deposited horizontally. Thus, any strata that were not horizontal were deformed by subsequent processes.
The presence of fossils in a rock led him to recognize that the fossil had to be deposited first and then the sediment around it had to be deposited- the fossil could not have grown in place, otherwise it would have been distorted by the preexisting materials.
This applied also to sedimentary beds, where the stratigraphically lower beds are seen to shape the layers above them. Steno recognized that this meant there was a definite order to the deposition of sedimentary layers, and led to his second law:
2) Law of Superposition- layers of rock are formed in a time sequence with the older rocks on the bottom and the younger ones on top.
This law is so fundamentally important to geologists that is does not only apply to sedimentary rocks; it also applies to metamorphic and igneous rocks, and to geologic structures. It is the basis for our being able to work out the geologic history of an area.
3) Law of Lateral Continuity. strata are not only laid down in order, but they are laterally extensive as well (a uniform physical process).
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Uniformitarianism- James Hutton (1726-1797).
“The present is the key to the past”
The physical processes that govern our world today can also explain the geologic history. However, what was needed was a LOT of time.
Plutonism – James Hutton (1726-1797)- rocks all formed from molten interior.
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(William Smith- 1769-1839)
Principle of Faunal Succession
William Smith- first geologic map
“Map that changed the world”
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Charles Lyell (1797-1875).
First Geologic Time Scale
Principle of Inclusions
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Rationalizing stratigraphic practices.
Field in chaos before the 1960s
No uniform practice in how to name stratigraphic layers, or what to call them.
I’ve seen some of this in the Italian literature today!
Hollis Hedberg- the development of stratigraphic codes.
Petitioned prominent scientists at the time with a substantial questionnaire to work towards formalizing the field.
This led to the development of the stratigraphic codes.
Outline specific procedures to follow in classifying and naming geologic units.
Several exist:
International Stratigraphic Code
North American Stratigraphic code.
These have tried to be in mutual agreement, and mostly they are.
Depending on what characteristic of the strata you are studying, you will need a different nomenclature to document it with.
Traditoinally 4 Categories of stratigraphic units
Lithostratigraphic- based on lithic characteristics (Navajo Sandstone)
Biostratigraphic- based on the fossil content (Discoaster Interval Zone)
Chronostratigraphic- based on age relations- a reference for all rocks
deposited at the same time (Devonian System)
Geochronologic- based on a division of time (Devonian Period)
Lithostratigraphic
Lithodemic- don’t follow the law of superposition (i.e. intrusions)
Magnetostratigraphic
Biostratigraphic
Pedostatigraphic- based on presence of soil horizons
Allostratigraphic- based on bounding discontinuities
2) Distinguished by Geologic age
Temporal categories
Geochronometric units- based on division of Geologic Time Scale
Difference between chronostratigrahic and geochronologic
Chronostratigaphic = relative ages
Geochronologic = absolute ages
Chronostratigraphy is the sand passing through the hourglass.
Geochronology is how much sand has passed through.
Requirements
S06.326.L2
Stratigraphic relationships and lithostratigraphy
Reading: Boggs Ch. 13
Topics:
Facies
Walther’s Law
Contacts
Lithostratigraphic units
Environment: suite of physical parameters that operate to produce a distinctive body of sediment that is characterized by textural, structural and compositional properties.
Facies: the distinctive body of sediment deposited in a particular environment.
Depositional environments generate sedimentary facies.
[[NASC Categories of stratigraphic units]]
Walther’s Law of succession of facies: Strata that occur in vertical succession also occur in lateral succession
[[Fig. 13.2]]
Def: Facies that occur in conformable vertical succession also occur in laterally adjacent environments
As environments migrate in response to shoreline migration, facies can migrate to the point of stacking on top of sediments from adjacent facies.
Transgressions: movement of a shoreline in a landward direction
=retrogradation
Leads to a fining upward sequence
Regressions: Movement of shoreline in seaward direction
=progradation
-leads to a coarsening upward sequence
[[e.g. buildout of a delta consider drawing something like Figure 13.14. However, consider whether you agree with the vertical presentation.]]
Note that the practical application of Walter’s Law offers the stratigrapher: strata are often studied in vertical succession rather than in horizontal succession; therefore, Walter’s Law provides you with the framework to study vertical successions and to identify unconformities. (i.e. stacked facies that are not from adjacent environments indicates removal).
Contacts
Lithostratigraphic units separated by Contacts: plane or irregular surfaces between different types of rocks.
[[Figure of contacts]]
Note: lithostratigraphic boundaries occur at lithic breaks. As such, they can occur vertically or laterally – a formation can occur below another one, or it occur adjacent to it where lateral facies changes occur.
Conformable contacts: Continuity of deposition, generally planar surface, can be abrupt or gradational (depending on rapidity of changing environmental conditions).
Between laterally adjacent lithostratigraphic units:
gradational
intertonguing- pinching or wedging out within another formation.
Progressive gradational contacts: slow, uniform change in lithology
Intercalated contact: gradational contacts with increasing number of thin interbeds up section.
Landward- truncate against erosional surface
Seaward-
Pinchouts: progressive thinning of units into extinction
Intertonguing: split into many thin units that pinch-out independently
progressive lateral gradation: same as progressive vertical gradation.
Unconformable: surface of erosion or nondeposition and represents a hiatus: a break in the geologic record
[[Figure 13.5: Cartoons showing unconformities]]
4 types
1) Angular unconformity- angular change
2) Disconformity- erosional surface
3) Paraconformity- disparity in age, contact obscure
4) Nonconformity- nature or rocks under the unconformity (igneous or metamorphic)
Lithostratigraphic Units
Unit: Informal name for a body of strata that are unified by some characteristic (e.g. sandstone unit identified as being made of sand).
If you have reviewed Appendix B, you will have read that the NASC subdivides the study of sedimentary rocks into different types of stratigraphy in order to focus on specific interests- such as the grains, fossil content, etc. which may address different questions about their history.
We’re going to start with lithostratigraphy: the study of rocks based on their lithic content.
Lithology- size, composition, rock type and color, clast shape
Lithostratigraphy- study of stratigraphic relationships of units based on lithology
Lithostratigraphic units include: sedimentary, metasedimentary, extrusive igneous rocks
Generally follow the law of superposition. (recall that lithodemic units don’t conform to the law of superposition).
(recall that lithodemic units do not follow superposition)
Based on establishment of stratotypes- the designated type unit.
Lithosome: units with uniform character and intertonguing relationships with adjacent masses of different lithology.
[[Slide showing Walther’s Law]]
Example: Lithosomes from a clastic delta might consist of the sandy delta front lithosome, the prodelta mud lithosome, and the gravel distributary channel lithosome.
Lithostratigraphic Classification
Naming Lithostratigraphic units
Formation= Fundamental unit of lithostratigraphic classification.
Mapable, lithologically distinctive unit (i.e. a lithosome).
alternatively: Formation may consist of several lithosomes. If so subdivided as follows:
Member: distinct interval that needs to be identified (i.e. the shale member of the Wilson Grove Formation).
Bed: smallest formally designed lithostratigraphic unit (e.g., Roblar Tuff bed in the Wilson Grove Formation).
On the grander scale:
Formations with unifying characteristics can be grouped into:
group: two or more formations
supergroup: two or more groups
Vertical and lateral successions of strata
Vertical successions
1) Lithologic uniformity- tends to occur with fine sediments in deep oceanic conditions
2) Lithologic heterogeneity- poorly sorted materials
3) Cyclic successions- many processes- varves, migrating environments, turbidites
autocyclic: changes that occur within a basin (e.g. storms)
allocyclic: changes that occur external to basin (e.g. climate and tectonic events)
[[Order of cycles]]
Different scales of allocyclic cycles [[Table 13.1]]
first-order cycles: 200-500 million years- continental scale events
Two during the Panerozoic
second-order cycles: 10s to hundreds of Myr.- volume changes in ocean basins
Sloss identifies 6 sequences in North America during Phanerozoic.
(see Boggs figure 13.9)
Tejas (youngest)
Zuni
Absaroka
Kaskaskia
Tippecanoe
Sauk (oldest)
third-order cycles : 1-10 Myr.- origin unclear due to poor regional correlation (perhaps caused by basin accommodation limitations??)
fourth-order cycles: duration ~0.2-0.5 myr. (I bet these don’t exist)
fifth-order cycles: 0.01-0.2 Myr. Ice age cycles. Astronomical in origin.
(Should I talk about here or later??)
Sedimentary Facies
Facies: Sediments of one type that grade laterally into another type in a laterally contiguous part of a depositional setting. (e.g. shale facies, sandstone facies).
Common practice to break up use of facies into:
1) Lithofacies: dividing the rocks based on lithologic characteristics
2) Biofacies: “ “ fossil content, irrespective of lithologic change
Ultimate goal of facies analysis: Environmental interpretation
Effects of climate and sea level on sedimentation patterns
Whole laundry list of causes and effects listed in Table 13.2. Note that time scales are different for each.
S06.326.L3
Chapters
Ch. 8: Environmental Interpretation
Ch. 9: Continental Environments
Depositional Environment: The geomorphic setting in which a particular set of physical, chemical and biological processes operates to generate a certain kind of rock.
and facies are the rocks produced by that environment.
As stratigraphers, we can go into the field and study facies in order to interpret environment.
Individual facies often cannot pinpoint an environment. It is necessary to study all facies that occur in a stratigraphic succession. Through this detailed study we can start to understand the environment.
Vertical Successions of facies:
Fining upward- refers to general fining of facies, as one would expect from a marine transgression.
Coarsening upward- general coarsening of facies- expected from a marine regression and progradation of facies.
1) Gross lithology
2) facies associations
3) sedimentary structures
4) fossils
1) Continental
2) Marginal marine
3) Marine
[[Table 8.2: Classification of depositional environments]]
4 types of continental environments
1) Fluvial (alluvial fans and rivers)
2) Desert
3) Lacustrine
4) Glacial
1) Fluvial (= alluvial) deposits
3 environmental settings for fluvial deposits
1a) Alluvial fans
1b) braided rivers
1c) meandering rivers
1a) alluvial fans
characteristics: steep slopes (1.5-25 degrees), poorly sorted sediments,
upper-, mid- and distal-fan
1a1) debris-dominated fans
1a2) braided fluvial fans
1a3) low sinuosity/meandering fluvial fans
-these three different types refer to how the sediment is transported across the top of the fan.
[[Fig. 9.5 alluvial fan cross section]]
note the convex upward cross section of alluvial fans
concave upward from head to toe profile.
distribution of sediment in radial channels
deposits characterized by coarsening upward sequences as fan progrades
1b) Braided river environment
low sinuosity
(sinuosity = length river/length river valley)
braided rivers form from very high sediment load, low sinuosity, many channels
[[Figure 9.9- braided river structure]]
high sediment load leads to deposition along side of river (lateral bar) in middle of river (longitudinal bar) and extending from edge into the stream channel (transverse bar)
forms sediments that have many superimposed channels filled with trough cross bedding.
Cross beds dip downstream
Note about cross bedding:
Two general shapes of cross beds:
1) tabular cross beds
2) trough cross beds
Note that these refer to the shape of the contacts, not the shape of the foreset beds within the cross beds.
1c) Meandering River
[[Fig. 9.13- meandering stream characteristics]]
features: single major channel, migration side to side, causes trough cross-bedding to form. Adjacent environments get vertically stacked: lag deposits, point bar deposits, overbank deposits.
5 Depositional settings within meandering river system:
1) main channel – coarse grained
2) point bars- sloping surface on the convex edge of the river- cross-beds
3) natural levees- form on concave side of meander loops- floods dissipate energy quickly and deposit sediment.
4) floodbasin- overbank deposits during floods or following a crevasse splay (breach of the levee)
5) oxbow lakes (abandoned channels) generall fine sediments from overbank flooding.
Thalweg = deepest part of stream channel
2. Eolian desert systems
locations: deserts are in tropics N and S of equator, and in rain shadows
Why in tropics? Hadley circulation. Equatorial low = rising moist air, travels away from equator and is deflected by coriolis to the east. Dry air descends, return flow to equator dry.
Deposits:
1) dust = loess transported in suspension to far away places
2) sand = well sorted, transported by saltation
3) lag deposits = gravel and larger = residue after deflation = desert pavement
subenvironments of desert:
[[Figure 9.18- shows all three together]]
1) dune
2) interdune
3) sand sheet
Dunes = primary source of wind transport and deposition
interdune= can contain other types of sediment transport (e.g. streams)
sand sheet- form on the margin of the dune fields.
Identified by wind directions
[[Figure 9.20- graphic of dunes by wind directions]]
Barchan dunes = unidirectional wind. Well developed lee side, single slip face.
Transverse dunes = Barchan with higher sediment supply
Blowout, parabolic = controlled by vegetation cover
linear, reversing dunes= two directions of wind
star dune= many directions of wind.
Interpreting environment from the number of wind patterns an interesting idea. Presently there is a winter and a summer Monsoon over Africa and so there are many reversing dunes there.
Wet = high water table which stops the transport of sand= preservation of foresets.
Dry = low water table which doesn’t affect the transport of sand.
3. Lacustrine environmentsOpen lakes- outflow= mostly siliclastic deposition
Closed lakes- no outflow- mostly chemical sedimentation
Depositional processes similar to marine environment
Exceptions:
1) wind energy level low- wind doesn’t have the fetch
2) sedimentation rates higher in lakes
3) lakes are tideless (no oscillatory ripples, tidal flats)
Varves- alternating light and dark colored layers
Depositional facies of lake look like a regressive process- the shoreline progrades into the basin. Coarsening upward sequence
Recognition of lake environments- absence of marine fossils and presence of fresh-water fossils, desiccation cracks.
[[Figure __ Gilbert-style delta]]
4. Glacial environments
Are composite environments- include fluvial, eolian, lacustrine environments
Glacial environment- all areas in contact with glacier
top= supraglacial
bottom= subglacial
within= englacial
around edge= ice contact zone
Proglacial environment: in front of the glacier
Glacio-fluvial, lacustrine, marine environments
Glaciofluvial= braided streams
glaciomarine= dropstones from ice-rafted debris
Diamicts: deposits from glaciers: extremely heterogeneous
Glacial Facies:
Continental ice facies: unstratified diamicts- moraines
[[Figure 9.37 end, lateral, medial, ground moraines]]
S06.326.L4
(This lecture only covered deltas; the other depositional settings were covered in the next lecture)
Principal depositional settings:
A. delta
B. beaches, strand plains (multiple parallel beaches w/o lagoons or marshes), barrier bars (barrier islands)
C. estuaries
D. lagoons
E. tidal flats
Generalitiesabout the Marginal marine environment (this should be a discussion with the students- ask them to help fill in the following information about forces and geometries in the marginal marine environment)
Boundary between terrestrial and marine environment is a line.
[[draw line on board and label one side terrestrial and the other marine]]
Note that the geometry of marginal marine environments tend to have long dimensions parallel to the shoreline.
Many of the directional indicators will be perpendicular to the shoreline.
Physical controls on marginal marine environment
Terrestrial controls: fluvial input (on diagram above this becomes a vector pointing from terrestrial toward marine environment
Marine control: wave activity; tidal variations (on diagram above this becomes a vector pointing from marine toward terrestrial environment.
Think about the magnitude of these competing vectors. The one that is largest will determine how sediment delivered to the marginal marine environment is deposited (and potentially reworked)
Some of these depositional settings are associated with coastline migration
Some of these are characteristic of transgressive (retrograding) coasts (estuaries and lagoons)
Some are characteristic of prograding coasts (deltas)
Note: nonalluvial deltas= those deposited by volcanic processes, such as lava flows and pyroclastic flows.
1. River deltas
[[take figure from old version of Boggs that shows these three]]
Main controls: river (and thus sediment) input
wave energy
tidal energy
These controls lead to the classification of deltas as:
a) Fluvial-dominated
b) Tide-dominated
c) Wave-dominated
a) Fluvial-dominated delta- how water and sediment interact with basin is based on density:
i) homopycnal flow (equal density) lead to rapid mixing and sediment fallout to form Gilbert-style delta.
[[Figure 10.6- Gilbert Style delta with topset, foreset and bottomset beds]]
Note that deltas forming in a lacustrine environment will be Gilbert-style deltas. The absence of tides, significant wave activity, leave the control up to the sediment supply.
ii) Hyperpycnal flow (occurs at base of water column- forms turbidity currents.
[[sketch plume of sediment-laden water flowing near base of water body as turbidite]]
iii) Hypopycnal flow (occurs on top of the water column- leads to flocculation (neutralization of – charged clay by cations in water)
sediment dispersal over a larger are so forms a lower angle foresets than do Gilbert-style deltas (i.e. about 1o vs. 10-25o).
[[sketch plume floating at water surface out over the delta]]
b) Tide-dominated deltas
Result in bidirectional flow
distributary mouth bar reworked into series of ridges that extend into the subaqueous deltafront platform. (these are intratidal channels)
c) Wave-dominated deltas
distributary mouth deposits are reworked by waves
get redistributed along delta front by longshore currents to form wave-built shorelines such as barriers, beaches and lagoons.
d) Side-category of delta : Fan delta
This involves an alluvial fan intersecting the shoreline and forming a delta.
[[Figure 10.15. Shows the main environments and subenvironments of a delta]]
Coarsest level is divided into two parts:
A) Subaerial delta plain - >>(bigger than) subaqueous part.
Range = all parts of the delta above low tide level
Upper delta plain (supratidal- only river processes)
environments: braided+meandering channels, lacustrine, floodplain
Lower delta plain (intertidal- both river and marine processes)
Note that width is determined by tidal range
environments: distributary channels, natural levees, interdistributary bays, swamps, marshes, crevasse splays
[[Figure of Delta Front from old Boggs]]
B) Subaqueous delta plain
Range = below low tide level
Delta front (from below low tide to wave base (~10 m)
high-energy wave processes- sediment reworked and winnowed, leading to normal grading.
Deposits: sand ± gravel as distributary mouth-bar deposits grading seaward into finer lithofacies. Strong wave activity will rework distributary deposits into cross-bedded delta-front sheet sands.
Prodelta- the deeper part of the delta that is unaffected by wave activity.
Fine silt and clay derived from the winnowing at the delta front.
Mud diapirs occur here- mud squeezed upward by sediment loading.
Differences between lacustrine and marine subaqueous delta plains: less tidal and wave activity in lacustrine environments leads to less sediments winnowing at the delta front.
Progradational phase- coarsening upward vertical succession of facies.
Transgressive phase- as ocean crosses the delta plain, erosion by waves and tidal currents become dominant
sediment influx from river decreases as it is trapped landward.
Useful criteria:
1) geometry- wedge of lens shaped in cross section
2) lateral facies relationships- shallow marine sand grades landward into nonmarine sediments and basinward into fine-grained clay.
3) Vertical successions of facies- stacking of facies shows progradation or retrogradation progression.
4) sedimentary structures and fossils- river-dominated = unidirectional ripples (asymmetric), tide-dominated = bidirectional ripples (symmetric). Marine, brackish or nonmarine fossils.
5) structural attitude of deltaic sequences: observable in seismic profiles as “clinoforms”
S06.326.L5
Karner ate pavement this day; no notes.
S06.326.L6
Principal depositional settings:
B. beaches, strand plains (multiple parallel beaches w/o lagoons or marshes), barrier bars (barrier islands)
C. estuaries
D. lagoons
E. tidal flats
Final word on deltas
Useful criteria:
1) geometry- wedge of lens shaped in cross section
2) lateral facies relationships- shallow marine sand grades landward into nonmarine sediments and basinward into fine-grained clay.
3) Vertical successions of facies- stacking of facies shows progradation or retrogradation progression.
4) sedimentary structures and fossils- river-dominated = unidirectional ripples (asymmetric), tide-dominated = bidirectional ripples (symmetric). Marine, brackish or nonmarine fossils.
5) structural attitude of deltaic sequences: observable in seismic profiles as “clinoforms”
Summary of modern beach deposits:
geometry: narrow body of sand
Primarily well-sorted medium to fine sand
Parallel laminations dipping seaward, striking along coastline
Trough cross beds dipping landward, seaward, and alongshore
± worm burrows, shells
[[Figure 10.29 cross section of the beach and nearshore zone with sedimentary structures shown below]]
beaches: attached to land
barrier islands: separated from land by lagoon, estuary or marsh
Strand plain: Seaward building series of beaches that result from very high sediment supply: beaches prograde as a consequence of high sediment supply coupled with relatively stable sea level
lagoon: shallow stretch of seawater with partial communication with open ocean- blocked by barrier island.
estuary: fresh water-salt water mixing involved (the geomorphic cause for the lagoon is the drowned river valley).
marsh: subareal
Effects of tides: High tidal ranges (macrotides) lead to dispersal of wave energy and so barrier islands do not form.
Backshore: landward of the beach berm (eolian processes dominate)
Foreshore: intertidal zone (swash zone) (+backwash)
form parallel laminae dipping several degrees seaward.
Shoreface: (aka nearshore) below low tide
Upper (aka surf zone): multidirectional trough cross-bed sets (due to bidirectional translation waves and longshore currents leading to migration of ripples). Basically, the upper shoreface has a lot going on!
Middle (aka breaker zone): Like the upper shoreface, the middle shoreface has bidirectional translation waves plus longshore transport; this is where the longshore sand bars will develop.
Lower (aka outer shoaling zone): Significantly lower energy conditions here lead to development of nearly horizontal planar laminated sand and landward dipping ripples.
Offshore transition affected by storm waves only. Hummocky cross-stratification also occurs here (thought to be storm-driven cross beds where high sediment delivery and oscillatory waves create hummocks (convex-upward) and swales (concave upward) cross stratification.
Offshore: unaffected by storm waves: Pelagic sedimentation ± turbidites
[[Figure 10.30]] Vertical succession of facies in a prograding beach
Use this figure to give students a guide for assessing the different beach environments, noting that there are overlapping characteristics in adjacent subenvironments.
Back-barrier deposits:
Washover deposits: These are landward of the beach and backshore subenvironments. These develop during storm surges which overtop the eolian dunes, generating planar beds and landward-dipping cross beds.
Tidal Channel deposits: Where tidal channels breach the barrier island
Ebb and tidal deltas: Form by tidal processes, combined with sediment delivery source.
Tidal Flats:
If wave-dominated: Longshore currents (upwash directed at angle, backwash = gravity)
geometry, lateral contacts, brackish fauna can identify them.
General shape: Elongated perpendicular to coastline.
-extends from tidal facies to seaward limit of coastal facies.
[[Figure 10.35. the 7 types of estuaries- based on physiographic characteristics]]
depends on the amplitude of tides: the higher the tide, the better the vertical mixing of fresh and salt water in the estuary.
External factors affecting estuary very similar to those affecting deltas: can be tide or wave dominated estuaries, or a combination of the two.
Features associated with each:
Wave-dominated: Forms barrier across inlet which restricts wave action into the estuary. Leads to a large quiet area where fine sediment is deposited
Tide-dominated: Tidal scour prohibits the formation of a full barrier- tidal influence in the estuary is stronger than in a wave-dominated estuary, and so coarser materials deposited throughout.
Wave and tide dominated- you get a combination of features of the two.
Note that estuaries also have fluvial deltas in them- an example would be the Chorro Creek delta in Morro Bay.
General shape- elongated parallel to coastline.
less affected by fresh water than estuaries (i.e. they are not restricted to drowned river valleys).
Degree of communication with ocean used to define them as choked, restricted, or leaky. Choked can lead to increased salinities and deposition of evaporite minerals.
Fauna in Lagoons often low diversity, as many open-ocean fauna cannot handle changes in salinity which might occur in a lagoon.
Low energy environment, low terrigenous input, can produce chemical sediments (evaporites).
Distinguishing features of lagoon vs. estuary. Estuaries have greater clastic input, whereas lagoons have primarily marine sedimentation.
D. Tidal Flat: sand to mud
exposed by the rise and fall of tides.
sediment commonly is delivered by the sea rather than the land.
gross morphology: flat tops except for tidal channels
Three zones:
subtidal: highest tidal velocities- coarsest material deposited here in tidal channels
intertidal: subaerially exposed daily- this is where you’d get deposition of ripple cross-laminated sand (during flood tide) and mud (during slack tide). Source of sediment (either land or sea) determines shape of ripple marks.
supratidal: finest material- only deposited during extreme high tides. Often subareally exposed…leads to highly evaporitic Sabkha environment in arid regions.
Common features of tidal flat environments:
Mudcracks (supratidal and intertidal zones)
lenticular and flaser bedding: alternating sand and mud (latter-slack tide)
ripples (shape and direction of cross bedding depends on the strength of the ebb and flood tides, and from where the sediment is being delivered.)- can be herringbone, or have reactivation surfaces.
[[Figure 10.50: reactivation surfaces in tidal ripples.]]
Note that tidal flats are fed from the sea and not the land. This means that sediment is delivered toward the shoreline, not away from it. Consequently, Shinn described them as river deltas turned inside out.
Channels in tidal flats accumulate laterally and meander just like meandering streams. Resuspension of sediment in tidal channels during flood tides is the source of sediment that gets deposited in the intertidal and supratidal zones. Consequently, the closer you get to shore (and hence farther from the tidal channels) the finer the sediment that accumulates.
Dan Jones once asked what the interiors of symmetric ripples might look like. The answer depends on the sediment source. In tidal flats the sediment is provided primarily by the sea, and so cross-beds form dipping inland and are formed during flood stage.
Vertical succession of facies:
[[figure 10.51- progradational stacking of tidal flats]]
Note that this leads to a fining upward succession, which is unlike what you get from a prograding delta
This also addresses Dan Jones’ question about the interior of tidal flats. Herringbone cross-stratification forms when sediment is provided during both flood and ebb tides.
Identifying an ancient tidal flat system:
1) Bimodal current directions
2) Tidal rhythmites
3) Channels and mudflats adjacent to each other
4) Reactivation surfaces, flaser bedding
5) Erosional contacts frequent
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Ch. 11: Siliciclastic marine environments
[[Fig. 11.1: Cross section of active vs. passive margins]]
Passive continental margin:
shelf – about 130 m deep, 1 degree dip
shelf break- shelf margin barrier
slope- 4 degree dip, submarine canyons
rise- accumulation of turbidites
Ocean floor = abyssal plain volcanic hills and seamounts
mid ocean ridge
also note: tectonic deformation = normal faulting = basin formation. Leads to lower input of clastic sediment to shelf. We’ll see this is sequence strat. Lab
Active continental margin
narrow shelf
rise is missing
trench instead
also note: tectonic deformation = convergence, so compressive deformation and high clastic input to shelf
Environments:
Based on water depth:
1. Neritic zone (shelf)
Neritic environment
pericontinental (marginal) sea
Epicontinental (epeiric sea)- when seas cover continents.
difference in sediment source: Pericontinental receive sediment from one side, epicontinental get is from most sides.
Note that modern environment not representative; sediment transport, grain sizes more variable because of sea level changes and glacial processes.
Several mechanisms:
1) Waves and storms (80% of modern shelves)
2) Tides (17% of modern shelves)
3) Oceanic currents (3% of modern shelves)
When in equilibrium, shelf wedge of sediment builds up to wave base, then additional sediment bypasses shelf.
Shelves are mostly in sedimentary equilibrium conditions, where new supplies of sediment bypass the shelf and are deposited on the slope. However the delivery of sediment in the seaward direction has been difficult to explain.
Sedimentary features: Oscillatory flow and longshore transport leads to multidirectional cross-bed bedforms.
2) Tide-dominated shelves
Bidirectional currents will drive sediment in the direction of the stronger current.
If currents subequal, Herringbone cross stratification develops.
Sedimentary features:
sand waves: very large scale dunes several meters tall, 10s of meters wide, cross sectional shape determined by strength of tidal currents.
Tidal sand ridges: sand ridge 40 m tall, 5 km wide, 60 km long, long axis parallel to current direction
3) Ocean current-dominated shelves
Unidirectional Surface currents directed by coriolis forcing into clockwise-rotating gyres in the northern hemisphere
e.g. Gulf Stream along eastern coast of the US
Sedimentary features:tend to affect the outer shelf mostly with unidirection bedforms.
Identification of ancient shelf sediments
Note that present-day shelves very young and have had lots of relict sediments left on them from the last Ice Age.
How Identify? Tabular shape, lateral dimensions, relatively uniform bedding, storm beds (hummocky cross-stratification, large and small scale cross beds (large = dunes), bioturbation features (Look at Fig. 11.12).
Facies changes on shelf associated with sea level change
Transgressions= fining upwards successions,
Regressions= coarsening upwards successions. Removal of accommodation space causes more sediment to bypass shelf.
2. Oceanic zone (from shelf break and deeper)
Oceanic Deep Water environment
[[Fig. 11.26 = geographic distribution of deep-sea sediments]]
[[Table 11.1- principal kinds of deep sea sediments]]
Sources:
terrigenous sediments
hemipelagic muds- nepheloid and plume deposition ± wind-blown materials and biogenic sediments (siliceous and calcareous). Poorly laminated bedding with abundant bioturbation; deposited on slopes where terrestrial and marine materials mix…landward of pelagic environments.
turbidites- sole markings, Bouma sequence (Fig. 2.18), cone-shaped fans. Recall upper and lower flow regimes identify the upper planar laminated beds and antidunes from the lower flow regime ripple laminated beds and planar laminated beds. E= interturbite. Fans have their own set of facies which are very similar to alluvial fan systems from the terrestrial environment: channel-levee system, overbank sheet deposits, mqass-transport complexes, slumps and debis flows.
glacial-marine- Ice-rafted debris, poorly sorted, striated and angular clasts.
slump and slide deposits.
pelagic sediments- organic materials slowly settle out of water column
oozes- significant amounts of biogenic remains
calcareous ooze- forams, coccoliths <4500m
siliceous ooze- diatoms (hi lat.), radiolaria (lo lat.) >4500m
allochthonous deep-sea carbonates: storm reworking from shallow areas into basin.
ancient deep sea sediments- poor record because many subduct, and are deformed when accreted to continents. General shape = large tabular or blanket shaped deposits which may be underlain by ocean crust.
associated with ophiolite sequences Peridotite, dunite, gabbro, sheeted dikes, pillow lavas.
Modern deep sea sediments studied by deep sea cores and seismic profiling, which identifies unconformities, changes in lithofacies,
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Biostratigraphy
Boggs Chapter 17
Recall that William Smith used fossils along with lithology to work out the early srtatigraphic principle of faunal succession:Fossils are deposited in a definite order
Biostratigraphy- Characterization and correlation of strata based on fossil content.
Biostratigraphic unit- a body of strata unified by its fossil content which distinguishes it from adjacent strata.
Not all rocks contain fossils and so the use of biostratigraphy has its limits.
The lateral and vertical extent of an identifying fossil of a unit must be identified. Any strata within that zone, regardless of fossil content, can be assigned to it.
life or death position? Are bivalves preserved as articulated or disarticulated specimens
in place or transported?
Reworked into other assemblages?- both into younger and older strata by bioturbation.
Objectives:
1) to subdivide lithostratigraphic units into smaller intervals of “time” for correlation. Formations can be long-lived and boundaries can be time-transgressive.
2) to place those strata into chronostratigraphic position (Principal of Faunal succession)
3) to characterize uniquely the depositional environment (note that lithostratigraphic units may not identify unique environment)
Note that Charles Lyell first subdivided the Tertiary into four subdivisions based on the ratio of living to extinct fossils in the rocks.
Newer Pliocene: 90% extant
Older Pliocene: (More recent) 33-50% extant
Miocene: (less recent) 18%
Eocene: (dawn of Recent) 3.5%
Now:
EPOCH
(cene=recent)
Paleocene= (old recent)
Eocene= dawn of the recent
Oligocene= slightly recent
Miocene= minor recent
Pliocene= more recent
Quaternary= Ice Age time we live in.
Pleistocene= most recent
Holocene = most most recent
Biostratigraphic unit boundaries may not coincide with lithologic boundaries.
This is particularly true for planktonic organisms… their living environment is the surface water, and so what gets deposited on the sea floor doesn’t affect them.
d’Orbigny door bin ee
Stage- group of strata containing the same fossil assemblage.
originally boundaries were last occurrence data, interpreted to be global catasptropic evens.
Detailed study required subdivision of stages into smaller units
Zone- invented by Oppel. Studied the ranges of individual species through rock units and recognized that strata were composed of overlapping ranges of fossils.
Joint occurrence of species was his ‘Zone’
The coexistence of a group of species probably represents a well-defined, narrow interval of geologic time.
[[Figure 17.2]] or just draw it
Oppel used index fossils- distinctive fossils- to name the zone
Delineated at top by first occurrence of index species of the stratigraphically higher zone
Delineated at base by first occurrence of index species of that zone
Zones are geographically limited: where they occur= biogeographic province
Biozone- the fundamental biostratigraphic unit (akin to formation in lithostratigraphic units).
Subbiozone (subzone)- recognized subdivision of biozone
NASC subdivision
3 Types of biozones: Interval, assemblage and abundance zones
[[Figure 17.3: Principal kinds of interval zones]]
1) Interval Zone: the interval of rock between the lowermost and uppermost occurrence of taxa:
a) Interval between the lowest and highest occurrence of a single taxon (ISG= Taxon range zone)- both upper and lowermost occurrence based on single taxon.
good time correlation with taxon range zone if the taxon is short lived.
b) Interval between lowest occurrence of one taxon and highest occurrence of another taxon (a.k.a. b) Concurrent range zone- based on first occurrence on one taxon and last occurrence of another taxon, or c)Partial range zone= last occurrence of one and first of another…with no overlap = barren zone)
c) Interval between successive first or last occurrences of two taxa
good time markers because the interval between the two occurrences will be synchronous at two localities. Note that the FAD are better than the LAD because radiation tends to be faster than extinction.
2. Assemblage zone- a biozone characterized by the association of three or more taxa.
particularly good for working out environments, since it is based on multiple taxa.
[[Figure 17.4- assemblage zone and Oppel assemblage zone]]
a. composite assemblage zone- incorporates two contemporaneous assemblage zones.
b. Oppel zone- assemblage zone whose boundaries are based on two FOD and/or LOD of the characterizing taxa.
3. Abundance zone- identified by a quantitatively distinctive maxima of abundance of one or more taxa.
Not necessarily good for time correlation as maxima can be caused by local ecological conditions.
[[Figure 17.5- abundance zones]]
Outlined in Appendix B
Compound name including the kind of biozone and the name of a characteristic taxa that are restricted to that zone
e.g. Globorotalia truncatulinoides abundance zone
Taxonomic Classification
Biological definition of species (a community of interbreeding things that pass along genetic material….difficult to apply to fossils.
So use skeletal morphology. Problem- not all members of a species look the same.
What causes species to change?
1) natural selection- chance mutations
2) environmental change- induces stress.
example- Neogloboquadrina pachyderma
cold water = left coiling
warm water = right coiling
First appearance
Last appearance
Extinction- all dead
pseudoextinction- evolves into new species so lineage retained.
Criteria: independent of their environment (they are robust), fast evolving, geographically widespread, abundant, readily preserved, easily recognizable.
Gradualism (phyletic evolution): evolution occurs gradually- steady transformation
Punctuated equilibrium: species unchanged for long time (stasis) and then change rapidly from small, isolated populations
-may be function of environment- stressed environment may lead to isolation and need to change.
Depending on which you believe, it could be difficult to delineate boundaries between species.
To get some idea of species longevity:
[[Table 17.2: Mean species duration]]
[[Fig. 17.9]]
Ord-Sil = brachs and bryozoans hit hard
Late Devonian= corals
end Permian= trilobites, fusulinids
end Triassic= conodonts
K-T= dinos, ammonites
Paleobiogeography
Biogeographic provinces- areas where certain taxa exist.
Physical and chemical barriers restrict dispersal
e.g. land restricts marine organisms, deep oceans restrict benthic foraminifera
temperature, sea level changes, plate movements
3 types
planktonic- suspended in shallow water- not so useful for environmental interpretation because fall out of their environment when die- they reflect habitat of pelagic realm.
nektonic- animals able to swim freely.
benthonic- on bottom- useful for environmental interpretation because found in their habitat.
assemblage zones and abundance zones may be diachonous when traced laterally
Assemblage zones correlation can be difficult because above and below the limits are transition zones that contain a part of the assemblage.
Abundance zones often refer to local conditions that favor growth and preservation.
interval zones (esp. with FAD) can be nearly synchronous.
[[Figure 17.18. Graphical correlation method]]
Compare two stratigraphic columns on opposing axes.
Mark the intersections of biohorizon (FAD, LAD) and then fit a line to these data.
Straight line= uniform sedimentation rate is not always a reasonable assumption, particularly if there are changes in gross lithology.
Deviations from the line could mean:
1) inaccurate stratigraphic data
2) different species appear and disappear at different times from the sections
3) Identification of hiatuses
you can look for patterns to see what’s going on.
Note: Can be used to find best FAD for taxon!
[[Fig. 17.19: variations in correlation line]]
Biogeographic Correlations: [[Fig. 17.21]]
Right vs. Left abundances of Globorotalia truncatulinoides tell you about geographic regions with similar environmental conditions.
Asides in the chapter: Deterministic (cause and effect) vs. probabilistic (random or stochastic) evolution.
My guess: Both. Impacts and plate tectonics are deterministic effects, whereas mutations are probabilistic.
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Boggs Ch. 16: Magnetostratigraphy
Inclination: The angle that the magnetic field is dipping into the Earth (at 90 degrees at the poles (+ for down and - for up), and 0 degrees at the magnetic equator).
Declination: the angle between the geographic north pole and the magnetic north pole, measured clockwise.
Normal Polarity: If the magnetic field is aligned like today
Field lines point in at North Pole and out at South Pole
Reversed Polarity: If the magnetic field is aligned opposite to today’s field.
Magnetostratigraphy: the study of rocks based on the remnant magnetic properties.
Magnetostratigraphic unit: a body of rock that is unified by some magnetic property (could be reversal, could be something else).
What causes remnant magnetization?
The ferromagnetic minerals: Fe2O3 Fe3O4 FeS2 plus some others (griegite)
Natural remnant magnetization(NRM): fossil magnetism.
In igneous Rocks:
Curie temperature: the temperature at which magnetite seals in its magnetic signature ~540C for magnetite.
In igneous rocks this is known as: thermal remnant magnetization(TRM) If magnetite is in melt, the atomic-scale magnetic fields within the grain can align themselves with the Earth’s magnetic field (note that the grains do not physically rotate).
Other rocks can be heated above the Curie Temp. and so lose their NRM.
In sedimentary rocks: detrital remnant magnetization mineral grains are already below the Curie temperature and so the grains behave like bar magnets…if the grains are free to rotate, then the magnetic properties will rotate the grains until they are aligned with the magnetic field. Sometimes this is known as Depositional Remnant Magnetization(DRM).
Then, if the sediments compact, the grains will seal in the magnetic field.
Note that this is called the “lock in depth”. The interesting question for doing high-resolution paleomag work on the B-M reversal is addressing this lock-in depth question.
Growth of new minerals that record a different field of magnetization. (Chemical remnant magnetization (CRM).
How sampled
1) oriented core samples
2) oriented hand samples
3) well core samples (presumed to be taken vertically but without declination use).
Many rocks have subsequent magnetization imposed on them.
Magnetometer: a device that incrementally eliminates the magnetic fields in a rock. This is done in order to find the original magnetic field direction preserved in the rock (the assumption is that the original signature is the strongest, and so will be there if the other components are removed. If not, then the samples do not provide good geomagnetic field data, which happens.)
Spinner Magnetometer: sample and or magnetic field applied in randomized way to leave magnetization homogeneous distributed.
Several ways that this can be done:
1) thermal demagnetization: increase temperature to cause low-temp. minerals to realign with the present magnetic field.
2) alternating field demagnetization: This randomizes the magnetic field in mineral grains, thereby removing their contribution to the paleomagnetic signature.
3) Chemical demagnetization: if the mineral that is affecting the signature is identifiable (say by microprobe) then it could be possible to treat the sample with acid to remove that component,
Utility of paleomagnetism:
Synchronous worldwide- not subject to local conditions, which affect other types of stratigraphy (i.e. biostrat, lithostrat.). Even isotope strat, can have delays from one place to another of 2 kyr. ocean mixing time).
Nomenclature and classification
Magnetostratigraphic Unit: general term for any body of rock unified by some magnetic property.
Magnetostratigraphic polarity unit: based on the magnetic polarity.
Hierarchy:
subzone, zones, superzones.
Polarity zone- the fundamental polarity unit for stratigraphic sections
Brunhes, Matuyama, Gauss, Gilbert zones – refers to magnetostratigraphic polarity units
Chronostratigraphic equivalent: Brunhes chronozone
Geochronologic equivalent: Brunhes chron
Other measured Properties:
Magnetic susceptibility: the ratio of induced magnetization to the inducing magnetic field, is directly proportional to the quantity of strongly magnetic minerals in a sample.
Paleoclimate uses of magnetic susceptibility: Climate response can lead to different effects on magnetic susceptibility. production rate of magnetic minerals is related to climate. In some environments cold weather can lead to anoxic bottom water conditions in lakes, which will keep Fe in its reduced form (soluble). Then when the lake warms the bottoms get oxygenated and so the Fe goes into oxides. Hence, alternating climates can be mirrored by the magnetic susceptibility.
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Magnetostratigraphy continued
Examples:
Loess: This correspondence showed clearly that these windblown (eolian) deposits potentially contain a high-resolution record of climate change. Relatively high magnetic susceptibility characterizes the paleosols that formed under relatively warm, humid interglacial periods, and it correlates with negative O values in deep-sea sediments. In contrast, relatively low magnetic susceptibility characterizes loess deposited under relatively cold, dry glacial periods, and it correlates with positive O values in deep-sea sediments. (O is the per mil deviation of the relative proportion of the isotopes O and O in a sample from that in a water standard, the Standard Mean O
Geomagnetic Polarity Time Scale
Things to discuss: The Astronomically-derived geomagnetic polarity time scale (APTS).
The use of orbital cycles to date magnetic field reversals.
Preservation of orbital cycles in the sedimentary record is now used to “date” the geomagnetic polarity time scale.
SEISMIC AND SEQUENCE STRATIGRAPHY
Boggs Chapters 14 and 15
Purpose: To use the stacking patterns and geometries of strata to interpret their depositional history. Originally applied to seismic reflection data, and later tied to outcrop data.
Seismic stratigraphy uses seismic reflection correlation patterns to identify:
depositional sequences
predict lithology
characterize sedimentary architecture to interpret sea level changes.
Depositional sequence- a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities (Mitchum, Vail and Thompson (1977)). Because sequences are based on their unconformities or their correlative conformities, they are not primarily dependent on determination of rock type, fossils, or depositional processes.
Thus a sequence represents one cycle of deposition through a cycle of rise and fall of sea level- from of subaerial erosion to the next period of subaerial erosion.
Recall that seismic reflection used to identify layers of earth’s interior- changes in density=changes in velocity, which causes waves to reflect or refract.
Seismic Profiles: Provide 2-D and 3-D maps of subsurface stratigraphy, which can be used to interpret environment, structural setting, hydrocarbon reservoirs, sea-level history, etc.
How done is generate seismic waves artificially and set out geophones to pick up reflections. You get 2-way travel times that tell you position of reflectors in subsurface.
[[Fig. 14.1 Cartoon of procedures to generate artificial seismic lines]]
- Identify shot points, geophones, Limestone beds A and B, reflection seismogram with arrival times from initial shots, and reflections from layers A and B.
Seismic waves travel time in sedimentary media
Shale = 3.6 km/sec
Sandtone = 4.2 km/sec
Limesone = 5.0 km/sec
When amplitudes surpass a threshold, the wave is colored black- superimposing such patterns identifies seismic discontinuities, which then require interpretation based on the geometries of bedding surfaces and their areal distribution.
Surface exposures: rare, but the patterns can be combined with lithostratigraphic analysis to interpret depositional environments.
These can be matched with well data to provide lithostratigraphic information about the layers identified in the seismic profile.
Seismic reflections generated by density-velocity changes in subsurface
Causes: Lithologic change (bedding surfaces)
Unconformities- change in lithology and/or bedding attitudes
Interpretations of seismic data:
Seismic reflection configurations
- the gross stratification patterns identified in seismic records
There are four types
[[Fist two = Fig. 14.6]]
1)Parallel patterns
includes subparallel and wavy patterns: Interpretation: uniform sedimentation processes.
2) Divergent patterns
wedge-shaped package of strata that thicken in one direction
3) Prograding configurations [[Fig. 14.5]]
sigmoid= superposed “S” shapes
Oblique tangential = approaches lower surface tangentially but upper surface approaches boundary at an angle.
Parallel- beds uniform in thickness but approach upper and lower boundaries at an angle.
Complex sigmoid-oblique- combination of the two.
Shingled – rotation of the basin?
Hummocky clinoforms = storm-generated prograding bedforms??
4) Chaotic reflections or reflection-free areas- bedding not identifiable due to deformation or lack of beds (e.g. igneous rocks).
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SEISMIC AND SEQUENCE STRATIGRAPHY (lecture 2)
Boggs Chapters 14 and 15
Reflection terminations
[[Fig. 14.9]]
Criteria used to recognize sequence boundaries:
1) Time significance- all strata within a sequence were deposited during a given broad interval of time by the marked boundaries of the sequences where they became conformities (i.e. unconformity becomes conformity laterally, and this will tell you where the end of the hiatus is, and the length of time it represents).
2) Internal relationships- strata making up a depositional sequence may be either concordant or discordant- either maintaining parallelism or not, with the sequence boundaries.
Concordance- strata above and below boundary are parallel, so there is no readily observable hiatus.
Discordance- is the most important physical criterion in determining sequence boundaries. In seismic profiles you will locate unconformities, truncations and lapout relationships.
Two types of discordance:
(top discordance) = Truncation- lateral termination of strata owing to being cut off from their original depositional limits by erosion. Truncation occurs on the upper boundary of a sequence. Truncation can be either erosional or structural. Minor effects of erosion may be evidenced by dipping bedding being abruptly cut off at the upper boundary, rather than approaching it asymptotically.
(base discordance) = Lapout- lateral termination of strata against a boundary at their original depositional limit. This is a nondepositional, rather than erosional, contact.
[[Figure 14.14 Internal relationships of strata]]- may want to draw these by hand.
Baselap- occurs on the lower boundary of the sequence.
Onlap- strata terminates against a surface of greater inclination
Downlap- initially inclined stratum terminates downdip against an initially horizontal or inclined surface.
Proximal onlap and distal downlap indicate the lateral beginning and ending of a depositional stratum.
Toplap- lapout at the upper boundary of a depositional sequence. Toplap results from a depositional base level, such as sea level, being too low to permit the strata to extend farther updip. It will often approach the upper limit asymptotically. Toplap is often associated with shallow marine environments, such as deltaic complexes.
[[Boggs 14.15- shows depositional sequences and their relationships to sequence boundaries]]
[[Boggs 14.16. Diagram illustrating sequence boundaries]]
Depositional sequence- a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities (Mitchum, Vail and Thompson (1977)). Because sequences are based on their unconformities or their correlative conformities, they are not primarily dependent on determination of rock type, fossils, or depositional processes.
Thus a sequence represents one cycle of deposition through a cycle of rise and fall of sea level- from of subaerial erosion to the next period of subaerial erosion.
Accommodation space- space available for sediments to accumulate. This changes throughout a depositional sequence cycle and so affects where sediments are deposited.
can reflect eustatic sea level, tectonism, or change in sediment supply.
Unconformity- observable discordance in a given stratigraphic section that shows erosion or nondeposition and represents a hiatus.
Conformity- surface separating younger and older strata between which there is no physical evidence of erosion or nondeposition or a hiatus.
Identification of depositional sequences Can be determined in outcrop, or in the subsurface using well data or seismic stratigraphy.
Well Data- utilizes the correlation of stratigraphy between neighboring wells to interpret the location of depositional boundaries. Problem is that this is not a laterally continual record of the strata and therefore may miss some of the important features of the subsurface.
Seismic stratigraphy- consists of utilizing seismic data to delineate stratigraphic sequences and their boundaries. This aids in exploration of areas which have little well data, and can provide explorers with information "ahead of the drill". Seismic stratigraphy is limited by the resolution of the imaging. Therefore, stratigraphic units which are thinner than several 10s of meters may not be resolvable. Additionally, there is no actual record of the strata, it is only a guess of what is truly down there.
Sequence stratigraphy (Chapter 15)
[[Table 15.1 shows the hierarchy of sequence-stratigraphic units]]
Type 1 sequence boundary- subaerial exposure, stream rejuvenation, basinward shift in facies, downward shift in coastal onlap, onlap of overlying strata (when sea level rises again).
occurs when the rate of sea level fall exceeds basin subsidence and so depositional shoreline break migrates basinward.
Type 2 sequence boundary- subaerial exposure and downward shift in coastal onlap landward of the depositional shoreline break. Lacks stream rejuvenation or basinward shift in facies.
occurs when the rate of subsidence exceeds the rate of eustatic fall, and so there is no major change in the depositional shoreline break or in accommodation space.
Depositional System: the entire three-dimensional assemblage of lithofacies enclosed within sequence boundaries.
Depositional system is subdivided into smaller packages of strata that correspond to different periods of a sea level cycle:
Systems Tracts: Define portions of a depositional sequences with respect to sea level.
Parasequences and parasequence sets: each systems tract is divided into parasequences or parasequence sets.
Parasequence: A relatively conformable succession of genetically-related beds or bedsets bounded by marine flooding surfaces. Parasequence bundaries are sharp (and hence show up well in seismic reflection lines), and can be used as timelines which separate older from younger strata.
Each parasequence is deposited during a single episode of submergence. Successive parasequences are deposited when there is an abrupt increase in water depth.
Depending on whether or not sediment supply outpaces accommodation space, parasequences can be stacked in a prograding or retrograding parasequence set.
[[Figure 15.3- systems tract deposits]]
A- lowstand systems tract (LST). Erosion, fans
B late lowstand- wedge builds up.
C- Transgressive systems tract (TST)- landward stepping of deposits. deeper part of system receive no sediment. Accommodation space > sediment supply
D Highstand Systems Tract (HST)- delta progadation, Holocene-esque conditions. Accommodation space < sediment supply.
E- Shelf-margin systems tract (SST) sit on a type-2 sequence boundary. Occur when sea level fall is slower than subsidence at the shelf break, and so nearshore areas exposed, but shelf-break experiences a slight sea level fall.
Highstand Systems Tract (HST)- Forms in the late stage of sea level rise, the sea level standstill and the start of sea level fall. Occur immediately below a type 1 or type 2 sequence boundary. Basinward, HST consists of layers draping over the depositional surface with relatively uniform thickness. Landward, the HST fills accommodation space and then begins to prograde in a basinward direction, with evidence of downlap. Top surface truncated by unconformity due to subaerial exposure during sea level fall.
Lowstand Systems Tract (LST)- Occurs when relative sea level is low. Is on top of a type 1 sequence boundary. Shelf often exposed, and so show signs of erosion (truncation). Sediments bypass the shelf and are deposited in a prograding sequence that downlaps onto older strata. (e.g. basin fans)
Shelf margin systems tract- Lower boundary is a type 2 sequence boundary. Represents time when eustatic sea-level fall is slower than subsidence at the shelf break, but NOT slower than fall at the shoreline break. Hence, nearshore deposits are subjected to erosion, but stream regeneration does not occur. Vertical aggradation rather than progradation occurs at this time. Very difficult to identify in seismic reflection data!
Transgressive Systems Tract (TST)- occurs with rapid relative sea level rise. Stops fluvial incision, so sediments deposit in a landward stepping fashion, with lower surface onlapping strata below.