Basic Concepts Concepts in Stratigraphy Lithostratigraphy
Sea level and sediment supply
Consequences of changes in sea level
Types of sequences
Other Types of Stratigraphy
Younger Basic Principles
Steno (1669)
Principal of original horizontality Older Sediments deposited as essentially Younger horizontal beds
Principal of superposition
Each layer of sedimentary rock (sediment) in a tectonically undisturbed Older sequence is younger than the one beneath it and older than the one above it
Basic Principles Basic Principles
Hutton (1700s) Walther (1884)
Principle of Uniformitarianism: The Walther’s Law: Only those facies and processes that shaped Earth throughout facies-areas can be superimposed geologic time were the same as those primarily which can be observed beside observable today each other at the present time
“The present is the key to the past” Only applies to conformable successions – i.e., no major breaks in sedimentation Sometimes there are Vertical successions do not always reproduce environments/conditions that do not horizontal sequence of environments have good modern analogues
1 Several km, 10s of km
Foreshore sandstones
Shoreface sandstones
Interbedded sandstones/shales 10s of m 10s
“Distal” shales
“Shalier (fining) upward” succession
Lithostratigraphy Lithostratigraphy
Formation Formation Generally considered to be tabular in Fundamental unit of lithostratigraphic geometry classification Large enough to be mappable at the A body of rock identified by lithic Earth’s surface or traceable in the characteristics (composition, colour, subsurface
sedimentary structures, fossils, etc) and Existing formations range from a few m stratigraphic position to several 1000s of m thick
Traceable for a few km or several 1000 km
Lithostratigraphy
Formation Todilto
Names (unfortunately...) may change at political boundaries or from one region Entrada to another
Names generally based on geographic Chinle locations
Contacts between formations established at obvious lithologic changes (sharp or gradational; lateral or vertical)
2 Lithostratigraphy
Members
Subdivisions of formations
Possess characteristics that distinguish it from other parts of the formation
Not all formations are subdivided into
Coarsening-upward Sandier-upward Shoaling-upward members Beds
Smallest formal lithostratigraphic unit
Used only if official designation is useful
Lithostratigraphy Lithostratigraphy
Groups Units described at “type sections”
Two or more formations related Outcrops, well logs
lithologically Independent from inferred geologic history
Component formations may change Based on objective, identifiable characteristics
laterally (e.g., due to facies changes) Interpretations of geologic history may change with time Supergroups Diachronous to some extent Assemblage of related or superimposed Produced by shifting depositional environments groups
Sometimes useful for regional syntheses
Several km, 10s of km
Foreshore sandstones
Shoreface sandstones
Interbedded sandstones/shales 10s of m 10s
“Distal” shales Sand T1 Shale
3 Sand Sand T2 T3 Shale Shale
Problems with Lithostratigraphy
Different facies represent different depositional environments
As laterally contiguous environments Formation “A” T4 shift with time, facies boundaries shift Formation “B” so that the facies of one environment lie above those of another environment
Walther’s Law
Problems with Problems with Lithostratigraphy Lithostratigraphy
Timelines cross lithologic boundaries Quest: Find/define “timelines” that will permit depositional histories to be defined Obscures genetic relationships when with precision we are reconstructing geologic history Timelines: stratigraphic surfaces generated by the interplay of tectonics, eustacy (global sea level) and sediment supply
Use links between sea level, sediment supply and other factors to develop predictive models
4 Sequence Stratigraphy Sequence Stratigraphy
Definition: Key concepts: The analysis of stratigraphic successions in “Genetically related strata” – different terms of genetically related packages of environments, deposited contemporaneously (“systems tracts”) strata, bounded by discontinuities “Bounding discontinuities” – 3 principal types of surfaces (unconformities, flooding surfaces, maximum flooding surfaces)
Relate sequence development to interplay of 3 first-order controls (global sea level, local tectonic movements, sediment supply)
Sea Level and Sediment Sequence Stratigraphy Supply
In previous lectures we looked at how Problems: changes in global sea level and Different schools of thought/competing regional subsidence/uplift combine to approaches create changes in relative sea level “Gurus” and “dogma” in some groups
Tends to be “jargon intensive” Now let’s start adding sediment
Solution (this course):
“Generic approach”
Adapt (not adopt) EXXON terminology (most widely practiced form of sequence stratigraphy)
Eustasy + High Subsidence By Adding Sediment, We Can Cause Changes in Water Depth, Regression Without Changes in Sea Level
Eustasy + Moderate Subsidence Relative SeaRelative Level ->
Eustasy
Time ->
5 Cyclical Sedimentation Water Depth Relative Sea Level • Transgression Sediment Subsidence/Uplift – Landward movement of the shoreline Accumulation • Regression Eustatic Sea Level – Basinward movement of the shoreline • Progradation – “Outbuilding” of shoreline (deposition) • Retrogradation Center of the Earth – “Backstepping” of shoreline (deposition)
Cyclical Sedimentation Cyclical Sedimentation
• Progradation is a type of regression • Whether an area will see transgression or – Not all regressions involve progradation regression (progradation or retrogradation) • Retrogradation occurs during transgression depends upon the interplay of two factors: – Not all transgressions involve retrogradation – Rate of sediment supply (clastic, carbonate) – Rate at which “accommodation space” is made available/removed
Cyclical Sedimentation
• Accommodation space: Space available for sediment to accumulate vertically • Approximately equal to water depth in marine settings – Changes in relative sea level create/remove accomodation space • Other types of accommodation (e.g., fluvial “base level”) Curtis, 1970
6 Sed supply > Accommodation
Shelf
Sed supply < Shelf break Accommodation Ramp
Sed supply = Accommodation Lateral changes in accommodation vary from place to place – influenced by physiography
Van Wagoner et al., 1988
Consequences of Sea Level Stratal terminations Change • Based on Embry (2002) • Describe geometric relationships between a • Changes in “depositional trend” reflection/marker and the surface against • Two main possibilities: which it terminates • Lapout - lateral termination of a reflection 1. Change from deposition to erosion and vice (“bedding plane”) at its depositional limit. versa – E.g., toplap, downlap, etc. 2. Change from shallowing upward trend to – Based on geometry alone deepening upward trend and vice versa • Truncation – implies surface originally extended further but was “cut” – E.g., erosional truncation, fault truncation – Can be based on interpretation
Reflection terminations
• Toplap – termination of inclined reflections against an overlying, lower angle surface – Assumes termination is original depositional limit • Erosional truncation – termination of reflections against an overlying erosion surface – Erosion surface may be marine (e.g., submarine • Different types of stratal terminations may channel) or non-marine (fluvial channel) be identified on seismic sections or log • Distinction between toplap and erosional cross-sections. They provide clues that truncation sometimes involves interpretation may be used to define depositional histories.
7 Reflection terminations
• Baselap – lapout of reflections against an underlying seismic horizon • Two types: downlap and onlap – Downlap – dip of the underlying horizon is less than that of the terminating reflections – Onlap – dip of the underlying horizon is greater • Toplap and erosional truncation than that of the terminating reflections • Downlap almost always indicates a marine setting • Onlap may be marine or non-marine Sea level ->
Time -> • Baselap A relative sea level curve
Let’s walk through one cycle of fall/rise
Consequences of Sea Level Basin Margin Change Basinward
• Base level fall: Erosion Deposition – Sediment accumulation ceases along basin margin, subaerial erosion surface expands basinward – Sea-floor erosion on inner shelf in advance of prograding shoreline
8 Erosion Deposition Erosion Deposition
Unconformity
A surface separating younger from older
Unconformity strata along which there is evidence of Erosion Deposition subaerial erosion and truncation (and in some instances correlative submarine erosion) and subaerial exposure along which a significant hiatus is represented Van Wagoner et al., 1988
Unconformity Unconformity
• Starts to form when base level falls • “Bypass surface” – Earth’s surface exposed to erosion to fluvial • Channel incision -> incised valley action and wind formation • Expands basinward as sea level falls and • “Interfluves” -> soil horizons the basin edge is progressively exposed • All strata below a subaerial unconformity are older than all strata above it
9 Unconformity - Recognition
• Truncation of strata below –Seismic – Logs • Onlap of strata above –Seismic – Logs Unconformity
Posamentier and Allen
Seismic Stratigraphy
• Unconformities are produced by subaerial erosion associated with a drop of relative sea level. Different amounts of time may be associated with these surfaces. They are recognized by erosional truncation of underlying stratigraphy.
Cant, 1994
The image above shows a significant unconformity (yellow line) between The image below shows two “nested” unconformities in a Lower Cretaceous Devonian carbonates and Lower Cretaceous clastics for an area of western North offshore section. Each unconformity is probably associated with a fourth-order America. Approximately 250 million years is missing at the unconformity. Note sequence superimposed on a third-order fall of relative sea level. The section the reflection truncations below the unconformity. has been flattened on an underlying horizon (green) for clarity.
10 Consequences of Sea Level Unconformity - Recognition Change • Incised valleys/channels • Base level rise: – Can we distinguish “significant incision” from – Accumulation of non-marine strata spreads localized channel scour? in a landward direction above a subaerial – Other factors can cause “significant incision” erosion surface • Increase in fluvial discharge/power (climate?) • Rising water table • Decrease in sediment load – Change from regressive trend to a • Tectonic uplift transgressive trend in marine deposits • Soil development on interfluves – Deposition ceases at shoreline and erosion – Not all soil horizons are at unconformities at shoreline starts • Changes in channel stacking patterns, – Change from transgressive trend to a sandstone amalgamation, etc. regressive trend
Shoreface Erosion
Shoreface Shoreface Erosion Erosion
11 Flooding Surface Flooding Surface
• Surface across which there is evidence of • Aka transgressive/transgression surfaces an abrupt deepening • Shallowing upward cycles overlain by • Formed during transgression deepening upward cycles – Shoreface erosion: “Ravinement surface” • May be capped by transgressive lag – Can remove 10-20 m of strata • Early diagenesis immediately below may – Erosion may cut down through underlying be apparent unconformity • Offshore: “abandonment”, marine erosion?
“Erosional shoreface retreat”
12 Barrier islands may form during transgression but are not always preserved in the stratigraphic record
Barrier pushed landward
La goonalf acies
Simmons 'A' 1 well, Yazoo Co., MS
SW NE
• Baselap Cliff House Sandstone: Preserved transgressive barrier complex Note interfingering with over/underlying strata
Marine Onlap – Tertiary, S.E. Asia
Coastal Onlap – Paleocene, Offshore N.S.
13 Maximum Flooding Surface Maximum Flooding Surface
• End of transgression, start of regression • Recognition • Shallowing-upward trend overlying a – Downlap surface log cross-sections deepening-upward trend – Downlap surface seismic images • May be a surface of non-deposition or – “Hot shales” on gamma ray logs marine erosion • May be an interval of very slow deposition – “Condensed section” – not really a surface
ale” t sh “Ho
Bhattacharya and Walker, 1994 Downlap – Paleocene, Offshore N.S.
Seismic Stratigraphy
• Downlap surfaces are present at the base of prograding packages. They are commonly associated with maximum flooding surfaces produced by a rise in relative sea level, but may be present elsewhere, such as in deltaic settings where they separate packages generated by autocyclic lobe switching.
The image above shows a downlap surface (“Green Surface”) separating two different deltaic lobes in a young lowstand deltaic setting. A shale horizon at the downlap surface acts as a vertical barrier to fluid flow, separating two stacked reservoir intervals.
14 Sequences Sequences
• The surfaces just described may be used • The surfaces just described may be used to define “sequences” to define “systems tracts” – Subaerial erosion surfaces – Linkage of contemporaneous depositional – Flooding surfaces environments • Transgression surfaces/maximum regressive – Form during specific portions of the relative surfaces sea level curve – Maximum flooding surfaces
Sequences Exxon School
• Different “schools” of sequence • Sequence boundary defined using stratigraphy define sequences in different unconformity and its “correlative ways conformity” • We will look at two: • Can we recognize the unconformity? – “Exxon school” – Most people will agree • Use subaerial unconformity and “correlative – ?Some tendency to “over-interpret” conformity” • Not every channel base is a sequence boundary – “Embry school” • Use subaerial unconformity and transgression surfaces (aka “maximum regressive surfaces”)
Exxon School Key Surfaces
• What is the correlative conformity? • Define “systems tracts” based on where • Forms basinward of subaerial strata are with respect to 3 key surfaces: unconformity – Highstand Systems Tract (“Progradational • Deposition is continuous Systems Tract”) • No consensus on what it is, how to • Between MFS & SB recognize it, when it forms, etc. – Lowstand Systems Tract • Between SB & FS – Transgressive Systems Tract • Between FS & MFS
15 Sequence Stratigraphy The Exxon Model
Coastal Plain Shoreface Sand
Sea level -> Marine Shale
Time -> Maximum Flooding Surface Highstand systems tract: progradation on shelf
Sequence Stratigraphy The Exxon Model
Sequence Boundary Sequence Boundary (Unconformity)
Regressive Surface Subaerial Unconformity Correlative Flooding Surface of Marine Erosion Conformity Sea level ->
Highstand Systems Tract Time -> Lowstand systems tract: submarine fans, Basin-floor fan prograding wedges, bypass/erosion on shelf
Sequence Stratigraphy The Exxon Model
Flooding Surface (Maximum Regressive Surface) Maximum Flooding Surface Sea level ->
Time -> Lowstand Systems Tract Transgressive systems tract: non-deposition, coastal plain aggradation, marine shale
16 Sequence Stratigraphy The Exxon Model
Transgressive Systems Tract Maximum Flooding Surface Sea level ->
Time -> Cycle begins anew – Highstand systems tract
Sequence Stratigraphy The Exxon Model
Sequence Boundary
Highstand Systems Tract Correlative Subaerial Unconformity Conformity Sea level ->
Time -> Lowstand systems tract
Depositional Sequence
Flooding Surface Bounded by Subaerial Unconformity, Regressive Surface Lowstand Systems Tract (Maximum Regressive Surface) of Marine Erosion, Correlative Conformity
17 The Exxon Model Parasequences A (Shared) Personal Perspective • The Exxon model is a useful tool for • Shoaling-upward stratigraphic units understanding the depositional history of a bounded by flooding surfaces, or their package of sedimentary rocks correlative surfaces (Van Wagoner et al. 1990) • It is a useful starting point – do not accept • Considered to be the building blocks of it as “absolute truth” sequences – Be flexible • Best defined in shallow-marine deposits • It is associated with a lot of complex • Parasequence stacking patterns differ terminology (“jargon”) between systems tracts – Focus on the concepts, rather than the – Progradational, aggradational, terminology retrogradational
SB Sed supply > Accommodation
MFS
Sed supply < Accommodation
Sed supply = Accommodation
Correlation of flooding surfaces to define parasequences Cretaceous clastics, Alberta Van Wagoner et al., 1988 Cant, 1994
Parasequences
• May be formed during transgression or regression • May be related to changes in sea level or “autocyclic” phenomena like lobe switching in a delta
18 Transgressive-Regressive Sequences • Embry and Johannessen, Embry 2002 Transgressive-Regressive Sequence • Sequence boundary is subaerial unconformity on shelf Bounded by Subaerial Unconformity and Maximum Regressive Surfaces • Basinward sequence boundary corresponds to “maximum regressive surface” – Change from “shallowing up” to “deepening up” trend – Maximum regressive surface also known as “transgressive surface”
Transgressive-Regressive Transgressive-Regressive Sequences Sequences • Sequences divided up into two systems • Advantages: tracts: – Simple, less jargon (e.g., “forced regressive – Transgressive systems tract: between systems tract”) sequence boundary (base) and maximum – Bounded by surfaces that can be objectively flooding surface (top) defined (subaerial unconformity, maximum • Deepening upward trend regressive surface) – Regressive systems tract: between maximum • Disadvantages: flooding surface (base) and sequence – Not widely used boundary (top) • Shallowing upward trend
Carbonate Sequence Carbonate Sequence Stratigraphy Stratigraphy
Many similarities to siliciclastic “Keep up” sequences Carbonate production able to keep up with rise in sea level – water never BUT-> sediment typically produced deepens locally “Catch up”
Therefore need to consider relative Sea level rises, water deepens then rates of sediment production (not carbonate aggradation catches up to s.l. supply) and relative sea level “Give up” (drowning)
Sea level rises, carbonate factory shut down – water stays deep
19 Jones and Desrochers, Facies Models
Carbonate Sequence Stratigraphy Sequence Stratigraphy
Karst surfaces develop when Traditionally 2 aspects:
carbonate platforms are exposed Global sea level
Dissolution of carbonates by acidic less emphasized now rain/surface water/groundwater Problems with correlation and mechanisms above 2nd order cycles Small-scale -> large scale Methodology for studying sedimentary Sinkholes, caves, valleys, etc. rocks
Started with seismic, then logs and outcrops
Think about relative sea level
20 Sequence Stratigraphy Sequence Stratigraphy
Avoid using sequence stratigraphic Focus on principles
models as “templates” They are fairly simple
Don’t “force-fit” observations Concepts applicable to Watch out for different carbonate/clastic, modern/ancient, approaches/jargon small/large scales
e.g., “Depositional sequences” vs “Genetic sequences”
Summary Summary
Traditional lithostratigraphy not ideal Stratigraphic record controlled by for understanding/defining earth interplay between three main history variables: “Timelines” cross lithostratigraphic Global (“eustatic”) sea level change contacts Local/regional subsidence/uplift Many stratigraphic successions show Sediment supply cyclicity Eustatic sea level change and local Different scales of cyclicity may be present tectonic movements produce relative sea level change
Summary Summary
Relative sea level change, Carbonates show similar patterns to sedimentation, and basin shape define siliciclastic systems
accommodation space “Keep-up”, “Catch-up”, “Give-up”
Three key surfaces: sequence Global sea level changes occur at a boundaries, flooding surfaces, variety of magnitudes, rates
maximum flooding surfaces Many different processes responsible
Key surfaces used to define systems Use sequence stratigraphy as a guide, tracts not a template
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