„ Basic Concepts Concepts in Stratigraphy „ Lithostratigraphy

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„ Sea level and supply

„ Consequences of changes in sea level

„ Types of sequences

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„ Other Types of Stratigraphy

Younger Basic Principles

„ Steno (1669)

„ Principal of original horizontality Older „ deposited as essentially Younger horizontal beds

„ Principal of superposition

„ Each layer of (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 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/ 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

, , 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

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 (, 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 /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 • – “Outbuilding” of shoreline () • 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 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 , 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//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

21