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Western North America Stratigraphy and Depositional

Environments

Brief overview of sequence stratigraphy and the influence of the western basins on

current stratigraphic models

What’s the plan?

• Overview of the sedimentary patterns and depositional changes from Cambrian to near present

• Brief review/introduction of sequence stratigraphy

• Examples from around the Cordillera highlighting the development of seq-strat

Cambrian - Devonian

Sediments are typically:carbonates (on shelf and shallow

environments) or pelagics and black shales (basins and similar ocean floor environments)

Why?No clastic input

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Mississippian

• Antler orogeny generates clastic input for basins adjacent to the thrusts – otherwise not too much difference from the previous billion years.

Permian

• Evaporite basins towards the east lead to salt-tectonics and the creation of small basins

• Non-marine clastics on the eastern extent of the Cordillera are derived from Appalachian and Ouachita orogenies.

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Jurassic

• Clastic sedimentation in full force• Uplifted central Cordillera contains non-

marine depositional environments from alluvial plains to eolian

Y

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Cretaceous

• Central Cordillera dominated by Western Cretaceous Interior Seaway– Extends from Alberta to Arizona– Sedimentary depositional environments range

from deep water to non-marine– Structural basins, arches and faults strongly

influence depositional patterns.

Ord-Sil Dev

Miss Perm -Tri

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Sequence Stratigraphy

• “Sequence stratigraphy is a methodology for analyzing and predicting the distribution of rocks and their properties within a framework of time equivalent or chronostratigraphically significant surfaces” – John Van Wagoner, 1999

What is It?

• Examination of regional stratigraphic units within a framework of interdependent variables:– Eustacy– Subsidence– Sediment Supply– Climate

Transgressive SurfaceMaximum Floofing Surface

Lowstand Wedge

Lowstand Fan

Canyon & Canyon Fill

Sequence Boundary

Lowstand Systems Tract(Incised Valley Fill)

Transgressive Systems TractRetrogradational Parasequence Set

Highstand Systems Tract ofolder sequence

Highstand Systems TractAggradational to Progadational

Parasequence set

200 ft

10 miles

Terrestrial fluvial/alluvial sediments

Shoreface sedimentsEstuarine/ Fluvial sands within Incised Valleys

Shelf and Slope sedimentsTurbidite sands

Flooding surface

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Cyclicity

• Precursor of sequence stratigraphy lies in the 1950’s and 1960’s with the concept of continent-spanning sequences (Sloss) and global cycles (Vail – (Sloss’ student)

• Rhythmic patterns and repetitions observed in nature at various time-scales from hourly (tides) to yearly (summer-winter alternations) and larger

Terms & Definitions• Accommodation – space created by sea-level rise;

subsidence or a combination of the two in which sediment can be deposited

• Flooding surface – separates younger from older strata across which is evidence of an abrupt increase in water depth

• Parasequence – relatively conformable succession of genetically related beds bounded by flooding surfaces or one sequence boundary

• Sequence - relatively conformable succession of genetically related beds bounded unconformities or their correlative conformities

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• Aggradational – facies of successive parasequences are deposited above one another without significant lateral shifts; rate of deposition equals the rate of accommodation

• Progradational – facies of successive parasequences are deposited farther basinward; rate of deposition is greater than the rate of accommodation

• Retrogradational – facies of successive parasequences are deposited farther landward (backstepping); rate of deposition is less than the rate of accommodation

Stacking Patterns

• Highstand Systems Tract – bounded below by the downlap surface and above by the next sequence boundary

• Early Highstand – commonly consists of aggradational parasequences

• Late Highstand – commonly consists of progradational parasequences

• Lowstand Systems Tract – bounded below by the sequence boundary and above by the first major flooding surface

• Transgressive Systems Tract – bounded below by the transgressive surface (the first major flooding surface above the sequence boundary) and above by the downlap surface or maximum flooding surface

Many of terms and concepts are derived from early seismic stratigraphy – using seismic lines to correlate units across basins. On the scale of seismic resolution, facies and intertonguing relationships are lost and only unconformities and the flooding surfaces would be seen as reflectors.

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• Eustacy – synchronous global changes in sea-level• Relative change in sea-level – change in sea-level

measured to a predetermined datum – generally not global in scale regional uplift and subsidence will play a major role

• Regression – basinward movement of the shoreline• Transgression – landward movement of the shorelineNote – both regression and transgression are not

necessarily related to sea-level change

Sea-level change

Transgressive SurfaceMaximum Floofing Surface

Lowstand Wedge

Lowstand Fan

Canyon & Canyon Fill

Sequence Boundary

Lowstand Systems Tract(Incised Valley Fill)

Transgressive Systems TractRetrogradational Parasequence Set

Highstand Systems Tract ofolder sequence

Highstand Systems TractAggradational to Progadational

Parasequence set

200 ft

10 miles

Terrestrial fluvial/alluvial sediments

Shoreface sedimentsEstuarine/ Fluvial sands within Incised Valleys

Shelf and Slope sedimentsTurbidite sands

Flooding surface

Example 1 – Book Cliffs of Utah – excellent exposure of marine – non marine depositional facies transition

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Subaerial response to sea-level changes – what happens above the beach?

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Example 2 – Shannon SandstoneDebated whether it is a sand-ridge or a transgressively incised shoreface

Example 3 – Cretaceous sediments of Alberta

Why are these examples being used? Isolated sandstone bodies form the major oil and gas reservoirs in the Western Interior Seaway. The coarse sediment is surrounded by less porous marine shale and forms petroleum traps.As a result, these units contain thousands of well-logs, core samples, and seismic lines run by the major oil companies and subsequently published in the major sedimentary/stratigraphic journals.

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Hornbeck

Carrot Creek

Burnstick

Raven River

Waskanigan

Dismal Rat

Bickerdike

Karr

Nosehill

Kakwa -HKakwa - BKakwa - N

BLACKSTONE FM.

Moreau

AmundsonKarrNE

SW WAPIABI FM.

approx. 200 km

terrestrial depositsincised valley fill depositslowstand shoreface deposits

highstand shoreface depositsoffshore/marine deposits

The Cardium Fm., contains examples of both incised valley facies and transgressively incised shoreface deposits. This is the type example for transgressive incised shoreface, described by Roger Walker and others 1987 and onward.

Example 4 – Gallup and Tocitosandstones of the San Juan basin.

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