Introduction To Sequence Stratigraphy - The Basics

Other "On Line Lectures" on sequence stratigraphy can be accessed by clicking on the highlighted text.

The purpose of this page is to provide access to a chalk board lecture that is focused on the ideal Vail et al 1977 sequence, and considers this in terms of the component system tracts, and their gross geometries as products of changes in relative sea level (base level) and the evolving accommodation' space.

This video taped lecture introduces the concepts of sequence stratigraphy from the perspective of evolving sediment geometry through time and relative sea level changes. The geological setting described in the lecture is hypothetical, and represents the first of a series of other hypothetical and real geological examples of clastic and carbonate sequences that are described in the pages that follow.

The movie makes the following points:

The lecture seen in the movie involves a geologic model that makes the following assumptions::

• Sea level position varied
• Subsidence was constant
• Sediment supply was constant

The sequence is divided by surfaces system tracts. Each systems tract is represented by a collection of the sediments of the associated sedimentary depositional systems that were active during the different phases of base level change. Thus system tract sediments can be considered as sedimentary units that were deposited synchronously and can be mapped as being enclosed by continuous surfaces that extend from sub-aerial and to sub-aqueous settings.

The systems tracts defined in order of deposition to form the ideal sequence are:

• Early Phase Lowstand System Tract
• Late Phase Lowstand Systems Tract
• Transgressive Systems Tract
• Highstand Systems Tract

Early Phase Lowstand System Tract is associated with:

• Falling stage of relative sea level induced by eustasy falling rapidly and/or tectonic uplift outpacing the rate of change in sea level position
• Fluvial incision up dip with formation of an unconformity or sequence boundary and the focus of sediment input at the shoreline
• Forced regressions induced by the lack of accommodation producing stacking patterns of downward stepping prograding clinoforms over the condensed section formed during the previous transgressive and highstand systems tracts
• Slope instability caused by the rapid deposition of sediment from the fluvial systems
• Basin floor fans formed from sediment transported from the shelf margin when this fails under the weight of the rapid sediment accumulation associated with the forced regression
• Shelf margin and slope fans form when rates of sedimentation slows and slope instability is reduced so sediment is not displaced so far downslope
• Onlap of sediments onto the prograding clinoforms below the shelf break
• The lower bounding surfaces of the Early Phase Lowstand System Tract are the updip unconformity and the top of the downdip condensed section. These surfaces form by different mechanisms and have different time significance
• The top of the downdip condensed section immediately underlies the downlapping prograding clinoforms of the forced regression
• The top of the Early Phase Lowstand System Tract in theory is marked by an initial onlap onto the often eroded surface of the prograding clinoforms of the forced regression

Late Phase Lowstand Systems Tract is associated with:

• A slow relative sea level rise is induced when eustasy begins to rise slowly and/or tectonic uplift slows
• Sediment is now outpaced by an increase in accommodation and in response the sediment begins to onlap onto the basin margin
• River profiles stabilize
• Valleys backfill
• Prograding lowstand clinoforms form and are capped by topset layers that onlap, aggrade, become thicker upward and landward

Transgressive Systems Tract is associated with:

• A rapid relative sea level rise above the shelf margin occurs when eustasy begins to rise rapidly, exceeding the effects of any tectonic uplift
• Condensed sequences are often composed of sediment layers rich in the tests of fauna that are no longer masked by sediment accumulation because sedimentation rates are very slow in response to the greater area of sea floor exposed to sedimentation
• Ravinement erosion surface formed when the transgressing sea reworks either the prior sequence boundary or the sediments that may have collected during the forced regression that may have followed the formation of that sequence boundary.
• Maximum flooding surface forms when the last fine-grained widespread transgressive sediment collects before the High Stand builds out over it.

Highstand Systems Tract is associated with:

• Slow rise of relative sea level followed by a slow fall; essentially a still stand of base level when the slower rate eustatic change balances that of tectonic motion
• Sediment outpacing loss of accommodation
• River Profiles stabilize
• River valleys are dispersed laterally in a position landward of the shelf margin.
• Prograding highstand clinoforms develop capped by aggrading topsets that become thinner upward.

To view carbonates and clastics with different inputs we refer you to the Quick Time Movies on our page.