Sequence stratigraphy

III. Sequence stratigraphy

Examines sedimentary packages over a large area (say, across the entire continental shelf or ancient sedimentary basin) to unravel the entire geologic history that led to their formation. Although this work can be done using data from outcrops and drill holes, a huge amount of this work is done using seismic stratigraphy.

A.      Sequence stratigraphy focuses on the relationships between sequences of conformable layers and the unconformities that bound them. Although lithologic and biostratigraphic information are certainly useful, it is the geometric relationships between unconformities and conformable strata, generally as seen on seismic records, that is the basis for sequence stratigraphy.

1.        It is convenient to divide the entire stratigraphic record at a given place into a number of depositional sequences.

Depositional sequence: a stratigraphic unit composed of a relatively conformable succession of genetically related strata that is bounded at its top and base by unconformities or their correlative conformities. Memorize this definition. It is a mantra. Its meaning will be made clear

below.

Sequence boundary: an unconformity and any correlative conformities that marks the base or top of a depositional sequence.

2.       Note that the layers are numbered 1 through 25. Wherever the numbers are consecutive, the sediments are conformable; wherever there is a gap in numbers between beds, there is an unconformity. The heavy line "A" represents the basal sequence boundary. From left to right, it is defined by an angular unconformity, then a correlative conformity (the boundary between beds 10 and 11 is of the same age as the angular unconformity), and grades out to a paraconformity. Note that on the right side the layering skips from bed 10 of the underlying sequence to anything between beds 12 and bed 16. Beds 11 through 19 suggest a prograding shelf sequence. The upper horizontal beds are likely to be fairly shallow water sediments (inner shelf/beach) whereas the lower horizontal beds are probably outer shelf sediments. The upper sequence boundary is marked by the angular relationship between Bed 19 and Beds 20 through 24. On the left is a paraconformity (gap between 17 and 24 over most of its length) and in the deep part of the basin is a correlative conformity (horizontal boundary between 19 and 20).

B.       Again, the fundamental basis of sequence stratigraphy is the physical relationships of conformable strata and unconformities. It does not depend on rock types, fossils, or inferred depositional processes.

C.       Time significance of boundaries: boundaries between adjacent conformable strata are regarded as essentially synchronous. Ages of sediments above and below unconformities varies widely along strike.

D.      Vocabulary to describe geometric relations among strata and bounding surfaces

All relations between strata and sequence boundaries are either:

1.       Concordant: layers are parallel to a sequence boundary. This boundary may be horizontal, inclined, or uneven (see figure).

2.        Discordant: layers are not parallel to a sequence boundary Discordance is essential to recognizing sequence boundaries. (A paraconformity by itself would never be distinguished from conformable strata in a seismic record!)

3.        Truncation: where strata are terminated by erosion, either regionally or locally, along the upper boundary surface.

4.        Apparent Truncation: where down-dip ends of strata terminate at the upper surface in what was probably a surface of non-deposition. There is no clear evidence for scouring.

5.        Lapout: when strata terminate against a sequence boundary at their original depositional limit. No significant erosion is involved. Lapout occurs at either the upper or lower boundaries. There are 4 terms for different types of lapout (6 through 9 below):

6.       Toplap: lapout at the upper sequence boundary. This results from sediments filling up a basin and being forced to prograde outwards. For example, they could be the foreset beds of a delta or of beach and inner shelf sediments. While there is probably some erosion along a toplap surface, this erosion was associated with the original deposition of the sequence.

7.       Baselap: lapout at the lower sequence boundary. Two types:

8.       Onlap: baselap in which successively younger strata march up an inclined surface, as would happen during a transgression.

9.       Downlap: baselap in which inclined strata terminate downdip against a horizontal or inclined surface, as would happen as delta front strata prograde across a basin. You can see similar relationships in cross-bedding in an outcrop, which is entertaining, but the above vocabulary is reserved for sequence stratigraphy.

E.       Relationship between seismic surfaces and lithology: somewhat indirect. Reflectors indicate some density contrast between layers. Often occurs across an unconformity, of course, but also often occurs when there is a sudden change in relative sea level. This puts suddenly coarser or finer sediments on top of older sediments, thus creating a contrast.

F.        Sequence stratigraphic units: built on geometry of discordant surfaces

1.        Sequence boundary: marked by seaward shift in facies, downward shift in costal onlap, subaerial exposure, ± erosion by stream down-cutting (Boggs Fig. 15.2). (They used to distinguish Type 1 and Type 2 sequence boundaries, but no longer.)

2.       Depositional Systems: 3-dimensional depositional sequences: assemblages of genetically related lithofacies bound between unconformities and their correlative conformities. Depositional systems are made up of one or more systems tracts:

3.       There are 4 kinds of Systems Tracts: (Boggs Fig. 15.3)

a. Highstand Systems Tracts: Progradational sequences that form during the late part of a sea level rise, stillstand, or earliest part of a fall. They lie immediately below the sequence boundary formed by a succeeding sea level fall.

b. Lowstand Systems Tracts: Formed during a sea level fall and during the earliest rise. If sea level drops below the shelf edge, rivers will cut into the exposed surface and allow sediments to by-pass the shelf. Thus, submarine fans form at the base of the continental slope (i.e. on the continental rise). Eventually, relative sea level begins to slowly rise due to either a slowing rate of sea level fall, which allows subsidence to outpace the falling sea level, or the beginning of a sea level rise. In this case, the incised river valley may fill with deltaic sediments, which prograde out to the shelf edge to feed the still growing submarine fans with gravity deposits (turbidites!).

c. Transgressive Systems Tracts: Formed during a rapid sea level rise that floods the shelf. Beach and inner shelf sediments move landward in parallel with the advancing shoreline. The outer shelf tends to be relatively sediment starved during this phase of rapid rise and thus accumulates "condensed section deposits" (Boggs Fig. 15.2). These include such things as limestone or phosphatic hardgrounds and/or thin shales hosting phosphatic nodule horizons and/or high concentrations of horizons of bored fossil materials.Following this period of rapid sea level rise comes a slower period of sea level rise, then stillstand, then the start of the next fall. During this period, beach, shelf, and fluvial/deltaic sediments can prograde seaward to produce the next Highstand Systems Tract (see above). If the next sea level fall is not as fast or far, instead of a lowstand systems tract, we get the following

d. Shelf-Margin Systems Tracts: With a slow rate of fall, fluvial, coastal plain, and delta plain sediments prograde seawards across the highstand systems tract. At the shoreline, sediments rapidly fill the accomodation space such that aggradation is as important as progradation. G. Sequence Stratigraphy and Eustatic Sea Level. When Peter Vail and his associates at Exxon developed sequence stratigraphy in the 70's, they assumed that eustatic (global) sea level changes were the main factor influencing the geometry and timing of systems tracts and sequence boundaries. Local tectonic (uplift (+erosion) and subsidence) and climate (erosion rates) variables were assumed to be far less significant than global sea level. As a result, they figured that they could generate a high-resolution global sea level curve from their data. Handout: Boggs Figs. 15.4, 15.5, 15.6 Without going into much detail, they looked at (Unfortunately, most of these data were locked up in the proprietary archives of Exxon and thus could not be independently evaluated by other geologists. This many found extremely irritating.) People seem to be generally happy with the first order sea level curve (several major fluctuations over the last 500 million years) and the general second order sea level changes (changes over 10s to 100 Myr), many people do not think that the third order sea level variations (10 to a few Myr) are global. Although this interpretation of sequence stratigraphy in terms of global sea level variations is hotly disputed, it is true that many many people, especially those exploring for oil, find that sequence stratigraphy is an extremely useful way for understanding the local and regional history of a basin. It is only the global extrapolations that seem a bit off.