Boggs S. Principles of Sedimentology and Stratigraphy

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13.2 Seismic Stratigraphy

43

Figure 13.2

Example of a seismic record displayed using the variable-density method of printing. The

vertical scale is given in two-way seismic-wave travel time (in seconds) rather than depth.

Note the presence of an unconformity (arrow), at a depth of about 1.7 seconds, separat

ing folded strata below from nearly horizontal strata above. [From Brown et al., 1995, Se

quence stratigraphy in offshore South African divergent basins: MPG Studies in Geology

41, Fig. 43, p. 54, reproduced by permission.]

early years of marine operations, a half-pound block of TNT was tossed over the

ip's side every 3 minutes to provide a continuous seismic record. This method

was potentially dangerous and damaging to fish and other ocean life. It has now

been largely replaced by teciques that use acoustic sources such as airgtms,

which produce sound energy by releasing highly compressed air.

Early marine operations were severely hampered by problems of accurately lo

cg the shot point and detector positions and the operations had to be carried out

wi sight of land so that locations could be determined by land-based surveying

methods. The development about 1949 or 1950 of radio navigation methods made

possible operations  the ocean away from land. Subsequent development of satel

H navigation methods (so-caHed CPS systems) that "home in" on orbing satellites

to x the position of ships at sea (or positions on land) now allows the position of the

survey ships in the open ocean to be accurately and continuously detered. An

other significant development that came about as a result of new advancements dur

g World War  was the invention of the floating streamer cable, which allowed

detectors (called hyrophones) to be towed in a floating cable behind the ship. Stream

e may be up to several kilometers  length. These technical advances made possi

ble rapid progress  marine seismic surveying methods in the years followg. The

general principles of marine seismic profiling are illustrated in Figure 13.3.



438

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Sound source

Streamer cable (eel)

Figure 13.3

Diagram illustrating the princi



ples of marine seismic survey

ing. Only a few of the possible

paths for travel of seismic waves

are shown. [Based on Kennett,

1982, Fig. 2.13, p. 40.)

Subsequent progress in both marine and land-based seismic surveying tech

niques have improved energy sources and devised sources that do not require ex

plosives; developed new detection equipment and procedures; and improved

treatment and analysis of seismic data.  particular, computer analysis of seismic

data has allowed a quantum leap forward in filtering and enhancing seismic sig

nals and in methods of displaying and interpreting seismic data.

Application of Reflection Seismic Methods

to Stratigraphic Analysis

The science of seismic stratigraphy was developed largely by petroleum compa

nies out of pragmatic necessity to locate petroleum deposits in deep, unexplored

basins both on land and offshore. Geologists have not yet discovered a successful

geochemical method for directly detecting oil or gas in deep subsurface forma

tions, although some progress has been made in direct detection of hydrocarbon

deposits by seismic methods. Therefore, the successful search for petroleum sll

requires that explorationists locate and drill petroleum traps such as anticlines

and salt domes. Because most shallow petroleum traps were located and tested

long ago during the earlier phases of petroleum exploration, petroleum compa

nies have been forced to extend exploraon efforts to deeper formations and fron

tier basins, which are undrilled or sparsely drilled basins, on land and offshore.

Inasmuch as successful oil finding depends upon knowledge of stratigraphic rela

tionships as well as structural anomalies, and because poorly explored basins lack

suicient well control for stratigraphic analysis, new techniques had to be devel

oped that would allow stratigraphic information to be extracted from seismic

data. Thus, seismic stratigraphy was bo in the 1960s as a tool that made possible

the integration of stratigraphic concepts with geophysical data-that is, a geolog

ic approach to stratigraphic interpretation of seismic data (Payton, 1977; Berg and

Wolverton, 1985; Vail, 1987; Cross and Lessenger, 1988; Whittaker, 1998).

Seismic reflections are generated by physical surfaces in subsurface rocks. In

the conventional structural application of seismic data, seismic reections a

used to identify and map the structural attitudes of subsurface sedimentary layers.

By contrast, seismic stratigraphy uses seismic reection patterns to identify depo

sitional sequences, to predict the lithology of seismic facies by interpreting depo

sitional processes and environmental settings, and to analyze relative changes in

sea level as recorded in the stratigraphic record of coastal regions. Seismic stratig

raphy thus makes possible many types of stratigraphic interpretations, such as ge

ologic time correlations, definition of genetic depositional units, and thickness

and depositional environment of genetic units.

Parameters Used in Seismic Sttigraphic Interetation

To accompsh the objecve of terpreting stratigraphy and deposional facies fm

seismic data, geologists must ideny characteristic features of seismic reection

13.2 Seismic Stratigraphy

439

records (seismograms) and relate these features to the geologic factors responsible

for the reflections. An understanding of the factors that generate seismic reflec-

tions is therefore critical to the entire concept of seismic stratigraphy. Fundamen-

tally, primary seismic reflections occur in response to the presence of significant

density-velocity changes at either unconformity or bedding surfaces. Reflections

are generated at unconformities because unconformities separate rocks having

different structural attitudes or physical properties, particularly different litholo-

gies. The density-velocity contrast along unconformities may be further enhanced

if rocks below the unconformity have been altered by weathering. Reflections are

generated at bedding surfaces because, owing to lithologic or textural differences,

a velocity-density contrast exists between some sedimentary beds; however, not

every bedding surface will generate a seismic reflection. Also, a given reflection

event identified on a seismic record may not be caused by reflection from a single

surface, but it may represent the sum or average of reflections phased together

from several bedding surfaces, particularly if beds are thin.

The seismic records produced as a result of primary reflections from uncon

formities or bedding surfaces have distinctive characteristics that can be related to

depositional features such as lithology, bed thickness and spacing, and continuity.

Tw o particularly important seismic parameters that have stratigraphic signifi

cance are reflection configuration and reflection continuity (Table 13.1).

Reflection Configuration. Reflection configuration refers to the gross stratifica

tion patterns identified on seismic records. As illustrated in Figure 13.4, parallel,

divergent, and prograding patterns can be interpreted in terms of primary

-i'1�·

��ic ei parameters commoy ed



ismic

$aph

�

.�

e g�o

oc sicanc

 

paramers

Seismic facies parameters

Reflection configuration

Reflection continuity

Reflection amplitude

Reflection frequency

Interval velocity

External form and areal association of seismic

facies units

Geologic interpretation

Bedding patterns

Depositional processes

Erosion and paleotopography

Fluid contacts

Bedding continuity

Depositional processes

Ve locity-density contrast

Bed spacing

Fluid content

Bed thickness

Fluid content

Estimation of lithology

Estimation of porosity

Fluid content

Gross depositional environment

Sediment source

Geologic setting

urce: Mitchum, R. M., Jr., P. R. Va il, and J. B. Sangree, 1977, ismic stratigraphy and global change of sea level,

Part 6: Stratigraphic interpretation of seismic reflection patterns in depositional sequences, in C. E. Payton (ed.),

ismic stratigraphy�Applications to hydrocarbon exploration: Am. Assoc. Petroleum Geolog1sts Mem. 26, Table 2,

p. 122, reprinted by permission of PG, Tulsa, Okla.

0

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Figu 13.4

Schematic illustration of seis

mic reflection configuration

and reflection continuity.

[Based on Mitchum, R. M.,

)r., P. R. Vail, and ). B. San

gree, 1977, Stratigraphic in

terpretation of seismic

reflection patterns in deposi

tional sequences, in Payton,

C. E. (ed.), Seismic stratigra

phy-Applications to hydro

carbon exploration: Am.

Assoc. Petroleum Geologists

Mem. 26, Fig. 4, p. 123; Fig.

5, p. 124; Fig. 2, p. 119; Fig.

3, p. 120, reproduced by

permission of AAPG, Tulsa,

Okla.]

REFLECTION CONFIGURATION

Pattern

Parallel-subparallel

Divergent

�

Parallel

�

Sigmoid-oblique

Progradmg

Chaotic-deformed

Chaotic-no

stratal pattern

Reflection free

Interpretation

Sediment deposited at a uniform rate

on a uniformly subsiding shelf or

stable basin

Lateral variations in rates of deposition

or progressive tilting of the sedimentary

surface during deposition

Strata deposited by lateral outbuilding

or progradation, forming gently sloping

surfaces called clinoforms

So-sediment deformation or possibly

deposition of siment in a highly

variable high-energy envinment

Highly disorded arrangement of

reflecting surfaces

Few or no reflections in seismically

homogeneous strata, e.g., igneous masses,

thick salt deposits, steeply dipping strata

REFLECTION CONTINUITY

Laterally continuous reections

Discontinuous reflections

Top discordant

Base discordant

depositional conditions or characteristics (the upper part of Fig. 13.2 further illus

trates parallel reectors). Chaotic patterns are the result of soft-sediment defor

mation or other kinds of deformation that produce a disordered arrangement of

reflecting surfaces. So-called reection-free patterns display no identifiable

stratal reflections; reflections appear simply as random "noise."

Note that prograding patterns display sloping surfaces called clinoforms.

e terms undaform, clinoform, and fondaform were introduced by Rich (1951) to

describe depositional environments in relation to wave base (Fig. 13.5A). e

undaform is the more or less flat topographic surface that exists in an aqueous en

vironment above wave base where bottom sediments are moved or stirred by

waves and currents, particularly during storms. The clinoform is the sloping sur

face extending from wave base down to the generally flat floor, called the

fondaform, of the water body. Bedding characterized by initial dips owing to de

position in the dinoform zone is referred to as dinoform bedding. The sloping

strata deposited on delta fronts constitute an example of clinoform bedding, as 

lustrated in the seismic record in Figure 13.5B.

Refleon Contlnu. Reflection continuity depends upon the continuity of the

density-velocity contrast along bedding surfaces or unconformities. It is closely

associated with continuity of strata, and it provides information about deposition·

al process and environment. Continuous reflections, such as some of those in e

upper part of Figure 13.2 that extend for more than 20 kilometers, characteri�tically

indicate stratified deposits that are continuous over large areas. In contrast to con

tinuous reflections, reflection pattes showing reflection terminations (e.g., some

reflections in the lower-left part of Fig. 13.2) indicate stratigraphic relationships

Land

Undaform Clinoform

Figure 13.

I�

1 km

Fondoform

13.2 Seismic Stratigraphy

441

Sea level

A. Sketch illlusating the meaning of the terms undaform, clinoform, and fondoform as

used by Rich (l 9Sl ); after Rich, J. L., 1951, Geol Soc. America Bull, v. 62, Fig. 1, p. 3. B.

xampe of clinotorm (prograding) bedding in a seismic record from the Rhone Delta,

South€ast France. After Posamentier and Allen, 1993, Va riability of the sequence strati

graphic model: Effects of local basin factors: Sedimentary Geology, v. 86, Fig. 8, p. 98;

reproduced by permission.

such

as onlap, downlap, and top lap (discussed subsequently) that occur in coastal

regions in response to transgressions and regressions.

Other Reection Parameters. Other reflection parameters that have some

significance in ismic stratigraphic interpretation are listed in Ta bre 13.1. The

amplitude of seismic waves displayed on seismograms is, among other things,

an indication of bed thickness and spacing. (As mentioned, amplitude is equal to

one-half the height of the wave above the adjacent trough.) If bed thickness is less

than the wavelength of the seismic wave-for example, one-fourth of a wave

length-the reections from the top and base of the bed can be phased together to

give exceptionaUy large amplitudes. On the other hand, when beds are very thick

(commonly >50 m), the reflections from the top and base of the beds are com

pletely separate and wave amplitude may be small (Sheriff, 1980). Analysis of

wave amplitudes allows geophysicists to calculate the thickness of beds, if a near

by contact with a layer thicker than l wavelength is present for calibration

(Christie-Blick, Mountain, and Miller, 1990).

The amplitude of reflected seismic waves can be affected also by fluid con

tent of sedimentary beds or by accumulations of gas  the beds. The presence of

hydrocarbons in beds can produce a marked increase  amplitude of waves that

shows up on seismic records as so-called "bright spots." These bright spots actu

lly apar blacker than surrounding events, so the meaning of the name is not

clear; perhaps it simply means that they stand out clearly on seismic records.

Bright-spot analysis was introduced in the petroleum industry in the early 1970s

and is now used as a method for direct detection of hydrocarbon deposits.

Reflection fre

uency refers to the number of vibrations or oscillations of

seismic waves per second. It is numerically equal to wave velocity divided by

wave length. The frequency of a seismic wave is commonly expressed in hertz (Hz)

or kilohertz (kHz). A hertz is a unit of frequency equal to one cycle per second; a

kilohertz is 1000 hertz. The frequency of seismic waves affects both the depth of

penetration of the waves into the subsurface and the resolution of the seismic

cords, that is1 the sharpness with which details of the seismograms c be distin

guished. Lower frequencies give greater depth of penetration but less resolving

442

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

power. The frequency of seismic waves is induced by the particular energy sound

source used to create the waves. As the waves pass downward through subsurface

formations and are reflected back to the surface, the initial induced frequency is

attenuated by bed thickness, which controls the spacing of reflectors. Thus, atten

uations of the initial induced frequency of seismic waves is related to bedding

characteristics. Frequency is also affected by lateral changes in uid content of

beds (the presence of hydrocarbon accumulations, for example) and by lateral

thickness changes in beds.

Inteal velocity refers to the average velocity of seismic waves between re

flectors. Seismic wave velocity is affected by several factors, especially porosity,

density, external pressure, and pore (uid) pressure. Porosity has a particularly

significant effect on velocity, which increases as porosity decreases. Thus, because

porosity commonly decreases with depth, velocity increases with depth. Ve loci

also increases with density of the rocks and with increasing overburden pressure.

For example, the velocity for a typical sandstone increases from about 4 km/s at

the surface to more than 5 km/ s at a depth of 5000 m. Velocity decreases wi in

creasing interstitial fluid pressure; the presence of gas at low saturations  e

pore spaces of the rocks also causes a decrease in velocity. Seismic velocity is of

particular interest because of the possibility at different rock types, which are

characterized by different densities, porosities, pore fluid pressures, and other

characteristics, can be differentiated on the basis of seismic velocity.

The

exteal fo or geometry of stratigraphic bodies that generate seismic

reflections can be interpreted from seismic data (e.g., Fig. 13.13). Thus, these data

can be used to identify "seismic facies/' which may be interpreted in terms of de

positional environments of the lithologic analogs of these seismic facies. This pro

cedure of interpreting the external form of stragraphic packages from seismic

data is part of the process of seismic facies analysis, which also provides informa

tion on sediment source and geologic setting, including major facies changes. Seis

mic facies analysis is an extremely important aspect of seismic stratigraphy and is

discussed in greater detail in the following paragraphs.

Procedures in Seismic Stratigraphic Analysis

The significance of the seismic stratigraphic approach to the study of subsurface

sedimentary rocks lies in the fact that it permits geologists and geophysicists to in

terpret stratigraphic relationships and depositional processes as well as to use seis

mic data for conventional structural mapping. Interpretation is a subjective

process, but when seismic stratigraphic analysis is pursued in a logical manner

and interpretation is based upon analogy with established stratigraphic and depo

sitional models that have been generated by other types of studies, seismic stra

graphic analysis becomes an extremely valuable tooL Seismic stratigraphy can

thus provide insight into such stratigraphic and depositional factors as lithofacies

changes, relief and topography of unconformities, paleobathymetry (depth rela

tionships and topography of ancient oceans), geologic time correlations, deposi

tional history, and subsidence and tilting history (burial history). The procedures

for interpreting stratigraphy from seismic data involve three principal stages: seis

mic sequence analysis, seismic facies analysis, and interpretation of depositional

environments and lithofacies (Vail, 1987). Seismic stratigraphic analysis is applied

also to interpretation of ancient sea-level changes.

Seismic Sequence Analysis

The

term sequence is often used informally by geologists to refer to any grouping

or succession of strata. Sequence is also used in a more restricted sense to idenfy

disnctive stratigraphic units that are commonly bounded by unconformities (i.e.,

similar to allostratigraphic units, described in Appendix C). Sloss (1963) considered

13.2 Seismic Stratigraphy

443

sequences to be major rock-stratigraphic units of interregional scope that are sep-

arated and delimited by interregional unconformities. He recognized and named

(using Indian names) six major sequences on the North American craton (see Fig.

12.9), each separated by demonstrable regional unconformities that can be traced

from the Cordilleran region of western North America to the Appalachian Basin in

the east. Each succession or sequence represents a major cycle of transgression and

regression, that is, advance and retreat of shorelines. Recognition of the sequences

is based on physical relationships among rock units, although Sloss indicates that

the sequences also have time-stratigraphic significance.

The sequence concept was subsequently extended and redefined by Mitchum,

Vail, and Thompson (1977). These authors define a depositional sequence as "a

stratigraphic unit composed of a relatively conformable succession of genetically

related strata and bounded at its top and base by unconformities or their correla

tive conformities." Sequences as thus defined differ from Sloss's sequences in that

they may be much smaller rock units (a few tens of meters to as much as a thou

sand meters; Wilson, 1992). Also, because they are bounded by interregional un

conformities and their equivalent conformities, they may be traceable over major

areas of ocean basins as well as continents. Distinct, related groups of deposition

al sequences superposed one on another are designated by Mitchum, Vail, and

Thompson (1977) as supersequences. These supersequences are of the same gen

eral order of magnitude as Sloss's original sequences. The basic concept of depo

sitional

sequences is illustrated in Figure 13.6. Because sequences are defined on

the basis of physical relationships of the strata, that is, bounded at the top and

base by unconformities or their correlative conformities, they are not primarily

dependent for recognition upon determination of rock types, fossils, or deposi

tional processes.

Inteal Relationships. The strata that make up a depositional sequence may be ei

ther concordant, that is, essentially parallel to the sequence boundary, or discor

dant, lacking parallelism with respect to the sequence boundaries. Concordant

B f----

A --

Figure 13.6

UNCONFORMI CONFORMI

--( S U

F









�







�

S



ITI



J ---------

---(N

HiUS)

- 1

UNCONFORMITY

(SURFACE OF EROSION

AND NONDEPOSITION)

CONFORMITY



(NO HIATUS)

UNCONFORM



iTY

�

=�



(SURFACE OF

NONDEPOSITION)

Illustration of the concept of depositional sequences. A depositional sequence is com

posed of relatively conformable, genetically related strata bounded at its base (A) and top

(B) by unconformities that pass laterally to correlative conformities. Individual units of stra

 1 through 25 are traced by following stratification surfaces; they are assumed to be

conformable where successive strata (no missing strata) are present. Where units of strata

are missing, hiatuses are present. [From Mitchum, R. M., ]r., P. R. Vail, and S. Thompson,

Ill, 1977, Seismic stratigraphy and global change of sea level. Part 2: The depositional se

quence as a basic unit for stratigraphic analysis, in Payton, C. E. (ed.), Seismic stratigra

phy-Applications to hydrocarbon exploration: Am. Assoc. Petroleum Geologists Mem.

26, Fig. 1, p. 54, reproduced by permission of AAPG, Tulsa, Okla.]

444

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

-

Upper Bounda



Erosional truncation

To p lap

Concordance

Lower Boundary



·.

On lap

Downlap 3.

Concordance

 Baselap 

Figure 13.7

Relations of strata to the (A) upper boundary and (B) lower bounda of a depositional se

quence. A 1. Erosional truncation: Strata terminate against the upper bounda mainly

owing to erosion. A2. Toplap: Initially inclined strata terminate against an upper bound

a created mainly as a result of nondeposition (e.g., foreset r ata terminating against an

overlying surface where no erosion or deposition took place). A3. Top concordance:

Strata at the top of a sequence do not terminate against an upper bounda. B1. Onlap:

Strata terminate updip against an inclined suace. 82. Downlap: Initially inclined strata

terminate downdip progressively against initially horizontal or inclined surfaces. 83. Base

concordance: Strata at base of a sequence do not terminate against lower boundary.

[From Mitchum, R. M., jr., P. R. Vail, and S. Thompson, Ill, 1977, Seismic stratigraphy and

global change of sea level. Part 2: The depositional sequence as a basic unit for strati

graphic analysis, in Payton, C. E. (ed.), Seismic stratigraphy-Applications to hydrocarbon

exploration: Am. Assoc. Petroleum Geologists Mem. 26, Fig. 2, p. 58, reproduced by per

mission of MPG, Tulsa, Okla.]

relations can occur at either the upper or the lower boundary of a sequence and

may  expressed as parallelism to an initially horizontal, inclined, or uneven sur

face (Fig. 13.7). Discordance is the most important physical criterion used in deter

mining sequence bodaries. Strata may display discordance with overlying beds

(top discordance) or underlyg beds (base discordance), as shown in Figure 13.8.

Erosional truncation is the lateral termination of strata because erosion has

cut them off from their original depositional limits. Truncation occurs at the

upper boundary of a sequence and may be of either local or regional extent.

Top lap (Fig 13.7 A.2, 13.88) is lapout (lateral termination of strata against a bound

ary at their original depositional limit) at the upper boundary of a depositional se

quence, for example, the lateral termination updip of the foreset beds of a deltaic

complex. Toplap is evidence of a nondepositional hiatus. Mitchum, Va il, and San

gree (1977) suggest that toplap results from a depositional base level, such as sea

level, being too low to permit the strata to extend farther updip, thus allowing

sedimentary bypassing and possibly minor erosion to occur above base level

while prograding strata are deposited below base level.

Baselap occurs at the lower boundary of a depositional sequence and may it

self  of two types. Onlap (Fig. 13.7B.l, 13.8C) is baselap in which an initially hori

zontal or inclined stratum terminates against a surface of greater inclination.

Downlap (Fig. 13.7B.2, 13.80) is baselap in which an initially inclined stratum ter

nates downdip against an inially horizontal or inclined surface. lap and

downlap indicate nondepositional hiatuses and not erosional breaks in deposition.

13.2 Seismic Stratigraphy

445

Figure 13.8

TOP DISCORDANT



2 mi

3 km

BE D!SCORDANT.

I I

3 km

To p-discordant seismic reflection patterns: A. Erosional truncation (a rrow). B. To plap (arrow). Base

discordant seismic reflection patterns: C. On lap (arrow). D. Downlap; downlap surfaces shown by

small, blak arrows. [A, B, C after Mitchum, R. M., Jr., P. R. Va il, and J. B. Sangree, 1977, Stratigraphic in

terpretation of seismic reflection patterns in depositional sequences, in Payton, C. E. (ed.), Seismic

stra�igraphy-Applications to hydrocarbon exploration: MPG Mem. 26, Fig. 2, p. 119; Fig. 3, p. 120, re

produced by permission; D after Posamentier and Allen, 1999, Siliciclastic sequence stratigraphy

Concepts and applications: SEPM Concepts in Sedimentology and Paleontology 7, Fig. 3.55, p. 94,

reproduced by permission.]

Toplap and baselap relations of the strata in a depositional sequence should

not be confused with the foreset bedding of cross-laminated units, which form

parts of beds rather than sequences. Figure 13.9 diagrammatically illustrates in a

regional setting the relationships of strata in depositional sequences to sequence

boundaries.

identification of Depositional Sequences. Depositional sequences c be identified

both in outcrop sections and in subsurface sections by searching for unconformi

ties

(erosional surfac, truncation of strata, missing strata). Subsurface identifica

on depends largely upon the use of instrumental well logs, such as electric logs

that measure resistivity of rock units (Chapter 12) and seismic data to locate and

trace

unconformities, truncations, and lapout relationships (see Van Wagoner

et al., 1990). In fad, the sequence concept, as conceived by Vail and his co-workers

at Exxon, was developed initially from seismic data.

Because sequences are defined as strati

hic units separated by unconfor

miti or their rrelative cohformities (Mitchum, Va il, and Thompson, 1977), the

mapping of unconformities is thus the key o seismic sequence anatysis. Seismic

sequence

analysis aims to identify major reflection "packages" that can be delineat

ed by recognizing surfaces of discontinuity. We commonly th of discontinuities

as unconformities, which are surfaces of erosion or nondeposition that represent



446 Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

To plap

(Overlying

unconformity) \

Erosional

truncation

Figure 13.9

Internal

convergence

(Underlying

;

unconformity)

Terminology for relations that define unconformable boundaries of a depositional sequence.

[After Mitchum, R. M., Jr., P. R. Vail, and J. B. Sangree, 1977, Stratigraphic interpretation of

seismic reflection patterns in depositional sequences, in Payton, C. E. (ed.), Seismic stratigra

phy-Applications to hydrocarbon exploration: Am. Assoc. Petroleum Geologists Mem. 26,

Fig. 1, p. 118; Fig. 3, p. 120, reproduced by permission of MPG, Tu lsa, Okla.]

major hiatuses, and we identify four different kinds of unconformities (see Fig.

12.5).  seismic stratigraphic analysis, however, two kinds of discontinuities are

recognized:

1. Erosional unconfoi surfaces that represent a significant hiatus owing to

subaerial or subaqueous erosional truncation

2. Unconformable surfaces called downlap surfaces, which are marine surfaces

representing a hiatus but without evidence of erosion.

Discontinuities are generally good reflectors and also commonly separate rock

units having different dips, at least on a regional scale. Discontinuities may thus

be recognized by interpreting systematic pattes of reflection terminations along

the discontinuity surfaces. Tw o patterns, onlap and downlap, occur above discon

tinuities. Three pattes, truncation, toplap, and apparent truncation (Fig. 13.10),

occur below discontinuities (Vait 1987).

Seismic resolution is generally not adequate to delineate minor sedimentary

sequences because the practical vertical resolution is on the order of 10 to 50 m. 

Figure 13.10

Sequence

bounda

Down lap

��

Downlap suace

Apparent

truncation

Diagram illustrating sequence boundaries (unconformities), downlap (maximum flooding)

surfaces, and various kinds of reflection terminations. Apparent truncation refers to termi

nation by depositional thinning . [From Vail, P. R., 1987, Seismic stratigraphic interpreta

tion using sequence stratigraphy, in Bally, A. W. (ed.), Seismic stratigraphy: Am. Assoc.

Petroleum Geologists Studies in Geology 27, Fig. 1, p. 2, reproduced by permission of

MPG, Tu lsa, Okla.]