Sequence Stratigraphy on the Northwest European Margin, Proceedings of the Norwegian Petroleum Society Conference

559 Pages·1995·84.969 MB·1-608\559

by  R.J. Steel, V.L. Felt, E.P. Johannessen and C. Mathieu (Eds.)

Preface
This book contains many of the papers presented at the international conference "Sequence
Stratigraphy: Advances and Applications for Exploration and Production in North West Europe"
held in Stavanger, Norway on 1-3 February, 1993. A number of other papers, including some from
poster presentations at the same conference, have been solicited in order to maintain a reasonable
balance between the various sections of the book.
With the decline in the discovery of large structural traps in North West Europe, it has become
increasingly urgent to have a more powerful, conceptual predictive tool to aid the exploration for,
and the exploitation of hydrocarbons. Sequence stratigraphy now has this potential, aimed at a
more accurate prediction of the time and space distribution of sandstones and shales. The recent
revolution in our approach to stratigraphy also has another side effect. It encourages the "geologist"
to play a more aggressive role in the team search for the "difficult-to-map" resources.
The aims of the conference were: (1) to provide an update on the conceptual aspects of sequence
stratigraphy, with weight on alternative opinions; (2) to highlight the importance of tectonics in
sequence generation; and (3) to provide a review of Carboniferous to Tertiary examples, where
sequence stratigraphy has been applied in North West Europe.
This volume contains 29 papers, with topical subjects including:
(cid:12)9 Modelling and alternative approaches to sequence stratigraphic theory (papers 1-4),
(cid:12)9 Sequence definition in fluvial strata (papers ,5 6),
(cid:12)9 Upper Paleozoic carbonates, northern regions (papers 27, 28),
(cid:12)9 Forced regression, valley incision and other mechanisms for "extending" Lower Jurassic
sandbodies (papers ,7 ,8 ,9 25, 26),
(cid:12)9 Middle Jurassic correlation, including views on Brent pinch-out (papers 10-14),
(cid:12)9 Syn-rift and Upper Jurassic sequences (papers 15-18),
(cid:12)9 Transgressive and Low-stand models for Cretaceous sands (papers ,91 29),
(cid:12)9 North Sea Tertiary succession: sequence development (papers 20-24).
stnemegdelwonkcA
The editors of this book are indebted to a host of reviewers, whose efforts have been invaluable in
improving manuscripts and shaping the final product. We wish to thank:
B. Beauchamp J. Gjelberg R. Mj0s M. Rye-Larsen
G. Butenko .W Helland-Hansen B. Noble A. Ryseth
.T Cross R. Hodgkinson J.E Nystuen L.J. Skjold
A. Dalland R. Johansen .S Olaussen J. Sneider
.T Dreyer I.L. Kristiansen .T Olsen .T Spencer
A. Embry E Livbjerg N. Parkinson D. Stewart
L.M. Ffilt A. Lcnoy M. Rider .S V~gene
G. Farrow .T Marjanac J. Riven~es .P van Veen
E. Fjellanger O. Martinsen A. Roberts D. Waltham
A. Forsberg O. Michelsen J. Roseway D. Worsley
J. Gerrard N. Milton J.L. Rubino
R. Steel, .V Felt, E.E Johannessen and C. Mathieu
(Editors)
VII
List of Contributors
Mobil Exploration Norway Inc., .O.P Box 510, 4001 ,regnavatS Norway
K. BARNES
A/S Norske Shell, eslekOsrednU go Produksjon, Risavikveien 180, Post-
L. BOLLE
boks ,04 4056 ,regnanaT Norway
M. CECCHI Mobil Exploration Norway Inc., Exploration Department, .O.P Box ,015
1004 ,regnavatS Norway
EG. CHRISTIANSEN Geological Survey of Greenland, Oster edagdloV ,01 1350 Copenhagen
,K Denmark
.P DE CLARENS Elf Aquitaine Production, Tour Elf, 2 place ed al Coupole, La D~fense ,6
92400 ,eiovebruoC ecnarF
J.H. COCKINGS Marathon Oil, UK. Ltd., Capital House, 25 Chapel ,teertS London NW1
5DQ, UK
A. DALLAND ,liotatS Sandslihaugen ,03 5020 ,negreB Norway
G. DAM Geological Survey of Greenland, Oster edagdloV ,01 1350 Copenhagen
,K Denmark
M. DANIELSEN Department of Earth Sciences, University of Aarhus, 8000 Arhus ,C
Denmark
R.J. DIXON BP Exploration, Farburn Industrial Estate, Dyce, Aberdeen AB2 ,BP0
UK
.T DREYER Norsk Hydro Research ,ertneC Sandsliveien ,09 5020 ,negreB Norway
M. DYCE BP ,yawroN .O.P Box ,791 4033 ,suroF Norway
Present address: BP Exploration, Farburn Industrial Estate, Dyce,
Aberdeen AB2 0PB, UK
G. ELVEBAKK agaS Petroleum ,.s.a Postbox 1134, 9401 Harstad, Norway
A.E EMBRY Geological Survey of Canada, 3303-33rd .tS N~, ,yraglaC Alta. T2L
,74,2 Canada
R.B. FA~RSETH Norsk Hydro ,.s.a .O.P Box 200, 1321 Stabekk, Norway
H. FJELLBIRKELAND Norsk Hydro Production, Sandsliveien ,09 5020, ,negreB Norway
E. FUGELLI Statoil Exploration Division, .O.P Box 300, 4001 ,regnavatS Norway
J.H. GAMBER Amoco Production Co., .O.P Box ,2903 Houston, TX ,35277 ASU
J. GJELBERG Norsk Hydro Research ,ertneC Sandsliveien ,09 5020 ,negreB Norway
C.M. GRIFFITHS cihpargitartS Research International, Sigurd Slembes .v ,21 7562 Hund-
hamaren, Norway
E HADLER-JACOBSEN BP/Statoil Research and Development Collaboration, Trondheim, Nor-
yaw
C. HEILMANN-CLAuSEN Department of Earth Sciences, University of Aarhus, 8000 Arhus ,C
Denmark
IIIV List of srotubirtnoC
.W HELLAND-HANSEN Norsk Hydro Research Centre, 5020 ,negreB Norway
EM. HINES .P.B Norway, Exploration Department, .O.P Box 197, 4033 Forus, Sta-
,regnav Norway
Present address: Conoco Norway Inc., Exploration Department, .O.P
Box 488, 4070 ~grebadnaR ,regnavatS Norway
.T JACOBSEN ,liotatS .O.P Box 300, 4001 ,regnavatS Norway
E.E JOHANNESSEN ,liotatS .O.P Box 300, 4001 ,regnavatS Norway
H. JORDT Department of Earth Sciences, University of Aarhus, 8000 2trhus ,C
Denmark
Present address: Geological Institute, Oslo ,ytisrevinU Boks 1047, Blin-
,nred 0316 Oslo ,3 Norway
L.G. KESSLER, II Marathon Oil Company, .O.P Box 3128, Houston, TX ,8213-35277 USA
W.N. KREBS Amoco Production Co., .O.P Box 3092, Houston, TX ,35277 USA
M.K. LA COE Amoco Production Co., .O.P Box 3092, Houston, TX ,35277 USA
G.V. LAURSEN Department of Earth Sciences, University of Aarhus, 8000 Jrhus ,C
Denmark
I. LAURSEN Statoil Exploration Division, .O.P Box 300, 4001 ,regnavatS Norway
K.S. LERVIK Statoil Exploration Division, .O.P Box 300, 4001 ,regnavatS Norway
.T LILJEDAHL Norsk Hydro a.s., .O.P Box 200, 1321 Stabekk, Norway
.T MARJANAC lacigoloeG Institute, ytisrevinU of ,negreB 5007 ,negreB Norway
J. MARKELLO Mobil Exploration and Production Technical ,retneC .O.P Box ,232056
Dallas, TX ,2320-56257 ASU
O.J. MARTINSEN Norsk Hydro Research ,ertneC 5020 ,negreB Norway
A. MATHIESEN Geological Survey of Denmark, Thoravej ,8 2400 Copenhagen ,(IN
Denmark
T.A. MAZZA Presidio Oil Co., 5613 DTC ,yawkraP Englewood, CO 80111, USA
O. MICHELSEN Department of Earth Sciences, University of Aarhus, 8000 suhrt2~ ,C
Denmark
N. MILTON BP ,yawroN .O.P Box ,791 4033 ,suroF Norway
R. MJOS ,liotatS .O.P Box 300, 4001 ,regnavatS Norway
I. NILSSON IKU Petroleum Research, 7034 Trondheim, Norway
.T OLSEN Geological Institute, University of Bergen, All~gaten ,14 5007 ,negreB
Norway
Present address: Statoil, Petek, .O.P Box 4035, 4001 ,regnavatS Norway
T.R. OLSEN Geological Institute, University of Bergen, All~gaten ,14 5007 ,negreB
Norway
D.N. PARKINSON .P.B Exploration, Research and Engineering Centre, Chertsey Road,
Sunbury-on- Thames, Middlesex TW16 7LN, UK
Present address: Atlas Wireline ,secivreS 554 London Road, Isleworth,
Middlesex TW7 5AB, UK
J. PEARCE Timelines Stratigraphic Consultancy, 08 Kings Rd., Farncombe, Surrey
GU7 3ES, UK
D. RENSHAW ,liotatS .O.P Box 300, 4001 ,regnavatS Norway
A.D. REYNOLDS BP Research and Engineering Group, Chertsey Road, Sunbury-on-
List of srotubirtnoC XI
Thames, Middlesex TW16 7LN, UK
L.A. RILEY B VR International Ltd., Ormond House, 2 High St., Epsom, Surrey
KT19 8AD, UK
A. RYSETH Norsk Hydro Research Centre, Sandsliveien ,09 5020, Bergen, Norway
B.E. SAUAR Norsk Hydro a.s., .O.P Box 200, 1321 Stabekk, Norway
M. SCHWANDER A/S Norske Shell, eslekOsrednU og Produksjon, Risavikveien 180, Post-
boks ,04 4056 ,regnanaT Norway
G. SHANMUGAM Mobil Exploration and Production Technical Center, P.O. Box 650232,
Dallas, TX ,2320-56257 USA
I.K. SINCLAIR ytisrevinU of Aberdeen and Canada-Newfoundland Offshore Petroleum
Board, 140 Water Street, .tS John's, Nfld., A1C 6H6, Canada
T.S. SJOBLOM Norsk Hydro a.s., .O.P Box 200, 1321 Stabekk, Norway
M. SKIBELI Mobil Exploration Norway Inc., .O.P Box 510, 4001 ,regnavatS Norway
J.S. SNEIDER Rice ,ytisrevinU .O.P Box 1892, Houston, TX 77251, USA
Present address: R.M. Sneider Exploration, 11767 Katy Freeway 300,
Houston, TX ,08077 USA
R.J. STEEL Geological Institute, University of Bergen, All~gaten ,14 5007 Bergen,
Norway
J.A. STEIN Amoco Production Co., .O.P Box 3092, Houston, TX 77253, USA
L. STEMMERIK GrCnlands Geologiske ,eslegOsrednU Oster Voldgade ,01 1350 Copen-
hagen K, Denmark
D.J. STEWART A/S Norske Shell, eslegCsrednU og Produksjon, Risavikveien 180, Post-
boks ,04 4056 ,regnanaT Norway
.T STRAUME Mobil Exploration Norway Inc., .O.P Box 510, 4001 ,regnavatS Norway
E SURLYK Geological Institute, University of Copenhagen, Oster Voldgade ,01 1350
Copenhagen ,K Denmark
S.E. SYVERTSEN Mobil Exploration Norway Inc., .O.P Box 510, 4001 ,regnavatS Norway
.P THI~RIAULT Geological Institute, University of Bergen, All~gaten ,14 5007 ,negreB
Norway
E. THOMSEN Department of Earth Sciences, University of Aarhus, 8000 Arhus ,C
Denmark
.T TJELLAND Norsk Hydro a.s., .O.P Box 200, 1321 Stabekk, Norway
ER. VAIL Rice ,ytisrevinU .O.P Box 1892, Houston, TX 77251, USA
L. WAITE Mobil Exploration and Production Technical Center, P.O. Box 650232,
Dallas, TX 75265-0232, USA
M. WIIG Statoil DDB, Sandslihaugen ,03 5020 Bergen, Norway
Sequence boundaries and sequence hierarchies:
problems and proposals
Ashton E Embry
Significant problems are being encountered yb stratigraphers sa they attempt to apply Exxonian sequence analysis to the
depositional record. The most serious problem si one of consistent and objective boundary recognition. The unconformable
portion of the boundary usually can be recognized with reasonable objectivity but a major problem occurs when the boundary si
a "correlative conformity". The Exxon model defines such a surface sa the depositional surface which existed at the time of the
end of base level (relative sea level) fall. In many cases this theoretical surface sah no apparent lithologic expression and cannot
be recognized objectively. Thus correlation of a depositional sequence throughout a basin si either impossible or si an extremely
subjective exercise.
oT remedy the problem of boundary recognition it si proposed that a sequence boundary be placed at the subaerial unconformity
and at the correlative transgressive surface. The transgressive surface si ideal for the conformable portion of a sequence boundary
because: )1( it si very distinctive lithologically and occurs ni both ramp and shelf/slope settings; )2( it has only minor diachroneity
ni most cases; )3( it merges with the basinward termination of the unconformable portion of the sequence boundary. This
methodology results ni a practical, genetic unit (T-R sequence) which can be objectively correlated.
A second problem with current sequence stratigraphic practice si the use of a sequence hierarchy scheme which si based on
frequency of boundary occurrence. This system si very subjective ni nature and si prone to circular reasoning.
oT counter the hierarchy problem, a hierarchical arrangement of T-R sequence boundaries has been established using boundary
characteristics which include: )1( extent of the boundary, )2( extent of the unconformable portion of the boundary, )3( degree of
deformation of strata directly underlying the boundary, )4( magnitude of deepening across the boundary, )5( degree of change
of the depositional regime across the boundary, and )6( degree of change of the tectonic regime across the boundary. These
characteristics reflect the magnitude of base level changes which generate sequence boundaries and this linkage allows the
establishment of a hierarchy. Five distinct orders of sequence boundaries are recognized ni the hierarchy and vary from ts1 order
boundaries which are widespread subaerial unconformities associated with significant deformation, to 5th order boundaries which
are transgressive surfaces which can be correlated only .yllacol
Introduction graphic reconstruction are much better accomplished
using a sequence framework than a lithostratigraphic
The general usage of the term sequence in strati- one.
graphic analysis follows the definition of Vail et In the past five years, detailed sequence strati-
al. (1977, p. 53), "a stratigraphic unit composed of graphic models have been published with the model
genetically related strata bounded at the top and from Exxon scientists being the most popular and
bottom by unconformities or their correlative con- well publicized (Jervey, 1988; Posamentier et al.,
formities". This definition provides us with a ge- 1988; Van Wagoner et al., 1990; Haq, 1991; Vail et
netic stratigraphic unit which has unconformities, al., 1991). The type of sequence advocated in this
across which significant changes in the depositional model is known as a depositional sequence which is
regime occur, on the boundaries rather than within. bounded by a subaerial unconformity and a confor-
It also provides for a unit which potentially can be mity which represents the depositional surface which
mapped over an entire basin. For the purpose of existed at the end of relative sea level (base level)
basin analysis, such a unit has obvious advantages fall. The application of this model to a variety of
over a standard lithostratigraphic unit (formation, stratigraphic successions has greatly improved the
member) which may contain significant unconformi- understanding of the depositional history of these
ties within it, which can be mapped only over a areas (e.g. Van Wagoner et al., 1990). However, in
portion of the basin and which commonly has very di- my attempts to use the Exxon model in the Devonian
achronous boundaries. Facies analysis and paleogeo- and Mesozoic successions of the Canadian Arctic Is-
Sequence Stratigraphy on the Northwest European Margin edited yb R.J. Steel et .la NPF Special Publication ,5 .pp 1-11, Elsevier,
Amsterdam. (cid:14)9 Norwegian Petroleum Society (NPF), .5991
2 A.E Embry
lands, I have consistently encountered two problems. in the marine realm at this time of change from
These are: (1) the objective recognition and correla- base level fall to base level rise is the correlative
tion of the conformable portion of the boundary, and conformity.
(2) the establishment and application of a sequence It should be noted that lately Posamentier et al.
boundary hierarchy. The main objective of this paper (1992) and Jervey (1993) have advocated that the
is to discuss each of these problems and to propose correlative conformity should be the marine deposi-
solutions to counteract them. tional surface which existed at the start, rather than
at the end, of base level fall. Hunt and Tucker (1992)
have commented on the inappropriateness of such
Delineating the conformable portion of a
a contact with the main objection to the practice
sequence boundary
being the resulting inclusion of much of the subaerial
ehT problem unconformity within the sequence rather than on its
boundary (see Fig. .)1 This defeats one of the main
As noted earlier, a sequence boundary consists of purposes of sequence delineation (a coherent genetic
an unconformable portion (subaerial unconformity unit without significant internal breaks) and such an
and/or ravinement surface) and a conformable por- unfortunate departure from the established defini-
tion. The use of unconformities as boundaries is the tion of the correlative conformity will not be further
crux of sequence analysis because it is essential that considered herein.
unconformities, across which there are depositional This still leaves us with the question: "Can the
and/or tectonic shifts, are not included in genetic correlative conformity, as originally defined, be rec-
units. In fact, it was just this type of reasoning which ognized with reasonable scientific objectivity?" I be-
led Sloss et al. (1949) to define a sequence in the lieve it cannot because it is basically a theoretical
first place and, in their usage, a sequence boundary time surface which has no lithologic expression. The
consisted only of an unconformity. The addition of a reason for the lack of lithologic change at such a
conformable portion to boundary definition by Vail et horizon is that there is no significant shift in sedimen-
al. (1977) was a significant improvement to sequence tary patterns or supply rates to the marine shelf and
analysis because it potentially allowed a sequence basin at the change from base level fall to base level
boundary to be correlated throughout a basin. Such rise. Regressive sedimentation on the shelf, slope and
a widespread genetic unit is ideal for facies analysis basin simply continues across this time boundary.
and makes the description, interpretation and com- The fact that there is no lithological change at the
munication of the depositional history of a basin a boundary at most localities is not surprising. Exxon
much less complex exercise. scientists selected such a boundary for the correlative
The identification of the unconformable portion conformity because it was independent of sedimen-
of a sequence boundary is not problem-flee, but in tation effects and would thus allow eustatic control
most cases it can be done in a relatively objective of sequence boundary origin to be tested (Cross and
scientific manner by the application of stratigraphic, Lessenger, 1988).
paleontologic, sedimentologic and geochemical tech- The proponents of the depositional sequence
niques (see Shanmugam, 1988). The conformable model have been very vague regarding criteria for
portion of a depositional sequence boundary is de- recognizing the correlative conformity as defined by
fined as the depositional surface which existed at the Jervey (1988) and described above. One suggestion
time of the end of base level fall (maximum rate provided has been: k~' change from rapidly prograd-
of eustatic fall in the model of Jervey, 1988), or ing parasequences to aggradational parasequences
conversely, at the beginning of base level rise. The marks the boundary" (Haq, 1991, p. 12). The ap-
main reason for such a definition is that it provides plication of such a criterion is extremely difficult
for a single continuous surface with the correlative especially in shale-siltstone successions which dom-
conformity beginning at the basinward termination inate marine successions. In practice, the boundary
of the subaerial unconformity and extending through is commonly placed at the contact between interbed-
the conformable marine succession. Such a boundary ded sandstones and shales below and more massive
is portrayed in Fig. ,1 a stratigraphic cross-section sandstones above (see fig. 31 in Haq et al., 1988).
which displays the various stratigraphic surfaces and However, such a boundary is commonly a very di-
horizons which are developed during a cycle of base achronous facies boundary and this horizon has no
level rise and fall. The subaerial unconformity is theoretical linkage to the change from base level
developed and migrates seaward during base level fall to base level rise. Other lithologic boundaries
fall and reaches its maximum extent at the end of which have been used to approximate the correlative
the fall. As noted earlier, the depositional surface conformity are discussed in the next section.
Sequence boundaries and sequence :seihcrareih problems and slasoporp
.giF .1 stratigraphic A schematic noitces-ssorc which swohs the between the relationships xis secafrus of stratigraphy-subaerial sequence
,ytimrofnocnu ravinement ,ecafrus evisserger erosion, of surface evissergsnart ,ecafrus mumixam surface flooding dna slope onlap -ecafrus
dna clastic .seicaf The noitces-ssorc saw a using constructed constant to of sediment break with a a supply shelf/slope basin distinct dna
a fo elcyc base level rise dna various Using fall. secafrus sa unit three boundaries, main of types secneuqes have been defined sa .swollof
)1( bounded sequence Depositional yb and/or ravinement a unconformity subaerial a ,ecafrus dna the time line to start equivalent of base
level rise (end of base level fall) (Jervey, .)8891 ehT slope onlap surface si oftuesne d sa na of the approximation theoretical time .enil
)2( Genetic stratigraphic bounded sequence yb mumixam flooding secafrus ,yawollaG( .)9891 )3( T-R bounded sequence yb a subaerial
and/or surface unconformity ravinement a dna a evissergsnart surface (this paper).
In summary, the lack of any distinctive lithological over all or most of its extent; (2) it has minor di-
change in regressive marine strata at the time of a achroneity in relation to the duration of the base
change from base level fall to base level rise makes level rise-fall cycle; (3) it merges with the basinward
the objective recognition of the correlative confor- termination of the subaerial unconformity so as to
mity (sensu Exxon) next to impossible. This, in turn, form a continuous stratigraphic boundary.
prevents the objective recognition of depositional The most radical solution yet proposed to resolve
sequence boundaries over much of a basin and sig- the dilemma of the invisible correlative conformity
nificantly reduces the practical usage of depositional was the definition of the genetic stratigraphic se-
sequences for basin analysis. quence by Galloway (1989). This type of sequence
uses a maximum flooding surface for each bound-
Proposed solution ary. Such a stratigraphic surface marks the change
from transgressive strata below to regressive strata
The solution to this problem of recognizing a above and it is lithologically very distinctive (Gal-
sequence boundary in the conformable marine suc- loway, 1989). A maximum flooding surface commonly
cession lies in the identification of a stratigraphic varies from being a conformable surface to an un-
surface which has three characteristics. These are: (1) conformable one with the unconformity being caused
it is characterized by a distinctive lithological change by net submarine erosion in areas of very low sed-
4 A.E Embry
imentation (Thorne and Swift, 1991). The genetic does not, in some cases, join the seaward termination
stratigraphic sequence fits the general Vail et al. of the subaerial unconformity as interpreted from
(1977) definition of a sequence. In fact, it is worth seismic. Rather, it is truncated by the regressive sur-
noting that Vail et al. (1977) used maximum flooding face of erosion (Plint, 1988) which itself is truncated
surfaces (downlap surfaces) as sequence boundaries by the subaerial unconformity. These relationships
although this practice was later dropped (e.g. Vail are shown in Fig. 1 and they result in part of the
and Todd, 1981). subaerial unconformity lying above, rather than on,
Although use of genetic stratigraphic sequence the designated sequence boundary (slope onlap sur-
boundaries resolves the boundary recognition prob- face).
lem, this solution is not without problems. The main (3) The most serious drawback of the usage of
drawback of using such boundaries is that a signifi- the slope onlap surface is that it does not develop
cant subaerial unconformity, across which changes in in many circumstances. These circumstances include
the depositional regime occur, is commonly included ramp settings where there is no well defined slope
within such a sequence. This results in the portion (Fig. 2) and situations in which the shoreline does
of the genetic stratigraphic sequence which is on the not migrate close enough to the shelf/slope break to
basin margin being composed of two very different, cause a significant shift in the depositional pattern
genetically unrelated units. during base level fall.
As described earlier, another attempt to resolve In summary, a slope onlap surface may be a
the problem has been to place the correlative con- reasonable choice for the correlative conformity in
formity at the contact between shelf-lower shoreface some specialized situations, but it is unsuitable for
strata below and mid-upper shoreface strata above usage in a general sequence model which is meant
(Haq et al., 1988). This is an entirely unsatisfactory to be applicable to a wide variety of stratigraphic
solution due to the large diachroneity of such a sur- situations.
face, its limited areal extent and the lack of any From my experience I have found that the most
theoretical linkage to the defined correlative confor- suitable stratigraphic surface for the conformable
mity. portion of a sequence boundary is the transgressive
Another surface which has been used as an ap- surface. This surface is illustrated in Figs. 1 and 2
proximation of the correlative conformity is the slope and is defined as a conformable surface which sepa-
onlap surface. This surface is illustrated in Fig. 1 and rates regressive strata below from transgressive strata
it develops in slope deposits during base level fall. above. The surface adequately meets the previously
When the shoreline has migrated near or all the way stated requirements for the conformable portion of
to the shelf/slope break, there is a significant change a sequence boundary. It is characterized by a dis-
in sedimentation pattern with the sediments being tinctive lithologic change which can be recognized
channelled mainly down submarine canyons. This re- in most marine successions and in both ramp and
suits in sediment starvation over much of the slope shelf/slope settings. Even in areas of the marine
and eventually the slope is onlapped by expanding basin which received only clay and silt, boundary
channel-fed accumulations. placement can be made commonly at the base of
The slope onlap surface is commonly recognized an interval of increased clay and organic matter
on seismic lines (e.g. Greenlee and Moore, 1988) content. Embry (1993) illustrates examples of such
and on such data it appears that the surface merges transgressive surfaces in both subsurface and surface
updip with the subaerial unconformity. Thus it would sections. The transgressive surface may be slightly di-
appear that the slope onlap surface is a suitable achronous, because transgression may begin later in
candidate for the conformable portion of a sequence areas of high sediment supply, but such diachroneity
boundary. Unfortunately the usage of this surface has is minor in relation to the duration of the base level
three drawbacks. In order of increasing severity these rise-fall cycle.
drawbacks are: Finally, in most cases, the transgressive surface
(1) Although the surface is clearly discernible on merges with the unconformity at its basinward ter-
seismic lines, the lithological change at the surface mination (see Figs. 1 and 2). This relationship is
is very subtle. As a compromise, the contact is most controversial and is detailed in Fig. .3 Figure 3A
commonly drawn at the first sandstone or siltstone illustrates a case in which the transgressive surface
unit overlying slope shales. does not join the seaward termination of the sub-
(2) As elegantly shown by Cartwright et al. (1993), aerial unconformity. In this case regressive fluvial-
the poor vertical resolution of seismic data leads brackish deposits have onlapped the subaerial uncon-
to an oversimplification of stratigraphic relationships formity between the time of the start of base level
portrayed on a seismic line. The slope onlap surface rise and the start of transgression. This results in
Sequence boundaries and sequence :seihcrareih problems and slasoporp
.giF .2 A citamehcs cihpargitarts noitces-ssorc ralimis ot that ni .giF 1 tpecxe that eht nisab skcal a epols/flehs break (ramp .)gnittes nI siht
gnittes eht slope onlap surface si ton generated tub eht other evif secafrus fo ecneuqes yhpargitarts era present. ehT noitaeniled of -oped
lanoitis ecneuqes boundaries ni eht elbamrofnoc strata a of ramp noisseccus cannot eb done with yna ecnalbmes of ytivitcejbo (see .)txet
the transgressive surface lying above the subaerial to be much more common in the stratigraphic record
unconformity within the fluvial-brackish water strata. than that in Fig. 3A. This assessment is based on my
From my experience this stratigraphic situation is the stratigraphic studies in the Canadian Arctic, discus-
exception rather than the norm, which is illustrated in sions with colleagues and a failure to find any docu-
Fig. 3B. As shown in the latter case, the transgressive mented examples of the 3A relationships in the lit-
surface merges with the ravinement surface which erature. The stratigraphic relationships shown in Fig.
then merges with the subaerial unconformity. This 3B develop in most cases because, during transgres-
situation results in one continuous boundary consist- sion, shoreface erosion most commonly cuts down
ing of an unconformable portion and a conformable through all of the onlapping, regressive, fluvial strata
portion and satisfies the requirement for a practical which were deposited between the time of the start
sequence boundary. of base level rise and the time of the start of trans-
The stratigraphic situation in Fig. 3B would appear gression. Shoreface erosion also removes a portion of
6 A.E Embry
Fig. 3. (A) Schematic stratigraphic cross-section on which the conformable transgressive surface does not merge with the basinward
termination of the unconformable portion of the sequence boundary. In this case regressive, fluvial to brackish strata onlap the subaerial
unconformity and, as a consequence, the subaerial unconformity and transgressive surface do not form a single through-going boundary.
(B) Schematic stratigraphic cross-section on which the conformable transgressive surface merges with the basinward termination of
the unconformable portion of the boundary and the surfaces form a single boundary. In this case all of the regressive, fluvial strata
which initially onlapped the subaerial unconformity were eroded by shoreface erosion. This results in a ravinement surface forming the
basinward edge of the unconformable portion of the sequence boundary. Data from Mesozoic sequences of the Canadian Arctic and a
literature survey indicate that the stratigraphic relationships shown in (B) are much more common than those in (A).
the subaerial unconformity at this time. Thus, in this In summary, the use of a transgressive surface
situation, all of the preserved fluvial to brackish wa- as the conformable portion of sequence boundary
ter strata which onlap the subaerial unconformity are provides a practical solution to the problem of ob-
transgressive strata and are not regressive lowstand jectively extending a sequence boundary through a
strata as proposed by Van Wagoner et al. (1990). This conformable succession of marine strata. A sequence
point was forcefully made by Thorne and Swift (1991) which is bounded by subaerial unconformities and/
who used data from Pleistocene and Quaternary sedi- or ravinement surfaces and their correlative trans-
ments. gressive surfaces is called a T-R sequence. Such a