Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level'fall

Sedimentary Geology, 81 (1992) 1-9 1 Elsevier Science Publishers B.V., Amsterdam

ExpresSed

Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level'fall

D a v e H u n t * a n d M a u r i c e E . T u c k e r

Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK

(Received June 16, 1992; revised version accepted September 14, 1992)

ABSTRACT

Hunt, D. and Tucker, M.E., 1992. Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall. Sediment. Geol., 81: 1-9.

To overcome inconsistencies in the Exxon sequence stratigraphic model as applied to siliciclastic and carbonate shelf margins, it is proposed that an ideal sequence should consist of four systems tracts. In addition to the transgressive and highstand systems tracts, developed during rising base-level, it is suggested that there should be two systems tracts associated with falling and lowstands of relative sea-level. These are: the forced regressiue wedge systems tract formed during falling base-level, bounded below by the 'basal surface of forced regression' and above by the sequence boundary, representing the lowest point of sea-level fall, and the lowstand prograding wedge systems tract, developed as relative sea-level begins to rise after sequence boundary formation. This systems tract downlaps the basin-floor forced regression deposits in a basinwards direction and onlaps forced regressive wedge sediments on the slope. The forced regressive wedge systems tract consists of shallow-water stranded parasequences deposited on the upper slope to the shelf, and basin-floor fan or apron sediments, deposited at the toe-of-slope and derived from erosion of the stranded parasequences and/or erosion of the previous highstand shelf and shelf-margin sediments.

Introduction

In r ecen t years app l i ca t ion of sequence s trat i -

g raph ic mode l s to s e d i m e n t a r y bas in fills has in-

c r ea sed dras t ica l ly wi th the gene ra l a ccep t ance of

the sequence s t r a t ig raph ic a p p r o a c h (e.g. p a p e r s

in P o s a m e n t i e r et al., 1993). T h e driving cont ro ls

u p o n indiv idual sequences , no tab ly the ro le of

eusta t ic , tec tonic , depos i t i ona l a n d / o r envi ron-

men ta l factors , however , r ema in controvers ia l . A

cri t ical cons ide ra t ion o f the concep ts of s equence

s t ra t ig raphy, pa r t i cu la r ly as it is app l i ed to the

s ed imen t s d e p o s i t e d dur ing base- leve l fall and

sea- level lowstand, reveals some incons is tenc ies

in the gene ra l mode l , as e x p o u n d e d by the Exxon

Correspondence to: D. Hunt, Department of Geology, The University, Manchester M13 9PL, UK.

g roup and its d isc iples (e.g. V a n W a g o n e r et al.,

1990). This shor t p a p e r addres ses the p r o b l e m of

s ed imen t s d e p o s i t e d dur ing a base- leve l fall on a

she l f -marg in s lope and in the basin, and of the

ch ronos t r a t i g raph ic pos i t ion of the sequence

boundary . The p r o b l e m s with the cu r r en t Exxon

mode l a re e n c o u n t e r e d in a reas of good exposure

o f she l f -marg in and s lope facies, whe re s ed imen t

body geome t r i e s and re la t ionsh ips a re clear , and

the b ios t r a t ig raph ic cont ro l is excel lent .

Sequence stratigraphy terminology

In the concep ts of sequence s t ra t ig raphy as

advanced by the Exxon group (e.g. Van W a g o n e r

et al., 1988; P o s a m e n t i e r e t al., 1988; Van Wag-

one r et al., 1990), a s equence is de f ined as a

re la t ively con fo rmab le gene t ica l ly r e l a t ed succes-

0037-0738/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

2 D. H U N T A N D M.E. " F U C K E R

sion of strata bounded by unconformities or their

correlative conformities. A sequence is divided

into systems tracts, deposited during specific in-

tervals of the relative sea-level curve. In a type 1

sequence, deposited after a base-level fall which

exposed the shelf and during the subsequent base-level rise, three systems tracts are usually

recognised: the lowstand, transgressive and high-

stand systems tracts. The lowstand systems tract

is divided into a lowstand fan (LSF) deposited

during the base-level fall and a lowstand wedge

(LSW) deposited at the lowstand and in the early

part of the base-level rise, before the transgressive systems tract is established with the in-

creased rate of sea-level rise (see Fig. 1). Al-

though its definition has changed in a subtle

fashion in the last few years (see Schlager, 1991),

a sequence boundary is regarded as an uncon-

formity (and its correlative conformity) separating

younger from older strata, along which there is

evidence of subaerial erosional truncation or sub-

aerial exposure with a significant hiatus. A se-

quence boundary of this type wilt be best devel-

oped on a siliciclastic shelf or carbonate platform,

with a correlative conformity developed in the

basin. Sequence boundaries in the Exxon model

occur beneath the lowstand systems tract so that in terms of chronostratigraphy, the one point in

time represented along the whole length of the

sequence boundary, from the inner shelf to the

basin, has to be during the early part of the

base-level fall. On the inner shelf, the hiatus of

the sequence boundary could be a significant

unconformity representing the late highstand

through to the transgressive systems tract of the

next sequence, whereas in the basin, the correla-

tive conformity may not be a hiatal surface at all.

In the toe-of-slope region, the sequence bound-

E X X O N S Y S T E M A T I C S 1. S T R A T A L P A T T E R N S sb ~ . ~ p o s u r e and subaer~l diagenesis ~ . s.Lt

L_. i T i i / , l I ~ ~ ~ ~___~ d . . . . . ing expo . . . . . . d -- ' " - '~ : . : .~c . .~ ;~ ..... 1\ subaeriat diagenesis " ~

H~lhstand systems tract ~ ~ - ~ ~ i~T lk . I \ of preceding seque

Th ~ s e q u once s ~ ~ ~ • -..z ... ................ . : . : ~ 7 \

These are placed below the sequence LSF boundary.

sb \AIIochthonous debris derived from

Relative sea-level collapse of the slope as sea-level falls. This is equivalent to the Iowstand fan of silioiclastic shelves and is at present

H S T / ~ T q T placed above the sequence boundary / ~ ' ~ ' \ Z ~ 2. C H R O N O S T R A T I G R A P H Y in the Iowstand systems tract.

l ~= I~ , , , , . . . . ,,~:~?!~:i:!~i;:~i!~:::i~iJ:?~i!:~i~i~i~ - - - - . . . . . . \ +~ . . . . boundary -~LSF

K E Y Fac,es; [ ~ ' . . . . . he. facies [ ] sheff-marginfacies [ ] foreslopefacies ~ slumps/debris flows

] allochthonous debris (mega breccia & slumDs) ~ meteoric diagenesis

Surface; =..~sb sequence boundary

Fig. 1. Cross-section of a carbonate sand-shoal rimmed-shelf showing facies, stratal patterns, chronostratigraphy and relationship to relative sea-level change for sediments deposited during a third-order base-level fall with smaller-scale sea-level rises and falls superimposed upon it. As relative sea-level falls, slope collapse supplies sediment to the basin-floor (allochthonous debris) and autochthonous slope wedges (stranded parasequences) site progressively lower down the slope. However, in the Exxon terminology,

the sequence boundary is placed beneath the basin-floor fan (LSF), but above the stranded parasequences.

STRANDED PARASEQUENCES AND THE FORCED REGRESSIVE WEDGE SYSTEMS TRACT 3

ary will occur beneath the coarse sediments of the lowstand basin-floor fan, which represent the first deposits of the base-level fall.

Deposition during base-level fall

Slope bypass, basin-floor fans and megabreccias

In many instances, there is no sediment deposited on the upper part of a slope during a relative sea-level fall. In siliciclastic depositional systems, the shelf and shelf margin are typically subjected to subaerial erosion during a lowstand. Incised valleys may be cut, and vast quantities of sediment discharged onto the lower slope to form a lowstand basin-floor fan (LSF). The upper slope is typically bypassed. The basin-floor fan consists of fluvially derived sandstones and mudrocks, deposited largely by debris flows and turbidity cur- rents.

In a carbonate system, exposure of a shelf rarely results in mechanical reworking of the shelf, but more typically in a 'chemical reworking' (cementation/dissolution) in the form of subaerial, surface-related diagenesis that will tend to be climatically controlled (e.g. humid-karstification, arid-dolomitization) (see Tucker, 1992). Expo- sure of the carbonate shelf thus does not normally result in an increased sediment supply to the adjacent slope/basin, but the reverse, as neg- ligible sediment is supplied off the shelf top. During the lowstand, slope and basinal periplat- form ooze sedimentation rates may decrease drastically as the shelf top produces no carbonate mud (Droxler and Schlager, 1985; Wilber et al., 1990). During a lowstand, collapse of the exposed carbonate platform margin, or of the shoulder of a distally steepened ramp, now in shallower water, commonly takes place with the deposition of megabreccias in a toe-of-slope apron. Megabrec- cias tend to form upon steep slopes (> 25°), and as such are more likely to form upon mud-free, grain-supported slopes, or those subject to early cementation/framebuilding (Kenter, 1990). Ex- amples of such lowstand deposits include the Raisby slide breccia, Zechstein (Upper Permian) of NE England (Tucker, 1991), the lower Carnian Marmolada breccia (Triassic) of the Dolomites in

northern Italy (Bosellini, 1984; Doglioni et al., 1990), Late Cretaceous-Eocene platform-margin breccias of southern Italy (Bosellini, 1989), and the 80,000-120,000 yr B.P. debrite of Exuma Sound described by Crevello and Schlager (1980). Caution should be used if attempting to use carbonate megabreccias as lowstand 'predictors' as they are not specific to times of falling/lowstands of relative sea-level. Aggradation during the transgressive systems tract can lead to oversteep- ening and collapse of the shelf margin, as in the Cambrian of the southern Rocky Mountains, Canada (Mcllreath, 1977) and Permian Bone Spring Formation of the Delaware Basin (Sailer et al., 1989). Faulting can also generate megabreccias, especially in rift basins (e.g. Colaci- cchi et al., 1975; Eberli, 1987). Megabreccias have occasionally formed in areas of low-angle slopes (e.g. mid-Cretaceous Urgonian platlorm, Aravis Range, French Alps, Spence and Tucker, 1992).

Stranded parasequences, slope-perched deltas and forced regression

During third-order (0.5-3 Ma) relative sea- level falls, higher-order cycles of sea-level change (i.e~ 4-5th order, 10,000 yr to 0.5 Ma) may be superimposed upon the general base-level fall. This results in acceleration and deceleration of the third-order base-level fall. During times of decelerated fall, parasequence-scale sediment bodies may be deposited on the upper slope and then subsequently abandoned, exposed and incised as the third-order fall continues. Such deposits are termed 'stranded' parasequences (Posamentier et al., 1990; Van Wagoner et al., 1990) and the relative sea-level fall is termed a forced regression. Stranded parasequences will be best developed on low-gradient foreslopes to shelf margins. They are unlikely to form on high- angle shelf margins, typical of many reef- dominated carbonate platforms, or on fault- bounded shelf margins, as may occur in rift and early extensional basins.

In siliciclastic. ~ystems, stranded parasequences are typically shallowing-upward units of shoreface to beach facies or coarsening-up deltaic sediments, deposited at the mouths of incised valleys.

4 D . H U N T A N D M . E . T U C K E R

They are generally not very thick (several metres), as a result of the decreasing accommodation space on the upper slope during the base-level fall, and there is little or no coastal plain facies updip on the shelf. Examples of slope-perched deltas occur in the Cretaceous of the Western Interior Sea- way, Utah and Colorado, and in the Tertiary of the subsurface of the Gulf Coast region, Louisiana (Van Wagoner et al., 1990).

Upper-slope, shallower-water siliciclastic environments may supply sediment to the toe-of-slope and basin through sediment gravity flows. The sediment is likely to be texturally mature, even compositionally mature, as a result of the extensive reworking in the beach, nearshore and delta mouth bar environments. The resedimented deposits will generally be poorly structured sands, with a vague lamination and some water-escape structures. Such deep-water massiue sands are a particular facies type of deeper-water siliciclastic successions (e.g. Stow, 1992). It is suggested here that they are significant in terms of sequence

stratigraphy and base-level change, reflecting the development of stranded parasequences on the upper slope.

In carbonate environments, stranded parasequences deposited on upper slopes to platform margins during forced regression will typically consist of grainstones or reefs. These facies types are the most likely to develop in view of the relatively high-energy conditions that would exist on the upper foreslope facing an open ocean. In most carbonate systems, the shelf is exposed during the lowstand and subject to karstification and meteoric diagenesis. Carbonate sediment is not normally eroded from the shelf, which is usually cemented up, although in some situations, siliciclastic sediment is carried across the shelf and deposited at the toe of the slope. This was the case in the Permian Delaware Basin of New Mexico and Texas, in the lowstand Cherry and Brushy Canyon Sandstones (Sarg, 1989; Sailer et al., 1989). Examples of stranded carbonate parasequences occur in the mid-Cretaceous Ur-

NEW SYSTEMATICS Forced regressive wedge systems tract (slope component) comprised of three

Exposure and, suba~ial di_agene._sis j s b _ _ " T autochthonous slope wedges ~ s.L1

. . . . . . . . "':'::i:~(ii?!~i!};:f::. r . . . . • . . . . . . ~ S.12

surfaces of forced / - - - - - - regression / - . ~ ~ a-er~°~i~°rce° / Forced reg . . . . . edge systems tract

• ~ ~ k ~ . ~ . (basin floor component) comprised ~ m ~ d ~ ~ ~ . . . . . . . . ] /" of aUocthonous debris

HST ~ ~ ~ = ~ V ~ /

FRWS ST ~ /

~ ~ LPWST ~ . : . . ~ . . . . . . . . . . ~ ................................................. _ . - ~

LPWST m i / the time of sequence boundary formation ~.::!~!::!~ ........ ~ . ~ . ~ , ? . ~ _ T ~ - ~ . ~ - | Lr'WST I

m__,0 . . . . : _ _ , . , . . . . . . . . . . . . . . J F R W S T ~ FRWST[- ~ . ~ , - A .~\Jt = 1 F R W S T I ~

VA . . . . . t - A : . ~ . : . : . : ~ , ~ - ~ ~ ~ .e. ,¢ . t - . ~ ~ -.! - k

new position of sequence basal surface of bourldst3/on the basin floor sequence boundary forced regression

superimposed upon basal (downlap surface) surface of forced regression

KEY Facies; as in Fig 1

Surfaces /sb New position of sequence boundary developed at time of lowest sea-level

Basal surface of forced regression

Fig. 2. Similar cross-section to Fig. 1 but showing a different interpretation, with the stranded parasequences and the basin-floor

fan comprising the forced regressive wedge systems tract beneath the sequence boundary and the lowstand prograding wedge the

first systems tract of the new sequence.

STRANDED PARASEQUENCES A N D THE F O R C E D REGRESSIVE W E D G E SYSTEMS TRACT 5

gonian of the Vercors in the French Alps, where a bioclastic sand body developed on the upper foreslope to a platform during the early stages of base-level fall (Hunt and Tucker, 1993), and in the Miocene of the Nijar region of Almeria, SE Spain (Dabrio et al., 1981; Fransen and Mankiewicz, 1991), where several downstepping reefal parasequences were deposited during a 3rd-order base-level fall.

During episodic base-level fall, stranded parasequences will be subjected to erosion, and in a carbonate environment, exposure and karstification. Sediment will be supplied to the basin- floor fan or apron. In a carbonate system, the basin-floor fan will consist of 'fresh' grains derived from an active, perched sand shoal or reef on the upper foreslope. In addition to the fresh grains, there may well be some lithoclasts derived from a higher abandoned parasequence or from the earlier platform-margin highstand facies. Basin-floor fans of this type occur in the lower Barremian (Cretaceous) Urgonian of the Vercors in the French Alps (Hunt, 1992).

The new terminology

The stratal patterns and chronostratigraphy of both 'stranded' parasequences and the lowstand systems tract in relation to the 'Exxon'-defined sequence boundary are summarised in Fig. 1. On the slope, 'stranded' parasequences are placed below the sequence boundary, i.e. the sequence boundary is forming close to the lowest point of sea-level (Van Wagoner et al., 1990). However, in the Exxon scheme the basin-floor time-equivalent deposits to the 'stranded' slope parasequences, allochthonous debris in the basin-floor fan (LSF), are placed above the sequence boundary. This means that the formation of the sequence boundary on the basin-floor occurs prior to the lowest point of sea-level (SI 2, Fig. 1) and is older than the sequence boundary on the foreslope above the stranded parasequences. Thus, in this model, the position of the sequence boundary in relation to geological time is somewhat contradictory and ambiguous. This requires a revision of the model and this is provided here in Fig. 2.

The revised scheme (Fig. 2) is based on the

models of a type 1 sequence of Van Wagoner et al. (1990) and earlier authors. The most significant revisions are the subdivision of the current lowstand systems tract into two newly named systems tracts, the forced regressive wedge systems tract and the lowstand prograding wedge systems tract, and the alteration of the position of the sequence boundary on the basin-floor to above deposits formed as base-level fell. In this way, the sequence boundary is now everywhere coincident with the lowest point of relative sea- level. The new systems tract boundaries are cho- sen to coincide with changes of both the rate and direction of relative sea-level change. In reality, this new scheme is academic and no easier to use than its predecessors for real geological situations, but it does eliminate the contradictions and ambiguities associated with failing and lowstands of relative sea-level in the previous models.

The forced regressive wedge and lowstand prograding wedge systems tracts

Sediments deposited during forced regression (i.e. falling relative sea-level), but prior to the lowest point of relative sea-level, are placed within the forced regressive wedge systems tract (FRWST) (Fig. 2). The base of this systems tract is the basal surface of forced regression (BSFR) (Fig. 2), a chronostratigraphic surface separating older sediments of the preceding highstand systems tract, deposited during slowing rates of relative sea-level rise and stillstand, from younger sediments, deposited during the base-level fall (Fig. 2). The systems tract has a slope component, termed the forced regressive slope wedge (after which the systems tract is named), and a basin-floor component, the forced regressive basin-floor fan~apron. Both components are schematically illustrated in Fig. 2.

The slope-wedge component of the FRW systems tract consists of one or more 'stranded' parasequences bounded below by the basal surface of forced regression and above by the sequence boundary (Fig. 2). Thus, the upper and lower surfaces are common to both components of the systems tract (Fig. 2). Slope and basin-floor elements of the forced regressive wedge systems

6 D. H U N T A N D M.E. T U C K E R

tract (as depicted in Fig. 2) are not necessarily developed together during an individual forced regression; the FRW systems tract may be just represented by the slope wedge or just .the basin- floor apron. Alternatively, the systems tract may be totally absent as a result of non-deposition during the forced regression (e.g. the sea-level fall is too rapid to allow deposition or the slope is too steep to sustain carbonate production).

The upper surface of the forced regressive wedge systems tract is the sequence boundary; this represents the lowest point of relative sea-level, and is thus the most extensive unconformity (Figs. 1 and 2). The position of the sequence boundary on the shelf top and upper slope is unchanged from previous models, but on the basin-floor, it should be placed above sediments (if any) deposited during the forced regression (i.e. above

the lowstand fan) so that it is now truly a chronostratigraphic surface in that all sediments below it are older and those above it are younger (e.g. compare positions of the sequence boundary in Figs. 1 and 2). Any sediments deposited at, or after, sea-level has reached its lowest position are above the sequence boundary and are thus part of the lowstand prograding wedge (LPW) systems tract, developed from the time that relative sea- level is at its lowest point and beginning to rise, but prior to the transgressive systems tract. The LPW systems tract downlaps the sequence boundary in a basinwards direction and onlaps it landwards (Figs. 2 and 3). The size of the LPW systems tract reflects slope angles and the ratio of the rate of relative sea-level rise to that of sedimentation (see Fig. 3). Its upper surface marks the beginning of the transgressive systems tract.

Rimmed she l f : LST models

Control: inherited slope morphology

exposure and subaer i , I diagenesis

z" aut~hmonous wedge . . . . . . . . . . . . . . . . . . . . . . . . ?':!.):i;:::;:?.:¢!:2:1

AIM All2

exposure and subaerial diagenesls - -

eo,oc.,, co,,.0 . . . . . ~ . ~ : ~ : : : ~ i ~ : ~ i ~ ` : ~ : ~ j ~ : ~ i ~ ! ~ : i ~ i i ~ ~

All1

f lI~s-IOO'S f metres

Fig. 3. Two end-member models for lowstand prograding wedge systems tracts for a carbonate platform with a low-angle mud-supported foreslope (above) and a high-angle grain-supported foreslope (below). As base-level falls, slopes undergo mass-wasting through slumps and debris flows to give a basin-floor fan (All. 1), of the forced regressive wedge systems tract. Upon exposure, the shelf undergoes chemical 'reworking'. When sea-level reaches its lowest point the sequence boundary is formed and a lowstand prograding wedge of autochthonous material may then develop and build out from the slope. The potential for sediment production is directly related to the available source area (P), and this increases as slope angles decrease. If, as modelled here, carbonate production rates remain constant then as the wedge builds out, topography increases and a bypass margin to the autochthonous wedge can develop (All. 2). Such secondary basin-floor allochthonous debris formed after the lowest point of relative sea-level fall sits above the sequence boundary. The lowstand prograding wedge will show a progradation to aggradation pattern of bedding geometries as the amount of accommodation space increases when base-level begins to rise more rapidly. The two slope end-members, high angle and low angle, develop contrasting styles of lowstand prograding wedge. Low-angle slopes develop wide, volumetrically significant lowstand prograding wedges, whereas high-angle slopes develop narrow autochthonous wedges. These Iowstand prograding autochthonous wedges onlap the slope and downlap basinwards onto the sequence boundary.

STRANDED PARASEQUENCES AND THE FORCED REGRESSIVE WEDGE SYSTEMS TRAC'F 7

A LPW systems tract will typically show a parasequence stacking pattern or bedding/clinoform

geometry of progradation to aggradation, as the amount of accommodation space begins to in- crease. In a carbonate system, much new shallow-water sediment can be created in a LPW systems tract. This was the case in the lower Barremian Urgonian of the Vercors, French Alps (Arnaud-Vanneau and Arnaud, 1990; Jacquin et al., 1991; Hunt and Tucker, 1993).

Since the FRW systems tract lies below the sequence boundary, it becomes the fourth and final systems tract of a sequence. The first three systems tracts (lowstand prograding wedge, transgressive and highstand systems tracts) of a sequence are now all formed during times of rising base-level (Fig. 2) after the lowest point of relative sea-level (represented by the sequence boundary, Fig. 2). The development and distinc- tion of each of these systems tracts (LPW, TST and HST) will depend upon the ratio of sedimentation rate to the rate of base-level rise, so that their boundaries can form at different points on the relative sea-level rise curve. The fourth systems tract of a sequence (FRW) forms during the time of falling relative sea-level (forced regression) and is terminated by the sequence boundary representing the lowest point of base-level (Fig. 2). Thus, in the scheme presented here, the upper and lower bounding surface to a sequence, the sequence boundary, is more precisely defined everywhere to form at the lowest point of relative sea-level.

It could be argued that the megabreccias which commonly develop during the lowstand of a carbonate rimmed shelf should be attributed to the forced regressive wedge systems tract. In most cases, these megabreccias are derived entirely from the highstand shelf margin and they proba- bly form during the relative sea-level fall. In a sense they also represent the final stage in a cycle of deposition, before any new sediment is gener- ated. They should therefore be regarded as the end of one sequence rather than the beginning of the next.

In siliciclastic depositional systems the times of falling and lowstand of relative sea-level are syn- onymous with increased sedimentation rates and

the deposition of extensive lower-slope and basin-floor coarse clastics. In carbonate depositional systems, the response is typically more complex, although in terms of sedimentation rates, times of failing and lowstand of relative sea-level are normally associated with a decrease or even cessation of sedimentation, as the shelf is exposed and the carbonate factory closed down. In special situations, normally where there is a strong antecedent topographic control or tectonic influence, and /or high production rates as a result of oceanographic factors (such as leeward/windward effects), the lowstand prograding wedge can become a major location of shallow-water carbonate production and deposition. Bypass may lead to fans and aprons on the basin floor ahead of the wedge, as shown in Fig. 3. This also occurred in the lower Barremian (Urgonian) LPW of the Vercors, French Alps (Arnaud-Vanneau and Arnaud, 1990; Hunt and Tucker, 1993).

Conclusions

This short paper has drawn attention to some inconsistencies in the concepts of sequence stratigraphy and has suggested that sequences should be discussed in terms of four systems tracts: lowstand prograding wedge, transgressive, highstand and forced regressive wedge systems tracts. The last systems tract is introduced here to cover those sediments deposited during base-level fall, when stranded parasequences may be deposited on the upper slope of a shelf margin and a basin-floor fan or apron may be deposited in a toe-of-slope location. The lowstand prograding wedge is the first systems tract of a sequence. A sequence boundary is defined as a chronostratigraphic surface developing at the lowest point of sea-level. These modifications to the increasingly used general sequence stratigraphic model re- move the inconsistencies inherent in the original Exxon model.

The existing Exxon-derived sequence stratigraphic model can be applied to many carbonate and siliciclastic successions. However, the prob- lems arise where sediments were deposited on the upper slope to a shelf during base-level fall

8 D. HUNT AND M.E. TUCKER

and during the lowstand, and where exposure is sufficiently good to see the geometric relation- ships.

Acknowledgements

D. Hunt gratefully acknowledges the receipt of NERC grant GT4/88 /GS/30 .

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Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level'fall