Boggs S. Principles of Sedimentology and Stratigraphy

Подождите немного. Документ загружается.

13.2 Seismic Stratigraphy

447

the other hand, major depositional units or systems such as progradational delta-

slope systems, carbonate shelf-margin systems, or marine offlap-onlap systems

can be identified. Once depositional sequences have been correlated basinwide,

these sequences provide a first-order stratigraphic framework within which seis-

mic facies can  studied in more detail. Figure 13.11, a seismic section from offshore

Newfoundland, provides an example of sequences delineated by unconformities

that separate different depositional units.

The procedure for carrying out seismic sequence analysis thus involves the

following steps:

1. Picking unconformities in a given area by recognizing reflection terminations

along their surfaces.

2. Extending or extrapolating these boundaries throughout the complete section,

including areas where the reflectors are conformable, to define the sequences

completely.

3. Repeating the process of delineating sequence boundaries on seismic records

from other parts of a basin or region and correlating the sequences throughout

the seismic gd to produce a three-dimensional framework of successive strati

fied seismic sequences separated by unconformities or correlative confoities.

4. Mapping sequence units on the basis of thickness, geometry, orientation, or

other features to see how each sequence relates to neighboring sequences.

Details of seismic sequence mapping and its application to geologic problems are

given by Hubbard, Pape, and Roberts (1985a, 1985b), Vail (1987), and Vail et al.

(1991). The sequence concept has become so important in stratigraphic studies

that it has taken on the role of a specialized branch of stratigraphy called sequence

stratigraphy. Sequence stratigraphy is further discussed in Section 13.3.

Seismic Facies Analysis

Seismic facies analysis takes the interpretation process one step beyond seismic

sequence analysis by examining within sequences smaller reection units that

may be the seismic response to lithofacies. In seismic facies analysis, the most

common reflection characteristics used to distinguish one seismic facies from an

other are the geometry of reflections or reection terminations with respect to the

two unconformity surfaces bounding the sequence, the exteal geometry of the

facies, and the inteal configuration and character of the reflections. A seismic fa

cies unit is a mappable, areally definable, three-dimensional unit composed of

seismic reflections whose characteristic reflection elements differ from those of ad

jact units. It is considered to represent or express the gross lithologic aspect and

stratication characteristics of the depositional unit that generates the reflections.

Some of the more important seismic reection patterns that constitute seismic fa

cies are olap, onlap, submarine mounds, channel/overbank complexes, slope

front fill, toplap, and drape (Figs. 13.10, 13.12).

ocedures for Interpreting Seismic cies. The objective of seismic facies analysis

is regional interpretation of lithology, depositional environments, and geologic

history. The interpretation process proceeds in several distinct steps (Mitchum and

Vail, 1977; Vail, 1987).

The first step is recognizing and delineating seismic facies units within each

sequence on all of the seismic sections in the region being mapped. The most use

ful seismic parameters in seismic facies analysis are the following:

 The geometry of reflections (Fig. 13.12) and reflection terminations

2. Reection configuration (e.g., parallel, divergent)

Three-dimensional form (Fig. 13.13)





gure 13.1 1

Depositional sequences as defined from seismic records. In this example from offshore Neoundland, seismic sequence boundaries are shown by solid

black lines. The vertical scale on this rord

.is

given in seismic wave two-way travel time  rather than depth. [From Hubbard, R. H., J. Pape, and

D. G. Roberts, 1985, Depositional sequence mapping to illustrate the evolution of a pas�ve continental margin, in Berg, 0. R., and R. G. Wolverton

(eds.), Seismic stratigraphy 11-An integat approach: Am. Assoc. Petroleum Geologists Mem. 39, p. 104, Fig. 8, reprinted by permission of MPG,

Tu lsa, Okla.]

Sigmoid

olap

Aggradational

offlap

Oblique

offlap

Apparent

truncation

Channel/overbank

complex

Slope front

fill

Mound

13.2 Seismic Stratigraphy

449

Figure 13.12

A simulated seismic section il

lustrating some common seis

mic facies patterns that can be

identified from seismic records.

[From Vail, P. R., 1987, Seismic

stratigraphic interpretation

using sequence stratigraphy, in

Bally, A.  (ed.), Seismic

stratigraphy: Am. Assoc. Petro

leum Geologists Studies in Ge

ology 27, Fig. 6, p. 6,

reproduced by permission of

AAPG, Tulsa, Okla.]

Figure 13.13

External form of soe

stratigraphic bodies as inter

preted from seismic facies

units. [After Mitchum, R.

M., Jr., P. R. Vail, and ). B.

Sangree, 1977, Stratigraphic

interpretation of seismic re

flection 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. 12, p. 131,

reproduced by permission

of AAPG, Tu lsa, Okla.]

450

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Rgure 13.14

Reflection terminations and configuraons can be analyzed visually from two-di

mensional seismic profiles. External three-dimensional geometry must be deter

mined by mapping based on many different seismic profiles. Each seismic fa

unit is distinguished from adjacent units on the basis of these parameters.

After the disibution (geometry) and thickness of the reection packages

have been mapped, the next step in seismic facies analysis is to combine this in

formaon th any other distinctive seismic information, such as interval velocity,

and any available nonseismic data, such as well and outcrop data, that shed light

on the regional geology. Vail et a!. (1991) provide an in-depth overview of e

procedures.

Interpretation of Lithofacies and Depositional Environments. Once the objective as

pects of delineating seismic sequences and fades have been completed, the final

objective is to interpret the facies in terms of lithofacies, depositional environ

ments, and paleobathymetry. For example, seismic facies that show prograding re

ection characteristics commonly indicate deltaic deposits. Reflection patterns

showing laterally adjacent undaform (above wave base), clinoform (seaward slop

ing), and fonda form (at basin floor) beds (see Fig. 13.5) suggest changes in wa

depth from shelf to slope to deep basin. Parallel reflectors that extend over large

areas suggest shelf deposits or possibly deeper water deposits in a stable basin.

To a large extent, enviroental interpretation of seismic facies is a process

elimination (Brown and Fisher, 1980). e interpreter may be able to immediately

eliminate

certain lithofacies or depositional environments because of obvious in

consistencies with available data or because of personal knowledge of the basin or

region under study. Additional analysis involving study of relationships to oer

units, reflections characteristics, and other properties commonly allows further 

duction of the remaining options unl only one or two depositional or lithofaes

models fit the available data. Lateral facies equivalents must be given special atten

tion in the interetation process and the interpreter must be expeenced in and

have a

good owledge of depositional processes and systems, that is, lithofacies

composition, geometry, and spatial relationships. Even so, it may not always be

possible to arrive at a final, unique interpretation, and the interpreter may have to

settle for the best conclusion that can be made consistent with available data.

Figure 13.14 demonstrates the process of seismic facies interpretation by

showing how the seismic reflecon pattes depicted in Figure 13.12 are intered

Sigmoid

offlap

Shelf

edge

Oblique

offlap

Aggradational

offlap

Schematic illustration of litho

logic and environmental in

terpretation of the simulated

seismic facies patterns shown

in Figure 13.12. [From Vail, P.

R., 1987, Seismic stratigraphic

interpretation using sequence

stratigraphy, in Bally, A. W.

(ed.), Seismic stratigraphy:

Apparent

truncation

Channel/overbank

complex

Drape

Slope front

fill

Mound

Am. Assoc. Petroleum Geolo

gists Studies in Geology 27,

Fig. 9, p. 10, reproduced by

permission of AAPG, Tulsa,

Okla.]

Deep water sand

Marine silt mudstone

Marine shale and mud

� Carbonate shelf facies

D Carbonate slope facies

 Carbonate basin facies

13.3 Sequence Stratigraphy 451

 terms of liofacies and environmental setting. The first step in making such an

interpretation is to lea as much as possible about the regional geology from well

and outcrop controL Such nonseismic data can commonly show whether the sedi-

mentary section consists of carbonates (and/ or evaporites), siliciclastics, or mixed

carbonates and siliciclastics. e seisc facies patterns can then be interpreted.

For example, the depositional unit lying directly above the sequence boundary

(i.e., the unconformity shown by the wiggly line)  Figure 13.12 is interpreted as

mainly siliciclastic; the channel/ overbank complex consists of marine silt and clay

(mudstone); and the mound is composed of deep-water sand. The depositional

unit lying immediately below the sequence boundary consists of carbonates,

which a shelf, slope, or basin carbonates depending upon their relation to the off-

lap patte (Vail, 1987). For additional examples of seismic sequence analysis, see

Light et al., 1993.

13.3 SEQUENCE RATIGRAPHY

Fundamental Principles

Sequence satigraphy is based on the premise that sedimentary successions can

be divided into unconformity-bounded units (sequences) that form during a sin

gle, major cycle of sea-level change, as mentioned. Sequences can be split into

smaller units, which are genetically linked and which form during different stages

of a single sea-level cycle (Table .2). The entire three-dimensional assemblage of

lithofacies enclosed within sequence boundaries is referred to as a depositional

stem. Depositional systems, in m, are made up of smaller stratigraphic units:

stem tracts and parasequences (which are arranged into parasequence sets). Se

quce stratigraphy sets out to place these stratal units into a predictable, chronos

tratigraphic framework by demonstrating how their generaon is related to

accommodation space (Vincent, Macdonald, and Gutteridge, 1998).

Accommodation space is the space available at any point in time in which

sediments can accumulate (Jervey, 1988). Fundamentally, it is the space between a

nceptual equilibrium surface at separates erosion from deposition in which

potential sediment can accumulate.  the marine realm, this equilibrium surface,

called base level, is sea leveL Accommodation in marine settings is goveed by

eustasy

(sea-level rise and fall) and tectonism (subsidence/uplift) (Fig. 13.15). Rel

ative rise in sea level or subsidence of the seaoor creates more sedimentation

space. Fall in sea level or elevation of the seafloor decreases accommodation.

ese accommodation variables and rate of sediment supply govern water depth,

and us transgression and regression, in several ways (Emery and Meyers, 1996).

 sediment is added while accommodation remains constant (e.g., static sea level

and no subsidence) or the sediment supply is greater than the rate of relative sea

level rise, water depth will decrease (regression of facies belts). lf sediment supply

is less than the rate of relative sea-level rise, water depth increases (transgression

of facies belts).  sediment supply is so great that it fills the accommodation to sea

level, a drop in relative sea level owing either to eustatic fall or to uplift of the

seafloor can cause sediment to be eroded.

Fundamental Units of Sequence Stratigraphy

Parasequences and Pa rasequence Sets

Parasequences are the smallest facies units of depositional sequences and range in

thickness om about 10 to 1 m. A parasequence is a small-scale succession of beds

or bed sets bounded by flooding surfaces, which are surfaces that separate younger

sata om older strata, across which there is evidence of an abrupt increase in

452

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Figure 13.15

Diagram illustrating accom

modation, the space avail

able for sediment

accumulation, in the marine

realm. [Based on Posamen

tier, jervey, and Vail, 1988].

Hierarchy of sequence-stratigra

hic uni

Depositional Sequence�genetically related strata bounded by surfaces of erosion or

nondeposition or their correlative conformities.

Stratal units within sequences include:

Depositional Syste�a three-dimensional assemblage of lithofacies, genetically

linked by active (modern) or inferred (ancient) processes and environments (e.g., flu

vial, deltaic, barrier-island).

System tract�a subdivision of a depositional system. Four main kinds are recog

nized: highstand (sediment deposited during high sea level), falling-stage (sedi

ment deposited as sea falls from high to low), lowstand (sediment deposited

during low sea level and early rising sea level), and transgressive (sediment

deposited during rising sea level).

Parasequence Set-a succession of genetically related parasequences that form a

distinctive stacking pattern that is bounded, in many cases, by major marine

flooding surfaces and their correlative surfaces.

Parasequence-a relatively conformable succession of genetically related beds

or bedsets (within a parasequence set) bounded by marine ooding surfaces

or their correlative surfaces.

Marine flooding surface-a surface that separates younger from older strata,

across which there is evidence of an abrupt increase in water depth.

water depth. They commonly form in coastal environments where the rate of in

crease of accomodation space is less than the rate of sediment supply. Under these

conditions, sediment entering the sea will fill nearshore aas first, then move sea

ward (prograde) into more distal areas. The result is a shallowing-upward, and

commonly coarsening-upward, vertical succession of facies. If this stage is fol

lowed by an incase in accomodation space (owing to eustatic sea-level rise or

decrease in rate of sediment supply), transgression of the sea over the succession

will take place, effectively terminating shallowing-upward deposition.

Subsequent decrease in accommodation space, owing to eustatic sea-level fall

or increase in rate of sediment supply, can initiate deposition of a new parase

quence. Short-term changes in rate of sediment supply can cause each succeed

ing parasequence to start prograding at a different point. Ve rtical stacking of

parasequences owing to these variations in relative sea level thus generates a

parasequence set (Fig. 13.16). In the case of the parasequences described here,

each succeeding parasequence will advance in a seaward direction, forming a

progradational parasequence set. Under different conditions of relative sea-level

rise and fall, parasequences may advance in a landward direction to form

retrogradational parasequence sets or build verticay to generate aggradational

Flooding surfaces

Coastal-Plain

Sandstones & Mudstones

Figure 13.16

Progradational Parasequence Set

(Deposition > accommodation)

.. .

Pas�. 4

Increasing water depth -

13.3 Sequence Stratigraphy

453

0 Shallow-Marine

Sandstones

Shelf Mudstones

Illustration of parasequences and parasequence sets formed under prograding conditions

(rate of deposition exceeds the rate at which accommodation space is being created).

Flooding surfaces are surfaces that separate younger strata from older, across which there

is evidence of an abrupt increase in water depth. [After Van Wagoner et al., 1990, Silici

clastic sequence stratigraphy in well logs, cores, and outcrops: PG Methods in explo

ration series, No. 7, Fig. 4, p. 12, reproduced by permission.]

parasequence sets (e.g., Coe and Church, 2003). For example, carbonate parase

quences are commonly aggradational and also shallow upward.

System Tr acts

As shown in Ta ble 13.2, the strata that make up a depositional sequence are re

ferred to collectively as a depositional system. A depositional system constitutes

the sediments deposited during a complete cycle of sea-level change, from high

sea level to low sea level and back to high sea level. The sediments deposited dur

ing particular parts of this cycle are referred to as system tracts, which, in turn, are

composed of parasequences. Thus, system tracts may lie immediately above or

below a sequence boundary or lie in the middle part of a sequence. Four kinds of

system tracts are recognized, depending upon the sea-level conditions under

which they formed.

Highstand system tracts lie immediately below a sequence boundary. They

form during the late part of a sea-level rise, during a sea-level standstill, or during

the early part of a sea-level fall (Fig. 13.17 A). They form under conditions that

allow progradation to aggradation. Alluvial and coastal plain sediments charac

terize the later parts of highstand system tracts, which grade seaward into shal

low-marine (that may include deltaic sediments) and offshore-marine deposits.

Highstand system tracts are terminated by the unconformity produced by the

next eustatic sea-level fall.

Falling-stage system tracts (Fig. 13.17B), referred to by some authors as

early lowstand system tracts, form as sea level falls from a highstand position.

Falling sea level, due either to the rate of eustatic sea-level fall exceeding the rate

of tectonic subsidence or the rate of eustatic sea-level rise being less than the rate

of tectonic uplift, brings on a condition referred to as forced regression. During

forced regression, accommodation space is reduced as the shoreline moves in a sea

ward direction (progrades) and also moves lower down the depositional profile.

As a result, the coastal plain is not a site of deposition but rather a zone of sedi

mentary bypass, forming an unconformity surface. That is, sediment eroded on

shore by fluvial incision will bypass the coastal plain and be deposited in a more

seaward position. Falling-stage system tract deposits include shallow-marine sed

iments, offshore-marine sediments, and submarine-fan sediments.

454

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Figure 13.17

Schematic illustration of

system tracts formed at dif

ferent stages in a cycle of

eustatic sea-level change

from high sea level to low

sea level and back. Note

that each system tract com

prises several small-scale re

gressive-transgressive cy cles

(i.e., parasequences). At

each phase of normal

shoreline regression within

a parasequence, fluvial ac

commodation is created and

deposition occurs, except

during falling-stage condi

tions, when erosion occurs.

[After Posamentier and

Allen, 1999, Siliciclastic se

quence stratigraphy

Concepts and applications:

SEPM Concepts in sedimen

tology and paleontology

No.7, Fig. 2.41 , p. 41, re

produced by permission.]

I Highstand System Tr act I

f .. :::: :I

Alluvial Deposits

igh

I::: :::I Nearshore Marine

 Oshore Marine

Relative

Sea Level

I Sea-Level Rise

Progradation

Falling-Stage System Tr act

I Lowstand System Tr act

!Transgressive System Tract

t Relative

Sea-Level Rise

Lowstand system tracts (Fig. 13.17C) begin to form after relative sea level

has fallen to its minimum and begun to rise, creating a small amonnt of accomo

dation space. As sea level continues to rise, marine sediments are deposited and

the fluvial system ceases to incise. Thus, a lowstand system tract is the package of

sediments deposited between minimum relative sea level (reduced accoodation

space) and subsequent pronounced increase in accommodation space. The sedi

ment consists of progradational to aggradational parasequence sets that can con

tain alluvial and coastal-plain sediments, shallow-marine sediments (including

deltaic sediments that can form and fill previously incised river valleys), offshore

marine sediments, and submarine-fan sediments (Coe and Church, 2003).

Continued rise in sea level creates conditions whereby the rate at which ac

commodation space is created is greater than the rate of sediment supply, which

brings on transgression. The site of deposition shifts in a landward direction, gener

ating retrogradational parasequences or parasequence sets. Tr ansgressive surfaces

13.3 Sequence Stratigraphy

455

may be marked by marine sediments overlying nonmarine sediments. The sedi-

ment deposited under these conditions forms a transgressive system tract (Fig.

13.170). Transgressive system tract sediments may contain alluvial and coastal

plain sediments, shallow-marine sediments, and offshore marine sediments, but

they generally do not include submarine-fan sediments.

When sea level approaches its maximum, the rate of sedimentation eventu

ally exceeds the rate of sea-level rise and aggradation to strong progradation gen

erates a new highstand system tract (Fig. 13.17E). Durg this stage, coastal-plain

and deltaic sedimentation predominates and these facies may prograde out over

underlying lowstand deposits.

The above discussion presents only the very basic elements of sequence

sag

raphy. For additional details, terested readers may wish to consult Coe and Church

(2003), Posamentier and Allen (1999), or the excellent Sequence Stratigraphy Web Site

mataed by the University of South Carolina (http:/ I strata.geol.sc.edu/).

Methods and Applications of Sequence Stratigraphy

Methods

The preceding discussion indicates that e concepts involved in sequence stratig

raphy are fairly complex. Students can be reasonably expected to ask at this point

exactly what advantage sequence stratigraphy has over other stratigraphic meth

ods that makes it worthwhe to develop an understanding of these concepts. The

fundamental aim of sequence stratigraphy is to provide a high-resolution chrono

satigraphic (time-stratigraphic) framework for carrying out facies analysis. Ve r

tical facies analysis must be done within conformable packages of stratal units to

accurately correlate coeval (equivalent age), lateral facies relationships along a

single depositional surface. Other researchers have accomplished this end for

many years by using transgressive and regressive cycles of strata for regional cor

relation of time and facies. The proponents of sequence stratigraphy maintain that

sequence stratigraphy is a much better way of doing this (e.g., Van Wagoner et al.,

1990, p. 6). A fundamental aspect of sequence sagraphy is the recognition that

sedimentary rocks are composed of a hierarchy of stratal units including laminae,

beds, bedsets (see Chapter 4), parasequences, parasequence sets, and system

tracts. With the exception of laminae, each of these its is a genetically related suc

cession of strata bounded by chronostratigraphically significant surfaces. Facies

above these surfaces or boundaries have no physical or temporal (time) relation

ship to the facies below. Therefore, correlation of ese surfaces provides the high

resolution chronostratigraphic framework necessary for facies analysis.

The actual practice of sequence stratigraphy requires that stratigraphers be

able to recognize the stratal expression of parasequences, parasequence sets, sys

tem tracts, and sequences. Early work relied mainly on seismic sequence and seis

mic facies analysis, as discussed in e preceding section-recognion of sequence

boundaries on the basis of stratal terminations, for example. Seismic stratigraphy

alone does not offer the necessary precision to recognize and analyze smaller scale

sedimentary units; therefore, well logs, cores, and outcrops are also used to ana

lyze

sequences. Identification of shallowing upward units allows recognion of

parasequences. Groups of parasequences can be observed to stack into retrogra

dational, progradational, and aggradational pattes to form parasequence sets,

which correspond roughly to a system tract (Van Wa goner et al., 1990, p. 3). Sys

tem tracks are idenfied by distinct associations of facies and position within a se

quence. Thus, using an appropriate combination of seismic data, well logs, cores,

and outcrop information, it  possible to generate a high-resolution chronostrati

graphic framework of sequences and parasequence boundaries, defined solely by

the relationship of the strata.

456

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Environmental Applications

Sequence stratigraphic concepts were originally applied primarily to analysis of

siliciclastic sediments deposited along continental margins, because these silici

clastic environments are particularly affected by cycles of relative sea-level

change. As sea level swings from highstand to lowstand, a succession of system

tracts are laid down, as documented in Figure 13.1 7. Subsequently, attempts have

been made to extend the concepts of sequence stratigraphy to carbonate and evap

orite environments, deep-sea environments, epicontinental (cratonic) marine en

vironments, and even fluvial systems (see Emery and Meyers, 1996; Witzke,

Ludvigson, and Day, 1996; Vincent, Macdonald, and Gutteridge, 1998; Coe,

2003). Although extending sequence-stratigraphy techniques to these environ

mental settings is apparently possible, important differences exist between sedi

mentation patterns in these environments and the siliciclastic marine shelf-slope

environment.

For example, the rate of production of carbonate sediment in carbonate envi

ronments is typically much higher than the rate of accumulation of siliciclastic sedi

ment in siliciclastic settings. Consequently, the rate of carbonate producon generally

exceeds the rate at which accommodation is created, causing the basins to fill to 

level and generating a shallowing-upward succession of facies (Chapter 11). There

fore, the pattern of system tracts in carbonate sediments may not be quite the same

as that in siliciclastic sequences. Also, sequence boundaries are commonly more

difficult to distinguish in carbonate successions than in siliciclastic deposits. Fur

thermore, the eects of subaerial exposure on carbonate platforms depends on eli·

mate. Humid climates will cause widespread dissolution and reprecipitation of

carbonate; arid climates will cause less carbonate diagenesis but tend to promote

precipitation of evaporites.

The deep-marine environment is affected far less by changes in relative sea

level of a few hundred meters than is the shelf environment. Nonetheless, sea·

level changes do affect deposition in deep-ocean basins, particularly deposition of

turbidites in submarine fan systems. Ahough submarine fans can develop dur·

ing sea-level highstands, turbidity currents appear more likely to move sediment

from shelf environments to the deep ocean basin during lowstands of sea level

than during highstands (Fig. 13.17). Analysis of deep-marine turbidite systems ap·

pears to be the main application of sequence-stratigraphy methods to the deep-sea

environment (e.g., Emery and Meyers, 1996, p. 178).

Although sequence-satigraphic concepts have been applied to marine epi

continental (cratonic) environments (e.g., Witzke, Ludvigson, and Day, 1996),

problems arise with such applications because of the low rates of subsidence of

cratonic areas. Sloss (1996) points out that vast areas of cratonic platforms appr

to have subsided at rates of 5 m/m.y., or less, for epoch and period-length spans of

time. Under such conditions, the bathometric relief required for clinofos,

downlap surfaces, lowstand tracts, and other characteriscs of basins with a shelf

break are rarely attainable except under special circumstances. Sloss suggests at

meaningful progress is inhibited by forcing cratonic stratigraphy to conform 

principles, definitions, and practices developed for a different set of conditions.

Application of sequence-stratigraphic techniques to fluvial systems presen

particular problems because the base level for fluvial sedimentation, and thus ac

commodation, is more difficult to define than that for marine systems. The con

ceptual equilibrium surface that defines the upper limit of accommodation space

(Fig. 13.15) in uvial systems is commonly taken as the graded profile, or profile

of equilibrium, of a stream. (A graded profile is the longitudinal profile of a grad·

ed stream or of a stream whose gradient at every point is just sufficient to enable

the stream to transport the load of sediment made available to it.) The level to

which

a stream can ultimately grade is called the bayline, which is effectively sea