7.2 Basic Concepts
buildup or removal of eontinental iee and, to a minor
degree, by rising or falling lake levels and groundwater tables on land, and the temperature of oeean water. Global eustatie sea-Ievel ehanges oeeur by definition simultaneously but do not have the same amplitude everywhere. Some of the diffieulties result from
the fact that, for example, in a subsiding area the sedimentary environment may "feei" global sea-Ievel
rise later than in a teetonieally stable area. Or, loeal
rapid subsidenee of the basin floor may suggest sealevel rise even during times of generally falling sea
level. Nevertheless, ehronostratigraphie evidenee for
the synehroneity of high-amplitude Neogene and
Quatemary sea-Ievel fluetuations is very good for
many regions but often poor for older eras.
Amplitudes of sea level, AMP. The regional amplitudes (Fig. 7.3b) of the "absolute" global sea-Ievel
fluetuations are strongly affeeted by mass flow proces ses in the mantle and, to a minor degree by temporal changes in the geoid determining the shape of the
ocean surface. Some authors therefore recommend
using the term "equivalent" sea level, being everywhere the same at the same time, instead of the
eustatie sea level whieh may deviate from loeation to
loeation (e.g. Lambeck 1994). Some of these problems are avoided by using the term "relative sea-Ievel
ehanges".
Relative sea-Ievel change, RSL, is the gain or loss
of vertieal space to aeeommodate sediment, ACC
(Fig. 7.3c). It is defined as the differenee between
eustatic change, EU, and subsidence, SUB, or uplift,
UPL. It is RSL = EU + SUB or EU - UPL. This is
valid for a eertain location within the basin and for a
certain time period. In a eorrect sense one should
always define "rates .of relative sea-Ievel change ete.
In a basin with low subsidenee close to the coastline
and stronger subsidenee basinward (differential subsidence), RSL-fall in shallow water ean oecur at the
same time as RSL-rise in deeper water (Fig. 7.3e).
For the diseussion in the following sections it is important to note that a relative sea-Ievel fall takes
plaee only when the rate of sea-Ievel fall is higher
than that of subsidence.
Accommodation space, ACC, is the space available
between sea level and the subsiding basin floor or, in
continental systems, between the highest base level
and the sediment surfaee on land. This spaee varies
with time and is redueed by vertical sediment aeeumulation (aggradation), whieh ultimately controls the
water depth in the basin or the potential for further
sediment buildup (Seetion 7.2.3).
Transgression and regression of the sea, T -R cyeies. These old terms have not always been used
unanimously and may sometimes even be misleading.
301
During rising sea level the coastline does not always
retreat landward (transgressive sea), but ean also
pro grade seaward, when the influx of terrigenous
sediment is high in this area. Here, the terms transgression and regression refer to the landward or
basinward shifting of the eoastline (T-R cycles).
They do not neeessarily include a rising or falling sea
level. With eonstant relative sea level the eoastline
may migrate either landward or seaward depending
on the sediment influx and hydrodynarnie energy of
the basin. High sediment influx ean eause "normal"
regression whereas relative sea-Ievel fall leads to
"foreed" regression (Posamentier 1992) regardless of
the amount of sediment supply.
Systems tracts or facies tracts, ST, are genetically
linked depositional systems showing characteristic
vertical and lateral facies sueeessions. In most cases
a systems tract eonsists of either (basinward)
prograding or retrograding (baekstepping) facies
units (Fig. 7.3g and h). These units may eomprise
sediments of eontinental, coastal, and deeper marine
environments. Vertieal seetions of systems traets display either shallowing or deepening-upward trends.
The boundaries of the systems tracts for the most part
do not eoineide with the peaks or lows of the eustatic
sea-Ievel curve (cf. Figs. 7.11a and 7.17a). After the
peak of the eustatie eurve, for example, ongoing subsidence delays the onset of relative sea-level fall.
Facies tracts: instead of three systems traets aecording to the EXXON model, some authors only use
two facies tracts, prograding and aggrading or
baekstepping (e.g. Cross 1998; cf. Figs. 7.3e and
7.33) which seems to be appropriate in partieular for
fluvial systems.
U nconformities are surfaees along whieh there is
evidenee of sub aerial and/or submarine erosion, indicating a signifieant stratigraphie gap (hiatus).
Sequence boundaries, SB (Fig. 7.3f, g and h). Type
I sequence boundaries (SB 1) are regional surfaces
whieh are associated with a basinward shift of the
faeies and coastal onlap of overlying strata. They
develop in times and areas of relative sea-Ievel fall.
Sequences ean also be bounded by regionally extended Type 2 sequence boundaries (SB 2), whieh
lack both signifieant erosion and a basinward shift in
facies. They oceur in areas where the rate of eustatie
sea-Ievel fall is less than the rate ofbasin subsidence.
If subsidenee inereases basinward, a Type 1 boundary
may therefore pass into a Type 2. Type 2 boundaries
mark the transition from the late highstand to the early
lowstand in deep water or from the highstand to the
shelf margin systems tract (Fig. 7.3g and h). Type 2
boundaries are less distinet than Type 1 boundaries, and
are diffieult to identify in the field and in eores.
buildup or removal of eontinental iee and, to a minor
degree, by rising or falling lake levels and groundwater tables on land, and the temperature of oeean water. Global eustatie sea-Ievel ehanges oeeur by definition simultaneously but do not have the same amplitude everywhere. Some of the diffieulties result from
the fact that, for example, in a subsiding area the sedimentary environment may "feei" global sea-Ievel
rise later than in a teetonieally stable area. Or, loeal
rapid subsidenee of the basin floor may suggest sealevel rise even during times of generally falling sea
level. Nevertheless, ehronostratigraphie evidenee for
the synehroneity of high-amplitude Neogene and
Quatemary sea-Ievel fluetuations is very good for
many regions but often poor for older eras.
Amplitudes of sea level, AMP. The regional amplitudes (Fig. 7.3b) of the "absolute" global sea-Ievel
fluetuations are strongly affeeted by mass flow proces ses in the mantle and, to a minor degree by temporal changes in the geoid determining the shape of the
ocean surface. Some authors therefore recommend
using the term "equivalent" sea level, being everywhere the same at the same time, instead of the
eustatie sea level whieh may deviate from loeation to
loeation (e.g. Lambeck 1994). Some of these problems are avoided by using the term "relative sea-Ievel
ehanges".
Relative sea-Ievel change, RSL, is the gain or loss
of vertieal space to aeeommodate sediment, ACC
(Fig. 7.3c). It is defined as the differenee between
eustatic change, EU, and subsidence, SUB, or uplift,
UPL. It is RSL = EU + SUB or EU - UPL. This is
valid for a eertain location within the basin and for a
certain time period. In a eorrect sense one should
always define "rates .of relative sea-Ievel change ete.
In a basin with low subsidenee close to the coastline
and stronger subsidenee basinward (differential subsidence), RSL-fall in shallow water ean oecur at the
same time as RSL-rise in deeper water (Fig. 7.3e).
For the diseussion in the following sections it is important to note that a relative sea-Ievel fall takes
plaee only when the rate of sea-Ievel fall is higher
than that of subsidence.
Accommodation space, ACC, is the space available
between sea level and the subsiding basin floor or, in
continental systems, between the highest base level
and the sediment surfaee on land. This spaee varies
with time and is redueed by vertical sediment aeeumulation (aggradation), whieh ultimately controls the
water depth in the basin or the potential for further
sediment buildup (Seetion 7.2.3).
Transgression and regression of the sea, T -R cyeies. These old terms have not always been used
unanimously and may sometimes even be misleading.
301
During rising sea level the coastline does not always
retreat landward (transgressive sea), but ean also
pro grade seaward, when the influx of terrigenous
sediment is high in this area. Here, the terms transgression and regression refer to the landward or
basinward shifting of the eoastline (T-R cycles).
They do not neeessarily include a rising or falling sea
level. With eonstant relative sea level the eoastline
may migrate either landward or seaward depending
on the sediment influx and hydrodynarnie energy of
the basin. High sediment influx ean eause "normal"
regression whereas relative sea-Ievel fall leads to
"foreed" regression (Posamentier 1992) regardless of
the amount of sediment supply.
Systems tracts or facies tracts, ST, are genetically
linked depositional systems showing characteristic
vertical and lateral facies sueeessions. In most cases
a systems tract eonsists of either (basinward)
prograding or retrograding (baekstepping) facies
units (Fig. 7.3g and h). These units may eomprise
sediments of eontinental, coastal, and deeper marine
environments. Vertieal seetions of systems traets display either shallowing or deepening-upward trends.
The boundaries of the systems tracts for the most part
do not eoineide with the peaks or lows of the eustatic
sea-Ievel curve (cf. Figs. 7.11a and 7.17a). After the
peak of the eustatie eurve, for example, ongoing subsidence delays the onset of relative sea-level fall.
Facies tracts: instead of three systems traets aecording to the EXXON model, some authors only use
two facies tracts, prograding and aggrading or
baekstepping (e.g. Cross 1998; cf. Figs. 7.3e and
7.33) which seems to be appropriate in partieular for
fluvial systems.
U nconformities are surfaees along whieh there is
evidenee of sub aerial and/or submarine erosion, indicating a signifieant stratigraphie gap (hiatus).
Sequence boundaries, SB (Fig. 7.3f, g and h). Type
I sequence boundaries (SB 1) are regional surfaces
whieh are associated with a basinward shift of the
faeies and coastal onlap of overlying strata. They
develop in times and areas of relative sea-Ievel fall.
Sequences ean also be bounded by regionally extended Type 2 sequence boundaries (SB 2), whieh
lack both signifieant erosion and a basinward shift in
facies. They oceur in areas where the rate of eustatie
sea-Ievel fall is less than the rate ofbasin subsidence.
If subsidenee inereases basinward, a Type 1 boundary
may therefore pass into a Type 2. Type 2 boundaries
mark the transition from the late highstand to the early
lowstand in deep water or from the highstand to the
shelf margin systems tract (Fig. 7.3g and h). Type 2
boundaries are less distinet than Type 1 boundaries, and
are diffieult to identify in the field and in eores.