7.8 Sedimentary Cycles
in field exposures of limited extent are 4th and higher
order cycles. Third order cycles take an intermediate
position; they are weIl represented in seismic records,
but in exposures on land they can usually be seen only
in large exposures. Otherwise, these sequences have to
be identified by combining information from neighboring outcrops and drilling holes of some depth.
7.8.2 Continental Encroacbment (Flooding)
Cycles (Plate Tectonic Megacycles)
Long-term trends and, to some extent, cyclic developments in the Earth's history have been of primary interest in geology and stratigraphy since their beginning.
This topic concems various specialized fields, such as
tectonic evolution and orogenesis, volcanism and
magmatism, climatic trends including greenhouse and
icehouse effects, the development of the atrnosphere
and the oceans, black shale events, rock cycling as a
result of subduction and mantle convection, geochemical cycles, etc., and last but not least the history oflife.
Plate tectonic megacycles resulted from the assemblage ofindividual continents and subsequent break-up
of super-continents. The last two megacycles ofEarth's
history are weIl established (see below), but probably
there were also earlier megacycles of this type (e.g.
Salop 1983; HoffmannPF 1989). Duringtimesofslow
ocean spreading and continental assemblage, associated with subduction, collision, andmountain building,
mid-oceanic ridges became smaller and lower and the
average depth of the ocean basins deeper (Pitrnan
1978; Heller and Angevine 1985). Hence, the global
sea level fell if it is assumed that the volume of ocean
water remained constant. In contrast, rapid spreading
rates generated high, broad oceanic ridges and thus
reduced the ocean basin volume. Consequently, the sea
level rose and flooded neighboring continental lowlands and partially flooded uplands of limited elevation. In addition, other factors such as submarine
magmatism and changing sediment influx from the
continents may have modified these general trends.
Tbe Last Two Megacycles (Overview)
Continental assemblages to super-continents occurred
in the late Proterozoic and Permian, and breakups in
the early Cambrian and early Jurassic. The time period
of a plate tectonic megacycle from one break-up to the
next one was 250-350 Ma. The most important effects
of the two youngest megacycles are summarized in
Figure 7.36.
The rifting and breakup ofthe Pan-African supercontinent in
the latest Proterozoic and early Cambrian (e.g., Porada 1989)
and subsequent rapid ocean spreading led to a major era of
flooding, with sea level highstand and widely extended marine sediments on continental crost in the eariy Ordovician.
355
The peak in sea level may have been 100 to 150 m above the
present level (Bond et al. 1988). It was followed by a longterm, slow sea-level fall which reached its deepest level with
the completion ofPangea at the end ofthe Permian.
The late Permian and Triassic represent an epoch of
radical change from falling to rising sea level. With the
break-up ofPangea, the overall climate changed from
an icehouse state with polar ice caps and large latitudinal temperature gradients to a greenhouse state (e.g.
Veevers 1990). In the Triassic, large volumes ofsediment did not reach the oceans, but were deposited as
nonmarine strata in incipient rift zones (Sects. 12.1
through 12.3) and in lowlands adjacent to the sea. The
base level was low and sediment supply exceeded the
space being created by slow relative rise in sea level,
resulting in major regression (Vail et al. 1991). The
tendency for arid climates favored the deposition of
evaporites, red beds, and eolian sediments.
In the oceans, relatively narrow, low-latitude zones were
characterized by a high supersaturation with respect to calcium carbonate and a high Mg/Ca ratio (Mackenzie 1990).
As a result, carbonate platforms were dolomitized to a great
extent (e.g., in the Triassie of the Alps), and aragonite was
the dominant mineral forming pore-lining cements and ooids
in the so-called aragonite ocean (Sandberg 1985; Opdyke
and Wilkinson 1990). Several major groups of organisms
be~ame extinct, but others persisted with modifications; reefbUilders developed new types of frarnework.
Since the Upper Jurassie, nannofossil carbonates accumulated in widely extended areas on shelves and in epicontinental seas. The Mg/Ca and Sr/Ca ratios in sea water were
relatively low. The dominant abiotic carbonate mineral in
enlarged, low-Iatitude ocean regions (so-called calcite
oceans) becarne calcite; reef growth was achieved by a few
species of reef builders; evaporites were rare, and glaciers
were, if at all present, restricted to high mountain ranges on
the continents.
Tbe Greenbouse State of tbe Eartb
During the Jurassic and Cretaceous, accelerated ocean
spreading led to sea-level rise. The global sea level
reached an absolute high represented by several peaks
in the period from the Turonian up to the Campanian.
The amount of elevation of the sea level above its present
level, however, is still controversial. It was probably less
than 200 to 250 m as assumed in earlier papers (e.g.
Kauffinan 1983; Hag et al. 1987) as pointed out, arnong others, by Sahagian et al. (1996; cf. Fig. 7.48). In any case, large
areas of the drifting continents were flooded and covered by
shallow-marine sediments.
In the early Cretaceous, the sea-level rise was accompanied in the Pacific by the extrusion oflarge volumes
of basaltic magma (Schlanger et al. 1981; Arthur et al.
1985). Both the high sea level and the release of large
amounts of CO 2 into the atrnosphere were probably
in field exposures of limited extent are 4th and higher
order cycles. Third order cycles take an intermediate
position; they are weIl represented in seismic records,
but in exposures on land they can usually be seen only
in large exposures. Otherwise, these sequences have to
be identified by combining information from neighboring outcrops and drilling holes of some depth.
7.8.2 Continental Encroacbment (Flooding)
Cycles (Plate Tectonic Megacycles)
Long-term trends and, to some extent, cyclic developments in the Earth's history have been of primary interest in geology and stratigraphy since their beginning.
This topic concems various specialized fields, such as
tectonic evolution and orogenesis, volcanism and
magmatism, climatic trends including greenhouse and
icehouse effects, the development of the atrnosphere
and the oceans, black shale events, rock cycling as a
result of subduction and mantle convection, geochemical cycles, etc., and last but not least the history oflife.
Plate tectonic megacycles resulted from the assemblage ofindividual continents and subsequent break-up
of super-continents. The last two megacycles ofEarth's
history are weIl established (see below), but probably
there were also earlier megacycles of this type (e.g.
Salop 1983; HoffmannPF 1989). Duringtimesofslow
ocean spreading and continental assemblage, associated with subduction, collision, andmountain building,
mid-oceanic ridges became smaller and lower and the
average depth of the ocean basins deeper (Pitrnan
1978; Heller and Angevine 1985). Hence, the global
sea level fell if it is assumed that the volume of ocean
water remained constant. In contrast, rapid spreading
rates generated high, broad oceanic ridges and thus
reduced the ocean basin volume. Consequently, the sea
level rose and flooded neighboring continental lowlands and partially flooded uplands of limited elevation. In addition, other factors such as submarine
magmatism and changing sediment influx from the
continents may have modified these general trends.
Tbe Last Two Megacycles (Overview)
Continental assemblages to super-continents occurred
in the late Proterozoic and Permian, and breakups in
the early Cambrian and early Jurassic. The time period
of a plate tectonic megacycle from one break-up to the
next one was 250-350 Ma. The most important effects
of the two youngest megacycles are summarized in
Figure 7.36.
The rifting and breakup ofthe Pan-African supercontinent in
the latest Proterozoic and early Cambrian (e.g., Porada 1989)
and subsequent rapid ocean spreading led to a major era of
flooding, with sea level highstand and widely extended marine sediments on continental crost in the eariy Ordovician.
355
The peak in sea level may have been 100 to 150 m above the
present level (Bond et al. 1988). It was followed by a longterm, slow sea-level fall which reached its deepest level with
the completion ofPangea at the end ofthe Permian.
The late Permian and Triassic represent an epoch of
radical change from falling to rising sea level. With the
break-up ofPangea, the overall climate changed from
an icehouse state with polar ice caps and large latitudinal temperature gradients to a greenhouse state (e.g.
Veevers 1990). In the Triassic, large volumes ofsediment did not reach the oceans, but were deposited as
nonmarine strata in incipient rift zones (Sects. 12.1
through 12.3) and in lowlands adjacent to the sea. The
base level was low and sediment supply exceeded the
space being created by slow relative rise in sea level,
resulting in major regression (Vail et al. 1991). The
tendency for arid climates favored the deposition of
evaporites, red beds, and eolian sediments.
In the oceans, relatively narrow, low-latitude zones were
characterized by a high supersaturation with respect to calcium carbonate and a high Mg/Ca ratio (Mackenzie 1990).
As a result, carbonate platforms were dolomitized to a great
extent (e.g., in the Triassie of the Alps), and aragonite was
the dominant mineral forming pore-lining cements and ooids
in the so-called aragonite ocean (Sandberg 1985; Opdyke
and Wilkinson 1990). Several major groups of organisms
be~ame extinct, but others persisted with modifications; reefbUilders developed new types of frarnework.
Since the Upper Jurassie, nannofossil carbonates accumulated in widely extended areas on shelves and in epicontinental seas. The Mg/Ca and Sr/Ca ratios in sea water were
relatively low. The dominant abiotic carbonate mineral in
enlarged, low-Iatitude ocean regions (so-called calcite
oceans) becarne calcite; reef growth was achieved by a few
species of reef builders; evaporites were rare, and glaciers
were, if at all present, restricted to high mountain ranges on
the continents.
Tbe Greenbouse State of tbe Eartb
During the Jurassic and Cretaceous, accelerated ocean
spreading led to sea-level rise. The global sea level
reached an absolute high represented by several peaks
in the period from the Turonian up to the Campanian.
The amount of elevation of the sea level above its present
level, however, is still controversial. It was probably less
than 200 to 250 m as assumed in earlier papers (e.g.
Kauffinan 1983; Hag et al. 1987) as pointed out, arnong others, by Sahagian et al. (1996; cf. Fig. 7.48). In any case, large
areas of the drifting continents were flooded and covered by
shallow-marine sediments.
In the early Cretaceous, the sea-level rise was accompanied in the Pacific by the extrusion oflarge volumes
of basaltic magma (Schlanger et al. 1981; Arthur et al.
1985). Both the high sea level and the release of large
amounts of CO 2 into the atrnosphere were probably