364
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
playa significant role (e.g. Buchem et al. 1995): the
redox cycles may be restricted to the deepest sub-basins, or they can pinch out laterally in a basin of little
changing water depth. The organic matter in the dark
layers is mainly of marine origin; sometimes terrestrial
sources play an important part.
Bed-scale successions ofthis type occur, for example, in the
middle Cretaceous of the Atlantic and Tethyan Oceans
(Weissert et al. 1979; Pratt 1984; Cotillon 1985; Dean and
Arthur 1986; Herbert et al. 1986; Ogg et al. 1987; Arthur and
Dean 1991; de Boer 1991; Sageman et al. 1991; Ricken
1994; Buchem et al. 1995; also see articles in de Boer and
Smith 1994; House and Gale 1995).
Taking into account the flux rates ofthe three major sediment components (siliciclastics, carbonate, organic matter),
it turns out that relatively minor variations in planktonic carbonate production and organic matter preservation can generate the limestone-marl or limestone-black shale couplets
(Fig. 7 .42b). Input of organic matter produced in surface waters may be even lower during black shale intervals than during limestone times if in the latter case an oxygen-depleted
deep-water zone is present preventing rapid degradation.
Relative sea-Ievel fall, enhanced water circulation, and increased dilution of carbonate by siliciclastics will lead to different sediment types and more or less prevent couplets as
described above.
The black shale beds are frequently thinly laminated,
including varve-scale (bio )laminations, but their
laminae often represent small sedimentological events,
or minor, short-terrn climatic variations rather than annual varves (Cotillon 1991; Gerdes et al. 1991). The
cyclic sequences commonly exhibit repeated transitions from aerobic to anaerobic conditions due to water
mass mixing and tumover. They therefore contain benthic and bioturbated intervals (Savrda et al. 1991;
Oschmann 1991). Similar to limestone-marl rhythrns,
black shale-carbonate rhythms are deposited in environments above the CCD or the lysocline (Fig. 7.39).
Rhythmic Bedding in Siliceous Sediments
Alternations between biogenic siliceous and non-siliceous marine beds and their origin were already mentioned in Sect. 5.3.5 (Fig. 5.6). Their rhythmic bedding
is generated by both cyclic and discyclic (event-related) depositional processes. In addition, calcareous
beds may contain layers of chert nodules which indicate primary alternations between beds poor and rich
in opaline silica.
In this section, cyclic phenomena in the range of the
Milankovitch frequency band are of primary interest.
Similar to limestone-marl alternations, cyclic sequences in siliceous sediments result from (1) variations in siliceous plankton productivity, (2) periodic
dilution of siliceous sediments by terrigenous material,
and (3) variations in dissolution of opaline silica in the
water colurnn and at the sea floor. However, cyclic
productivity appears to be more effective than cyclic
dilution in generating rhythmically interbedded chert
layers. Thus, climate-induced global changes in oceanic circulation and the recycling of nutrients through
upwelling maypredominantly control the generation of
beds rich inopaline silica. At the same time, carbonate
production should be low, or the position of the carbonate lysocline or CCD should be weIl above the sea
floor in order to prevent strong dilution of opaline silica by carbonate.
The occurrence of banded chert is not restricted to
deep water; rather, it indicates an environment ofhigh
siliceous plankton production and very limited
terrigenous influx.
Rhythmic bedding in siliceous sediments has been described
from manyregions, particularly in Mesozoic sediments ofthe
Tethys Ocean (Hein and Obradovic 1988) and in Tertiary
sequences around the Pacific (lijima et al. 1985), i.e., regions
of high fertility in siliceous plankton. Rhythmicity may be
expressed in annual varves or in banded ribbon radiolarites
(e.g., Jenkyns and Winterer 1982; Jenkyns 1986), which are
modified and enhanced by silica diagenesis (Sect. 5.3.5). The
most important facts about such alternations were sumrnarized, e.g., by Decker (1991).
Overall deposition of black shales and/or siliceous sediments may become overprinted by "oxygenation events"
caused by accentuated deep water circulation. Part of these
events is mediated by Milankovitch cyclicity, e.g. in the Miocene Monterey Formation along the Pacific coast of North
America (Ozalas et al. 1994).
Periodicities and Causes of Cyclic Bedding
Limestone-marl and limestone-black shale successions
commonly display cyclicities at different time scales.
Total successions (several tens to a hundred meters in
thickness) often represent time spans of one to several
millions of years. The periodicities of individual bedding cycles, including part of the siliceous bedding
couplets, are in the Milankovitch frequency band
(Sect. 7.9.2). This is true ofvarious types of carbonate
rhythms in the Quatemary as weIl as for pre-Quaternary examples which developed during times of little
to no continental ice. In some cases, sea level-induced
shaIlow-water carbonate cycles could be correlated
with cyclic bedding forrned in deeper waters (Sect.
7.9.5).
These findings support the assumption that subdued
glacio-eustasy also controlled cyclic carbonate bedding. On the other hand, at least part of the different
modes of cyclic bedding can be solely interpreted by
climatic change. Relatively weak orbital forcing can
bring about significantshifts of climatic belts, changes
in oceanic therrnohaline circulation, and modifications
within the global carbon cycle, including atmospheric
CO 2 and carbonate deposition. Thus, both planktonic
carbonate productivity and carbonate dissolution
(namely by rising or falling CCD) can vary with time.
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
playa significant role (e.g. Buchem et al. 1995): the
redox cycles may be restricted to the deepest sub-basins, or they can pinch out laterally in a basin of little
changing water depth. The organic matter in the dark
layers is mainly of marine origin; sometimes terrestrial
sources play an important part.
Bed-scale successions ofthis type occur, for example, in the
middle Cretaceous of the Atlantic and Tethyan Oceans
(Weissert et al. 1979; Pratt 1984; Cotillon 1985; Dean and
Arthur 1986; Herbert et al. 1986; Ogg et al. 1987; Arthur and
Dean 1991; de Boer 1991; Sageman et al. 1991; Ricken
1994; Buchem et al. 1995; also see articles in de Boer and
Smith 1994; House and Gale 1995).
Taking into account the flux rates ofthe three major sediment components (siliciclastics, carbonate, organic matter),
it turns out that relatively minor variations in planktonic carbonate production and organic matter preservation can generate the limestone-marl or limestone-black shale couplets
(Fig. 7 .42b). Input of organic matter produced in surface waters may be even lower during black shale intervals than during limestone times if in the latter case an oxygen-depleted
deep-water zone is present preventing rapid degradation.
Relative sea-Ievel fall, enhanced water circulation, and increased dilution of carbonate by siliciclastics will lead to different sediment types and more or less prevent couplets as
described above.
The black shale beds are frequently thinly laminated,
including varve-scale (bio )laminations, but their
laminae often represent small sedimentological events,
or minor, short-terrn climatic variations rather than annual varves (Cotillon 1991; Gerdes et al. 1991). The
cyclic sequences commonly exhibit repeated transitions from aerobic to anaerobic conditions due to water
mass mixing and tumover. They therefore contain benthic and bioturbated intervals (Savrda et al. 1991;
Oschmann 1991). Similar to limestone-marl rhythrns,
black shale-carbonate rhythms are deposited in environments above the CCD or the lysocline (Fig. 7.39).
Rhythmic Bedding in Siliceous Sediments
Alternations between biogenic siliceous and non-siliceous marine beds and their origin were already mentioned in Sect. 5.3.5 (Fig. 5.6). Their rhythmic bedding
is generated by both cyclic and discyclic (event-related) depositional processes. In addition, calcareous
beds may contain layers of chert nodules which indicate primary alternations between beds poor and rich
in opaline silica.
In this section, cyclic phenomena in the range of the
Milankovitch frequency band are of primary interest.
Similar to limestone-marl alternations, cyclic sequences in siliceous sediments result from (1) variations in siliceous plankton productivity, (2) periodic
dilution of siliceous sediments by terrigenous material,
and (3) variations in dissolution of opaline silica in the
water colurnn and at the sea floor. However, cyclic
productivity appears to be more effective than cyclic
dilution in generating rhythmically interbedded chert
layers. Thus, climate-induced global changes in oceanic circulation and the recycling of nutrients through
upwelling maypredominantly control the generation of
beds rich inopaline silica. At the same time, carbonate
production should be low, or the position of the carbonate lysocline or CCD should be weIl above the sea
floor in order to prevent strong dilution of opaline silica by carbonate.
The occurrence of banded chert is not restricted to
deep water; rather, it indicates an environment ofhigh
siliceous plankton production and very limited
terrigenous influx.
Rhythmic bedding in siliceous sediments has been described
from manyregions, particularly in Mesozoic sediments ofthe
Tethys Ocean (Hein and Obradovic 1988) and in Tertiary
sequences around the Pacific (lijima et al. 1985), i.e., regions
of high fertility in siliceous plankton. Rhythmicity may be
expressed in annual varves or in banded ribbon radiolarites
(e.g., Jenkyns and Winterer 1982; Jenkyns 1986), which are
modified and enhanced by silica diagenesis (Sect. 5.3.5). The
most important facts about such alternations were sumrnarized, e.g., by Decker (1991).
Overall deposition of black shales and/or siliceous sediments may become overprinted by "oxygenation events"
caused by accentuated deep water circulation. Part of these
events is mediated by Milankovitch cyclicity, e.g. in the Miocene Monterey Formation along the Pacific coast of North
America (Ozalas et al. 1994).
Periodicities and Causes of Cyclic Bedding
Limestone-marl and limestone-black shale successions
commonly display cyclicities at different time scales.
Total successions (several tens to a hundred meters in
thickness) often represent time spans of one to several
millions of years. The periodicities of individual bedding cycles, including part of the siliceous bedding
couplets, are in the Milankovitch frequency band
(Sect. 7.9.2). This is true ofvarious types of carbonate
rhythms in the Quatemary as weIl as for pre-Quaternary examples which developed during times of little
to no continental ice. In some cases, sea level-induced
shaIlow-water carbonate cycles could be correlated
with cyclic bedding forrned in deeper waters (Sect.
7.9.5).
These findings support the assumption that subdued
glacio-eustasy also controlled cyclic carbonate bedding. On the other hand, at least part of the different
modes of cyclic bedding can be solely interpreted by
climatic change. Relatively weak orbital forcing can
bring about significantshifts of climatic belts, changes
in oceanic therrnohaline circulation, and modifications
within the global carbon cycle, including atmospheric
CO 2 and carbonate deposition. Thus, both planktonic
carbonate productivity and carbonate dissolution
(namely by rising or falling CCD) can vary with time.