Cyclostratigraphy

Cretaceous Resources, Events and Rhythms Background and Plans for Research CYCLOSTRATIGRAPHY edited by R. N. Ginsburg Rosenstiel School of Marine & Atmospheric Science, University of Miami, Miami Beach, FL, U.S.A and Bernard Beaudoin Ecole des Mines de Paris, France P.L DE BOER Comparative Sedimentology Inst. of Earth Sciences Budapest/aan 4, NL 3508 TA Utrecht, The Netherlands A.G. FISCHER Department of Geological Sciences University of Southern California Los Angeles, CA 90089-0740 I. PREMOLI SILVA Departmento de Scienze della Te"a Via Mangiagalli 34 I-20133 Milan, Italy ABSTRACT. The sedimentary record of orbitally-forced variations in climate has the potential to provide high-resolution dating to levels of a few hundred thousand years of less. Easily-measured, bed-to-bed variations in various components give the basic data for defining these rhythms. The Cretaceous is a particularly appropriate period for testing both the validity and the application of cyclostratigraphy. Recommended specific short range objectives are: detailed comparative studies of time slices, in order to recognize the effect of orbital influences in different sedimentary environments, and to map the paleogeographic distributions of obliquitydominated patterns versus precession-eccentricity dominated ones. This approach should illuminate Cretaceous climates and oceanography, and the pathways along which sedimentary systems were forced. ACKNOWLEDGEMENTS This position paper was prepared with the help of A. Berger, D. Bottjer, P. Cotillon, W. Dean, J. V. Gardner, C.R. C. Paul, I. Premoli Silva, W. Ricken, P.H. Roth, W. Schwarzacher, A. Strasser, and G. P. Weedon. In addition, all the members of WG-4, who attended the meeting in Perugia, provided infonnation and ideas. Introduction Kluwer Academic Publishers Dordrecht I Boston I London Published in cooperation with NATO Scientific Affairs Division In 1895 G.K Gilbert suggested that certain rhythmic stratification patterns in the Cretaceous of Colorado (USA) reflect the response of sensitive sedimentary facies to orbital rhythms (Fischer, 1980). His concept was based on the work of astronomers of those days, such as d'Alembert who had discovered the precessional cycle and Leverrier who had discovered the variation in eccentricity - concepts which had 139 R. N. Ginsburg and B. Beaudoin (eds.). ｃｲｾｴ｡｣･ｯｵｳ＠ Resources, Evenls and Rhythms, 139-172. C> 1990 All Rights Reserved. Primed in the Netherlands. 141 140 already been applied to the ice-age problem by Adhemar and by Croll (Imbrie & Imbrie, 1979). Such signals should provide a basis for a geochronology - a burning topic in Gilbert's day, and still a matter of great interest. The recognition of orbitally induced rhythmic sedimentation and the environmental factors behind it would shed light on paleoclimatology, and on oceanic and sedimentary responses to changes of climate and the sensitivity of the various sedimentary systems. These topics have assumed increasing significance as we have come to recognize the environmental fragility of the outer Earth. In the end, they also might clarify the evolution of the Earth's orbital behaviour. Despite Gilbert's great challenge, the succeeding century did not see the development of a workable cyclostratigraphy. Many thought (and think) the concept far-fetched. The effects of orbital cycles on climates and sediments, in various sedimentary facies and through time are complex. Many sedimentologists were inclined to attribute all to allo- or autocyclic causes with stochastic timing. The level of stratigraphic detail needed was daunting, as was the prospect of analyzing large data sets. This has now changed. Insight into the orbital variations and their climatic effects has grown due to the work of Milankovitch (1941), Berger (1978a, 1988), and others. The sensitivity of climate and ocean to external forcing is becoming clear (Emiliani, 1955; Hays, Imbrie & Shackleton, 1976; Fischer, 1981). The case for rhythmic orbital control of the Pleistocene glaciations has become overwhelming (Imbrie et al., 1984). Thus the theoretical basis for a cyclostratigraphy has been established. So has the need for it. Man now feels imminently threatened by climatic changes, partly of his own making. Only cyclostratigraphy can yield the chronological resolution and the insight into ancient climates that are necessary to understand climatic change. At the same time, instrumental methods of scanning sedimentary sequences for variations in physical and chemical characteristics have come into use, as have the processing of large data sets and the analysis of time series by computer. Thus practical cyclostratigraphy is coming within reach. CRER Workshop 3 was designed to consider the following questions: 1. Do certain sedimentary sequences really record orbital variations? 2. How widespread are such orbital signals in sedimentary facies and in time? 3. How are they best identified and studied? 4. What can they tell us about geochronology, orbital variations, paleoclimates and sedimentology? 5. Is there hope to link them into a coherent cyclostratigraphy? 6. How can we best proceed toward the goal of establishing and testing a working cyclostratigraphy? well zoned by biostratigraphical means, and relatively free ?f ｳｨｯｲ･Ｍ｡｣ｩｾｴ､＠ tectonic and geomorphic noise. Thus, CRER provides a particularly appropriate framework for further research in cyclostratigraphy. In subsequent pages we briefly review the general basis of what is known _about ｴｾ･＠ orbital variations, their possible climatic effects, and the role of chmates m facies. We review _the nature _of influencing sedimentation in various ｳ･､ｩｾｮ｡ｲｹ＠ stratigraphic rhythmicity and methods by which ｾｴ＠ can be approached. Finally we rruse some practical problems; whether to proceed with a global program of research, and, if so, how this might be coordinated. Orbital Variations Centuries of detailed astronomical observations have provided the basis for calculating the characteristics of the Earth's orbit. Berger (1978b) charted the orbital changes for the Neogene that potentially could have affected climates, namely the distribution of insolation patterns by latitude and by season. These chan¥es are related to the Earth's cycle in axial obliquity, and with the cycle of precession and the changing eccentricity of the Earth's orbit around the Sun. ,/ I \ -- ' \ ＧｾＭＬＮ＠ '' , ］ｾ＠ ｾ＠ / ---- e \ ' 'I I I ',, ,,,,. -ＭＮｾＬ＠ ' " ' I _.../ eccentricity main period in years about obliquity 100,000 40,000 precession 19,000 - 23,000 Figure 1. Outline of the orbital variations. Whereas the effects of orbital forcing may be recorded in local sedimentary sequences, they can only be properly understood in a global context. An inventory of cycle patterns must proceed on a global basis. Cyclostratigraphy thus fits particularly well into the framework of GSGP. Work on cycles will continue throughout the stratigraphic column, yet a coherent global cyclostratigraphy calls for a time-focus. The Cretaceous period seems particularly promising for this endeavour. The wide expanse of the Cretaceous seas provided a large area for marine sediments, Obliquity. The Earth's obliquity changes through about 3.5 degrees of arc, with .a both. poles. m _ dominant period of about 41,000 years. The climatic signal ｡ｦ･｣ｾ＠ phase. With increasing obliquity, the mean annual latitudinal ､ｩｾｴｮ｢ｵｯ＠ of ｾｯｬ｡ｴＱＰｮ＠ becomes more uniform, but seasonality increases. The chmat1c effect is most pronounced in the polar regions, where increasing obliquity expands the extent and duration of the polar winter night. 142 143 Precession-eccentricity syndrome. The Earth's orbit would be circular except for the effect, of ｰｬｾｮＮ･ｴ｡ｲｹ＠ ｭ｡ｳｾ･Ｎ＠ The ｧｲｾｶｩｴ｡ｯｮｬ＠ attraction of the planets deforms the Earth s orbit mto an elhp.se of varymg eccentricity, and drags its axis through space. Be.rger's ｾＱＹＷ｡Ｉ＠ calculations show major periodicities in eccentricity at ca. 100 Ka (With maJor ｣ｯｭｰｾｮｴｳ＠ at. 98 and 126 Ka), 400, 1,290, 2,030 and 3,400 Ka. We may term ｴｨ･ｳｾ＠ ｾ｣･ｮｴｹ＠ ｰ･ｾｯ､ｳ＠ El, E2, E3, E4 and ES. However, by itself, the effect of eccentnc1ty on chmate 1s thought to be very slight. ... ;;a;m;;cm ﾷＭｾ＠ L lL t.02 o.o ••·• ca.1cu1rr n.a z:r.o -0.07 ll_ -0.02 0.04 o.o too. 200. 300. 400. TINE !K 600. too. 100. too. Yr• DQol Figure 2. ｡ｲｩｾｴ＼ｬＬＡｳ＠ｖ in eccentricity, obliquity, and the precession index ( e smw) over the past 800,000 years (Berger, 1978b ). At right: variance spectra calculated from these time series, with the dominant periods (Ka) of conspicuous peaks indicated. The Earth's axis precesses with a period of 26 ka Precession is related to spin rate, an? £?-USt the.refore: ｨ｡ｾ･＠ slowed during geological time (see below). Relative to the elliptical orbit (penhehon) the precessional period measured is not 26 ka, but varies between extremes of 14 and 28 ka, with a mean value of 21 ka, owing to the variable drag of the elliptical orbit through space. The gross "average" measured at the Earth's surface is 21 ka In a circular orbit, the precession cycle would have no climatic effect. ｾｯｷ｟･ｶＬ＠ the ｣ｯｭｾｩｮ｡ｴ＠ of elliptical orbit and precession causes changes in the d1stnbuuon of received solar energy. When a given hemisphere occupies the perihelia! portion of the orbit in summer, i.e., faces the Sun when closest to it, it experiences a hot short summer and a long, cold aphelia! winter, i.e., greaterthan-norrnal seasonality. At the same time, the opposite hemisphere, with an aphelia! summer and a perihelia! winter, experiences a reduced seasonality. The effect varies directly with the eccentricity. The result of this coupling is the generation of climatic cycles that have the period of the precession, but whose amplitude varies in response to the eccentricity - i.e. in response to a hierarchy of cycles with longer periods. This relationship is expressed in Berger's "precession index" (Fig. 2). In the precessional climatic cycles generated by the eccentricity-precession syndrome, the hemispheres are 18if out of phase. This results in a shift of the ｣｡ｬｾｲｩ＠ equator, which invades a given hemisphere during its perihelia! summers, carrymg with it low to mid-latitude climatic belts and the boundaries between them (de Boer, 1982). This not only shifts the zonal boundaries between wet and dry belts, but also disturbs the balance between zonal and monsoonal wind circulation (Kutzbach & Otto-Bliesner, 1983). The effects are thus especially pronounced in the climates of the mid-latitudes. Orbital Variations During The Geological Past (Berger & Loutre) Among the longest astrophysical and astronomical cycles that might influence climate (and even among all forcing mechanisms external to the climatic system itself), only those involving variations in the elements of the Earth's orbit have been ｦｯｾｮ､＠ significantly related to the Jong-term climatic data de?uced from ｴｾ･＠ ｧ･ｯｬＱｾ｡＠ records. The aim of the astronomical theory of paleochmates, a particular version of which being due to Milankovitch (1941), is to study this relationship between insolation and climate at the global scale. It comprises four different parts: the orbital elements, the insolation, the climate model and the geological data (Berger, 1988). During the first half of this century, Milankovitch regarded mild winters and cool summers as favouring glaciation. After Koppen and Wegener related Milankovitch's new radiation curve to Penck and Bruckner's subdivision of the Quaternary, there ｴｨｾ＠ Qu.aternary ｾｬ｡｣ｩﾭ was much scepticism if such changes in insolation can ･ｸｰｬｾｩｮ＠ interglacial cycles. In the 1970's, with the improvements m datmg, m ｡｣ｱｵｭｾｧ＠ and and with. the in interpreting the geological data: with the advent of_ ｣ｯｭｰｾｴ･ｲｳ＠ development of astronomical and climate models, the M1Jank0Vltch theory revived . . (Imbrie & Imbrie, 1979; Berger et al., 198:4)· . Spectral analysis of Quaternary paleochmatic records has proVlded ｳｵ｢ｴ｡ｮｾＱｬ＠ evidence (Berger, 1987) that, at least near the obliquity and precession ｦｲｾｱｵ･ｮ｣ｳＬ＠ a considerable fraction of the climatic variance is driven in some way by 1nsolat1on ＱＹＷ｡ｾＮ＠ ｾ･＠ changes forced by changes in the Earth's orbit (Hays et al., 1976; ｾｲｧ･Ｌ＠ fundamental astronomical and climatic frequencies are not only alike but the climatic series are also phase-locked and strongly coherent with orbital variations (Imbrie et al., 1984). . Models of different categories of complexity, from conceptual ones (lmbne &Imbrie, 1980) to 3-D atmospheric general circulati?n models (Prell & Kutzbach, 1987) and 2-D time-dependent models of the whole climate system (Be.rger et ｾｉＮＬ＠ 1988a) have now been astronomically forced in order to test the physical reality of the 145 144 astronomical theory. For example, seasonal models for simulating the transient response of the climate system to the astronomical forcing are developed and their output compa!es favourably with data of the past 400,000 years. Accordingly, the m?del predictions for the next 100,000 years are used as a basis for forecasting how cltmate would evolve when forced by orbital variations in the absence of anthropogeThe long-term cooling trend which began some 6,000 years ago will nic ｾｩｳｴｵｲ｢｡ｮ｣･Ｎ＠ contmue for the next 5,000 years; this first temperature minimum will be followed by an amelioration around 15 ka AP (after present), by a cold interval centered 23 ka AP and by a major glaciation at around 60 ka AP (Berger, 1980). The validity of the astronomical computation was also tested (Berger, 1984). The time series of eccentricity, obliquity, precession and related insolation developed by Berger (1976) were acceptable for the last 1.5 Ma (Berger & Pestiaux, 1984). For the accuracy in the time domain of the long-term variations of the astronomical ･ｬｾｮｴｳ＠ and of the insolation values, improvements required in the early 1980s for penods further back than 2 Ma BP (before present) are in progress (Berger et al., 1988b). It is expected that the new solution will be now reliable for at least 10 Ma. About the stability of the frequencies, the fundamental periods (around 40, 23 and 19 ka) do not deteriorate with time over the last 15 Ma but their relative importance for each insolation and each astronomical parameter is a function of the period considered. For the Pre-Quaternary times, it is roughly 100 years after Gilbert's suggestion ＨＱＸＹｾＩ＠ that. the astr?nomical frequencies start to be found with some degree of confidence m geological records of the Mesozoic (de Boer & Wonders, 1981; Fischer & Schwarzacher, 1984; Fischer, 1986; Olsen, 1986). The sensitivity of climate to the astronomical forcing is also becoming clear, in particular for the Cretaceous (Barron et al., 1985), so that there is an imperative need now to understand how and why the orbital frequencies change at the geological time scale. There is no hope, at present, to obtain a general solution of the planetary system valid over the whole Earth's history (Deprit et al., 1984). The equations can indeed not .be integrated over more than the Neogene period using the present way of solvrng them. Before this general solution becomes available, we will bypass the difficulty by assuming that the system is locally stable in time; it means that it keeps its general form for a given value of the slowly varying parameters of the EarthMoon system. Therefore, forgetting about the amplitudes and phases (which depend primarily upon unknown initial conditions), we may focus on the frequencies only and analyse how they respond to changes of these slowly varying parameters. Theoretical investigation of the frequencies of the orbital parameters has allowed to trace their origin (Berger, 1978a; Berger & Loutre, 1987). They are a linear combination of 17 basic frequencies, 16 (s; and g,., l:Si:S8) which are characteristic of the two fundamental sets of variables of the Earth's orbit (Berger, 1977b) and the last one, k, which is introduced through the Poisson's equations for precession and which present-day value is 50''439273/year. This frequency k has the following general form: 1 k ex: --- n C-A ------- ( E + c whereAis the rotational angular velocity of the Earth, A and C the Earth's moments of inertia around an axis contained into and perpendicular to the equator, ac the semi-major axis of the orbit of the Moon around the Earth, E and M are functions of the Earth and Moon orbital parameters which vary much faster thann, C, A and ｾ＠ at the geological time scale. In fact the obliquity, e , and the climatic precession, e sinw, are given by Berger (1978a) = E e sintU where f; and a; + E• = :E A; COS (lit :E P; sin ( a;t + 6;), ..g.), take one of the following forms: s;+k, 2(s;+k), s;+sj+2k, s;+gj+2k, gj+k, srsj and ｳｲｾﾷ＠ Among them, s; + k (i = 1, ... , 8) and g. + k (j = 1, ... , 8) are related to the m?st important periods of the obliquity (41,0oo and 54,000 years) and the precession (19,000 and 23,000 years), respectively. Consequently, the changes in f; and a; due to the changing value of k can be computed. Geological epoch Age Ma 0 Holocene 72 Upper Cretaceous 270 Lower Permian Upper Carboniferous 298 380 Middle Devonian 440 Lower Silurian ac (km) 384000 381690 374908 373892 370828 368493 J.o.d. 19000 (s) 23000 41000 54000 become 19000 18641 17545 17272 16562 16014 23000 22474 20868 20468 19428 18625 41000 39328 34227 32954 29649 27097 86162 85064 81972 81108 78876 77328 54000 51100 42250 40043 34309 29884 Table 1. Estimated values for the last half-billion years of the semi-major axis of the lunar orbit, of the length of the day and of the two most important periods of precession and obliquity. The past values of the length of the day (1.o.d.) and accordingly of , are taken from Stoyko (1970), the subsequent past values of (C-A)/C are those computed by Denis (1986) and the past value of ac is given by Walker & Zahnle (1986). Table 1 summarizes the results obtained for the last half-billion years (Berger et al., 1987). As it was foreseen by Bernard (1975), the lengths of the ｦｵｮｾ｡ｭ･ｴｬ＠ periods are ｧ･ｯｬＱ｣｡ｾ＠ past. Due to non- becoming smaller and smaller with time back into ｾｨ･＠ linearities in the relationship between the frequencies and the different parameters the longest periods decrease more rapidly than the others at a rate such that ｾＬＵＰ＠ Ma BP, the 41,000 yr obliquity period becomes equal to the 23,000 yr precess1onal one. 147 146 Geoclimatic Forcing Mechanisms The changes in caloric seasons and insolation patterns (Milankovitch, 1941) provided by the orbital variations are small, but the changes in the monthly values are much larger (up to 12%; Berger, 1978a). Some changes in half-year caloric insolation may be transferred to the sediment record directly but most forcing is probably amplified by non-linear responses, feedback mechanisms and threshold effects (Anderson, 1984, 1986), that are more pronounced at certain latitudes and in certain sedimentary environments than at/in others. The Pleistocene record of ice volume has been deduced from changes in the stable ｯｸｹｧ･ｾ＠ .isotope co.mposition of sea water as inferred from the composition of forarrumferal tests m deep-sea cores. The pathways by which the orbital cycles drove and the ice, are not at all clear, but certain aspects are: surely climate, the ｯ｣ｾ｡ｮｳ＠ the growth of ice was enhanced by the positive feedback of the Earth's increased albedo in. the sno":'-covered polar regions and by isostatic response (cf. Oerlemans, ＱＹＸｾＩＮ＠ EVJdently chmat:s were also modified by changes in atmospheric C02 content attnbutable to changes m the relative partitioning of carbon dioxide between the two main reservoirs - atmosphere and deep ocean (cf. de Boer, 1986). In pre-Pleistocene times we must reckon with these as well as with additional feedbacks and thresholds. The changes in insolation patterns drove climatic variations strong enough to change of ｯ｣･｡ｮｩｾ＠ and sedim:ntary systems. Examples of such possible changes the ｢･ｨｾｶｩｯｵｲＮ＠ and thelf sed1mentolog1cal effects mclude the following: Bottom-water fonnation and global marine circulation. Presently, deep waters are formed largely from the cold waters generated in the high latitudes. It now seems ｾｮ｣ｲ･｡ｳｩｧｬｹ＠ ｬｩｫｾｹ＠ that over large parts of geologic time, warm saline waters generated m the paratrop1cal belts were relatively more involved in deep-water formation (Roth, 1978; Southam, Peterson & Brass, 1982). Orbitally induced climatic changes, that triggered switches in circulation must have had massive consequences for the ecology and geochemistry of marine settings. The changes range from bottom water ｴ･Ｎｭｰｲｾｵｳ＠ to the availability of oxygen (redox cycles), bottom current strength, wmnowmg cycles, lysocline and calcite compensation depth (dissolution cycles), and recycling in the ocean (productivity cycles). It seems likely that deg;ee of ｮｵｴｾ･＠ ｾｴ＠ ｴｉ＿ｊ･ｾ＠ and. sites ?f delicate balance, the orbital variations played the crucial role m fl1ppmg Clfculat10n from one mode into another, triggering cyclic sedimentation responses (ROCC Group, 1986). ｓｨｩｦｴｾ＠ of boundaries between wet and dry belts. The precessional displacement of the tropical wet belt and the paratropical horse latitudes should leave certain latitudes ｰ｡ｲｴｩｾｬｹ＠ ｳｾ｣･ｰｴｩ｢ｬ＠ to arid-humid alternations (Kutzbach & Otto-Bliesner, 1982; ｒｯｳＱｧｮＮｬＭｓｴｾｫＬ＠ 1983; Barron et al., 1985). This would be reflected in the salinity ｡ｾ､＠ ｾｴｲ｡Ｑｦｩ｣ｯｮ＠ of lakes and shelf seas, in the supply of detrital matter (siliciclastic d1lut10n cycles) and other aspects of sedimentology, e.g. paleosols, desertification producing wind blown dust, etc. Changes in dominance of zonal. versus latitudinal circulation. Shifts of the caloric equator are likely to have caused cyclic variations in wind speed and direction, causing cyclic variations in upwelling, sediment supply, etc. Eustasy. The case for orbital control over the eustatic patterns of Pleistocene tiJ?e has already been made. The question of polar ice through geological time remams debated - even for the Cretaceous (Kemper, 1987). Growth and decay of even small ice caps and the chilling and warming of oceans could result in eustatic fluctuations of a few metres - enough to cause scour cycles (ROCC Group, 1986), emergence 1986; Strasser, cycles of carbonate platforms (Hardie; Bose!lini & . ｇｯｬ､ｨ｡ｭＱ＿･ｾＬ＠ 1988), small-scale progradational cycles m detntal settmgs, and d1luuon of ｾ｢ｯｮ｡ｴ･ｳ＠ in hemipelagic sediments. Moreover, possible effects of changes of the geo1d due to changing orbital forces (Momer, 1981) are still a subject of debate. . There is thus no dearth of possible mechanisms that may serve ｾｯ＠ amphfy .the climatic signal generated by orbital ｶ｡ｲｩｴｯｮｾ＠ .and to. ､ｲｾｶ･＠ ｳ･､Ｑｭｮｾｯｬｧ｡＠ expression. Productivity cycles, carbonate and ｳＱｨ｣｡ｾ･＠ d1lut10n cycles, wmnowmg progradat10.nal cycles, redox cycl.es, and cycles, dissolution cycles, emergence ｣ｹｬ･ｾＬ＠ ulumately colour banding in paleosols, all find possible explanations m ｾｲｯ｣･ｳ＠ tuned by orbital variations. To be sure, these effects are certam to have changed as through time with changes in length of orbital cycles (Berger & Loutre, ｡｢ｯｾ･ＩＬ＠ well as with changes in the physical disposition of continents ｡ｾ＠ oc:ans, 1!1 the chemistry of the ocean and the atmosphere, etc. - factors whose ongins he not m the planetary system, but within the Earth itself. Observed Stratigraphic Cyclicity: Various Facies Lakes. Lakes are particularly sensitive to climatic change, and they have furnished knowledge, no some of the best examples of stratigraphic cyclicity. While, to ｯｾｲ＠ cyclicity has been reported, to date, from Cret.aceous lake d<:pos1ts, rocks ?f other ｐ｡ｾｴＱ｣ｬｲｹ＠ ages have yielded fine examples of lacustnne ｣ｹｬｯｳｴｲ｡ｵｾｰｨＮ＠ noteworthy are Bradley's (1929) studies of the Eocene Green River Formation m the Rocky Mountains, and Van Houten's (1962, 1964) and.Olson's ＨＱＹｾＴＬ＠ 1986) work on the Triassic-Jurassic Newark Group. In the Green River Format10n, Bradley used varve counts to identify a precessional cycle in the .alternation ?f ｯｾｬ＠ shale and dolomitic marlstone. This cycle, that we now recogmze as an osc1Jlat10n betw:en lacustrine and playa conditions (Surdam & ｓｴ｡ｾｬ･ｹＬ＠ 1978): reflects an alter?at1on between humid and arid climatic periods. Climatic changes m the Newark senes are recorded not only at the level of the cycle of precession but at the level of 100 and 400 ka of the eccentricity cycle as well. Evaporite basins. Evaporite basins are similarly very sensitive to changes in ｾｨ･＠ precipitation-evaporation balance. The classical ｳｾ､ｹ＠ is the ?ne of the. Permian .600 m thick ｳｾ｣･ＱＰｮＬ＠ more Castile Formation by Anderson (1982, 1984). In ｾｨｩｳ＠ than 200,000 annual cycles were measured, yieldmg m sulphate thickness / year a clear record of the cycle of precession (Fig. 4). 149 148 ? DEPTH RANK LITHOLOGY ｾ＠ CALCITE DOLOMITE SULFUR (:t,) (:t.J (:t.) (:t.) rmm rrrn• 0 2 • 6 (ml TOC 0 2 0 20 •o 0 20 40 0 0.5 MUD CRACKS I0 flTT1 I I I I I 11111111111 - + - FISH + . 50K IOOK 150K ZOOK 250K YEARS Figure 4. Smoothed plot of absolute thickness of calcium sulphate in the Castile Formation. Smoothing is 8 ka. Dominant oscillation in the range of the precession and the 100 ka eccentricity cycle (from Anderson, 1984). Detrital systems. In the theoretical approaches to ､ｾｴｲｩｬ＠ MicrofamW'lated bt11ck clayslone lll!JiI] Mud cracked fabrtc ｾ＠ lamtn1l•d gray ctayston. ｾ＠ Intensely mud cracked tabtk: C=:::J Ttun bedded to massive gray siltstone - ｾ＠ Burrows ｅｐ｟ｩ｣ｲ｡ｴｯｮｾ＠ Supergroup, New Jersey - Pennsylvania. Cycles, Figure 3. !riassic-Jurassic ｎ･ｷｾｲｫ＠ inferred. to reflect the influence of the precession of the Earth's axis, show ｡ｬｾ･ｲｮｵＱＺＵ＠ between lacustrine fish bearing deposits and playa mudstones with reptilian tracks. The deposits evidence oscillations of lake level as the. result ?f changes in the precipitation-evaporation balance due to latitudinal shifts of the boundary between relatively wet and arid climate zones (from Olsen, 1986). (© 1986 by the AAAS). basif!S. ａｾ｡ｩｮＬ＠ we know no good Cretaceous examples of cyclicity in fac1es. The marine/non marine cyclothems of the Pennsylvanian nuxed ･ｰＱｾｲ｡ｴｯｭ｣＠ and Pernuan of the North American Mid-continent region, appear to have a timing of ca. _400 ｾ｡＠ •. and presumably record the E2 eccentricity cycle, related to glacio･ｵｾｴ｡｣＠ vanat10ns. In Kansas, where best developed, they show a 4-fold substructure which i:iresumably reflects the 100 ka El cycle (Heckel, 1977; Fischer, 1986). The shale-hmestone oscillations of the British Jurassic have been attributed to the obliquity cycle, and contain possibly signals from El and 21 ka (House, 1985; Weedon, 1986). systems, ｳｴｯ｣ｨ｡ｾ＠ models of random fluvial meandering and of delta-lobe switching have been dominant, but contrary views begin to appear. Apart from the activity of ｡ｵｾｯ｣ｹｬｩ＠ ｰｲ＿ｾ･ｳ＠ (e.g., channel switching), both fluvial and deltaic systems are obVIou_sly ｳｾｮＱｵｶ･＠ to b?th climate and eustasy, and rhythmicity in sandstone-shale :i-Iternat1?ns in the ｄ･ｾｯｲｵＳＺＱ＠ pro-delta facies of the Catskill (Van Tassel, 1987) and in the ｍＱｯ｣･ｮｾ＠ of ｃ｡ｨｦｯｲｾｴ＠ (Clifton, 1981) have been attributed to orbital effects. In places, ｦｬｵｾ｡＠ sandbod1es embedded in a rhythmical pattern within marls and paleosols (e.g., in the TrempGraus Basin, Spain; unpubl. data, de Boer) also invite to be ｩｾｴ･ｲｰ､＠ as responses to alternating wet and dry periods potentially forced by orbital effects. Marine carbonate platfonns. Studies of marine platform (emergence) cyclicity have mostly come from the Triassic of the Alps and Hungary. Here, Schwarzacher (1947, 1954), Fischer (1964), Schwarzacher & Haas (1966), Hardi_e. et ｾＡＮ＠ (1986) a?d with cycle ｲＮ｡ｾＱＰｳ＠ Goldhammer, Dunn & Hardie (1987) found an emergence ｣ｹｬＱｾ＠ of around 5: 1, suggestive of glacial control forced ｢ｾ＠ P and El (Fig. 5). In penudal environments of the Lower Cretaceous of France, Switzerland and England, Strasser ｲ･｣ｯｧｮｩｺｾ､＠ cycles ?f P, El and ｾＲＮ＠ (1988) and Strasser, Mojon & Deconinck ＨＱｾＸＩ＠ Of special interest to us and also to Working Group 4 1s the ｰ･ｮＭｬ｡ｴｦｾｲｭ＠ fac1es in which deposition continued during times of platform emergence, .but without the ready supply of fine carbonate ｴｨｾＮ｣｡･ｲｩｺ､＠ the subme_rgent episodes. Rando1!1 observations suggest that rhythm1c1ty 1s strong in these settings, but further study is needed. 151 150 ｾ＠ ｾＭ＠ Sm ." ,_.,.,,_, j Ｚｾ＠ ｾ＠ ｾ＠ 0 ..t r o.-. l 0.2 ｆｩｧｵｾ･＠ ｾ＠ 1::: r . r T o.4f 1 ｏＧＭｾＮＺ＠ ｾ＠ l-13'(23-5<) 0.2 0 5 10 15 20 25 30 • ｏＧＭｾＮＺ＠ 0 5 10 5. Fischer plot. ｯｦｾ＠ represen.tative sequence of 37 emergence cycles (Latemar limestone•. M. ｔｮｾｳＱ｣Ｌ＠ ｾｯｬｭＱｴ･ｳＩＮ＠ ｅｾ｣ｨ＠ cycle represents an upward transition to mterudal or suprat1dal conditions. "Bundles" or megacycles from ｳｾ｢ｵ､｡ｬ＠ ｡ｶ･ｲｾｧｭ＠ 5 ｣ｹｬｾｳ＠ are defined by exceptionally thick cycle(s) at the basis. Mean subsidence rate 1s plotted on the Y-axis, and the sequence of cycles is plotted on. the X-axis, assuming a constant period per cycle. Each cycle represents an episode of ｳｵ｢ｴＡｾ｡ｬ＠ ､･ｰｯｳｩｴｾＬ＠ followed by an emergence recorded as a cap that has been mod1f1ed by leachmg (enlarged pores), dolomitization and vadose cementation. Inset,autocorrelation graphs demonstrate the grouping of cycles into bundles of 5 (from Goldhammer, Dunn & Hardie, 1987). Hemipelagic systems. Hemipelagic systems such as those of the French "Fosse ｾｯ｣ｮｴｩ･Ｂ＠ and those _of the ｾ｡ｲｩｴｭ･＠ Alps show particularly striking cyclicity in which are probably largely the result of fluctuating limestone-shale ｡ｬＮｴｾｲｮｩｯｳＬ＠ ｣｡ｲ｢ｯｾｴＺ＠ ｰｲｾ､｣ｴｉｖ＠ and mud supply. Here ｢･ｾ､ｩｮｧ＠ cycles of possible precessional or obl.1qmty t1mmg (Fischer, 1986) are grouped mto larger cycles that reach into the domam of 1 - 2 Ma E3-E4 cycles (Cotillon, 1984; Cotillon & Rio, 1984; Ferry & Rubino, 1987). Somewhat similar patterns appear in the hemipelagic Niobrara chalk (Fig. 6) of the ｾｭ･ｲｩ｣｡ｮ＠ Western Interior (Bottjer et al., 1986; Pratt et al., in press). The ｢･､Ｑｾｧ＠ couplets ｡ｾ･＠ ｬｯｷＭ｣ｾｲ｢ｮ｡ｴ･＠ / high-carbonate oscillations, linked to changes (anoxia) through a Chondrites facies (dysaerobic) to from httle. or no Ｎ｢ＱＰｴｵｲ｡ｾｯｮ＠ the Planoiltes fac1:s (aerobic). These couplets are believed to reflect the precession, grouped mto 100 .ka cycles (see also Laferriere, Hattin & Archer, 1987). and ｡ｲｾ＠ These m turn are grouped mto less-well defined superbundles with possible rhythms Figure 6. Cyclicity in the Niobrara Formation (Coniacian - L Campanian) Colorado. . Wyoming, USA A. Resistivity microlog (rm) and neutron porosity log (np) (mmor. plots) of Golden Buckeye - Champlin core in Laramie <;ounty, Wyonung. B. Calcium carbonate curve (left) and gamma ray log (nght) ｯｦｂ･ｮｨｾ､Ｎ＠ 4 State near Fort Collins, Colorado (Pratt et al, in ｰｲｾＩＮ＠ C. Res1sUVIty log of basal Niobrara in another Colorado.well Ｈｾｦ･ｭｲ＠ et al., 1987). D. Detailed calcium carbonate curve showmg beddmg couplets (Pratt et · E2 al., in press). h t ese mto Bedding couplets (P) are grouped into El ｢ｾｮ､ｬ･ｳＬ＠ superbundles, best shown in gamma ray log and m C. at 400 and 1200 ka (E3) which in tum would fit into a 1,600 ka ｣ｹｬｩｾ＠ (L) that is not included in Berger's orbital periodicities. Hattin (1986) showed_ that m the ｬｯｾ･ｲ＠ Niobrara Formation (Fort Hays Member) the number of ｾ･､ｭｧ＠ couplets ma bundle, showing the normal 5 : 1 ratio in the trough of the ｢｡ｳＱｾ＠ decreases eastward by the absence or amalgamation of some couplets, ｡ｮｾ＠ ｭ｣ｲ･｡ｾｳ＠ v.:estward, presumably due to high-frequency influx of mud from tectomcally acuve highlands. The British Chalk (Hart, 1987) and the basal Lias of Britain Ｈｾ･､ｯｮＬ＠ 1986) show evidence of both the precessional and obliquity cycles. Cottle (m prep.) found 152 153 400 2 0 10 0 75 60 50 Ka 10 5 Figure 7. ｾｰ･ｲ＠ Alb!an pelagic sequence near Moria, Umbrian Apennines Italy of (precession-related) high-carbonate / low carb'onate with ｢ｵｾ､ｬｭｧ＠ couplets m bundles of 4 - 5 representing the El eccentricity rvcle (de Boer -; , 1983). ･ｶｩ､ｮｾＮ＠ of control of the foraminiferal fauna by the precession obliquity and (El and E2 cycles) in rocks which show no ｬｩｴｨｯｧ｣ｾ＠ cyciicity He corre ates presumed obliquity and El cycles over a distance of 80 km. · ･｣ｾｴｮＱｹ＠ Figure 8. Spectral curve of the thicknesses of 150 successive carbonate-rich beds (shaded curve) in the Albian succession near Moria (Fig. 7) in comparison to a spectral curve (with highest peak value) of the successively most northerly positions of the caloric equator calculated on the basis of Berger (1978b). Analysed data thus are not equally distributed in time or space, but represent successive maxima of the carbonate curve and of the precessional effect upon the position of the caloric equator, the latter having a variable spacing in time of 14 ka to 28 ka (from de Boer, 1983). ｾｮ･＠ 0 pelagic ｳｹｴ･ｾＮ＠ The cyclic structure discovered by Gilbert in the Cretaceous t e North American Western Interior Seaway has parallels in most other ｾｲ･ｴＮ｡｣ｯｵ＠ ｣ｾ｡ｬｫ＠ and marl sequences, but is more elaborate than Gilbert realized a.rt 1.cu ar ｭｾＱｨｴ＠ has ｢･ｾｮ＠ ｧｾｩｮ･､＠ ｢ detailed work on the Albian - ｃ･ｮｯｭ｡ｩｾ＠ ｾ｣ｩｳｴ＠ ｾｆＰＱ､＠ and Scagha Bianca (Fig. 7) in the Umbrian facies of central Italy (de oer ｯｾ､･ｲｳＬ＠ 1981, 1984; de Boer, 1982, 1983; Tornaghi, 1984; Erba, 1986; ｾ･ｲ｢＠ &_Fischer, 1986; Herbert, Stallard & Fischer 1986· Park & Herbert 1987· ｡ｾｨ＠ ｓｩｬｶｾ＠ Erba & 1:'ornaghi, 1989). Figure 8 shows the results of a spectrai refl ysis applied ｴｾ＠ the ｴｾＱ｣ｫｮ･ｳ＠ of the successive carbonate-rich beds, thought to m ect the ｰｲ･｣ｳＱｾｮＮ｡ｬ＠ signal, m comparison to a spectral analysis of the successively Ｈｬｾ［ｲｴｨ･ｹ＠ pos1t1ons of the caloric equator calculated on the basis of Berger Figure 9 shows the upper 20 m of this formation, as logged in the Piobbico core, in the proximity of Moria, while instrumental scans of the underlying beds (core depth 10 - 18 m) are shown in figure 10. Bedding couplets of marl/limestone, visible in the outcrop, appear as high frequency digitations in figure 10. They are grouped into bundles of ca. 5 (cf. Figs. 7, 8), defined by shales (commonly black) and labelled El in figure 10. These in turn are grouped into superbundles (E2). Applying the mean sedimentation rate for the Albian here (5 m / Ma) identifies the bedding couplet with the ca. 20 ka precessional cycle, the bundle with the El ca. 100 ka eccentricity cycle, and the superbundle with the ca. 400 ka E2 eccentricity cycle. Figures 10 D and E show the emergence of the ca. 100 ka El as the dominant peak in spectral plots, confirming earlier observations by de Boer & Wonders (1981) and by Schwarzacher & Fischer (1982) in underlying and overlying limestones. Park & Herbert (1987) showed that this peak is bimodal, with its two subsidiary peaks corresponding to the (presently) 98 and 126 ka peaks of the orbital signature. De Boer (1983) also shows the presence of the 2 Ma cycle (E4 ). 155 154 ｾ＠ ｾＡ＠ ｾ＠ ｩｾ＠ < Ａｾ＠ ｾ＠ ! ｾ＠ &'. ｾ＠ ::! 8 "ｾ＠ ｾ＠ . ｾ＠ ! 8 <.> ｾ＠ ｾ＠ ｾ＠ ..: ｾ＠ ｾ＠ ｾ＠ i An 8 m (1.6 Ma) Albian segment of the Scisti a Fucoidi, Umbria, Italy. A Darkness values by densitometry; B. Calcium carbonate curve (mean interval 2 cm); C. Expansion of carbonate curve; D. Adaptive multitaper spectrum of carbonate curve; E. Adaptive multitaper spectrum of darkness curve; P: inferred precessional signal; El and E2: inferred 100 ka and 400 ka eccentricity cycle. (from Premoli Silva et al., 1989) l.b. ｾ＠ ! ｾ＠ . . ｾ＠ j ｾ＠ -i z < IJJ - I<( aJ .J ;: . . .. --.. jj --..ｾ＠. i . .j . ｾ＠ J < ｾ＠ ;;. ｾ＠ ｾ＠ Erba & Tomaghi, 1989; Tornaghi, Premoli Silva & Ripepe, 1989). The similarity of carbonate, planktonic foraminifera, and colour index are evidentiated in figure 9. Limestone/shale alternations in the Lias of Breggia Gorge, Switzerland, also reflect the precession and eccentricity cycles (Weedon, 1989). Even more complex patterns are to be expected where the obliquity cycle finds stronger expression, and where dissolution, dilution and/or winnowing (cyclic or not) overprint the primary signals, as must be the case in much of the record of the deepsea (Dean, Gardner & Jansa, 1978; Dean. Gardner & Cepek, 1981; Dean & Gardner, 1986) and of marginal basins (e.g., Monterey Formation, Bramlette, 1946). ｾ＠ E3 Lleestone ｾﾷ＠ •) Cl'lert Magnetic signals. Magnetic properties (intensity, inclination and declination) measured noaults Figure 9. Upper part of Piobicco core; litho- and biostratigraphy % colour index %To ｃ｡ｾＳＬ＠ % planktonic foraminiferal and radiola;ian ｡｢ｵｮ､｣･ｾ＠ ( omagh1 et al., 1989). .The carbonate/marl rhythms are carbonate productivity cycles positive! correlated with the of bottom water aeration (de Boer, 1983; Stallard & Fischer, ｭｴ･ｲｶｾｬｳ＠ ｳ｡ｭｰｾ･＿＠ at 1 cm spacing produced rhythms in the distribution 1986). ｾｮｧ＠ ｾ｣ｴｲ｡＠ o planktomc ｦｾｲｭ･｡＠ and ｾｰｩｮｧ＠ planktonic faunas every mm resulted 1 spectra that clearly md1cate the obhquity and precessional cycles (Premoli Silva, ､･ｧｲｾ＠ 1.0 0 Ci I c1.r•ous e 1A'f tnd city ｾ＠ Figure 10. ｈ･ｲ｢ｾ＠ in the Umbrian sequence, are not correlated to lithology, but nevertheless show variations that fit well to the orbital signals observed in the lithology of the succession (VandenBerg, de Boer & Kreulen, 1983; Napoleone & Ripepe, in prep.). Earlier, such relations were suggested by Wollin, et al. (1971) and Wollin, Ryan & Ericson (1978) . The data suggest that magnetic secular variations may be related to orbital signals, not transmitted via climate but through some more direct physical mechanism. They thus constitute an independent approach to orbital cyclicity. In summary, many stratigraphic sequences of pelagic, hemipelagic, platform, lacustrine and evaporitic facies of various ages (from at least the Ordovician to the Present) are cyclic in structure, and a relation with the orbital cycles has been established with varying degrees of confidence. These are mainly sequences in which the cyclicity is apparent to the eye, and therefore attracted study. Many more such sequences remained unstudied as yet. There are many sequences in which no such regularity of bedding patterns is apparent on first sight. Indeed, in many sequences rhythmic patterns are striking at certain levels, and fade into others (e.g., Darmedru et al., 1982; Darmedru, 1984; Tribovillard, 1988; Cottle, in prep.). Such lateral 156 variations can result from fluctuations in the strength of the climatic signals, from varying receptivity of the sedimentary system, from the interference between different cycles, from the overprint by local signals of different origin (rhythmic or not), and from diagenetic overprint (Ricken, 1986, 1987). It may eventually be possible to separate orbitally induced cycles from others in such sequences, but only after we know more about the patterns of cyclicity from unalterated sections. The wide stratigraphic range in which orbital influences can be found, i.e., the whole stratigraphic column, means that techniques and results from CRER will be of general application and not just appropriate to the Cretaceous. Expression Of Cyclicity And Sampling Techniques VARIATIONS IN SEDIMENT The expression of cyclicity takes many different forms in various sedimentary sequences. In the Pleistocene deep sea record, for example, variations in oxygen isotope ratios in foraminifera have turned out to be a most useful signal of orbital cycles, despite the complex and roundabout pathway which first led to climatic change (no doubt very complex in itself), which led to the growth and decay of ice sheets, which in turn found expression in the isotopic composition of sea water, reflected in turn by the isotopic composition of foraminiferal and nannofossil carbonate tests. In other sequences it is the mineralogy which oscillates most conspicuously, as in the Green River Formation or the Scisti a Fucoidi. Many cycles involve the character of the bottom fauna. This may be expressed in the degree and type of bioturbation (Bottjer et al., 1986; Savrda & Bottjer, 1987) (Fig. 11), microfaunal content, megafossil biofacies such as the cyclic occurrence of megalodont clams, alternating with the peritidal gastropods and algal mats in the emergence cycles of Triassic platforms (Fischer, 1964). Sedimentary structures may become diagnostic, as for example fenestrae or shrinkage pores and desiccation cracks associated with platform emergence. Compositional differences commonly appear to the eye as differences in colour and/or bedding thickness, the latter often controlling the geomorphic expression. In short, many primary features in sediments and combinations of them may reflect orbitally driven cycles. The rhythmically bedded appearance of a sedimentary succession in the outcrop may represent a combination of such features, variously suppressed or enhanced by diagenesis, tectonism, weathering and erosion. 157 Figure 11. MBD IOC Paleo-oxygenation curve (IOC) constructed on the basis of trace fossil distribution and maximum burrow diameter (MBD, in ｾＩＮ＠ Fort Hays . Limestone, N10brara Formation, Colorado. (from Bottjer et al., 1986) L: laminated; B: bioturbated; C: Chondrites; Z: Zoophycos and/or Teichichnus; P: Planolites; T: Thalassinoides. Space versus time. To be sure, the appearance of cyclicity ｲｾｳｵｬｴ＠ not ｯｮＡｾ＠ from_ sue? variations, but also from their rhythmic and regular spacmg, a regulanty_which_ ｾ＠ commonly modulated by cycles of Ａｯｷ･ｾ＠ ｦ［･ｱｵｾｮＺｙＬ＠ as Ｎｾｨｯｷｮ＠ above. This ｾｰｴｩ｡＠ regularity led Sander (1936) to prop_ose his ﾷｾ･ｵｮｳｴＱ｣＠ rule that space rhythm - time rhythm, i.e., rhythmicity in the forcmg functions. Dia enetic overprints. This heuristic rule of Sander has been ｣ｨ｡ｬ･ｮｧｾ､＠ by a ?umber ｯｦ｜ｾｲｫ･ｳ＠ (e.g., Sujkowski, 1958; Hallam, 1986) who proposed that ､Ｑ｡ｧ･ｮｾｳ＠ could produce a "rhythmic unmixing" of initially homogeneous and. un?ed?ed sediment by way of a redistribution of calcium carbonate. That such a red1stnbut10n of carbonate 158 159 ｣ｾｮ＠ .occur on a massive scale is now clear. Especially marl-limestone sequences a affected by ､ｩＮ｡ｧ･ｮｾ｣＠ overprint Carbonate contents and bedding rhytru: are mfluenced ?Y. ｴｨｾ＠ d1ssolut10? of carbonate in marly layers and by partly or CO?Jplete ｾ･ｰｲ｣Ｑｴ｡Ｐｮ＠ of the dissolved carbonate as cement in the pore space of ｡､ｊ｣･ｮｾ＠ limestone lay.ers (e.g: Ricken, 1985, 1986; Hallam, 1986; Bathurst, 1987). The mfluen.ce of diagenes1s on carbonate and organic carbon content can be calculat.ed, usmg the carbonate compaction law and performing mass balance ｣｡ｬｵｴｾｯｮｳ＠ ｢ｾ･､＠ on numerical decompaction (Ricken. 1986, 1987). Cementation n:sults m an mer.ease of the carbonate content of limestone layers to 5% - 30% ｨＱｧ･ｾ＠ values, ｾｨｬ･＠ th.e ｣ｾｲ｢ｯｮ｡ｴ･＠ content of the marly beds in between is reduced. The highest vanatmns m d1agenetic carbonate content are obtained when the average carbonate c<?ntent of the sediment is low, around 25% to 50%. .'f!le ｢･､ｾｭｧ＠ ｲｨｴｾ＠ is severely affected only when the carbonate content of the ｾｮｧｭｉ＠ sediment is high. As predicted by the carbonate compaction law, originally curves are changed by differential compaction from sinusoidal smuso1dal ｣｡ｲ｢ｾｭｴ･＠ ｴｾ＠ angular, .while the marly beds are reduced in thickness. Additionally, primary ､Ｑｦ･ｾｮ｣ｳ＠ m carb?nate content are obliterated by shifting the carbonate content in ｴｨｾ＠ lirr.iestones to ?1gher, but rela.tively constant values. ｾｮｳ･ｱｵｴｬｹＬ＠ highly rhythmic, bnck-hke alternat10ns ｾ･＠ foun? I? cemented, carbonate-nch sequences. In carbonatepoor sequences, the diagenetJc mfluence on the original bedding rhythm is weak, although small amounts of cements will significantly change the amplitude of the ｣ｾｲ｢ｯｮ｡･＠ .curve and enhance the original bedding rhythm. It is thought that d1agenes1.s m carbonate:poor ｡ｬｴ･ｾｩｯｮｳ＠ will make power spectra more significant, alternat10ns power spectra will be severely distorted. whereas m ｣｡ｲ｢ｯｮｾｴ･ＭＮｨ＠ In. ｳｵｭｾｲｹ［＠ while pnmary ｳ･､ｩｭｾｮｴ｡ｲｹ＠ fabrics are overprinted to varying degrees by diagenes1s, th.e soi_ts of .cycles ?1scussed above, expressed in multiple ways and commonly orgamzed mto h1erarch1es of cycles, are not caused by diagenesis, though they may be strongly enhanced or obliterated by that process. ｳＱｾ｣｡ｮｴｬｹ＠ Obscuring factors. Sander's rule would work perfectly if sedimentation were continuous and proceeded at fixed rates. All sedimentary systems diverge from these ideals to a greater or lesser ､ｾｧｲ･＠ .. Many facies are riddled with hiatuses; in some of the finest examples of ｾｨｹｭＱ＠ sedimentation, such as the Scisti a Fucoidi, the cyclicity itself ･ｸｰｲｳ＿ｾ＠ ｶ｡ｮｴＱＰｾ＠ m the rate of sedimentation, chiefly in the supply of carbonate. any sedu:ientary sequence contains some elements of "noise", not related In ｡､ＱｴＰｾＬ＠ changes m ｴｾ･＠ sedimentary environment. Such noise may overpower the to the ＼［ｙ｣ｬｾ＠ rhyth.rruc ｳＱｾ｡ｬ＠ recorded Ｑｾ＠ ｭｾｮｹ＠ sed!n:ientary systems. Thus, cyclostratigraphy is only ｡ｰｨ｣｢ｾ･＠ if ＨＱｾ＠ the fac1es Ｑｾ＠ sensitive for environmental changes induced by astronomical ｶ｡ｾ｢ｬ･ｳＬ＠ (2) sedimentation was continuous and at relatively constant rates, and (3) disturbances by random events were minimal. ｊＡＧ｡ｴｾｭｳ＠ in time and space. Urgent questions concern the distribution of cyclic patterns m time and space: Orbitally induced stratigraphic rhythmicity has been established in detail for e.xample for ｴｾ･＠ Late Albian in Central Italy. A next logical step is to study the same part of the same basin, with similar methods and in similar time. 1i:terval m ｡ｮｾｴｨ･ｲ＠ detail, m order to discover the level of accuracy with which correlations can be made. Can the same precessional events be recognized, and can correlations at the 10 ka level be carried through? Another logical step would be to study the same time interval in similar fades, but in another basin. A study of the Late Albian portion of the Mames Bleues in Southeastern France, highly comparable, but with a higher rate of sedimentation. could be revealing. Indeed, any rhythmic-appearing Late Albian sequence offering close biostratigraphic control appears attractive as a target for comparative study. Due to dependence on close biostratigraphic control, and considering the fact that the precessional effect dominates at low latitudes, whereas at higher latitudes the influence of the obliquity cycle may be expected to interfere, such detailed comparisons for the Albian might, in the first instance, be restricted to the lower paleolatitudes - the Tethyan belt and the lower paleolatitudes of the Pacific. As stated, high latitude climates are likely to be affected more by the 40 ka rhythm of the obliquity, and the climates of low latitudes by the cycle of precession. It is therefore not surprising to find that the Albian-Cenomanian in Italy is dominated by the precession-eccentricity pattern, while that of England (Hart, 1987) and of Northern Germany (Kemper, 1987) seem to reflect the obliquity cycle. The paleolatitude of deposition is not the only factor of importance; for example, the bedding patterns, the productivity and redox cycles in the Albian and Cenomanian in Umbria are dominated by the precession-eccentricity pattern, but the abundance of planktonic foraminifera in the same beds suggest the presence of an obliquity signal (Premoli Silva et al., 1989). It is important to identify the nature of sedimentary rhythms within specific time slices, such as the Albian and Cenomanian stages, and to map the global distribution of rhythmic patterns relative to paleolatitude and to fades, using a variety of proxies. FIELD APPROACH The time-honoured method of field stratigraphers is to measure stratigraphic sections, describing them unit-by-unit. However, published field sections rarely provide measurements for each bed, and are thus generally not sufficiently detailed to discern Experience, cyclostratigraphic signals. A careful field approach is absolutely ･ｳｮｴｩｾＮ＠ e.g., in the English Chalk, shows that certain marker beds are very widespread and to count. ｲｨｹｴｾｳ＠ easily recognizable. Once these have been identified it is ｰｯｳｩｾｉ･＠ or flint bands between them and to achieve an even more precise correlation. This can only be done after the basic field logging and description of sections is completed and after key paleontological, sedimentological, and geochemical markers have been collected and identified. Another way to approach cyclicity in the field is to restrict measurements to a small but rigorously observed set of features, such as bedding thickness, which can be computer-processed. Different approaches in techniques are exemplified by Schwarzacher (1964) and Schwarzacher & Fischer (1982) who sampled carbonate content and bedding thickness at predetermined stratigraphic intervals, and by de Boer & Wonders (1981, 1984) who measured the thickness of successive individual carbonate-rich beds. The great advantage of field observations is that a great many features can be studied. Disadvantages are that, depending on the freshness of the exposure, diagenesis, weathering and cover provide differential overprints. The 160 geologist is forced to make decisions - such as what constitutes a bed - that ｾ･ｦｲ､＠ to a ｳｴ｡ｩｯｮｾｲｹ＠ base line, but are subjective judgements made ［｛ｾ＠ Ｚ Ｐ ｾ＠ ｾｮｳｴｲｵＺ･＠ - the ｢ｾ｡ｭ＠ - whose base lines are flexible, and much influenced b 1mmed1ately precedmg observations. y is ｴｾ＠ make instrumental measurements of fresh cores, which a Slillp!e curve with a stationary base-line. Examples are the work on the ｐｩｯ｢ｾＱ｣＠ (Umbnan Apennines, Italy) core (Herbert & Fischer, 1986, Herbert, Stallard & Fischer, 1986? Park&; Herbert, 1987). Two curves for this core, one of darkness produced by m1crodens1tometry scans, and one of CaCO values taken at mean ｭｴ･ｲｶ｡ｬｾ＠ of 2 cm, ､ｾｦｩｮ･＠ two rather similar curves that ｣｡ｾ＠ be analyzed in various ways ＨｾＱｧＮ＠ 10). C:ontmuous peels of such cores have also allowed the construction of essentially contmuous plots of fauna! composition that yield spectral periodicity patterns .comparable to those for the geochemistry (Tornaghi 1984· Erba 1986· ' ' ' ' Tornagh1 et al., 1989). close to and comparable with outcrops, is not overly expensive, Drilling such ｣ｯｾ･ｳＬ＠ but few laboratories are now set up to scan cores in a rigorous and rapid fashion, so that such procedures are time-consuming at present. ｰｾｯ､ｵＮ｣･＠ Cores. An ｾｴ･ｲｮ｡ｩｶ＠ Bore.·hole logs: Objective scans of physical properties of stratigraphic sequences are ｲｯｵｴｭｾｬｹ＠ ｾ｡ｭ･､＠ ?ut by Industry, in the form of bore-hole logs (Fig. 3). Such Jogs, taken m situ, .avoid the problem of non-recovery in cores. Logs record parameters as porosity or ｧ｡ｭ＿ＱＭｲｾｹ＠ counts that serve as proxies for the features which d!rectly resulted from chmat1c change. New logging techniques (as yet little used) yield ･Ａｭｮｴｾ＠ analyses. The chief problem with existing well logs is the limited ｲｾｳｯｬｵｴＱｮＬ＠ ｾｨＱ｣＠ generally fails to resolve the primary bedding. Logging methods of high ｲｾｳ＿ｬｶｭｧ＠ power are available, but they are not as yet being widely used. Even so, ex1stmg logs should prove enormously useful in tracking low-frequency cycles. ｳｾ｣ｨ＠ Recognition And Identification Of Orbitally Induced Cyclicities Wi:h certain exceptions, the identification of stratigraphic rhythmicities with specific orbital cycl.es has ｢･ｾ＠ based upon ･ｾｴｩｭ｡ｳ＠ of their periods. Three approaches have one, applicable t? (partially) varved sequences, relies on extrapolating been ｵｾ･､Ｎ＠ varve (1.e. year) counts to orbital cycles, the other on the calculation of sedimentation rates, and the third on the complicated but characteristic character of astronomical cycles. r-:'a':'es were ｵｳ･ｾ＠ by ｂｾ｡､ｬ･ｹ＠ ＨＱＹＲｾＩＬ＠ by Van ｈｯｵｴｾｮ＠ (1964), and by Olsen (1986) in Ｑｭｾｮｧ＠ｴ the strat1graph1c rhythms m the Green River Formation and the Newark Senes. Varve ｭｾ｡ｳｵｲ･ｮｴ＠ in a. long continuous set of evaporites allowed Anderson (1982, ｾＹＸＴＩ＠ :o 1dent1fy precess10n and eccentricity forcing in the Permian Castile Format10n (Fig. 4). Sedi'!1entati?n rates infe"ed ｛ｾｯｭ＠ bio- and magnetostratigraphy. The great majority of marme sediments lacks varvmg, and an approach to the timing of rhythms can be 161 made using stratigraphic datum levels based on biostratigraphy, calibrated by means of radiometric scales such as those of Harland et al. ( 1982), Kennedy & Odin (1982), Hallam et al. (1985), and others. Such calculations invariably contain an element of uncertainty: the duration of stages is commonly not well constrained, sedimentation rates during a stage may vary, and the sequence may contain stratigraphic gaps (Sadler, 1981; Anders et al., 1987), especially when there are notable lithological changes and/or discontinuities. Thus timing arrived at by this means must generally be considered as a tentative first approach until confirmed by other methods. The refinement of magnetic geochronology offers another approach to the duration of cycles in times of a frequently reversing magnetic field (Schwarzacher, 1987). The use of integrated stratigraphy, based on various mega- and microfossil groups, magneto- and chronostratigraphy (cf. Kauffman, 1988) holds a great promise for improved temporal resolution. Once orbital cycles in sediments have been clearly established,and the changes in orbital frequency through geological time are known, cyclostratigraphy may contribute to refining the chronostratigraphy, as was first attempted already by Gilbert (1895) in the last century. Suggestions for refinement and adaption of parts of the stratigraphic timescale, based on the analysis of orbitally induced cyclicities in sedimentary successions, are being made already, e.g., for part of the Turonian (Cottle, pers. comm.), for the Pliocene (Hilgen & Langereis, 1989) and for the Pleistocene (Shackleton, 1989). Interference patterns characteristic for astronomical cycles. In cases where a detailed time control is lacking, use can be made of the complexity of the orbital motions. The effects upon climate and sedimentary facies do not produce a simple sinusoidal pattern in time and space. Rather it is a complex pattern due to the interference of the different orbital motions, each of which is characterized by a long series of different frequencies and amplitudes. For the Umbrian sequence it clearly turned out that cyclicities in the pattern of bedding thicknesses show a pattern that is closely comparable to the pattern of maxima of cool spring equinoxes at the equator, modulated by eccentricity and obliquity patterns. De Boer & Wonders (1981) calculated the statistical incidence of bedding thicknesses (reflecting subsequent periods of cumulative positive influence of the precession cycle upon ocean water circulation, nutrient supply and carbonate production) for the Albian in the Umbrian sequence (Fig. 8), and Schwarzacher & Fischer (1982) for the Barremian and Cenomanian in Umbria and for Carboniferous limestones in Ireland. ANALYTICAL TECHNIQUES FOR IDENTIFYlNG CYCLIC SEDIMENTS In theory, periodicities in lithology, and others, elemental chemistry, isotope rati?s, etc., hidden to the eye, should emerge in spectral plots of several sorts - fast Founer transforms, power spectra, Walsh spectra, etc. (Schwarzacher, 1964, .1975, 1987; Ripepe, 1988; Weedon, 1989). Essential to this method is (1) the ex1stmg of an accurate time control, (2) a long-enough time span to provide a fair sampling of the . low-frequency cycles involved, and (3) a low diagenetic ｯｶｾｲｰｩｮｴＮ＠ rocks ｾｴｨ＠ In unlithified, porous rocks (e.g., DSDP/ODP cores) and m ｣ｯｭｰ｡ｾｴ･､＠ low diagenetic overprint (e.g., marly shales) power spectra can be directly apphed. 162 163 Spec.tral ai;alysis can provide an objective demonstration of regular cyclicity in a (although ､｡ｾｩｮｧ＠ can be poor) or of modulation of cycles by st.ratigraph1c ｳ･ｱｾ＠ higher ｾｲ､･＠ cychcltles: ｄ･ｾｯｮｳｴｲ｡Ｑ＠ of regular cycles in thickness can be a very useful piece of supportmg eVIdence that the observed sedimentary cycles are indeed cycles. Walsh spectral techniques (Schwarzacher, 1975; Weedon, related to the ＿ｲ｢Ｑｾ＠ 1986) are reqmred Ｑｾ＠ severely ｣ｾｭｰ｡ｴ･､＠ or st;ongly diagenetic sequences where the physical or geochemical properties of the section vary in a more nearly square-wave pattern ｴｨｾ＠ in a sinusoidal ｾｮ･Ｎ＠ Moreover, a better understanding of how power ｳｰ･ｾｲ｡＠ are mfluenced by stra.t1g;aph!c gaps (Sadler, 1981; Anders, Krueger & Sadler, 1987, ｗｾ･､＿ｮＬ＠ 1989), by vanat1?ns Ｑｾ＠ ｴｾ･＠ ｲｾｴ･＠ of.sedimentation, and by differential compact10n is needed ..ｳｾｴ･ｭ｡Ｑ｣＠ vanat1on m sedimentation rates between beds can ｢ｾ＠ detected by determmmg ｴｨｾ＠ ,t):'Pe depositional dilution (e.g., dominant dilution either throu&h the ".3-"bonate, srhctclastrc, or organic fraction) and by applying dilution formul.as (Ricken, m pres.s ). In car?onat7-rich alternations, considerable diagenetic ｯｶ･ｲｰｮｾ＠ due to cementat10n and d1ssolut10n processes is accompanied by differential the shape of the carbonate curves, and reduce the thickness compact10n; both ｡ｦｾ･｣ｴ＠ of the marl beds (Ricken, 1986, 1987, and "diagenetic overprint" above). of Summary ｒｨｹｴｾｩ｣･ｳ＠ ｡ｰｾｲｯｸｩｭｴｮｧ＠ .the ｰ｡ｴｾｲＮｮｳ＠ of Milankovitch forcing have been recognized Ｑｾ＠ pelagic, henupelagic, evapont1c and lacustrine facies from various parts of Earth's ｾＱｳｴｯｲｹＮ＠ Up to four hierarchies, ranging from the 20 ka precessional cycles to a cycle m the 2 Ma range, have been identified in a single sequence. These cycle patterns la;gely seem to match those of the Earth's orbital variations and thus ｳｵｰｾｯｲｴ＠ G.1lbert's original proposition. The case for an orbital ｲ･｣ｾ､＠ in the strat1&1'aph1c column now appears strong. Clearly this is a major frontier for stratigraphic work. ｃｹ｣ｬｾｳｴｲ｡ｩｰＭｨ＠ patterns may be expressed in paleontological, physical (e.g., ｭｾｧｮ･ｴＱ｣ＡＬ＠ mmeralogical. and sedimentological oscillations. Study of surface outcrops will ｣ｯｮｾｭｵ･＠ to be an important factor in cyclostratigraphical studies, but field measurements are advantageously combined with laboratory data obsei;ations ｾ､＠ that yield add1t1o?al ｮｵｭｾｲｩ｣｡ｬ＠ time series. Particularly advantageous can be the use which prov1?e. the opportunity of continuous paleontological, physical of core ｨｾｬ･ｳＬ＠ ｡ｾｬｹｳ･Ｎ＠ Ex1stmg borehole logs commonly lack the resolving power to and ｣ｨｾｭＱ｡ｬ＠ recognize the high-frequency Ｈ｢･､ｾｩｮｧＭ｣ｯｵｰｬＮｴＩ＠ cycles, but nevertheless they constitute a ｬ｡ｾｧ･＠ unexplored database of mformat10n on cyclostratigraphic rhythms. New loggmg methods hold great promise. ｾ､＠ timing of rhythms have to be refined, including Methods for both Ｎｩ､･ｮｴｦｾｯ＠ methods for modellmg ｳ･､Ｑｭｾｴ｡ｯｮ＠ ;ates, f?r improving the resolution of power spectra, and for the deconvolVIng of d1agenet1c overprint . That cyclostratigraphy was discovered in the Cretaceous is no accident. The ｷｩ､ｾｳｰｲ･｡＠ seas with. ｣ｬｩｭ｡ｴｾｹ＠ ?ensitive sedimentary systems predisposed this ｰｾｮｯ､＠ f?r the recordmg of climatic cycles. This, together with a well-developed b10str.at1graphy, makes the Cretaceous system particularly promising for the establishment of a coherent stratigraphy. The Cretaceous is also a time of particular interest for paleoclimatology. Its low latitudinal gradients suggest a greenhouse atmosphere, a state towards which ｾ･＠ may be heading now, due to anthropogenic carbon dioxide enrichment. Cyclostrat1graphy of the Cretaceous promises to bring insights into the functioning of Cretaceous atmosphere and oceans on the one hand, and on the other into the chronology needed to apprehend the rates of climatic change. Most of what we now know has been learned in the last two decades. New approaches have been developed and more will come along. As yet the rhythmic sequences analyzed are isolated fragments out of the geological record. Many more sequences, sufficiently well distributed ?ver the ＦＱＰｾＬ＠ have to be ｦｾ､＠ and analyzed, to provide rigorous counts of precess10nal, obhqmty and eccentnc1ty cycles for the zones and stages and polarity chrons of the stratigraphic record, in order to make a contribution to geochronology. A clear view on cycle patterns for. given t!me slices will contribute to paleoclimatology and paleoceanograph¥, and. will provtde. ｵｾ･ｦｬ＠ information for reconstructing the evolution of the Earths orbital charactensucs. The analysis of orbital cyclicities in sedimentary successions requires techniques ranging from log interpretation to time-series analysis, and is strongly depende.nt upon geochronological control, largely rooted in biostratigraphy and magnetostraugraphy. Accordingly, cyclostratigraphy calls for combinations of talents, for teamwork. Returning to the questions posed in the introduction: . . . . 1) The existence of stratigraphic sections that record orbital variations 1s now well established for rocks of many different facies and ages. 2) Orbitally forced rhythms are widespread ｢ｯｾ＠ in facies ｾ､＠ time, at least fr?m the Ordovician to the present day and m both manne and non-marme sediments. 3) Cyclic sequences are best studied by techniques which rapidly generate quantitative data for spectral analysis. . . Potentially they offer an enormous amount of mformauon, and a much refined 4) stratigraphy. . . Practical cyclostratigraphy is within reach, but will reqmre a great deal of 5) detailed sampling. . 6) The best method seems to define time horizons, and to attempt to fix segments of cyclic sequences in time. In the Cretaceous s.everal. good .mark7r beds are available, including tephra horizons, well-_constramed ＿Ｑｾｴｲ｡ｧｰｨ｣＠ and magnetostratigraphic boundaries, and promise the poss1b1hty of intercontinental stratigraphic correlation. Work on continuous and cyclic appearing sequences is encouraged, but particularly timely is work directed at answering any of three proble.ms: -What will cyclostratigraphy do for accuracy of ｣ｯｲ･ｬｾｴＱｮｳ＿＠ . . The answer to that question is to be sought by detailed cyclo.strat1gr:aph1c study of a time-slice in which a cyclostratigraphy has been resolved m ､･ｴｾＱｬＮ＠ . -What is the paleogeographic distribution of major cycle types - obhqmty versus precession/obliquity? 164 165 This can be approached by more general studies of global cycle patterns on the scale of a stage. Did cycle patterns change in time? The answer to this lies in the analysis of long sequences of cycles of appropriate and uniform facies. Prospects And Recommendations General comparative studies of longer stratigraphic sequences. Are there temporal patterns in orbitally driven cyclicity such as shown in the Pleistocene isotope record (cf. Pestiaux & Berger, 1984)? Are there regional or global alternations between episodes of marked cyclicity and others in which the orbital signatures are suppressed or absent? Any long and continuous stratigraphic section of appropriate and relatively uniform facies lends itself to this type of study. Ideally, such work should be done in close cooperation of sedimentologists, paleontologists, magnetostratigraphers, geochemists and geophysicists, ｨ｡ｾ､＠ in hand with astronomers studying changes of the character and of the frequencies of the orbital movements during the geological history on a theoretical basis. Every bore-hole for petroleum is now logged by various methods, and enormous numbers of such logs rest in the files of Industry and Governments. While most of these logs lack the resolution needed to identify the higher frequencies, they at least represent a largely unused source of data on the lower frequencies in the orbital variation spectrum. They will certainly become vital in linking the more detailed cornerstone studies. No one method of spectral analysis is ideal for any and all data sets. Continued research on mathematical resolution of rhythmicities in stratigraphic time series is highly desirable. At the same time, so Jong as ､ｩｦｾ･ｲＮｮｴ＠ workers process ｴｾ･ｩｲ＠ data in different ways as to achieve a power spectrum m different manners, their results are not comparable. It therefore is a matter of importance to devise some central data base in which numerical cyclostratigraphical data of all sorts can be collected and redistributed for availability to all who wish to process such data in comparable There is a gap be.tween traditional stratigraphic resolution (order of accuracy of Ma) and modern sedimentary phenomena observed within the limits of our personal eph.emeral experience. The Milankovitch rhythms partially fill this gap. For parts of ｳ･､Ｑｭｾｮｴ｡ｲｹ＠ ｳｵ｣ｾｩｯｮ＠ for ｷｨｩ｣ｾ＠ astronomical cycles have been clearly and reliably established, relative age determmations can be made, that is, relative within that ｳｵ｣ｾｩｯｮＬ＠ and with .great detail. The complicated pattern of frequencies and amplitudes of the orbital movements and their effects provide a basis for detailed cor:elations, with a resolution of a fraction of the 21,000 precessional period on a ｲ･ｧＱｾｮ｡ｬ＠ scale (de Boer, 1983), and also over the globe (Cotillon, 1984, 1987). Cyclostrat1graphy should not become a science 'per se', but it should contribute to a better ｵｮ､･Ｎｲｳｾ｡ｩｧ＠ of sedimentary ｾｮ､＠ climatic processes of the past. It also may allow pred1ct1ons of the future evolution of the Earth in relation to the activities of Man. The_refore, we call upon stratigraphers to identify Cretaceous sequences that show ｾ｣ＡＱ＠ ｰ｡ｴｾｲｮｳＬ＠ ｾｮ､＠ to commence studying them. Discovery of Milankovitch cyclic1ty Ｑｾ＠ ｳｴｲ｡Ｑｾｰｨ｣＠ sequences is a function of the degree of sophistication with which ｳｴｲ｡ＡｧｾｨＱ｣＠ sequences have been studied. It is presently restricted to sequences studied smce long. Therefore, prospects of finding other examples in more remote areas seem good. SPECIFIC SHORT-RANGE OBJECTIVES WHICH WE RECOMMEND: ｄ･ｴ｡ｩＯｾ､＠ corr:parative studies. One of the first questions to be settled is the suitability of orbitally mduced cycles for precision in stratigraphic correlations and geochrono!ogy. The most detailed cyclostratigraphy now in hand is that of the praeticinesis zone m the Late Albian of central Italy (Premoli Silva et al., 1989), and that zone is a target f?r ｳｩｭｾ｡ｲ＠ studies elsewhere, wherever it can be rigorously identified in ｡ｰｾｲｯｮｴ＠ fac1es. ｓｾ｣ｨ＠ work should include profiles in the same basin, in adjacent basms and m other, distant parts of the world. Other studies will reach the same level of detail, and may define other time slices of equal promise. General comf!arative studies of time slices. Also where such close stratigraphic control does not exist, cycle patterns in time slices such as the Late Albian and the Cenomanian should be resolved wherever the sedimentary facies permit, in order to map the paleogeographic distributions of obliquity-dominated patterns versus precession-eccentricity dominated ones. This should prove illuminating for our conception of Cretaceous climates and oceanography, and for the pathways along which sedimentary systems were forced. ｷｾ＠ identification of stratigraphic rhythmicities with specific orbital cycles depends primarily on their periods. A coherent cyclostratigraphy, ｬｩｫｾ＠ a ｭ｡ｾｮ･ｴｯｳｲｩｧｰｨｹＬ＠ can only be developed with the help of close b10straugraph1c control. Thus, improvements in geochronology and stratigraphic cor:elation ｾｲ･＠ of great ｩｭｾｯｲｴ｡ｮ｣･＠ to refine determinations of frequencies. Paleoecolog1cal studies are needed m order to better determine causes and pathways of cyclicities. Composition of, ｾｮ､＠ preservation of fossil assemblages, especially if ｣ｯｭ｢ｩｮｾ､＠ with .g.eochemical ｳｾｵ､Ｑ･Ｌ＠ allow a more realistic determination of oceanographic cond1t1ons and sed1mentological processes in the past (see e.g., Roth, 1986; Thierstein & Roth, 1989 for an . interpretation of Lower Cretaceous cycles). The present state of the art has convinced some. (but no.t all) straugraphers that a cyclostratigraphy is attainable. It calls. for. focus mg a w1d.e array ?f talents ｡ｾ､＠ techniques on microstratigraphy, and 1t will depend on ｭｾ･ｲｮ｡ｵｯｬ＠ purs.uit, cooperation and possibly coordination. All this places cyclostraugraphy squarely mto the CRER framework. A newsletter on (Cretaceous) cyclostratigraphy could serve as a clearing house to keep cyclostratigraphers informed into what is being done where. Walter Dean (US Geological Survey, Denver, Colorado, U.S.A.) has offered to ｬ｡ｵｮｾｨ＠ such a letter. The benefits derived from CRER do not end there. Cyclostrat1graphy must be with ｃｒｅｾＮ＠ coordinatio?,: ｲ･､ｯｾ＠ intimately linked with other stud.ies being ｣｡ｲｾｩ･､＠ cycles for example are directly linked to the mte.rests of the black shales group, emergence cycles are intimately tied to the question of .sea level change, and bear on the life history of carbonate platforms. All of these fit mto the broader framework 166 167 of Cretaceous Paleoclimates. Cyclostratigraphy depends upon geochronology, and holds promise for refining it. Thus CRER will serve not only a coordinating function for the cycle workers, but will insure their interaction with other endeavours focused on the Cretaceous. Our workshol? in Perugia has served to canvas the field of cyclostratigraphy, from theory to techruque, and from what we have learned to what may lie beyond. Under the aegis of CRER this can the beginning of a drive towards a COHERENT CYCLOSTRATIGRAPHY of the CRETACEOUS. References Anders, M.H., Krueger, S.W. & Sadler, P.M. 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