cretaceous pelagic black shales and red beds in western tethys: origins… · 2020-07-16 ·...

59CRETACEOUS PELAGIC BLACK SHALE AND RED BEDS

CRETACEOUS PELAGIC BLACK SHALES AND RED BEDS IN WESTERN TETHYS:ORIGINS, PALEOCLIMATE, AND PALEOCEANOGRAPHIC IMPLICATIONS

LUBA JANSAEarth Science Department, Dalhousie University, Halifax, Nova Scotia, Canada

AND

Geological Survey of Canada-Atlantic, Dartmouth, Nova Scotia B2Y 4A2, Canadae-mail: [email protected]

AND

XIUMIAN HUDepartment of Earth Sciences, Nanjing University, Nanjing 210093, China

e-mail: [email protected]

ABSTRACT: Sedimentological studies of Cretaceous deep-sea sediments deposited in the western Tethys allow at least rough analyses of theEarth Systems processes involved. The deep-sea evidence shows that the change from Lower Cretaceous pelagic carbonates to mid-Cretaceous black shales was not a sudden event, but took 15 Myr for a complete transition. This change was a response to thermalsubsidence of oceanic crust in the western Tethys. The mid-Cretaceous black-shale development was forced by high deposition of organicmatter in a response to a humid, warm climate, with the latter in turn resulting in decreased rate of deep water formation, poor ventilation,and lower content of dissolved oxygen in deep waters. However, the bottom waters remained oxic and/or variably dysoxic, as indicatedby presence of mid-Cretaceous greenish-gray claystones in the Moroccan basin, while elsewhere black shales were deposited, and by localbioturbation and intercalated reddish, oxic horizons.

Various processes were involved in the origin of oceanic anoxic events (OAE1a and OAE2). Sediments deposited during these eventswere laid down during onset of major transgressions in the central North Atlantic and thus can be viewed as condensed horizons relatedto flooding events. The high content of organic matter is most probably a phytoplankton response to ocean surface water fertilization byiron during extensive Aptian volcanic activity along North Atlantic continental margins. Less certain is the influence of the Caribbeanigneous plateau (93–89 Ma) and Madagascar flood basalts on the development of OAE2.

The reddish colors of the overlying Upper Cretaceous pelagic claystones and shales are of early diagenetic origin and not a depositionalfeature. Sediment oxidation was forced by low sedimentation rate of < 20 mm/kyr, driven by eustatic sea-level rise and increasedventilation of the basin. The red beds intercalated in mid-Cretaceous strata in western Tethys have variable origins, such as 1) an inflowof higher-salinity surface waters from the southern Atlantic when the equatorial seaway began to open; 2) reduction of sedimentation rateas a result of local tectonics; and 3) sea-level changes. Less certain is whether a temporary inflow of colder, more oxygenated bottom waterinto small tectonic sub-basins of the Mediterranean region could have resulted in bottom-sediment oxidation, because the validity of sucha hypothesis cannot be confirmed by studies of modern marine sediment.

INDEX WORDS: Cretaceous, pelagic red beds, CORBs, black shales, OAEs, depositional regime, paleoceanography, paleoclimate, EarthSystems

Cretaceous Oceanic Red Beds: Stratigraphy, Composition, Origins, and Paleoceanographic and Paleoclimatic SignificanceSEPM Special Publication No. 91, Copyright © 2009SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-135-3, p. 59–72.

INTRODUCTION

The Cretaceous Period was a time of upheaval in the EarthSystems, with major plate-tectonic movements, and changes inclimate, ocean circulation, and volcanicity, all recorded by deep-sea sediments. Pelagic carbonates dominated deep-sea deposi-tion in the western Tethys from the Late Jurassic to the EarlyCretaceous. In the Valanginian they become progressively inter-bedded with dark gray marls, which in the Aptian were suc-ceeded by noncalcareous “black” shales. The latter shales in turnwere followed rather abruptly by deposition of reddish clays andshales during Early Turonian time, deposition of which contin-ued in the central North Atlantic up to the Early Eocene (Lancelotet al., 1978; Jansa et al., 1979).

The mid-Cretaceous (124–93 Ma, Aptian–Cenomanian) blackshale facies contain several horizons enriched in organic carbon,which are more or less synchronous between different oceanicbasins and were termed oceanic anoxic events (OAEs). The EarlyAptian OAE1a (also known as the Selli Level, e.g., Schlanger andJenkyns, 1976; Arthur et al., 1990; Bralower et al., 1994) and OAE2,

occurring at the Cenomanian–Turonian boundary (the BonarelliLevel), have global distributions. Several more OAEs were pro-posed (Erbacher et al., 1996; Erba, 1988; Tornaghi et al., 1989), butit seems that they are of only local and/or regional significance.In addition to high content of organic matter (OM), OAE1a andOAE2 are recognized by positive shifts in carbon isotope values(e.g., Arthur et al., 1988; Hasegawa, 1997; Weissert et al., 1998;Tsikos et al., 2004).

Most of the theories that attempt to explain the origin of OAEsare based on interpretations of δ13C and δ18O isotopic ratio froma few localities or even a single locality, with the authors implyingglobal significance of their isotopic data. Few of these interpreta-tions consider the influence of major changes in the Earth systemsoccurring during Cretaceous time. Nor has the presence of red-dish clays and marly beds intercalated with the mid-Cretaceousblack shales been considered significant. Such horizons are re-gionally restricted and are found in the Aptian, Albian, andCenomanian strata in southern Europe and in some areas of thecentral North Atlantic (e.g., Lancelot et al., 1978; Sheridan et al.,1983; Norris et al., 1998; Hu et al., 2006). If the deposition of mid-

LUBA JANSA AND XIUMIAN HU60

Cretaceous black shales was driven by humid, warm climates(Fassell and Bralower, 1999) and if the intercalated oxic, reddish-colored horizons are similarly indicative of cooler climatic peri-ods, then the mid-Cretaceous climate could not have been asequable as proposed by many paleoceanographic studies (e.g.,Barrera and Johnson, 1999), but would have been more variable,with some of the changes on a scale of several tens of thousandsto several million years (Hu et al., 2006). Similar questions can beasked about the origin of Upper Cretaceous pelagic red beds,because they were deposited during a time which, from oxygenisotope studies, has been interpreted as a cooler climate period(Steuber et al., 2005).

The study of occurrences of Cretaceous pelagic red bedsdemonstrates clearly that they have different stratigraphic posi-tions in different areas (Hu et al., 2005), and therefore an intrinsicrelationship to climate change becomes doubtful. A caveat to thestudy of Upper Cretaceous pelagic red beds is that they representa major change in the location of the carbon repository becausecarbon was no longer accumulating in deep oceanic basins.

The aim of this paper is to combine the results of sedimento-logical studies of Cretaceous sedimentary strata in the westernTethys, in order to better understand the Earth system processesthat caused the deposition of mid-Cretaceous black shales, OAEs,mid-Cretaceous oxic horizons, and Upper Cretaceous pelagic redbeds (CORBs), represented by clays, shales, marls, and lessfrequently by chalks. Was the principal driver climate, oceancirculation, paleogeography, bioproductivity, sea-level change,tectonics, or a combination of any of the above? In the course ofthis study we found that not all of the CORBs were generated bythe same processes; therefore they had multiple origins. Becausepelagic red beds also occur intercalated with mid-Cretaceousblack shales (e.g., Sheridan et al., 1983; Hu et al., 2006), their originand that of the OAEs had to be included in this study, inasmuchas they are part of the Earth Systems variability.

The processes controlling type, amount, and pace of sedimentaccumulation will, however, be better understood if sedimenta-tion rate (i.e., one-dimensional mass accumulation rate) and itsvertical variations are accurately documented. Sedimentationrates are calculated as sediment thickness divided by duration.They are influenced by several factors, such as compaction,dissolution, erosion, and hiatuses. But, because most of the dis-cussed sediments were deposited below the CCD and, therefore,lack a carbonate component, and because most were deposited inabyssal depths out of reach of bottom currents and erosion, weuse sedimentation rate and accumulation rate in mm/kyr to cm/kyr interchangeably.

MID-CRETACEOUS BLACK SHALESIN THE WESTERN TETHYS

The dominant mid-Cretaceous deep-sea deposits in the west-ern Tethys are the so-called “black shales”. They were encoun-tered in numerous DSDP and ODP drilling sites in the Atlantic,where they are up to 300 m thick and are designated the HatterasFormation (Jansa et al., 1979). Some authors confuse the mid-Cretaceous “black shales” with black shales comprising the oce-anic anoxic events—OAEs. To limit potential confusion we usethe abbreviation HBS (Hatteras Formation–type black shales asdescribed by Jansa et al., 1979) for the dark-gray to black Aptianto Cenomanian shales deposited in the western Tethys.

Composition

Several sedimentological features are characteristic of theHBS: their dark gray, locally blackish color (Fig. 1B, C), lack of

carbonates, variable presence of terrigenous mineral particles,presence of both marine and terrestrial organic matter, andhighly variable sedimentation rates. The HBS sedimentation ratein the central North Atlantic basin averaged 3 to 5 mm/kyr, butnear the continental margin the sedimentation rate increased upto 12–19 mm/kyr, due to incorporation of turbidites (Jansa et al.,1979). HBS are composed of clay minerals dominated by wellcrystallized smectite, minor illite, kaolinite, chlorite, and mixed-layer clays (Chamley and Robert, 1982), together with quartz silt,which in some turbidite beds is up to 30%. Feldspar grains,zeolites, organic matter of both marine and terrestrial origin(wood fragments, leaves), and rarely glauconite are present(Lancelot et al., 1978; Jansa et al., 1979; Tissot et al., 1979), indicat-ing sediment transport from surrounding continents. Organiccarbon in the HBS is usually around 2% (Gardner et al., 1978).Lack of carbonate in these shales confirms deposition below thecarbonate compensation depth (CCD).

Cyclicity

The HBS in the North Atlantic commonly show faint cyclicalternation of greenish gray and dark gray to black-coloredbeds, with cycle thickness of ~ 20 to 40 cm (Dean et al., 1978).Such cycles are in many DSDP and ODP cores from the NorthAtlantic, and span without interruption from the Late Jurassicup to the Neogene (Lancelot et al., 1978; Dean and Gardner,1982). The periodicity of these cycles was calculated to be 30 to50 kyr (Dean et al., 1978), and thus they most probably are aresult of climatic changes related to the Earth’s obliquity cycleof about 41 kyr (de Boer and Smith, 1994). An important obser-vation is that the lighter or greenish-colored half-cycle in theHBS commonly shows traces of bioturbation (Fig. 1A, B), whichdisproves the theories of complete bottom-water stagnationduring deposition of mid-Cretaceous black shales (euxinic con-ditions; Bolli et al., 1978). Schieber (1994, 2003) challenged theanoxic paradigm by demonstrating that macroscopic bioturba-tion represents the activities of aerobically metabolizing meta-zoans. Thus these deposits do not necessarily represent epi-sodes of basin-wide stagnation and anoxia. Furthermore, howwould this stagnation be created and destroyed with greatfrequency and regularity over such a long time period? It is notdifficult to imagine abrupt changes in bottom-water circulationin restricted basins such as the Black Sea, the Baltic sea, or theMediterranean Sea, but it is difficult to imagine that completestagnation could develop in the Atlantic ocean, because it wouldhave to change dramatically from oxygenated to stagnant an-oxic bottom conditions on a scale of about one-half Milanko-vitch obliquity and precession cycle (20,000–12,000 yr) and thatcyclically continuing for several million years.

Hypothesis of Origin of Mid-Cretaceous Black Shales

While some authors have attributed the origin of the HBS toa combination of low productivity and enhanced preservationunder stagnant dysoxic–anoxic conditions (e.g., Bralower andThierstein, 1984; Premoli Silva et al., 1989), others have inferred avariety of other factors and their combinations. These are: changesin oxygen level of the water column, overall sedimentation rate,climate, formation of bottom waters, occurrence and intensity ofupwelling, distance from the coast, water depth, and burial ratesof organic matter (Berner, 1978; Calvert and Pederson, 1993;Calvert et al., 1996; Wortmann et al., 1999; Herrle et al., 2003;Wagner et al., 2004). In the search for the mechanism that led tothe deposition of HBS, most authors do not consider the potentialinfluence of broader changes in the Earth systems on deep-sea


FIG 1.—A) Bioturbation (marked by arrow) in lower Aptian black-shale facies, central North Atlantic (ODP Site 641C, core 11). B) Redbeds (on right) intercalated in the lower part of the Hatteras Formation composed of black shales (on left). Red bed subunit is 27m thick and Late Aptian in age. Note bioturbation in black shale facies (DSDP Site 534A, core 41). C) Sharp boundary betweenmid-Cretaceous black shale with overlying OAE2 (black bed), which in turn is overlain by Upper Cretaceous pelagic red clays(central North Atlantic, ODP Leg 103, Site 641, core 6-7). D) Upper Cetaceous Scaglia Rossa Formation, composed of pink andreddish marlstone and argillaceous limestones in northern Italy. The white band is a carbonate turbidite bed. E) Cyclic red-coloredmarlstones alternate with white chalk beds on the Blake Bahama slope (Leg Site 171, Site 1049C, core 13x, late Aptian), while inadjacent deep ocean basin were deposited black shales (e.g., Leg 76, Site 534A, core 43; middle Aptian).


deposition, because most current studies are based almost exclu-sively on interpretations of carbon and oxygen isotope data.

The basin-wide change from Lower Cretaceous pelagic car-bonates to mid-Cretaceous deep-sea shales in the western Tethyswas driven by thermal subsidence of the oceanic crust, which inthe western Tethys subsided below 4000 m water depth andbelow the CCD for the first time during Aptian time (Thierstein,1979; Jansa et al., 1979). This is contrary to the hypothesis ofLarson and Erba (1999), who suggested that the onset of HBS wasthe result of outgassing associated with a superplume event in thePacific Ocean. A change from pelagic carbonate deposition toblack shale deposition in the central North Atlantic was not asudden event, but as recorded by deep sea sediments took 15 Myrfor a complete transformation from pelagic carbonate depositionto noncalcareous black shale deposition, while the ocean crustsubsided from ~ -4000 to -5000 m (Ewing and Houtz, 1979).Concurrent was the climate change from arid and warm duringthe Early Cretaceous to a more humid temperate climate dominat-ing the mid-Cretaceous in the western Tethys, as demonstrated bynumerous oxygen isotope studies (Clarke and Jenkyns, 1999,Wilson and Norris, 2001; Steuber et al., 2005, and references therein).

HBS – The Result of Covariance of Several Processes

A review of the origins of the HBS suggests that they are theresult of the co-occurrence of several externally dependentprocesses. The major driving force was the change to a humidclimate (Frakes et al., 1994; Abreu et al., 1998), on which has beensuperimposed orbitally modulated climatic variations (Milanko-vitch cyclicity; obliquity and precession cycles). A humid cli-mate resulted in increased fluxes of terrigenous sediments intothe ocean, which, in combination with higher input of terrestrialand marine organic matter (Tissot et al., 1979) and nutrientsduring wet Milankovitch hemicycles, resulted in increased depo-sition of organic matter in pelagic sediments. The subsequentdegradation of organic matter within the sediment led to thedevelopment of anaerobic environments within the sediments,conditions possibly extending up to the sediment–ocean waterinterface (Gardner et al., 1978). As a result of microbial utiliza-tion of available oxygen during organic-matter degradation,H2S is produced as a by-product of bacterial sulfate reduction(Froehlich et al., 1979) and iron monosulfates are precipitated.These minerals, together with manganese oxides, impart a darkgray to black color to the sediment (Nagao and Nakashima,1992). Therefore, the dark color of black shales has to be consid-ered to be an early diagenetic and not a syndepositional feature(Gardner et al., 1978), and cannot be used as evidence for thepresence of anoxic bottom waters. That the bottom watersremained oxic is supported by the presence of local bioturbationwithin the green hemicycle in HBS and by the co-occurrence ofpelagic red beds intercalated within mid-Cretaceous strata (Fig.1B; to be discussed in more detail below). But more significantly,it is a lack of mid-Cretaceous black shale facies in the deepMoroccan basin (Lancelot et al., 1978), seldom considered in theliterature, that contradicts the stagnant-bottom-water theories(Bolli et al., 1978; Brumsack, 1980). In the deep Moroccan basingreenish gray shales rather than black shales were depositedduring the mid-Cretaceous. The presence of such shales isconclusive evidence that the deep waters in the central NorthAtlantic could not have been at any time stagnant and anoxic.However, in some smaller basins on the continental margins orin semi-restricted, small oceanic basins (Wagner et al., 2004),occasionally stagnant bottom conditions could develop as aresult of changes in the basin morphology, or sea level, as isdemonstrated by the Holocene data from the Black Sea, the

Cariaco Trench, the Saanich Inlet, the Santa Barbara Basin, andthe Gulf of California. But such conditions are not applicable tolarge oceanic basins such as the Cretaceous western Tethys.

It is concluded that mid-Cretaceous black shales (HBS) devel-oped in the western Tethys as a result of several simultaneouslyoccurring changes within Earth systems. First, it was a change toa humid and warm climate during the mid-Cretaceous, mostprobably driven by extensive volcanic activity and increases inatmospheric CO2 (Larson and Erba, 1999). Second, high input ofboth marine and terrestrial organic matter intrinsic to climatechange (Tissot et al., 1979) resulted in the development of ananaerobic environment within the sediments. The anaerobiccondition was the consequence of organic-matter degradationand oxidation and resulted in increased organic-matter preserva-tion. Finally, during wet Milankovitch hemicycles, warmer tem-peratures and a decrease in surface-water salinity could cause aslowdown in deep-water formation, resulting in decreased deep-basin ventilation with bottom waters having a lower content ofdissolved oxygen, because oxygen solubility decreases with in-creasing temperature. This could result in bottom waters beingperiodically dysoxic. However, the importance of decreasedsalinity and bottom-water formation is questioned, as for ex-ample in the present Norwegian Sea the recent bottom sedimentsare dark-gray silty clays, although the bottom waters are highlyoxygenated, containing 6 ml/l of dissolved oxygen (Bainbridgeet al., 1981). Another important similarity between the mid-Cretaceous central North Atlantic and the modern NorwegianSea is that both basins are of similar widths.

The sedimentation rate for some HBS in the central NorthAtlantic (Jansa et al., 1979) was similar to the sedimentation rateof modern pelagic sediments in the Nares and Madeira abyssalplains (as will be discussed later), which are oxidized and redcolored. Therefore the sedimentation rate alone has not been theprincipal driver in the dark color of HBS. Contributing factorswere lower content of dissolved oxygen in bottom waters due topoor ventilation of the deep ocean basin and, particularly, due toincreased influx of marine and nonmarine organic matter, intodeep-sea sediments.

OCEANIC ANOXIC EVENTS (OAES)

The OAE horizons are also composed of black shale or aclaystone bed, but those are mostly only several tens of centime-ters to meters thick. The OAE2 at ODP Site 641 is 30 cm thick (Fig.1C) and dominated by clay minerals (smectite with minor amountsof illite, palygorskite, and kaolinite). No detrital minerals weredetected, nor were carbonate, nor calcareous microfossils. TheOAEs have a much higher content of organic matter (OM) thanHBS, which are predominantly of marine origin. At some sites inthe central North Atlantic OM content is 10.5% (Thurow et al.,1988), but it can be as high as 22%, as in the Umbria–Marche basin(Arthur and Premoli Silva, 1982). The OAEs represent geologi-cally brief events. The OAE2 in the Umbria–Marche basin wasestimated to last 0.35–0.7 Myr (Arthur and Premoli Silva, 1982),and a similar duration was estimated for OAE2 drilled off GaliciaBank in the central North Atlantic (Boillot et al., 1987). Anexception is the Lower Aptian OAE1a in Umbria–Marche basin,which lasted from 0.5 to 1 Myr (Menegatti et al., 1998). Among atleast six black shale horizons considered to represent OAEs(Bralower et al., 1994; Erbacher et al., 1996), only two organic-carbon-enriched horizons, the Lower Aptian Selli Level (OAE1a)and the uppermost Cenomanian Bonarelli Level (OAE2), haveglobal distributions, and these are discussed here. Both are iden-tified by positive δ13C isotope excursions (e.g., Schlanger et al.,1987; Weissert et al., 1998; Hochuli et al., 1999), which convention-


ally are related to excess burial of organic carbon (Jenkyns, 1980;Scholle and Arthur, 1980; Schlanger et al., 1987).

Hypothesis of Origin of OAEs

Even though only minor lithological differences exist be-tween HBS and OAEs, these two types of shales had very differ-ent origins. Many different hypotheses for the origin of OAEshave been suggested, such as climate change, changes in oceancirculation, bioproductivity, and organic-matter preservation.

Ocean Circulation and Productivity.—

The so-called stagnant-ocean model (e.g., Brumsack, 1980;Bralower and Thierstein, 1984; Herbin et al., 1986) is based on theassumption that the high sea level and changes in thermohalinecirculation resulted in decreasing subsurface oxygen concentra-tions and an expansion of the oxygen-minimum zone. This wouldhave led to increased burial of organic carbon (e.g., Schlanger andJenkyns, 1976; de Graciansky et al., 1984; Arthur et al., 1990).Recently Herrle et al. (2003) suggested from studies of EarlyAlbian OAE 1b in the Vocontian basin that the black shales areprobably the result of monsoonally driven changes in tempera-ture and the evaporation–precipitation cycle. They concludedthat at least in the Vocontian Basin the increased preservation oforganic matter at the sea floor was more important for black shaledeposition than enhanced productivity. This is in contrast toWeissert (1989), Pederson and Calvert (1990), Calvert and Pederson(1993), Erbacher et al. (1999), Hochuli et al. (1999), and PremoliSilva and Sliter (1999), who attributed the formation of OAEs toincreased productivity during warmer and more humid climaticconditions. The latter authors suggested that increased runoffaccelerated the transfer of nutrients from the continents to theocean, inducing higher primary productivity in the surface wa-ters.

Role of Eustasy.—

Sedimentological studies of Cretaceous deep-sea strata andsynchronous sedimentary deposits on continental margins indi-cate that several other processes not considered by previoustheories were important in the formation of OAEs. Geologicstudies of the circum–North Atlantic continental margins (Jansaand Wiedmann, 1982; Jansa and Wade, 1975a, 1975b) revealedthat a major Aptian transgression drowned the Early Cretaceouscarbonate platform. Another transgression occurred during theLate Cenomanian–Early Turonian (Gradstein et al., 1990). TheOAE1a and OAE2 beds were deposited at the onset of these twomajor transgressions in the central North Atlantic. As demon-strated by numerous oil exploration–related studies, the onset oftransgression commonly leads to deposition of condensed hori-zons, characterized by high content of organic matter. From theviewpoint of sequence stratigraphy, both OAE1a and OAE2 inthe western Tethys can thus be considered to represent floodingevents. In addition, the sedimentological composition of OAE1aand OAE2 is similar to flooding horizons in that they are domi-nated by clays and lack coarser terrigenous components (Boillotet al., 1987; Thurow et al., 1988).

Premoli Silva et al. (1999), as well as Larson and Erba (1999),suggested that increased runoff led to accelerated transfer ofnutrients from the continents into the ocean, inducing higherprimary productivity of surface waters. The difficulty with thistheory is that increased runoff from continents should also resultin increased incorporation of terrigenous clastics in deep marinesediments (OAEs), which is not the case. As both OAE1a and

OAE2 developed during major transgressions, river-borne sedi-ment were kept near the coastline, therefore nutrients did notspread far offshore. In addition, longshore currents did spreadthe nutrients mainly along the coast, and they were not trans-ported into the deep ocean basin unless wind-driven gyres devel-oped at that time. This suggests that the cause of increasedbioproductivity during the above two events was internal changewithin the system, not from external sources.

Role of Pacific Volcanicity.—

It has been proposed that OAEs are ultimately caused bymassive volcanism associated with formation of large igneousprovinces (LIPs) because of chronological similarities, but noconsensus has been reached yet. In their seminal study Larsonand Erba (1999) documented that the igneous activity in thePacific leading to the construction of the Ontong–Java andManihiki Plateaus and to increasing global sea-floor spreadingwere broadly synchronous with the Selli Event in the Umbria–Marche basin. They suggested that increased concentrations ofmetals in the Cismon core at the Selli stratigraphic level, corre-sponding to 122–121.7 Ma, imply enrichment from hydrothermalplumes. They further hypothesized that some excess hydrother-mal metals, such as iron, might fertilize areas of normally lowbiological productivity, and cause fluctuation in microbiota andδ13C, and conversion from carbonate sedimentation to reducedblack shales. They further suggested that, conversely, or perhapsconcurrently, the excess carbon dioxide from volcanism mighthave increased atmospheric temperature and hence rainfall, lead-ing to excess runoff of nutrients from the continents and to anincrease in bioproductivity. The hypothesis of the influence ofvolcanicity on OAEs has become widely accepted. It is supportedby Menegatti et al. (1998), who suggested that negative isotopetrends prior to a positive excursion of δ13C at the OAE1a indicateintensified volcanic activity and enhanced CO2 emissions. Like-wise, isotopic compositions of bulk organic carbon and lead (Pb)from the Bonarelli Level in Italy (Kuroda et al., 2007) exhibitvalues characteristic of volcanic rocks from contemporaneousLIPs (Caribbean and Madagascar flood basalts). Therefore, it isvery probable that volcanic activity played one of the decidingroles in the origin of OAE1a and OAE2.

North Atlantic Volcanicity.—

Larson and Erba (1999) were most probably not aware ofextensive volcanic activity occurring along the eastern NorthAmerican margin during the Aptian (Jansa and Pe-Piper, 1985;Pe-Piper et al., 2007). Because both OAE1a and OAE2 are bestdeveloped in the western Tethys, the cause of the rapid increasein the bioproductivity was most probably located within and notoutside of this region. Due to sluggish ocean circulation duringmid-Cretaceous time (Arthur and Sageman, 1994), hydrothermalactivity from deep-water eruptions of LIPs or mid-ocean volcanicridges (MORBs) would probably not disperse volcanic materialfar from their sources. Massive subaerial or very shallow-watervolcanism is most likely necessary to supply such a large amountof volcanogenic material. Therefore, in agreement with the Larsonand Erba (1999) hypothesis and supported by modern fertiliza-tion experiments in the Pacific Ocean, it is conceivable that theincreased bioproductivity leading to OAE1a may have beensubstantially enhanced by iron fertilization of surface oceanicwaters. But, in contrast to Larson and Erba (1999), it is suggestedthat rather than from deep Pacific sources, the iron fertilization inwestern Tethys was the result of extensive circum–North Atlanticvolcanic activity, which was located mainly on the shallow shelves.


An additional important factor for the fertilization theory toconsider is that during Cretaceous time the central North Atlanticwas of width comparable to that of the modern Norwegian Sea(Scotese, 1991), and that a gyre for surface water circulation wasdeveloping in the central North Atlantic (Barron and Peterson,1990); thus the iron fertilization of surface waters could have beenachieved. Less conclusive evidence exists of volcanic input for theorigin of OAE2, even though isotope studies of bulk carbonateand lead from the Bonarelli Level in Italy (Kuroda et al., 2007)strongly suggest volcanic input, which could have been related tothe formation of the Caribbean igneous plateau (93–89 Ma) andMadagascar flood basalts.

Interplay of Earth System Processes Leading to OAE Origin

It is concluded that the origin of OAEs is the result of theinterplay of several processes, with the most significant being theeustatic sea-level rise, a pulse in extensive volcanic activity alongNorth Atlantic continental margins, and a period of a humid,warm climate. Transgressive events documented in the EarlyAptian and during the Late Cenomanian– Early Turonian in thecentral North Atlantic (Gradstein et al., 1990) resulted in thedeposition of condensed beds, which are known to containhigher percentages of organic matter, as the influx of terrigenousclastics from continents is cut off. Both OAE1a and OAE2 havesedimentological characteristics of such beds. Currently the besthypothesis is that rapid increase of organic matter in the OAEswas most probably forced by iron fertilization of ocean surfacewaters, even though that is difficult to prove. In contrast to Larsonand Erba (1999), volcanicity along the central North Atlanticcontinental margins may have been more important than theformation of the Ontong–Java and Manihiki Plateaus. It is sug-gested that a contributing factor for the feasibility of the fertiliza-tion theory never considered is that the central North AtlanticOcean during the mid-Cretaceous was only about half of itspresent width and comparable to the modern Norwegian Sea. Ifthis hypothesis is correct, then it can be concluded that OAEs arenot the result of the presence of anoxic bottom waters, but oforganic-matter preservation as a result of high rates of OMaccumulation on the ocean floor and of the development of ananerobic environment during organic-matter decomposition.However, because of a high amount of organic matter in the watercolumn during the formation of OAEs, it is very probable that thebottom waters may have been periodically dysoxic.

CRETACEOUS PELAGIC RED BEDS (CORBS)

Occurrences and Composition

Upper Cretaceous reddish or locally multicolored,noncalcareous claystones and shales directly overlie the HBS incores from the North Atlantic (Fig. 1C) and in outcrops of theTethys sedimentary strata (Hu et al., 2005). The boundary be-tween black shales and overlying reddish shales is in placestransitional over a few meters (Jansa et al., 1979), but in otherplaces it is sharp, as at Site 641 (Fig. 1C; Boillot et al., 1987). Theseuppermost Cenomanian to Eocene strata lack calcareous micro-fossils except for rare benthic foraminifera (Krasheninikov andPflaumann, 1978; Kuhnt, 1990), which documents their deposi-tion below the CCD. The red beds in the western North Atlanticwere named the Plantagenet Formation (see Jansa et al., 1979, forsynthesis) (Fig. 2). Correlative red shale facies were encounteredin the Cape Verde Basin (DSDP Site 367) (Fig. 2), off Galicia Bankoff Portugal ( ODP Sites 398, 641) (Fig. 2), off eastern Newfound-land (ODP Site 1276; Tucholke et al., 2004), in the Angola Basin in

the South Atlantic (DSDP Site 530; Dean et al., 1984) and in NewZealand (Hikuroa et al., this volume). In western Timor theCretaceous red clays containing ferromanganese nodules re-semble deep oceanic sediments deposited below the CCD. Audley-Charles (1972) explained these red clays as being scraped off theoceanic crust during subduction and emplaced as part of anolistostrome during the Miocene. Deep-sea pelagic red clays ofCretaceous age, mixed with fragments of radiolarites, basic vol-canic rocks, and limestone olistoliths tectonically wedged into anup to 1-km-wide suture zone in western Iran, similarly representremnants of pelagic sediments deposited below the CCD ineastern Tethys. On land subaerial exposures of CORBs can befound along the northern margin of Tethys from southern Spainup to southern Tibet (Hu et al., 2005).

Because of a paucity of microfossils in many deep-sea redclays, except for the rare occurrence of radiolaria or benthicforaminifera, these sedimentary deposits are difficult to date. Forthat reason, and because they occupy the deepest parts of oceanbasins, they were never primary objectives of deep-sea drilling inthe Pacific Ocean. Few exceptions exist, such as the early study ofthe Pacific Ocean east of the Japan trench, where DSDP Leg 20drilled holes in water depth of 6184 m (Heezen and MacGregor,1973). At DSDP Site 196, 623 m of deep-sea pelagic sedimentswere drilled, composed mostly of pelagic, noncalcareous, pre-dominantly red beds, extending in age from Early Hauterivian toQuaternary. The Upper Cretaceous strata are composed of red,zeolitic clays with Mn micronodules. Similar Upper Cretaceousdark yellowish-brown zeolitic clays contain 2% of iron oxides inthe Indian Ocean drill sites located west of Australia (DSDP Leg27, Sites 260 and 261; Veevers et al., 1974). Although only frag-mentary evidence is available for deep oceanic deposits in thePacific Ocean, the available data indicate that during the Creta-ceous most of the ocean floor of the eastern Tethys located belowthe CCD, distant from the continents, was covered by pelagic redclays similar to modern deep-sea red clays.

Variable Stratigraphic Position, Composition,and Occurrences of CORBs

A compilation of occurrences of Cretaceous pelagic red bedsfrom the western Tethys region shows that despite prevailingcolor similarities, differences exist in their composition, strati-graphic position (see Fig. 1 in Wang et al., this volume), environ-mental setting, and origin. Therefore, they can be separated intoseveral categories. (1) Age: in the western Tethys, CORBs occur inLower Aptian, Albian, Cenomanian, and Turonian to LowerEocene strata (Lancelot et al., 1978; Jansa et al., 1979; Hu et al.,2006). (2) Lithologic composition: a) noncalcareous, deep-sea redclays (e.g., Plantagenet Formation, central North Atlantic); and b)reddish/pinkish colored marls and argillaceous limestones (e.g.,Scaglia Rossa, Umbria basin, Italy). (3) Thickness and occurrence:a) strata of variable thickness from several tens of meters to overone hundred meters and mostly uniform in color, e.g., PlantagenetFormation in the central North Atlantic and the Capas RojasFormation in southern Spain (Vera and Molina, 1999), and b)isolated, single or multiple thin red beds, centimeters to meters,or up to several tens of meters thick, which are intercalated withstrata of the same or different lithology, with the main differencebeing the color, as in southern central North Atlantic basin(Lancelot et al., 1978; Sheridan et al., 1983), in the Umbria–Marchebasin (Hu et al., 2006), or the red beds at the Santonian–Campa-nian boundary in the Caucasus (Scherbinina, personal communi-cation). (4) Environmental setting: a) deep oceanic basin with thefloor below the CCD (Plantagenet Formation in the central NorthAtlantic), and b) shallower oceanic basins with the floor above the


CCD (Scaglia Rossa), and c) slopes of continental margins (BlakeNose, ODP Site 1049; Norris et al., 1998) (Fig. 2).

Some of the deep-sea shales and clays are not uniform in color,but are multicolored or variegated due either to a) alternating bandsof red-brown, yellow, orange, or olive-green on a scale of millime-ter to decimeters in the same lithology (e.g., DSDP Site 105,Lancelot et al., 1972; Fig. 2); b) variegated as result of alternatinglithology, such as alternation of red, gray, and greenish grayshales, marlstones, and even sandstone beds (e.g., RhenodanubianFlysch zone of Austria; Wagreich and Krenmayr, 2005).

Pelagic Red Beds—An Early Diagenetic Phenomenon

The variability in composition, environmental setting, andage of pelagic red beds suggests that not all of them had similarorigins. But all have one feature common: the red color. The redcolor is not depositional but an early diagenetic feature thatreflects a change in redox conditions at the sea bed. Many modelshave been proposed to describe the development of oxidationfronts in various deep-sea sediments (Calvert and Pedersen,1993; Buckley and Cranston, 1988; Glasby, 1991; and referencestherein). These models included cases for the development ofnon-steady-state diagenetic processes, in which downward-dif-fusing oxidants, such as oxygen and nitrate, result in the deple-tion of initially deposited organic carbon. Color difference is itself

a geochemical indicator reflecting the difference between oxi-dized (ferric) and reduced (ferrous) state of iron. It is well re-searched that the redox change in sediments deposited on oce-anic abyssal plains is controlled principally by two factors: supplyof Corg (governed by bioproductivity, which in turn is controlled bysupply of nutrients) and by the rate of supply of dissolved oxygen.Oxygen content in the ocean in turn is governed by exchange withthe atmosphere, transport as a result of water movement, photo-synthesis in the euphotic zone, and oxidation within the entirewater column. Furthermore, content of dissolved oxygen in thewater is climate dependent, in that oxygen solubility decreaseswhen salinity and temperature increase. Therefore, the origin ofpelagic red beds could be the result of an intricate interplay of avariety of oceanographic factors, tectonics, and climate.

Only a few studies have been devoted to the origin of UpperCretaceous pelagic red beds in the ocean basins. Lancelot et al.(1972) linked the occurrence of multicolored Upper Cretaceousclaystones in DSDP Site 105 (Figs. 1 and 3; Hollister and Ewing,1972) to either iron- and manganese-enriched basal sediments,such as along the actively spreading East Pacific Rise (Piper, 1973)or metal-enriched sediments associated with the Red Sea hotbrines (Hendricks et al., 1969). They concluded that volcanic orhydrothermal exhalations superimposed on slow rates of pelagicclay deposition could account for the color banding resultingfrom periodic precipitation of iron and manganese. However,

FIG 2.—DSDP/ODP sites in the North Atlantic which encountered the Cretaceous oceanic red beds (CORBs). Paleogeographic mapat 85 Ma was downloaded from the website https://www.odsn.de. Round dot shows Turonian–Maastrichtian CORBsdistribution; triangles represent Aptian–Albian CORBs.


Arthur (1979) suggested that metal enrichment in Upper Creta-ceous multicolored claystones could have resulted from diage-netic mobilization of redox-sensitive metals in anoxic pore watersof the Lower Cretaceous black shales and upward diffusion–advection into oxidized claystones. This could account for thevariable iron–manganese enrichment observed. The equivalentred clays in the Cape Verde Basin (Site 367), however, are notparticularly enriched in any metals, nor do the red and greenclays have different metal concentrations except for a slightlyhigher concentration of Fe in red clays. Furthermore, it is notrealistic to expect that anoxic pore waters would penetrate through> 100-m-thick red-bed strata and still cause metal enrichmentnear the surface. Upper Cretaceous cyclically alternating reddishand greenish-gray deep-sea shales exposed in Beskydy Mountainin the Czech Republic (Jiang et al., 2005) have also significantlydifferent ratios of only Fe2+ and Fe3+, but not the other elements,similarly to the red-bed strata in the Cape Verde Basin.

Modern deep-sea red clays constitute up to 49%, 26%, and 25% of the sediments in the Pacific, Atlantic, and Indian oceans,respectively (Anderson, 1986). An important contribution to theunderstanding of processes that result in development of deep-sea red clays is the study of Holocene sediments in the Nares andMadeira abyssal plains by Buckley and Cranston (1988). Bothabyssal plains are located in water depths > 5500 m, where pelagicsediments are intercalated with turbidite beds deposited belowthe CCD. These deep-sea sediments were oxidized and redcolored when the deposition rate was < 20 mm/kyr (Buckley andCranston, 1988). At this rate the diffusion of oxygen into thesediment exceeds its consumption during the degradation oforganic matter. Iron is therefore present in the sediments domi-nantly in the trivalent state. At higher sedimentation rates, asrepresented by the turbidite beds, sediments attain light to darkgray and/or greenish gray colors, because the sediment is ex-posed for a shorter time to oxygen diffusion from the bottomwaters into the deposited sediment. Studies of modern pelagicsediments from regions of moderate productivity and sedimen-tation rate below a threshold of 40 mm/kyr (Muller and Mangini,1980; Muller et al., 1988) document that the chemical environ-ment remains oxic. These studies provide convincing evidencethat the red color of abyssal clays is an early diagenetic featurecontrolled by low sedimentation rate and oxygen diffusion rates.

Variable Forcing Mechanisms for CORB Deposition

Four different CORB occurrences demonstrate their differentorigins and a range of paleoceanographic factors that can beinvolved in deposition of pelagic red beds: (1) basin-wide UpperCretaceous pelagic red clays in the central north Atlantic basin(Plantagenet Formation) or red limestones in the Umbria–MarcheBasin, Italy (Scaglia Rossa–type); (2) “thin” pelagic red beds at thetransition from Lower Cretaceous pelagic carbonates into blackshale facies in the Cape Verde and Blake–Bahama basins; (3)Aptian-Albian red marls and chalks on the Blake–Bahama conti-nental slope; and (4) red beds intercalated with mid-Cretaceousstrata in the Umbria–Marche basin in Italy and in the Alpine–Carpathian tectonic chains.

(1) Upper Cretaceous noncalcareous, reddish-colored pelagicclays of the Plantagenet Formation are up to > 100 m thick atseveral DSDP and ODP sites in the central North Atlantic. Theyrange in age from Late Cenomanian to Early Eocene (Jansa et al.,1979). Along the American side of the North Atlantic at severaldrilling sites (100, 101, 391, 417, 418) the Plantagenet red beds aremissing, having been removed by Cenozoic erosion. DeGraciansky et al. (1987) suggested that an ocean-wide nonconfor-mity separates the Hatteras and Planatagenet formations. New

biostratigraphic studies by R.W. Scott (this volume) support thedisconformity between the Hatteras and Plantagenet formationsin the central North Atlantic, with the oldest Plantagenet bedsbeing of Turonian age. The mineral composition of Plantagenetred shales is similar to that of modern deep-sea clays in the PacificOcean (Glasby, 1991). The Plantagent Formation–type red bedsare composed of clay minerals (illite, montmorillonite, minorkaolinite), zeolites (6–20%; clinoptilolite, phillipsite), with traceamounts of quartz and feldspars. Iron and manganese oxidemicronodules are present in some places. Agglutinated benthicforaminifera are very rare, but calcareous microfossils are absentbecause these sediments were deposited below the CCD. Anadditional feature common to both the Upper Cretaceousnoncalcareous red clays and modern deep-sea red clays in thePacific Ocean is the low sedimentation rate. The accumulationrate for the Plantagenet-type red beds in the central North Atlan-tic is 1–2 mm/kyr (Jansa et al., 1979), and it is > 1 mm/kyr formodern Pacific deep-sea red clays (McCoy and Sancetta, 1985).However, a different method of measuring results in rates up toseven times greater for the Plantagenet Formation (see Scott, thisvolume), but despite that, the rates remain on the scale of millime-ters per thousand years. Because the range of sedimentation ratesfor the Upper Cretaceous pelagic red beds in the North Atlanticis similar to that of modern pelagic red beds in the Pacific and wellbelow the rates for red-bed deposition in the Nares and Maderiaabyssal plains (Buckley and Cranston, 1988), the forcing mecha-nism for the origin of the Upper Cretaceous pelagic red beds in thecentral North Atlantic was the low sedimentation rate. A remain-ing question is what caused the change from the high sedimenta-tion rate during the mid-Cretaceous, when HBS were deposited,to a low sedimentation rate during the Late Cretaceous. Extensiveoil exploration on the North Atlantic continental margin providesample evidence that continental margins were affected by a majortransgressive event during latest Cenomanian, with the trans-gression continuing until the Maastrichtian (Gradstein et al.,1990). This allows us to argue that the low sedimentation rateduring the Late Cretaceous in the North Atlantic was in directresponse to eustatic sea-level rise.

A similar origin can be postulated for the Upper Cretaceouspink and reddish argillaceous limestones and marlstones of theScaglia Rossa in the Umbria basin in Italy (Fig. 1D). The LowerTuronian to Eocene Scaglia Rossa Formation is 345 m thick in theBottacione section of the Umbria basin (Premoli Silva and Sliter,1994). Scaglia Rossa facies were deposited in a shallower oceanicbasin, about 2 km deep (Kuhnt, 1990). The basin was adjacent toa continental slope, as indicated by the presence of carbonateturbidite beds. The latter are white in color and have sharpboundaries with underlying and overlying pinkish and reddishmarls (Fig. 1D), similarly to modern carbonate turbidites in theMadeira abyssal plain. Sediment accumulation rate for the ScagliaRossa was about 8–11 mm/kyr (Coccioni, 1996), within the rangefor bottom-sediment oxidation by percolating oxygenated bot-tom waters under a regime of low sedimentation rate. Therefore,it is concluded that the reddish color of Scaglia Rossa is an earlydiagenetic and not a depositional feature. Support for this conclu-sion is that some thicker beds show the most intense red color attheir tops, with coloration losing its intensity and eventuallydisappearing downward in the bed.

(2) More uncertainty is involved in establishing conditionsthat led to the oxidation of isolated, reddish-colored pelagic redbeds interbedded with strata of different colors. The color changemay record a variety of different oceanographic events, such asopening of a seaway, change in ocean circulation pattern,downwelling, a brief change in the climate (an increase in or-ganic-matter input), or local tectonics (e.g., Wagreich et al., this


volume) affecting the chemistry of ocean-bottom waters. Suchbeds were encountered in the Cape Verde basin (Site 367/core 24;Lancelot et al., 1978) and in the west-central North Atlantic (Site391C/core 10, Benson et al., 1978; Site 534A, cores 40–42, Fig. 1B,Sheridan et al. 1983; and Site 603B, cores 39–43, van Hinte et al.,1987). In the Cape Verde basin (Site 367) the base of the HBS isseparated from the underlying Lower Cretaceous pelagic carbon-ates by variegated, bioturbated claystone, which is reddish brownwith a zone of dark greenish gray to dark gray. Only 9.5 m of theseclays was drilled because the coring was not continuous, but thedrilling-log data suggest that these strata may be up to 100 mthick. Their age was interpreted as most probably Late Aptian(Lancelot et al., 1978). Similarly, in the western central NorthAtlantic at Site 603B (DSDP Leg 93) and at Site 534A (DSDP Leg76) (Fig. 2), predominantly reddish brown claystones with aminor greenish gray zone directly underlie the base of the mid-Cretaceous black shale facies. The reddish-colored zone at Site603B is 50 m thick and is underlain by 31 cm of organic-rich, blackclaystone, which in turn overlies the Lower Cretaceous pelagiccarbonates (van Hinte et al., 1987; Haggerty et al., 1987). A similar3-m-thick variegated claystone was penetrated at Site 391 and atSite 534A in the Upper Aptian. The petrographic composition ofthese variegated claystones is similar to that of the overlying darkgray claystones, which have a much higher Corg content. Thesimilarity in sediment composition and depositional environ-ment but the different sediment color suggests periodic sedimentoxidation, which could be due either to a higher content ofdissolved oxygen in bottom waters or to a longer exposure timeto oxygenated bottom waters. The geographic location of thesepelagic red beds in the southern part of the central North Atlanticand their Late Aptian age allows them to be interpreted as aconsequence of a sudden inflow of shallow, high-salinity, there-fore higher-density oxygenated waters from southern Atlantic,as the equatorial seaway between southern and northern Atlanticstarted to open. Because of the density contrast, such waterswould have become incorporated into North Atlantic bottomwaters and would have had a higher volume of dissolved oxygenand a high capacity for the oxidation of bottom sediments.

(3) The third case is represented by pelagic red marls cored onthe Blake Nose continental slope at ODP Site 1049. Twenty metersof alternating light gray nannofossil chalk and reddish brownand greenish gray nannofossil claystone were encountered in theSite 1049C cores 11X to 13X (Fig. 1E). The age of this variegatedsequence spans from Late Aptian to Early Albian (Norris et al.,1998).

Three hypotheses can be advanced to explain the occurrenceof pelagic red beds in the Aptian–Albian on the continental slopeof Blake Nose. According to first hypothesis, advocated byMacLeod et al. (2001), the red intervals, which are characterizedby low carbonate content, are the result of a combination of warmsurface water, low terrigenous input, and low nutrient flux. Acontributing factor could have been downwelling and/or up-welling along the margin. In contrast, the greenish beds, whichhave high carbonate content, were deposited during high inputof sediment and nutrients, relatively cool surface water condi-tions, and enhanced upwelling (MacLeod et al., 2001). The mid-Cretaceous paleolatitude position of Blake Nose was estimated tobe about 23° N (Norris et al., 1998). Special oceanographic condi-tions exist in subtropical regions (horse latitudes), where warmsurface water, high evaporation, and low rainfall result in in-creased salinity in surface waters and low bioproductivity andcould lead to downwelling.

A second hypothesis is based on the recognition that Aptianpelagic red beds on the Blake Nose are coeval with red beds in theadjacent deep ocean basin at Site 603B, Site 391, and Site 367 in

Cape Verde basin. An argument was made above that such redbeds reflect oxidation of ocean-bottom sediments as a result ofinflow of high-salinity and highly oxygenated surface watersfrom the southern Atlantic. If that is the case, then the presence ofred beds on the Blake Nose would indicate not only that dis-solved-oxygen content increased in bottom waters but also thatsuch an increase affected intermediate waters in the southernNorth Atlantic as well. Alternatively, because such red beds areknown only from sites drilled off the Blake Plateau, the presenceof such red beds on the lower continental slope could be anindicator of local upwelling.

The third hypothesis is that pelagic red beds at the Blake NoseSite 1049 are the result of periodic low sedimentation rates. Norriset al. (1998) estimated the sediment accumulation rate for theAptian to Lower Albian strata to have been about 6 mm/kyr, andthus in the range for oxidation of deep-sea sediments by bottomwaters, as discussed in more detail above. Therefore, the red colorwould be an early diagenetic overprint. Support for this conclu-sion is that at ODP Site 1052E, drilled at a shallower water depthof 1355 m, the correlative Albian–Cenomanian sediments aredark olive gray calcareous claystones, indicating deposition in anexpanded shelf-edge oxygen-minimum zone (Norris et al., 1998).This argues against the possibility of coastal downwelling in thisregion at that time.

(4) Eight reddish-colored intervals are embedded in mid-Cretaceous sedimentary strata in the Umbria–Marche basin (Huet al., 2006). The oldest is of Aptian age, located just above the SelliLevel, and the youngest is middle Cenomanian (see Fig. 1 inWang et al., this volume). The age span of each red-bed intervalis relatively short, with the longest being the one above Selli(ORB1) lasting about 4.54 Ma, and the shortest (ORB4) in themiddle Albian with a time span of only about 0.13 Ma (Hu et al.,2006). The reddish horizons are composed mostly of marls orclayey beds, and commonly they are less calcareous than thesurrounding lithologies (Tornaghi et al., 1989). Compilation ofthe occurrences of pelagic red beds in mid-Cretaceous sequencesof central and eastern Europe indicate random occurrences intime of these horizons, even though some of them have a broadregional extent (Hu et al., 2006). The latter authors related originof the above red beds to an inflow of colder, more oxygenatedwaters in response to local tectonics.

Origin of Cretaceous Pelagic Red Beds

Cretaceous pelagic red beds are widespread in the westernTethys, where they occur as relatively thin horizons intercalatedwith mid-Cretaceous black shales or marly deposits, and asmassive-bedded, red-colored strata, up to > 100 m thick, of LateCretaceous to Paleogene age. The composition of Upper Creta-ceous pelagic red clays of the central North Atlantic is verysimilar to that of modern pelagic red clays deposited in the PacificOcean. The latter occupy the niche below the CCD in the PacificOcean basin and cover an area extending from about 40° N toabout 40°˚S latitude, crossing several climatic zones. This indi-cates that climate is not the controlling factor for deposition ofpelagic red clays. On this evidence it is argued that deposition ofCretaceous pelagic red clays/shales in the western Tethys wasnot driven by global paleoclimate change. Although colder cli-mates could effect redox conditions in the deep ocean becausecolder water can contain more dissolved oxygen, as stated above,this influence is not considered to be the major driving forcebehind the change from pelagic black shales to red clays. Neitherare changes in the content of dissolved oxygen in deep abyssaloceanic waters considered to be the controlling factor in deposi-tion of red shales. For example, the recent deep waters at 4000 m


in the northern North Atlantic have 6 ml/l of dissolved oxygen(Bainbridge et al., 1981), and yet the sediments are dark grayclays. In contrast, modern pelagic red clays are being deposited inthe northern North Pacific Ocean, where the oxygen level is only3.2 ml/l (Craig et al., 1981). Therefore, the level of dissolvedoxygen in bottom waters is not the principal controlling factor forthe deposition of pelagic red clays.

The principal controlling factor of modern deep-sea red claysis the sedimentation rate (Buckley and Cranston, 1988; Glasby,1991). If the rate is less than 20 mm/kyr, then the dissolved-oxygen content of ocean bottom waters is sufficient to oxidize theorganic matter and establish an oxic, early diagenetic environ-ment resulting in the transformation of Fe2+ to Fe3+. The forma-tion of iron oxides is the cause of the red color of sedimentarybeds. If the sedimentation rate is higher, the sediment attains agray or dark gray color, depending on the content of enclosedorganic matter (Buckley and Cranston, 1988).

If the sedimentation rate is the principal driving force, thenwhat controls it? Several processes, such as climate, tectonics,ocean circulation, or sea-level change, can influence the rate ofsediment accumulation in the ocean. The Turonian–Maastrich-tian transgressions (Gradstein et al., 1990) caused terrigenousclastics to be kept near shore and distant basins became sedimentstarved. Therefore, the low sedimentation rate in the centralNorth Atlantic was a direct result of the sea-level rise. Eventhough sea-level rise was the major force, the potential influenceof Late Cretaceous global cooling remains un-established, ascolder and more arid climate could have increased bottom-waterformation and its higher oxygen content. Pucéat et al. (2007)suggested a latitudinal homogeneous cooling of about 7˚C ofsurface waters on the basis of middle and latest Cretaceousthermal gradients from fish-tooth δ18O data. Colder and morearid climates would have led to decreased fluxes of sediment andnutrients to the ocean, and therefore may have contributed todecreased sedimentation rates and lower bioproductivity. Im-portantly, colder waters would have higher contents of dissolvedoxygen, and the formation of deep water would increase. In anaddition, a major change in the ocean circulation in the AtlanticOcean occurred during the early Late Cretaceous, as the Southand North Atlantic basins became interconnected and globalocean circulation changed from latitudinal to longitudinal, re-sulting in increased ventilation of the North Atlantic basin. Couldthe hiatus between the Hatteras and Plantagenet formationsimplied by de Graciansky et al. (1987) and R.W. Scott (thisvolume) for several Deep Sea Drilling sites in the central NorthAtlantic be the evidence for increased bottom circulation?

An uncertainty remains in arriving at the origin of mid-Cretaceous “thin” pelagic red beds, intercalated with sedimen-tary strata of similar or different lithologic composition, andparticularly those embedded in the HBS. They could have origi-nated from a variety of Earth system processes such as changes inpaleoceanographic conditions (e.g., seaway opening, changes inocean circulation or in the input of organic carbon, downwelling,and/or changes in oxygen concentration in bottom waters), and/or tectonics (e.g., influx of turbidites; Wagreich and Krenmayr,2005).

As suggested in this paper, the Upper Aptian pelagic red bedspresent at the base of mid-Cretaceous black shales in the southernNorth Atlantic could be result of an inflow of high-salinity,oxygenated surface waters into the North Atlantic, because theequatorial gateway began to open. However, this hypothesis failsto explain the occurrence of pelagic red beds in a similar strati-graphic position in the Umbria–Marche basin in Italy, or theirabsence in correlative strata in the deep Moroccan basin. Simi-larly, mid-Cretaceous pelagic red beds correlative with those in

the Umbria–Marche basin are absent in the central North Atlan-tic. Aptian red beds are found in various areas of the westernTethys (Hu et al., 2006), which indicates strong influence of localor regional environmental conditions on the development ofgeographically restricted pelagic red beds. One feature commonto most mid-Cretaceous pelagic red beds is that they have lowercarbonate content than the surrounding beds. Carbonate contentcan be reduced during early diagenesis, because oxidation of 1%of organic carbon would result in dissolution of up to 22% ofCaCO3 in the oxic zones (Cranston and Buckley, 1990; referencestherein). Therefore, some of the differences in carbonate contentbetween red beds and surrounding strata could be due to diage-netic processes and would not be of environmental significance.

The low-sedimentation-rate hypothesis we propose for theorigin of Upper Cretaceous pelagic red shales in the westernTethys does not explain the origin of mid-Cretaceous pelagic redbeds in the Umbria–Marche basin. According to Hu et al. (2006)the deposition rate for the pelagic red beds and the surroundingbeds was similar, both being < 20 mm/kyr. One possibility is thatnon-red-colored beds represent periods of either increased con-tent of organic matter in deposited sediments or decreased dis-solved oxygen in bottom waters, due to either a period of climatewarming or a decreased rate of bottom-water formation and/orcirculation (Hu et al., this volume).

The rather random distribution of pelagic red beds in mid-Cretaceous strata in southern Europe and Alpine–Carpathianchains (Hu et al., 2006) points to overwhelming controls by localtectonics, causing an influx of turbidites into the adjacent basin(Skupien et al., this volume; Wagreich and Krenmayr, 2005),which would prevent development of red beds. Also, tectonicallyforced changes in the ocean-basin bottom topography couldresult in localized downwellings, periods of inflow of colder,more oxygenated bottom waters into various sub-basins of thecircum-Mediterranean, and oxidation of bottom sediments, asadvocated by Premoli Silva et al. (1989) and Hu et al. (2006).However, the deposition of such red beds could occur only ifspecial conditions were met, such as the sedimentation rate beingnear to the boundary conditions for CORB deposition. Undersuch circumstances, a period of climate cooling resulting inincreased oxygen content in bottom waters, and most probablyintrinsic to increases in bottom-water production, could result inoxidation of the deep pelagic sediment layer. If such a hypothesisis correct, then the appearance of isolated pelagic red beds couldbe an indicator of a colder climate, suggested for the Late Aptian–Early Albian by Hochuli et al. (1999) and Steuber et al. (2005).

That modern deep-marine sediments remain unoxidized evenif the bottom waters are highly oxygenated, as in the NorwegianSea, supports our hypothesis that the sedimentation rate is themost important controlling factor in development of CORBs.However, when the sediment deposition rate is near to theboundary conditions for development of CORBs (~ 20 mm/kyr),other factors, such as climate, oxygen content, and changes inocean circulation, become increasingly important.

CONCLUSIONS

Although the conclusions offered here suffer from the cus-tomary uncertainties and deficiencies encountered in any at-tempt to analyze a multivariate system, they do suggest, at leastcrudely, that a variety of Earth processes were involved in theorigins of Cretaceous pelagic sediments, such as black shales(HBS), OAE1a and OAE2, and Cretaceous pelagic red beds (CORBs).

Subsidence of oceanic crust is the cause of a change in compo-sition of deep-sea sediments in the western Tethys, from EarlyCretaceous pelagic carbonates to mid-Cretaceous black shales,


the latter deposited below the CCD. Completion of the changewas not a sudden event: it took more than 15 Myr. The mid-Cretaceous black shales (HBS; 123–93.5 Ma), being depositedbelow the CCD, represent an integrated response to a warm,humid climate and increased influx of mixed terrestrial andmarine organic matter. Because of the relatively small size of thewestern Tethys, a warmer and more humid climate may have ledto a decreased rate of bottom-water formation, with bottomwaters having a lower content of dissolved oxygen, becauseoxygen dissolution decreases with increasing temperature. Occa-sional bioturbation in the greenish gray “Milankovitch” hemicycleand the lack of black shale facies in the deep basin off Morocco,however, are evidence for oxic to dysoxic, but not anoxic, bottomwaters in the central North Atlantic during mid-Cretaceous time.

OAE1a (Early Aptian) and OAE2 (Late Cenomanian–EarlyTuronian) horizons bracket the HBS sequence. A sea-level rise,restriction of terrigenous sediment flux to the ocean floor, and apulse in volcanic activity are considered to be the major forcingmechanisms for development of these horizons. OAE1a andOAE2 developed at the onset of major transgressions in the NorthAtlantic, and therefore can be considered to represent “floodingevents” and condensed horizons, generally typical of high or-ganic-matter accumulation. Iron fertilization of surface watersduring Aptian explosive volcanic activity with contributionsfrom the North Atlantic margin (Jansa and Pe-Piper, 1985; Piperet al., 2007) may have been the major force behind the explosionin plankton productivity in the western Tethys and increases inCorg in horizons mentioned earlier.

Eight reddish/pink-colored horizons are embedded withinAptian–Cenomanian strata in the Umbria–Marche basin in Italy,with similar beds also present in Cretaceous strata in southernand central Europe (Hu et al., 2006). Lack of synchronous correla-tive red beds in the central North Atlantic is strong evidence thattheir origin was not a direct response to global climate change butwas forced by changes in local or regional environment. Tempo-rary inflow of colder, more oxygenated bottom waters due totectonically mediated changes in bottom topography of Mediter-ranean sub-basins is proposed as an explanation. However, aprerequisite for such a process is that the deposition rate ofpelagic sediments has to be near to the boundary conditions forCORB deposition (about 20 mm/kyr). In contrast, the origin ofUpper Aptian pelagic red shales in the southern North Atlanticmay have resulted from an inflow of high-salinity, highly oxy-genated surface waters from the southern Atlantic as the equato-rial seaway started to open.

The Late Cretaceous Atlantic CORBs are closely similar incomposition and environmental setting to modern deep-sea redclays of the Pacific Ocean. The red color is not depositional but is anearly diagenetic feature resulting from a decrease in sedimentationrates to few mm/kyr. A drop in sedimentation rate has been aresponse to the Late Cretaceous transgressions in western Tethys,which began during Late Cenomanian–Early Turonian and con-tinued in the central North Atlantic up to the Maastrichtian.

The sedimentological study of Cretaceous sedimentary stratain the western Tethys documents a multivariate, fluid systemforced by several Earth systems changes. The study points to theoverwhelming influence of eustasy and early diagenetic pro-cesses on Cretaceous deep-sea sediments, an approach greatlyunderestimated and mostly neglected in modern paleoceano-graphic studies.

ACKNOWLEDGMENTS

Luba Jansa is indebted to friends and colleagues, too many tobe thanked individually, who during past thirty years accompa-

nied him in field studies of Mesozoic sedimentary rocks in south-ern and central Europe and eastern Asia, providing help andadvice, and to colleagues involved in the deep-sea drilling in theNorth Atlantic, who shared their knowledge while on board thedrilling ships. The manuscript was improved by critical commentsby Frank Thomas and reviewers D.J.W. Piper, Robert Sheridan,and R.W. Scott. Financial support for Hu Xiumian was providedfrom MOST 973 Project 2006CB701402 and National Science Foun-dation of China Project 40332020. L. Jansa acknowledges supportby the Geological Survey of Canada.

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cretaceous pelagic black shales and red beds in western tethys: origins… · 2020-07-16 ·...

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