THE RECORD OF GLOBAL CHANGE IN MID-CRETACEOUS

8/9/2019 THE RECORD OF GLOBAL CHANGE IN MID-CRETACEOUS

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Journal of Foraminiferal Research, v. 29, no. 4, p. 418437, October 1999

THE RECORD OF GLOBAL CHANGE IN MID-CRETACEOUS(BARREMIAN-ALBIAN) SECTIONS FROM THE SIERRA MADRE,

NORTHEASTERN MEXICO

TIMOTHY J. BRALOWER 1, EMILY COBABE 2, BRADFORD CLEMENT 3, WILLIAM V. SLITER 4*, CHRISTOPHER L.OSBURN 5, and J OSE LONGORIA 3

ABSTRACT

Our current understanding of mid-Cretaceous globalchange is largely based on investigations of pelagic sec-tions from southern Europe and deep sea drilling sites.Much less information exists from other continents andfrom hemipelagic sections deposited on continental mar-gins. This investigation seeks to broaden our under-standing of mid-Cretaceous global change by focusingon the record from hemipelagic sections deposited alongthe continental margin of northeastern Mexico. The major goals are to compare the record, timing, and extentof the Oceanic Anoxic Events (OAEs) in Mexico and oth-

er areas, and to determine the relationship between theseevents and the global burial of organic material usingcarbon isotopes.

We have investigated four sections from the SierraMadre Oriental, integrating biostratigraphy, magneto-stratigraphy and carbon isotope stratigraphy. Carbonisotopes, measured on the organic carbon (C org ) fraction,show identical stratigraphic changes to curves from Bar-remian to lower Albian European and Pacic deep-seasections. Our results add new detail to the C-isotopestratigraphy of the middle and late Albian interval.Three abrupt peaks in C org content correlate with OAE1a(early Aptian), OAE1b (early Albian) and an event inthe late Aptian Globigerinelloides algerianus Zone. All

three events are marked by short-term, 0.53 per mildecreases in C-isotope values followed by increases of similar magnitude. The decreases may reect changes inthe type of C org , the nature of carbon cycling, or an in-crease in hydrothermal activity. The increases in C-iso-tope values reect widespread burial of C org . The similarshape of the C-isotope curves in Mexico and other areas,and the response of C-isotopes to the OAEs, indicate thatthe late Aptian episode was extensive, and that OAE1aand OAE1b were global.

The three anoxic events appear to correlate with risingrelative sea level. OAE1a also corresponds to majorchanges in nannofossil assemblages; the well-knownnannoconid crisis can be easily recognized in the Mex-

ican sections. This event is characterized by an increasein abundance of nannofossils and foraminifera in sedi-

1 Department of Geological Sciences, University of North Carolina,Chapel Hill, NC 27599-3315, USA.

2 Biogeochemistry Laboratories, Department of Geosciences, Uni-versity of Massachussets, Amherst, MA 01003, USA.

3 Department of Geology, Florida International University, Miami,FL 33199, USA.

4 U. S. Geological Survey, 345 Middleeld Rd., Menlo Park, CA94025, USA; *published posthumously.

5 Department of Earth and Environmental Sciences, Lehigh Univer-sity, Bethlehem, PA 18015, USA.

ments, possibly reecting a decrease in dilution as a re-sult of the rise in relative sea level.

INTRODUCTION: MID-CRETACEOUS GLOBALCHANGE

The mid-Cretaceous is known as one of the best ancientexamples of greenhouse climate (e.g., Barron and Washing-ton, 1982; Parrish and Curtis, 1982; Berner and others,1983). This interval was characterized by global warmth andlow latitudinal temperature gradients (e.g., Douglas andSavin, 1975; Brass and others, 1982; Barron and Peterson,1990; Huber and others, 1995; Norris and Wilson, 1998;Fassell and Bralower, 1999). The mid-Cretaceous green-house was coincident with a world-wide pulse in oceancrustal production (Larson, 1991, Tarduno and others, 1991;Arthur and others, 1991; Erba and Larson, 1991; Bralowerand others, 1994). This, the largest volcanic episode possi-bly in the past 250 m.y. of Earth history, included increasedrates of seaoor spreading and increased rates of formationof Large Igneous Province (LIP) oceanic plateaus, seamountchains and continental ood basalts (Hays and Pitman,1973; Schlanger and others, 1981). The release of mantleCO 2 from this enormous volcanic episode may have directlycaused mid-Cretaceous greenhouse warming (Arthur andothers, 1985a; Arthur and others, 1991; Larson, 1991).

Mid-Cretaceous oceanic environments favored the depo-sition and burial of organic carbon-rich sediments, knowninformally as black shales (e.g., Schlanger and Jenkyns,1976; Ryan and Cita, 1977; Arthur and Premoli Silva,1982). Warm deep-water temperatures and sluggish circu-lation led to widespread dysoxia and anoxia in deep watermasses (e.g., Brass and others, 1982; Wilde and Berry,1982). Stratigraphic investigations have led to the recogni-tion that mid-Cretaceous black shales were not depositedrandomly through time, but that accumulation was concen-trated in intervals known as Oceanic Anoxic Events(OAEs) (Schlanger and Jenkyns, 1976; Jenkyns, 1980; Ar-thur and others 1990; Bralower and others, 1993). The brief ( 1 m.y.) OAE that took place at the Cenomanian/Turonianboundary (OAE2) was global in extent (e.g., Schlanger and

others, 1987; Huber and others, 1999). The lengthy ( 20m.y.) Aptian-Albian OAE (OAE1) was characterized by atleast four separate phases of organic-carbon accumulation(e.g., Arthur and others, 1990; Bralower and others, 1993;Erbacher and Thurow, 1997), but only the rst of the four(early Aptian event [OAE1a]) is known to be global in ex-tent (e.g., Sliter, 1989; Bralower and others, 1994).

Carbon isotope records can be used to establish globalorganic carbon (C org ) budgets during these OAEs. The prom-inent positive carbon isotopic excursion that corresponds toOAE2 (e.g., Scholle and Arthur, 1980; Pratt and Threlkeld,1984; Arthur and others, 1987) suggests that the volume of


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419MID-CRETACEOUS GLOBAL CHANGE, NORTHEASTERN MEXICO

FIGURE 1. Map of northeastern Mexico showing location of sec-tions investigated.

Corg buried was a sizable part of the global carbon budget.Carbon isotope records for the Aptian-Albian interval arecomplex (e.g., Weissert and others, 1985, 1998; Pratt andKing, 1986) and their relationship with dysoxic/anoxic in-tervals is not fully understood. Carbon isotope values in-crease just above early Aptian OAE1a and remain high intothe Albian. Major discrepancies exist, however, in thechronostratigraphic correlation of the C-isotope record inMexico (Scholle and Arthur, 1980) and Europe (e.g., Weis-sert and Lini, 1991; Weissert and Breheret, 1991). Beforeshort-term changes in Aptian-Albian carbon budgets can befully explained, the age of major changes in C-isotope ratiosand their relationship with C org -burial events must be estab-lished.

Oceanic environments and their microplankton inhabi-tants are intimately related; the evolution of microfossilgroups is thought to have been profoundly affected by majoroceanic events such as OAEs (e.g., Roth, 1987; Leckie,1989; Leckie and others 1998). One of the most dramaticturnovers in the nannoplankton took place in the early Ap-tian around the time of OAE1a. This event involved thetemporary collapse of the nannoconids, a group of nanno-plankton that had dominated assemblages for the previous20 million years, and is known as the nannoconid crisis(Coccioni and others 1992; Erba, 1994). A possible expla-nation for the crisis was that a mantle superplume event(e.g., Larson, 1991) directly or indirectly caused a changein the thermal or nutrient structure of oceanic surface waters(Erba, 1994). While it is suspected that the nannoconid cri-sis was a global event, all of the supporting data come fromsections in Europe and the central Pacic Ocean that weredeposited in pelagic environments.

Much of our knowledge on the diverse aspects of mid-Cretaceous global change derives from investigations of theclassical Tethyan sections exposed in France and Italy (e.g.,Arthur and Premoli Silva, 1982; Weissert and others 1985;Premoli Silva and others, 1989; Coccioni and others, 1992).Yet, the sedimentary record of other areas clearly holdsclues to the causes of environmental and evolutionarychanges. The classic paper of Scholle and Arthur (1980) thatdemonstrated the potential of C-isotopes in stratigraphy waslargely based on the study of two sections from northeasternMexico, but this area has received little paleoceanographicstudy since. This paper describes an investigation of variousaspects of global change based on the study of sedimentaryunits in northeastern Mexico.

GEOLOGICAL SETTING OF STUDIED SECTIONS

TECTONICS AND PALEOGEOGRAPHY

The sections we investigated are exposed in ranges of theSierra Madre Oriental fold and thrust belt of the northernMexican Cordillera (Humphrey, 1956; Longoria, 1998) (Fig.1). During the Barremian-Albian, northeastern Mexico wasdominated by a series of interngering shallow basins andshallow-water carbonate platforms lying on the northwestmargin of a deeper seaway (e.g., Smith, 1981; Wilson,1989). Environments of deposition varied through time as aresult of tectonic processes, eustatic uctuations and varia-tions in detrital supply (e.g., Goldhammer and others, 1991).These sections provide a transect from pelagic (La Boca

Canyon) to hemipelagic (Canyon Los Chorros) deposition.Deformation of this region occurred in the Laramide orog-eny (e.g., Humphrey, 1956; De Cserna, 1956; Lopez-Ramos,1980; Tardy, 1980; Longoria, 1984; 1985; 1998), exposingthe sections in elongate anticlines that form the ranges.

BIOSTRATIGRAPHY AND LITHOSTRATIGRAPHY

The Barremian to Albian succession of northeastern Mex-ico contains abundant planktonic foraminifera with a well-established biostratigraphy (Longoria, 1974; Longoria andGamper, 1977; Longoria, 1984; 1998). However, investiga-tions of nannofossil stratigraphy in northeastern Mexicowere carried out before most taxonomy was developed andwere limited largely to the nannoconids. Trejo (1960, 1975)demonstrated that the nannoconids have signicant bio-

stratigraphic potential, at least on a regional basis.Lithostratigraphic nomenclature of the Lower Cretaceous

sedimentary units of northeastern Mexico is by no meansstandard (e.g., Ross, 1981; Smith, 1981; Wilson and others,1984; Longoria, 1998; Lehmann and others, in press). Thelithostratigraphy of the four sections investigated is de-scribed in detail in Longoria and others (in prep.) (Fig. 2).

San Angel Limestone

This unit consists of a monotonous succession of thick-to massive-bedded, dark-grey to black, carbonaceous lime-


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420 BRALOWER, COBABE, CLEMENT, SLITER, OSBURN, AND LONGORIA

FIGURE 2. General lithostratigraphic scheme for the Barremian-Cenomanian interval of the sections investigated (after Longoria,1998).

stones and marly limestones with interbedded chert (Lon-goria and Davila, 1979). The San Angel was deposited in a

deep-water setting and contains common planktonic fora-minifers which indicate a Barremian to Aptian age (e.g.,Longoria, 1998). This unit is often referred to as the lowerTamaulipas Limestone. The stratotype of this unit is exposedat Santa Rosa Canyon.

Cupido Formation

This unit is a platform equivalent of the San Angel Lime-stone found in the west of the study area (Fig. 2). The Cup-ido consists of homogeneous medium- to thick-bedded lightgrey limestone. Sedimentary facies suggest deposition on acarbonate ramp in supratidal to lagoonal environments (e.g.,Wilson, 1981; Longoria and Monreal, 1991; Lehmann and

others, in press).

La Pena Formation

The La Pena Formation consists of thin- to medium-bed-ded, dark grey and black limestone, marlstone, and shale.The unit contrasts lithologically with limestones above andbelow, representing an important paleoceanographic eventas well as a transgressive systems tract (Goldhammer andothers, 1991). This unit is rich in ammonites and planktonicforaminifera which indicate an age ranging from early Ap-tian to early Albian (e.g., Humphrey, 1956; Longoria, 1974,

1975; Longoria and Gamper, 1977; Longoria and Monreal,1991; Longoria, 1998).

Tamaulipas Limestone

The Tamaulipas Limestone is composed predominantly of grey, medium- to thick-bedded limestone with abundantblack chert layers, but contains a few mudstone and shale

intervals. The unit was deposited in a basinal setting (e.g.,Ross, 1981; Wilson, 1981). Planktonic foraminifera areabundant in the limestones of the Tamaulipas indicating thatits age ranges from early to late Albian.

Cuesta del Cura Formation

The Cuesta del Cura is composed of dark grey to black,thin- to medium-bedded limestone, with shale interbeds andchert lenses. The unit was deposited in a hemipelagic slopesetting (Ice, 1981). The limestones are rich in planktonicforaminifera that provide a late Albian to early Turonianage.

SECTIONS

INVESTIGATED

We investigated four sections in the Sierra Madre Oriental(Fig. 2). Detail on the locations of the sections is providedin Longoria and others (1998).

Santa Rosa Canyon

The Santa Rosa Canyon section is exposed in the easternfront range of the Nuevo Leone Cordilliera on both sides of Highway 58 between Iturbide and Linares. The lithostratig-raphy of the section has been described in detail by Ice(1981), Ross (1981), Wilson (1981), and Carslen (1998).

La Boca Canyon

The section is located in the Cerro de le Silla, a range of the isolated Sierras Tamaulipecas (Longoria and others,1998). The canyon dissects an elongate, narrow anticlinethat exposes the entire Mesozoic section. The lithostratig-raphy of the section has been described in detail by Lon-goria and Davila (1979). We sampled the uppermost SanAngel Limestone and La Pena Formation along the roadimmediately to the west of Rancho Paraiso.

Canyon Los Chorros

The section is located in the Sierra de la Nieve on oldHighway 57 from Matehuala to Saltillo. Exposure is contin-uous, but lithostratigraphy indicates that the sequence is

overturned: the Cupido Formation overlies the La Pena For-mation which rests on top of the Tamaulipas Limestone (seePlate 2 in Longoria and others, 1998).

Cienega del Toro

The section is located in the eastern front range of theNuevo Leone Cordilliera on a dirt road from San Pablo toRajones near the town of Cienega del Toro (see Fig. 19 inLongoria and others [1998] for detailed location). The sam-pled part of the section includes the San Angel Limestoneand the lower part of the La Pena Formation.


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FIGURE 3. Calcareous nannofossil biostratigraphy of the Santa Rosa Canyon section. A-abundant; C-common; F-few; R-rare.


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FIGURE 4. Calcareous nannofossil biostratigraphy of the La Boca Canyon section. A-abundant; C-common; F-few; R-rare.

RESULTS

BIOSTRATIGRAPHY

Nannofossils

Santa Rosa Canyon. Nannofossils are virtually absentin the lower and middle part of the San Angel Lime-stone. Abundance begins to increase in the uppermost

part of this unit, but occurrences remain rare (Fig. 3).Nannofossil abundance increases signicantly in the LaPena Formation and remains high into the middle partof the Tamaulipas Limestone allowing precise detectionof zonal events including the rst occurrence (FO) of Rucinolithus irregularis (base of Zone NC6) betweenSRB 15 and SRB 16 at 106.7m, the FO of Eprolithus oralis (base of Zone NC7) between SRB 46 and SRB47 at 142.2m, and the FO of Prediscosphaera colum-nata (base of Zone NC8) between SRC 4 and SRC 6 at215.8 m (Table 2). The unexposed interval in the lowerpart of the section (127 to 134 m) lies in the middle of

Zone NC6. Several useful non-zonal datum levels canalso be determined including the last occurrence (LO) of Nannoconus steinmannii between SRB 34 and SRB 34.5at 124.9 m, the FO of N . truittii between SRB 48 andSRB 49 at 143.1 m, and the LO of Micrantholithus hos-chulzii between SRB 85 and SRB 86.5 at 175.05 m.With the exception of the latter datum, which denes thebase of Subzone NC7C, none of the other subzonal unitsof Bralower et al. (1993) can be identied. The nanno-conid crisis interval (Erba, 1994) lies between 124.9 m(LO of N . steinmannii ) and 142.2 m (abrupt increase inabundance of nannoconids).

Preservation deteriorates markedly in the middle partof the Tamaulipas Limestone and nannofossils remainvery rare in the upper part of this unit and in the Cuestadel Cura Formation. Only one event can be detected withany certainty, the FO of Eiffellithus turriseiffelii (baseof Zone NC10) between SRC 61 and SRC 64 at 301.05m. The base of Zone NC9 cannot be determined due to


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FIGURE 5. Calcareous nannofossil biostratigraphy of the Canyon Los Chorros section. C-common; F-few; R-rare.

the absence of the nominate taxon, Axopodorhabdus al-bianus .

The Barremian/Aptian boundary is placed at 100 mat the base of magnetic Polarity Zone MO (Clement etal., in prep.). If the identication of MO is correct, thenthe true FO of Rucinolithus irregularis should lie some-what lower in the section than the level determined here(e.g., Coccioni et al., 1992). The Aptian/Albian bound-ary is placed just above the FO of Prediscosphaera col-umnata at 220 m.

La Boca Canyon . Almost all samples collected fromthe San Angel Limestone are barren of nannofossils.Nannofossils are rare and preservation is poor in mostsamples from the La Pena Formation (Fig. 4). The low-ermost part of this unit from 208.8 m to 244.15 m cor-relates with the lower Aptian Chiastozygus litterarius(NC6) Zone based on the presence of Rucinolithus ir-regularis and the absence of Eprolithus oralis . The as-semblage in the sample at 205.8 m is too poorly pre-

served for the absence of R. irregularis to be interpreted.The absence of Nannoconus steinmannii in this intervalindicates correlation to the nannoconid crisis interval inthe upper part of Zone NC6 (Erba, 1994). The top of thenannoconid crisis lies at 247.95 m as suggested by amarked increase in the abundance of nannoconids be-tween samples BCA 17 and BCA 18. The FO of Eprol-ithus oralis lies between samples BCA 15 and BCA16.1 at 244.15 m, and the E . oralis Zone (NC7) extendsto the top of the section (Fig. 4). Other events which canbe determined in the La Pena Formation are the LO of Micrantholithus hoschulzii (256.8 m), and the FOs of

Nannoconus truittii (247.95 m) and Prediscosphaeraspinosa (287.4 m). Nannoconids are rarer than in theother sections. The Barremian/Aptian boundary is ten-tatively placed at 200 m below the known occurrence of R. irregularis .

Canyon Los Chorros . Nannofossil abundance in sam-ples from Canyon Los Chorros ranges from rare to abun-dant and preservation from poor to moderate (Fig. 5). Amarked increase in abundance and improvement in pres-ervation occurs between the San Angel Limestone andLa Pena Formation. The paucity of nannoconids sug-gests that the base of the section lies within the nanno-conid crisis interval. The top of the nannoconid crisiscorresponds to the increase in nannoconid abundance be-tween samples LC 4.1 and LC 5.1 (12.5 m). The FO of Rucinolithus irregularis (base of Zone NC6) corre-sponds to the improvement in preservation at the baseof the La Pena Formation (between samples LC 4 andLC 4.1 at 9.15 m). This datum lies above the base of

the nannoconid crisis (opposite of other section [Table2]) and might actually lie somewhat above its true level.The FO of Eprolithus oralis (base of Zone NC7) isplaced at 12.5 m (between samples LC 4.1 and LC 5.1).Hence the La Pena Formation correlates with lower Ap-tian Zone NC6 and upper Aptian Zone NC7. Otherevents that can be determined include the FO of Nan-noconus truittii between samples LC 4.1 and LC 5.1(12.5 m) and the LO of Micrantholithus hoschulzii (be-tween samples LC 7.1 and LC 7.2; 27.95 m).

Cienega del Toro . Nannofossils are exceptionally rareand poorly preserved in most samples from the Cienega


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FIGURE 6. Calcareous nannofossil biostratigraphy of the Cienegadel Toro section. C-common; F-few; R-rare.

del Toro Section (Fig. 6). The FO of Rucinolithus irre-gularis (base of Zone NC6) is tentatively placed betweenCTB 6 and CTB 6.1 (3.72 m). Nannoconus steinmanniihas been found in most samples up to CTB 50.21. Theabsence of this species in the uppermost sample (CTB50.22) may result from the poor preservation of thissample. The overlap of R. irregularis and N . steinmanniifor much or all of this section suggests that it is restrictedto the lower Aptian and represents a high rate of sedi-mentation. Although nannoconids are rare throughoutthe section, the occurrence of N . steinmannii in the up-permost samples suggests that the section lies below thenannoconid crisis interval (Erba, 1994). The Barremian/ Aptian boundary is tentatively placed at 2 m just belowthe FO of R. irregularis .

Planktic Foraminifers

Planktic foraminifers were observed in samples from theSanta Rosa Canyon Section. The abundance of specimensvaries signicantly; the preservation is predominantly poor.The lowermost part of the SRB section belongs to the Glo-

bigerinelloides blowi Zone based on presence of the nom-inate taxon (Fig. 7). The base of the overlying Leupoldinacabri Zone is tentatively placed between samples SRB 43(139.2 m) and SRB 46 (142.0 m) based on the rst occur-rence of L. reicheli (Longoria, 1974; Sliter, 1992). However,specimens similar to L. cabri ( L. cf. L. cabri ), were notobserved below sample SRB 49 (143.3 m) and the lower-most L. cabri sensu stricto was observed in sample SRB54 (146.0 m). The top of the L. cabri Zone, and the baseof the overlying Globigerinelloides ferreolensis Zone can-not be dened precisely, and lie between samples SRB 64.5(154.3 m) (the uppermost specimen of L. cabri ) and SRB84 (172.7 m) (the rst occurrence of large, robust specimensof G . ferreolensis ). The lowermost occurrence of small,fragile specimens of G . ferreolensis is in sample SRB 70(159.7 m); the lowermost robust specimens of this speciesare observed in sample SRB 75 (163.5 m). The overlyingGlobigerinelloides algerianus Zone ranges from sampleSRB 91 (181.1 m) to SRB 107 (197.6 m) based on theoccurrence of the nominate taxon. This interval appears tolie in the lower part of the G . algerianus Zone based on theabsence of Hedbergella trocoidea . Sample SRB 114 (207.4m) contains favusellids and calcispheres including Colom-iella mexicana and C . recta suggesting a shallow water de-positional environment and correlation to the latest Aptianto earliest Albian (Longoria, 1998).

Planktic foraminiferal biostratigraphy and assemblagesindicate a sequence boundary between samples SRB 107(197.6 m) and SRB 114 (207.4 m) separating deeper-waterand shallow-water facies. The occurrence of favusellids andC . mexicana continues up to sample SRC 15 (230.8 m).However, a prominent dissolution facies is observed be-tween samples SRC 6 (217.3 m) and SRC 10 (223.3 m) thatcorresponds to OAE1b. Samples SRC 21 (239.8 m) to SRC56 (292.3 m) appear to correlate to the lower Albian Tici-nella primula Zone based on the presence of Hedbergellacf. H . rischi (in sample SRC 21 [239.8 m]) and the nominatetaxon (without younger planktic foraminiferal markers) insamples SRC 31 (254.8 m) to SRC 56 (292.3 m). The pres-ence of Clavihedbergella cf. C . simplex in sample SRC 61(299.8 m) suggests a correlation to the top of the T . primulaZone or the lower part of the Biticinella breggiensis Zone(e.g., Leckie, 1984; Tornaghi and others, 1989; Premoli Sil-va and Sliter, 1995). The uppermost sample observed, sam-ple SRC 81 (329.8 m), also appears to lie in the lower partof the upper Albian B. breggiensis Zone based on the oc-currence of the nominate taxon, T . primula , T . roberti , andthe T . roberti group (Leckie, 1984; Tornaghi and others,1989; Premoli Silva and Sliter, 1995).

Geochemistry

Organic carbon and carbonate content . Most of the sam-ples analyzed contain low ( 0.1%) TOC contents. Threemarked peaks (up to 2.7%) in TOC were found in the SantaRosa Canyon section (Fig. 7), in the lower Aptian, the upperAptian, and the lowermost Albian. Less marked increaseswere found in the other sections (Figs. 810).

Carbonate measurements at Santa Rosa Canyon indicatemajor facies changes between the San Angel Limestone andthe La Pena Formation (in the covered interval between 127


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TABLE 1. Rock-Eval data from TOC-rich intervals from the Santa Rosa Canyon.

Sample S1 S2 S3 TMAXTOC(%) HI OI S2/S3

SRB 26.5 0.01 0.05 0.42 *406 0.124 40.32 338.71 0.12SRB 34.5 0.02 0.18 0.76 *407 0.155 116.13 490.32 0.24SRB 37.5 0.02 0.09 0.24 *493 1.183 7.61 20.29 0.38SRB 39.5 0.01 0.04 0.65 *462 2.512 1.59 25.88 0.06SRB 40 0.01 0.04 0.18 *460 1.573 2.54 11.44 0.22

SRB 43.5 0.01 0.06 0.17 *538 1.394 4.30 12.20 0.35SRB 46 0.02 0.13 0.23 *580 1.894 6.86 12.14 0.57SRB 46.5 0.01 0.10 0.27 *573 1.854 5.39 14.56 0.37SRB 56.5 0.02 0.10 0.31 *478 1.316 7.60 23.56 0.32SRB 58.5 0.01 0.00 0.38 0.255 0.00 149.02 0.00SRB 68.5 0.02 0.10 0.21 *388 0.635 15.75 33.07 0.48SRB 82.5 0.02 0.06 0.19 *328 0.287 20.91 66.20 0.32SRB 98 0.00 0.02 0.32 *428 1.798 1.11 17.80 0.06SRB 99 0.01 0.09 0.32 *427 1.441 6.25 22.21 0.28SRB 100 0.02 0.17 0.24 *574 2.664 6.38 9.01 0.71SRB 101 0.03 0.13 0.17 *524 1.341 9.69 12.68 0.76SRB 106 0.00 0.07 0.79 *379 1.522 4.60 51.91 0.09SRB 107 0.02 0.09 0.59 *380 0.497 18.11 118.71 0.15

S1 mg. hydrocarbons thermally distilled from 1 gr. of rock.S2 mg. of hydrocarbons generated by pyrolytic degradation of the kerogen in one gram of rock.S3 mg. of CO 2 generated from a gram of rock during temperature programming up to 390 C.TMAX temperature at which maximum S2 hydrocarbons are generated.HI Hydrogen Index (quantity of pyrolyzed organic compounds in S2 relative to TOC).OI Oxygen Index (quantity of CO 2 in S3 relative to TOC).

TABLE 2. Meter levels of nannofossil datums in sections investigated.

Nannofossil eventBase of

zoneSanta Rosa

CanyonLa BocaCanyon

CanyonLos Chorros

Cienegadel Toro

FO Eiffellithus turriseiffelii NC10 301.05FO Prediscosphaera columnata NC8 215.80LO Micrantholithus hoschulzii 175.05 256.80 27.95FO Nannoconus truittii 143.10 247.95 12.50Top nannoconid crisis 142.20 247.95 12.50FO Eprolithus oralis NC7 142.20 244.15 12.50LO Nannoconus steinmannii

(base nannoconid crisis) 124.90 base base topFO Rucinolithus irregularis NC6 106.70 base 9.15 3.72

and 134 m). In addition, the three marked peaks in TOC arematched by decreases in CaCO 3 content (Fig. 7). Other in-dividual samples with low CaCO 3 content represent thinmarl and shale seams sampled for micropaleontology.

Stable isotopes . C-isotopes measured on the organic frac-tion ( 13Corg) show major stratigraphic uctuations (Figs. 710). The most detailed, extended record is from the SantaRosa Canyon section (Fig. 7). This section shows four long-term cyclic uctuations in 13Corg values. These uctuationsconsist of a marked, short-term 0.53 per mil decrease in

13

Corg values followed by a longer-term 0.53 per mil in-crease. The four decreases are: (1) in the lowermost Aptiannannofossil Zone NC6, top of the Globigerinelloides blowiplanktic foraminiferal Zone (139.2 m); (2) in the upper Ap-tian in the middle of nannofossil Zone NC7, lower part of the Globigerinelloides algerianus planktic foraminiferalZone ( 190 m); (3) in the lowermost Albian at the base of combined nannofossil Zones NC8-NC9, middle part of thecombined Hedbergella planispiraTicinella bejaouaensisZones ( 219 m); and (4) in the upper Albian in the upper-most part of combined nannofossil Zones NC8NC9 andthe top of the Ticinella primula planktic foraminiferal Zone(292299 m).

Carbon isotope stratigraphies of the La Boca Canyon andCanyon Los Chorros sections are limited to the upper Bar-remian-lower Aptian interval, contain less detail, but showthe same basic features (Figs. 8,9) as the contemporaneouspart of the Santa Rosa Canyon section. The Cienega delToro section shows highly uctuating, but gradually de-creasing 13Corg values for most of the section followed byincreasing 13Corg values (Fig. 10). This pattern appears tocorrelate to the lowermost cycle at Santa Rosa Canyon.

Rock-Eval . In general, the low organic carbon content of

the SRB samples resulted in S 1 and S 2 values that were toolow for condent interpretation (Table 1). As a result, theTmax values for the samples are not reliable. Likewise, thehydrogen index (HI) and oxygen index (OI) values for thesesamples plot close to the origin, revealing little about thesource or thermal history (Fig. 11).

DISCUSSION

CHRONOLOGY OF THE APTIAN -A LBIAN C-I SOTOPEEXCURSION

The classic paper by Scholle and Arthur (1980), based inpart on investigation of the Peregrina Canyon and Rancho


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FIGURE 7. Carbonate, total organic carbon (TOC), and 13Corg stratigraphy of the Santa Rosa Canyon section. Gaps in the record indicatesampling gaps. Nannofossil zones are from Roth (1978). Planktic foraminiferal zones are from Premoli Silva and Sliter (1995). Shaded area innannofossil stratigraphy represents the nannoconid crisis interval.

Jacalitos sections in Mexico, demonstrated the applicabilityof C-isotopes in regional and global correlation. The C-iso-tope measurements of Scholle and Arthur (1980) were madeon the carbonate ( 13Ccarb ) fraction (Fig. 12). Chronostrati-graphic control of the two sections was based largely onunpublished proprietary microfossil and ammonite biostra-tigraphy. However, the Peregrina Canyon section is almostentirely barren of nannofossils (T. J. Bralower, unpublisheddata). Subsequent C-isotope stratigraphic studies of the Bar-remian-Albian interval (e.g., Weissert and others, 1985;1998; Pratt and King, 1986) have reproduced the basicshape of the Scholle and Arthur (1980) curve, but have re-

vealed numerous differences in its chronostratigraphic cor-relation.The results of the present investigation allow detailed cor-

relation between nannofossil and foraminiferal biostratig-raphy and C-isotope stratigraphy for the late Barremian tolate Albian interval. Carbon isotopes, measured on the or-ganic carbon fraction, show identical stratigraphic changesto detailed 13Ccarb and 13Corg curves from European, Mid-dle Eastern and Pacic shallow-water and deep-sea sections(e.g., Weissert and others, 1985; Jenkyns, 1995; Erbacherand others, 1996; Vahrenkamp, 1996; Ferreri and others,1997; Menegatti and others, 1998; Grocke and others, 1999)

(Fig. 13). Most detailed curves from Europe and the Pacicterminate in the late Aptian or early Albian. The only curvethat extends through the late Albian is that of Erbacher andothers (1996). The present investigation adds new detail tothe Erbacher and others (1996) curve and to the C-isotopestratigraphy of the middle and late Albian interval.

High-resolution Aptian 13 C carb and 13 Corg stratigra-phies of sections from Alpine Tethys show several dis-tinct segments (Menegatti and others 1998). These in-clude: a signicant decrease in 13 C values (segment C3),followed by a marked increase (C4), an interval of level

13 C values (C5), further increase (C6), then somewhat

variable but overall constant values (C7), followed by de-creasing values (C8) (Fig. 13). In the Tethyan sections,segment C3 to lower segment C7 lie in the Chiastozyguslitterarius nannofossil Zone (NC6) and the Globigerinel-loides blowi planktic foraminiferal Zone (Menegatti andothers, 1998), and the remainder of the segments lie inthe Rhagodiscus angustus nannofossil Zone (NC7). Theupper part of segment C7 lies in the upper part of the Leupoldina cabri Zone and segment C8 begins in the L.cabri Zone and continues into the Globigerinelloides al-gerianus Zone (Fig. 13).

Carbon-isotope stratigraphy of the four Mexican sec-


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FIGURE 8. Total organic carbon (TOC), and 13Corg stratigraphy of the La Boca Canyon section. Nannofossil zones are from Roth (1978).Shaded area in nannofossil stratigraphy represents the nannoconid crisis interval.

tions show many of the same features as the Menegatti andothers (1998) curve, thus we have adopted the terminologyof these authors and extend it to the late Albian using therecord at Santa Rosa Canyon (Fig. 7). This record showsa sharp increase in 13Corg values (segment C9) in the G .algerianus planktic foraminiferal Zone and Rhagodiscusangustus nannofossil Zone, followed by level 13 Corg val-

ues (segment C10) for the upper parts of the same zonesextending into the combined Hedbergella planispiraTi-cinella bejaouaensis planktic foraminiferal Zones, sharplydecreasing (C11), then sharply increasing (C12) 13Corgvalues in the middle and upper part of the combined H . planispira T . bejaouaensis planktic foraminiferal Zonesand the lower part of the combined Prediscosphaera col-umnata (NC8) Axopodorhabdus albianus (NC9) nanno-fossil Zones, then fairly constant 13Corg values that extendto the top of the Ticinella primula foraminiferal Zone andthe combined P . columnata A. albianus nannofossil Zones(segment C13). Finally, a sharp decrease in 13 Corg values

(segment C14) occurs at the top of the T . primula Zoneand the combined P . columnata A. albianus nannofossilZones, and a slight increase in 13Corg values (segmentC15) occurs in the lower parts of the Biticinella breggien-sis foraminiferal and Eiffellithus turriseiffelii nannofossilZones.

The Santa Rosa Canyon 13Corg curve shows several of

the same features as the13

Ccarb curve from Peregrina Can-yon (Scholle and Arthur, 1980), but with a different chro-nostratigraphic correlation (Fig. 12). Since the early Aptiannegative-positive 13C excursions (segments C3 to C7) aredistinct, we use these features to anchor the Peregrina Can-yon 13Ccarb curve and tentatively identify each of the C-segments identied at Santa Rosa Canyon (Fig. 7). Usingthe correlation with Santa Rosa Canyon, we have moved theBarremian/Aptian and the Aptian/Albian boundaries up-wards at Peregrina Canyon (Fig. 12).

The similarity of C-isotope curves and a range of bio-chronologic data (e.g., Premoli Silva and others, 1989; Erba,


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FIGURE 9. Total organic carbon (TOC), and 13Corg stratigraphy of the Canyon Los Chorros section. Nannofossil zones are from Roth (1978).Shaded area in nannofossil stratigraphy represents the nannoconid crisis interval.

1991; Herbert, 1992; Erba, 1996) suggest that sedimentationrates in Italian Barremian-Aptian sections with 13C stratig-raphies (i.e., Cismon, Piobbico) were fairly constant. Carbonisotope data and general paleogeographic factors indicatethat the Mexican sections were characterized by rather dra-matic changes in sedimentation rates and that differencesbetween sections were marked. For example, the Cienegadel Toro section had much higher overall sedimentationrates during the earliest Aptian than the other sections. How-ever, particular details of the Barremian-Albian C-isotoperecords of the hemipelagic Mexican sections and pelagicsections from southern Europe and the deep sea are re-markably similar. Sufcient detail exists in the C-isotoperecord of this time interval (Fig. 13) that C-isotope che-mostratigraphy can provide precise age control in shallow-water limestone sections with poor biostratigraphic control(e.g., Follmi and others, 1994; Jenkyns, 1995; Vahrenkamp,1996; Lehmann and others, in press).

ORIGIN OF APTIAN -A LBIAN C-I SOTOPE VARIATIONS

Effect of C org Source and Diagenesis on 13 C org Values

Variations in 13Corg values can result from a variety of different factors. These include changes in the source of C org(e.g., Arthur and others 1985b), and selective diagenetic al-teration of particular samples. Before the paleoenvironmen-tal signicance of 13Corg stratigraphies can be established,the signicance of source and diagenetic changes must beestablished. The Rock-Eval data (Fig. 11) reveal little aboutthe source of C org . However, major changes in source seemunlikely given the hemipelagic and pelagic depositional en-vironment of the sequences investigated. Although the SantaRosa Canyon section was sampled in more detail than theother three sequences, and hence the C-isotope stratigraphyof this section is more detailed, the sections can be corre-lated with one another and with C-isotope stratigraphies of other sections (Fig. 13). Thus, although absolute 13Corg val-


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FIGURE 10. Total organic carbon (TOC), and 13Corg stratigraphy of the Cienega del Toro section. Nannofossil zones are from Roth (1978).

ues may have been altered during burial, diagenesis doesnot appear to be responsible for the major trends observed.

Carbon Isotope Shifts and Aptian-Albian Oceanic Anoxic Events

The Santa Rosa Canyon 13Corg curve (Fig. 7) shows aninteresting relationship with oceanic anoxic events, earlyAptian event OAE1a, earliest Albian event OAE1b (Arthur

and others, 1990; Bralower and others, 1993), and a newevent in the late Aptian. All three events correspond toshort-term, 0.53 per mil decreases in 13Corg values fol-lowed by increases of similar magnitude. The increases in

13Corg values and the long-term Aptian-Albian excursionlikely reect the burial of isotopically light C org from themarine reservoir (e.g., Scholle and Arthur, 1980; Arthur andothers, 1985a; Weissert and others, 1985; Weissert and oth-ers, 1998). The negative excursions have previously beenobserved in high-resolution records (e.g., Jenkyns, 1995;Menegatti and others, 1998; Grocke and others, 1999; Erbaand others, 1999) and their origin is not fully understood.

A change in carbon cycling (e.g., Menegatti and others,1998), an increase in hydrothermal activity (e.g., Arthur andothers, 1991; Bralower and others, 1994; Leckie and others,1998), or thermal dissociation of methane hydrates (Jahrenand Arens, 1998) are possible explanations.

Oceanic anoxic events OAE1a (segments C3 to C6) andOAE1b (segments C11 to C12) appear to be global in extent(e.g., Bralower and others, 1993). This is suggested by: (1)the occurrence of C org -rich horizons in the Santa Rosa Can-yon corresponding to levels identied as OAE1a andOAE1b in widespread European land sections and deep-seasections, and (2) the similar negative-positive response of C-isotope curves from Santa Rosa Canyon and other sec-tions. The lack of C org -rich horizons or signicant C-isotopeanomalies in the Axopodorhabus albianus nannofossil Zonein Santa Rosa Canyon suggests that OAE1c (Bralower andothers, 1993; Erbacher and others, 1996) was more regionalin extent.

The late Aptian event (segments C8 and C9) lies in thelong duration Rhagodiscus angustus nannofossil Zone


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FIGURE 11. Plot of oxygen versus hydrogen indices for select sam-ples across organic-rich intervals at Santa Rosa Canyon.

FIGURE 12. Barremian-Albian C-isotope stratigraphy of the Pere-grina Canyon section after Scholle and Arthur (1980). C-segments C1to C8 after Menegatti et al. (1998), C9 to C15 as dened in Santa RosaCanyon (Fig. 7). Stage boundaries determined by Scholle and Arthur(1980) and revised in this investigation are shown at right (see text fordetails).

(NC7) and the upper part of the Globigerinelloides alger-ianus planktic foraminiferal Zone (Fig. 7). Segments C8 andC9 can been recognized in other C-isotope records (e.g.,Weissert and Lini, 1991; Jenkyns, 1995; Erbacher and oth-ers, 1996; Vahrenkamp, 1996; Ferreri and others, 1997;Weissert and others, 1998; Menegatti and others, 1998;Grocke and others, 1999) (Fig. 12), however neither seg-ment in the other sections corresponds to C org -rich sedi-ments. The C org -rich interval in Santa Rosa Canyon liesstratigraphically above a C org -rich level in the Calera Lime-stone in the Globigerinelloides ferreolensis Zone (Sliter,1989); it lies below the C org-rich Jacob level and Level 113which are observed in the Ticinella bejaouaensis plankticforaminiferal Zone in sequences from the Fosse Vocontien,southern France, and the Umbrian Apennines of Italy, re-spectively (e.g., Tornaghi and others, 1989; Weissert andBreheret, 1991).

The late Aptian decrease in 13C values has previouslybeen assigned to an interval of cooling, lower sea level (e.g.,Haq and others, 1987) and reduced C org burial (Weissert andLini, 1991). However, this interval in Mexico clearly cor-responds to increased C org contents (Fig. 7) and rising rel-ative sea-level (Lehmann and others, in press). The absenceof C org-rich horizons in pelagic sequences from southern Eu-rope indicates that the late Aptian event is not global inextent, however, the negative-positive C-isotope responsesuggests that C org burial may have been widespread alongcontinental margins. Alternatively, the increase in C org con-tent in the Santa Rosa Canyon section may be a local phe-nomenon caused by a decrease in carbonate accumulation.Clearly, the late Aptian event warrants further investigation.

Carbon Isotope Variations: Understanding GlobalControls

Understanding the primary oceanographic, eustatic andbasinal processes responsible for detailed Aptian-Albian C-

isotope uctuations (Fig. 13) is difcult. Originally suchvariations were thought to result primarily from increasedburial and subsequent exhumation and oxidation of C org dur-ing OAEs (e.g., Scholle and Arthur, 1980). More recently,however, additional factors have been proposed. These in-

clude input of isotopically light ( 6 to 7 per mil) volcanicCO 2 (e.g., Arthur and others, 1991; Caldeira and Rampino,1991; Bralower and others, 1994), increased recycling ratesof 12C-rich intermediate water (e.g., Menegatti and others,1998), intensied ux of 12C-rich riverine DIC (e.g., Weis-sert, 1989), and thermal dissociation of methane hydrate(Jahren and Arens, 1998). Since several of these mecha-nisms are potentially at work, the causes of Aptian-Albian

13C uctuations are clearly complex (e.g., Menegatti andothers, 1998).

At least part of the negative C-isotope excursion that pre-cedes OAE1a can be explained by volcanism; the emplace-


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FIGURE 13. Correlation of carbon isotope stratigraphies from the Santa Rosa Canyon section ( 13Corg ), Peregrina Canyon section, Mexico ( 13Ccarbafter Scholle and Arthur, 1980), Cismon section, S. Alps, Italy ( 13Ccarb , after Menegatti et al., 1998), DSDP Site 463, Mid-Pacic Mountains ( 13Corgfrom D. Allard, unpubl. data), and ODP Site 866, Resolution Guyot ( 13Ccarb after Jenkyns, 1995). Depth scales show tick marks every 20 m forCismon, Site 463, and Santa Rosa Canyon, and 100 m for Site 866 and Peregrina Canyon.

ment of the massive Ontong Java LIP is synchronous withthe isotopic shift and sea-oor spreading rates also increasedat this time (Arthur and others, 1991; Larson, 1991; Erba,1994; Bralower and others, 1994). Although the correlationis less precise, OAE1b possibly correlates with the emplace-

ment of the Kerguelen LIP in the early Albian (e.g., Leckieand others, 1998). No known LIP episode correlates withthe late Aptian C-isotope event.

Explaining the negative C-isotope excursion by thermaldissociation of methane hydrate (e.g., Jahren and Arens,1998) is attractive from a volumetric point of view. The C-isotopic composition of methane is so negative ( 6065per mil) that a small proportion of the global methane res-ervoir can cause a sizable negative global excursion (e.g.,Dickens and others, 1995). Given the warm temperatures of mid-Cretaceous intermediate and deep waters (e.g., Douglasand Savin, 1975; Huber and others, 1995; Fassell and Bra-

lower, 1999), however, it is unlikely that a signicant meth-ane hydrate reservoir existed at this time.

Variations in carbon isotopes measured on the organicfraction of Aptian-Albian sediments are larger than thosemeasured on the carbonate fraction; this is thought to reect

increased atmospheric CO 2 contents (e.g., Hayes and others,1989; Popp and others, 1989; Menegatti and others, 1998).For example, at Cismon (Southern Alps, Italy) 13Corg var-iations are 7 per mil, 13Ccarb variations are 3 per mil; atRotter Satel (Swiss Alps) 13Corg variations are 5 per mil,

13Ccarb variations are 3 per mil (Menegatti and others,1998); at ODP Site 866 on Resolution Guyot, 13Corg vari-ations are 6 per mil, 13Ccarb variations are 4.5 per mil(Jenkyns, 1995; Baudin and Sachsenhofer, 1996). In Mexi-co, the size of carbon isotopic uctuations is smaller thanin these other locations: 13Corg variations are 3 per miland 13Ccarb variations are 2 per mil (Scholle and Arthur,


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1980; Fig. 12). We speculate that the amplitude of the Mex-ican 13Corg variations has been decreased by diagenetic al-teration accompanying burial and subsequent tectonic upliftof the Sierra Madre.

Menegatti and others (1998) showed that the decrease in13C values and the increase in 13C ( 13Corg - 13Ccarb ) pre-

ceded the onset of elevated C org contents in OAE1a. Such arelationship is not observed for any of the OAEs in theSanta Rosa Canyon Section (Fig. 7). However, the base of OAE1a is contained within the covered interval and thusthe relative timing of the decrease in 13Corg values and theincrease in C org contents cannot be observed. In addition,level 13C values that characterize segment C5 in the recordsof Menegatti and others (1998) cannot be observed in theMexican sections. One possibility is that this segment alsolies within the covered interval and that segments C3 andC4 have been misidentied.

CLUES TO THE ORIGIN OF OCEANIC ANOXIC EVENTS

Potential mechanisms for increased burial of C org in theAptian-Albian OAE were discussed by Weissert (1989). Heproposed that increased ux of hydrothermal CO 2 into theatmosphere caused warm and humid conditions which ledin turn to more intensive continental weathering and in-creased runoff. Heightened continental runoff increasedphosphorus and dissolved inorganic carbon ux to theoceans, which increased primary productivity. Warm andfresh surface waters led to increased water-column stabilityand deep-water stagnation that caused increased burial ratesof C org in sediments. Increased sea oor spreading rates andmid-plate volcanism (e.g., Larson, 1991; Tarduno and oth-ers, 1991) also led to eustatic rise in sea level which wouldhave increased the area of upwelling along continentalshelves. Although it is difcult to understand the relativerole of all of the different factors cited by Weissert (1989),subsequent authors (e.g., Arthur and others, 1990; Weissertand Lini, 1991; Bralower and others, 1994; Menegatti andothers, 1998) have proposed similar scenarios for the Ap-tian-Albian OAE. One major problem with many interpre-tations of pelagic sections is that the correlation of environ-mental changes with relative changes in sea level is indirectand thus subject to considerable error.

The Mexican sections investigated can be directly corre-lated with shallow-water sequences in which relative sealevel changes have been interpreted. An early Aptian sealevel rise has been proposed to have drowned the CupidoPlatform to the northwest of the study area (e.g., Goldham-mer and others, 1991). However, peak ooding, associated

with the deposition of the La Pena Formation and termi-nation of the platform, did not occur until the mid to lateAptian (Lehmann and others, in press). Platforms in Europeand the Middle East (Follmi and others, 1994; Vahrenkamp,1996; Ferreri and others, 1997; Weissert and others, 1998)and atolls in the Pacic (e.g., Rohl and Ogg, 1996) show asimilar relative sea level chronology.

The transition from the San Angel Limestone to the LaPena Formation at Santa Rosa Canyon is associated with anoverall decrease in CaCO 3 content (Fig. 7), but an increasein the abundance of planktic foraminifers and coccoliths(nannofossils excluding nannoconids). These changes may

reect the combined demise of the nannoconids, prolic car-bonate producers, and a decrease in dilution of carbonatedetritus derived from the platform to the west as a result of a relative sea level rise. Although sea level was clearly ris-ing, OAE1a appears to have taken place before peak ood-ing and before its associated condensed section (Loutit andothers, 1989). A similar relationship has been observed byFollmi and others, (1994), Vahrenkamp (1996) and Weissertand others (1998). In Mexico, the late Aptian anoxic eventand OAE1b appear to lie in intervals of rising relative sealevel, although subdivision of the transgressive part of thesea level cycle is not possible due to the lack of lithologicvariability in the La Pena Formation (Lehmann and others,in press). Other investigations show variable interpretationsof relative sea level change in these time intervals: Follmiand others (1994) and Vahrenkamp (1996) show generallyrising sea level; Weissert and others (1998) show generallyfalling relative sea level.

Thus the three anoxic episodes documented in the Mex-ican sections appear to lie somewhere within the transgres-sive systems tract suggesting a constant relationship with anexternal forcing mechanism, possibly volcanism (e.g., Ar-thur and others, 1985; Bralower and others, 1994).

THE APTIAN N ANNOCONID CRISIS IN MEXICO

The nannoconid crisis observed in lowermost Aptiansediments from Europe, and the Atlantic and Pacic Oceans(Coccioni and others, 1992; Erba, 1994; Bralower and oth-ers, 1994) involves a dramatic reduction in the number of narrow-canal nannoconid taxa (including Nannoconus col-omii and N . steinmannii ), a short interval with few or nonannoconids and high abundances of other taxa such as As-sipetra infracretacea and Rucinolithus terebrodentarius(the crisis interval), followed by an increase in the numberof nannoconid taxa with wide canals ( N . globulus , N . buch-eri , N . truittii , and N . kamptneri ). In the European and Pa-cic sections, the nannoconid crisis occurs within nan-nofossil Zone NC6, just above magnetic Polarity Zone M0and below organic-rich sediments of OAE1a (Erba, 1994).In Deep Sea Drilling Project (DSDP) Site 641, however, thecrisis occurs slightly earlier, at the base of Polarity Zone M0(Bralower and others 1994). The signicance of this minordiachroneity is not understood.

Although poor preservation in the Mexican samples hasreduced the number of nannoconids, it is still possible todetect the extinction of narrow-canal nannoconids and theappearance of the wide-canal taxa. The top of the nanno-conid crisis is much easier to detect than the base, corre-

sponding to a sharp increase in the relative abundance of nannoconids in the Santa Rosa Canyon, La Boca Canyon,and Canyon Los Chorros sections.

The crisis interval lies between 124.9 m and 142.2 min the Santa Rosa Canyon section, between the base of thesampled section (205.8 m) and 247.95 m in the La BocaCanyon section and between the base of the sampled sectionand 12.5 m in the Los Chorros Canyon section. The strati-graphic distribution of nannoconid taxa determined here issimilar to that described by Trejo (1975) in some of thesesame sections. We have found rare specimens of Nanno-conus wassallii , N . circularis , N . bucheri , and N . kamptneri


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Received 1 February 1999 Accepted 8 June 1999

APPENDIX: NANNOFOSSIL TAXONOMY

Lithraphidites alatus magnus Covington and Wise, 1987 Hayesites albiensis Manivit, 1971Parhabdolithus achlyostaurion Hill, 1976Corollithion achylosum (Stover, 1966) Thierstein, 1971 Rhagodiscus angustus (Stradner, 1963) Bralower, Erba and Mutterlose

in Bralower et al., 1994 Rhagodiscus asper (Stradner, 1963) Manivit, 1971Watznaueria barnesae (Black, 1959) Perch-Nielsen, 1968 Nannoconus bermudezii Bronnimann, 1955Watznaueria biporta Bukry, 1969 Nannoconus bonetii Trejo, 1959Watznaueria britannica (Stradner, 1963) Reinhardt, 1964 Nannoconus bucheri Bronnimann, 1955 Lithraphidites carniolensis Deandre, 1963 Microstaurus chiastius (Worsley, 1971) Bralower et al., 1989 Nannoconus circularis Deres and Acheriteguy, 1980 Markalius circumradiatus (Stover, 1966) Perch-Nielsen, 1968Prediscosphaera columnata (Stover, 1966) Perch-Nielsen, 1984Watznaueria communis Reinhardt, 1964Cretarhabdus conicus Bramlette & Martini, 1964 Biscutum constans (Gorka, 1957) Black ex Black & Barnes, 1959


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Grantarhabdus coronadventis (Reinhardt, 1966) Grun in Grun andAllemann, 1975

Tetrapodorhabdus decorus (Deandre, 1954) Wind and Wise in Wiseand Wind, 1977

Zygodiscus diplogrammus (Deandre & Fert, 1954) Gartner, 1968Cribrosphaerella ehrenbergii (Arkhangelsky, 1912) Deandre in Piv-

eteau, 1952 Zygodiscus elegans (Gartner, 1968) Bukry, 1969Parhabdolithus embergeri (Noel, 1959) Bralower, Monechi & Thier-

stein, 1989 Zygodiscus erectus (Deandre, 1954) Bralower, Monechi & Thierstein,

1989Percivalia fenestrata (Worsley, 1971) Wise, 1983 Eprolithus oralis (Stradner, 1962) Stover, 1966Tranolithus gabalus Stover, 1966 Nannoconus globulus (Bronnimann, 1955) subsp. globulus Micrantholithus hoschulzii (Reinhardt, 1966) Thierstein, 1971 Assipetra infracretacea (Thierstein, 1973) Roth, 1973 Rucinolithus irregularis Thierstein in Roth and Thierstein, 1972 Nannoconus kamptneri (Bronnimann, 1955) subsp. kamptneri Rotelapillus lafttei (Noel, 1956) Noel, 1973 Diazomatolithus lehmanii Noel, 1965Chiastozygus litterarius (Gorka, 1957) Manivit, 1971Cyclagelosphaera margerelii Noel, 1965 Eiffellithus monechii (Hill and Bralower, 1987) Crux, 1991Flabellites oblongus (Thierstein, 1973) Crux, 1982 Micrantholithus obtusus Stradner, 1963

Tranolithus orionatus (Reinhardt, 1966) Perch-Nielsen, 1968Watznaueria ovata Bukry, 1969 Manivitella pemmatoidea (Deandre ex Manivit, 1965) Thierstein,

1971 Braarudosphaera regularis Black, 1973 Bidiscus rotatorius (Bukry, 1969) Thierstein, 1973Cretarhabdus schizobrachiatus (Gartner, 1968) Bukry, 1969 Micrantholithus speetonensis Perch-Nielsen, 1979Prediscosphaera spinosa (Bramlette and Martini, 1964) Gartner, 1968 Rhagodiscus splendens (Deandre, 1953) Noel, 1969 Nannoconus steinmannii (Kamptner, 1931) subsp. steinmannii. We

combine this species with N. colomii as it is not always possible todiscern a central cavity.

Tegumentum stradneri Thierstein, in Roth & Thierstein, 1972Vagalapilla stradneri (Rood, Hay & Barnard, 1971) Thierstein, 1973Tegumentum striatum (Black, 1971) Taylor, 1979Cretarhabdus surirellus (Deandre, 1954) Reinhardt, 1970 Rucinolithus terebrodentarius Applegate et al., in Covington & Wise,

1987 Eiffellithus trabeculatus Gorka, 1957 Eiffellithus turriseiffelii (Deandre in Deandre and Fert, 1954) Rein-

hardt, 1965 Nannoconus truittii Bronnimann, 1955 Nannoconus vocontiensis Deres and Acheri teguy, 1980 Nannoconus wassallii Bronnimann, 1955Chiastozygus spp. Nannoconus spp.

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THE RECORD OF GLOBAL CHANGE IN MID-CRETACEOUS

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