the selli level of the gargano promontory, apulia, southern italy: foraminiferal and calcareous...
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Cretaceous Research (1999) 20, 255269Article No. cres.1999.0155, available online at http://www.idealibrary.com on
The Selli Level of the Gargano Promontory,Apulia, southern Italy: foraminiferal andcalcareous nannofossil data
*Miriam Cobianchi, Valeria Luciani and Alessandra Menegatti
*Dipartimento di Scienze della Terra, Universita degli Studi di Pavia, via Ferrata, 1, 27100 Pavia, Italy
Dipartimento di Scienze Geologiche e Paleontologiche, Universita degli Studi di Ferrara, Corso Ercole Io dEste, 32,
44100 Ferrara, Italy
Dipartimento di Scienze Geologiche e Paleontologiche, Universita degli Studi di Ferrara, Corso Ercole Io dEste, 32,
44100 Ferrara, Italy; current address: Department of Geology & Petroleum Geology, Meston Building,
Kings College, Aberdeen University, Aberdeen AB9 2UE, Scotland, UK
Revised manuscript accepted 17 November 1998
Two Aptian pelagic stratigraphic sections from the northern Gargano Promontory, Apulia, southern Italy, were investigated
on the basis of foraminifera and calcareous nannofossils. The successions are characterized by cyclically arranged marls and
marly/cherty limestones and can be referred to the Scisti a Fucoidi Formation. In the lower portion of this unit a thin black
shale segment was recognized. Planktonic foraminiferal and calcareous nannofossil biostratigraphic data enable the level to be
attributed to the upper part of the Globigerinelloides blowiand Chiastozygus litterariusZones of late Early Aptian age. These data
suggest that the black shale is equivalent to the Selli Level of the Umbria-Marche Basin, which is considered to be the
sedimentary expression of the global oceanic anoxic event OAE 1a. A perturbation of the biotic signal occurs across the Selli
Level. A crisis of Globigerinelloidids and nannoconids precedes and follows the anoxic episode, and a marked increase in the
eutrophic indicators (hedbergellids, Zygodiscus erectus, Biscutum constans, radiolaria) was observed. These critical conditionsassociated to the OAE1a are widely documented and generally related to a high fertility episode of surface water. However,
with respect to the Gorgo a Cerbara section (Umbria-Marche Basin), the Selli Level from the Gargano is not completelybarren of calcareous plankton, probably suggesting slightly less fertile conditions in the surface water or a shallower
environment. Moreover, the occurrence of the benthonic genus Spirillina indicates local dysaerobic conditions versus
complete anoxia on the sea floor. 1999 Academic Press
K W: Early Cretaceous; oceanic anoxic event; Selli Level; planktonic foraminifera; calcareous nannofossils;
integrated biostratigraphy; Gargano Promontory; southern Italy.
1. Introduction
The slope and basinal Cretaceous deposits of the
Gargano Promontory (Apulia, southern Italy) have
recently been investigated in detail (Luperto Sinni
& Masse, 1987; Coccioni & Luperto Sinni, 1989;
Luciani, 1993; Luciani & Cobianchi, 1994; Luperto
Sinni & Borgomano, 1994; Neri & Luciani, 1994;
Cobianchi et al., 1997). In the Lower Cretaceous
three formations have been recognized: the Maiolica,
the Mattinata and the Scisti a Fucoidi (Marne a
Fucoidi auct.). The last unit is considered to be the
equivalent of the Scisti a Fucoidi in the Umbria-
Marche Basin and similar units in the Southern Alps
of northern Italy (Scaglia Variegata Formation). The
Aptian-Albian Scisti a Fucoidi Formation consists
of a pelagic cyclic sequence of marls and marly
limestones. The correlation with the Umbria-Marche
unit is supported by various types of evidence besides
lithological similarity. The Umbria-Marche Basin and
the Gargano Basin border the same platform (the
Apulia Platform). Moreover, the Cretaceous pelagic
successions recognizable in the two areas are similar:
Maiolica Formation, Scisti a Fucoidi and Scaglia,
from the base upwards respectively. Furthermore,
both successions are characterized by anoxic episodes
documented by black shales that are correlatable in
the two basins (Urbino and Amadeus levels;
01956671/99/030255+15 $30.00/0 1999 Academic Press
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Cobianchi et al ., 1997). Therefore, the classic
Cretaceous successions of the Umbria-Marche Basin
can be extended farther to the south in the Gargano
Promontory (Cobianchiet al., 1997). The occurrence
in the Gargano succession of gravity-displaced
deposits, which laterally substitutes the Maiolica
Formation (Mattinata Formation locally), testifies
that this sector was closer to the margin of the
platform than the Umbria-Marche region.
A further black shale was recognized in the lower
portion of the Scisti a Fucoidi in two stratigraphic
sections near Vieste (Coppitella and Le Batterie,
northern Gargano). The aim of the study presented
here is the chronostratigraphical attribution of this
level on the basis of an integrated biostratigraphic
analysis (planktonic foraminifera and calcareous
nannofossils). The stratigraphic position of the black
shale suggests, however, that it could be equivalent to
the Selli Level in the Umbria-Marche Basin, which is
considered to be the sedimentary expression of the
global oceanic anoxic event OAE 1a (Schlanger &
Jenkyns, 1976;Arthur et al., 1990).
2. Geological setting and stratigraphical
framework
The transition between a Jurassic-Cretaceous
carbonate platform and basin is well exposed in the
Gargano Promontory (Figure 1). The slope, base-of-
slope and basin deposits are confined to the
north-eastern part of the promontory, while the
shallow-water carbonates, belonging to the Apulia
Platform, are represented in the western sector.
The Apulia Platform is part of the stable and
relatively undeformed foreland of the Apennine thrust
belt (Ricchetti et al., 1987). Its shallow water
carbonates pass eastwards and northwards into thinly-
bedded cherty pelagic mudstones, which can be
attributed to the Maiolica (Valanginian-Early Aptian),
Scisti a Fucoidi (Early Aptian- Late Albian) and
Scaglia (Late Cretaceous) Formations. The basinal
fringe of the platform, which is characterized by the
common occurrence of gravity-displaced deposits
such as turbidites, breccias and megabreccias, is
represented by the Mattinata Formation
Figure 1. Simplified geological map of the northern Gargano Promontory showing the location of the sections studied(modified fromCobianchi et al., 1997).
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(Hauterivian-Late Albian), the Monte S. Angelo
Megabreccia (Late Albian-Cenomanian) and the
Monte Acuto Formation (Late Cretaceous) (Luperto
Sinni & Masse, 1987; Coccioni & Luperto Sinni,
1989; Luciani, 1993; Luciani & Cobianchi, 1994;
Luperto Sinni & Borgomano, 1994; Neri & Luciani,1994; Cobianchi et al., 1997).
According toBoselliniet al. (1993),the Cretaceous
succession can be subdivided in two depositional
sequences (respectively Early and Late Cretaceous in
age), separated by a sequence boundary recognizable
at the base of the Monte S. Angelo Megabreccia. The
age and relative stratigraphic position of the basin and
slope deposits were controlled by the depositional
dynamics of the Apulia Platform. In the Early
Cretaceous, during periods of relative highstands, the
platform margin prograded basinwards, and clastic
deposits (Mattinata Formation) overlie the basinal
sediments of the Maiolica and Scisti a Fucoidi.
During transgressions, the platform retreated, its
margin drowned, and the export of clastics was
temporarily interrupted. The interval of Scisti a
Fucoidi represented in the two sections near Vieste
does not contain resedimented episodes. The onset of
deposition of the Scisti a Fucoidi in the basin, which
contains levels that indicate anoxia, is coeval with a
drowning of the Apulia Platform margin, related to
a transgression which was probably eustatically con-
trolled. Although precise relationships between a rise
in sea level and oxygen depleted waters is still prob-
lematic, according to Jenkyns (1991), the temporarydemise of carbonate sedimentation may have also
been related to a particularly thick column of deoxy-
genated water (Boselliniet al., in press).
3. Materials and methods
For a preliminary analysis aimed at a biostratigraphic
calibration of the successions and of the black shales,
34 samples from both sections were analyzed for their
calcareous nannofossil and foraminiferal content.
Samples were collected from different lithologies
(marlstone, limestone, silicified limestone, black
shale) as indicated in the stratigraphic columns of
Figures 2and5.
The nannofossil study was carried out by observing
300 fields of view (FOV) in random traverses of each
smear slide under a polarizing light microscope at a
magnification of 1250. Calcareous nannofossil
species abundances were semiquantitatively estimated
as reported inFigures 4and7.
For the foraminiferal analysis, the marly samples
were disaggregated using Desogen and washed
through a >38m-mesh sieve. The indurated samples
(limestone and silicified limestone) were analysed in
thin section. The range of the species identified are
plotted onFigures 3 and6 together with the state of
preservation of planktonic foraminiferal fauna, total
planktonic foraminiferal abundance and radiolarian
abundance.
4. Coppitella section
The Coppitella section extends along the State Road
N. 89 Garganica, from 104 km to 105 km, south-
west of the town of Vieste. It is 22.5 m thick (Figures
1,2). The outcropping sediments consist of cyclically
arranged couplets of bioturbated grey marlstones and
off-white marly limestones (sometimes silicified) with
black chert in nodules. The thickness of couplets is
c. 20 cm. This alternation shows a major hierarchical
arrangement in bundles (about 1 m thick) made up of
5 couplets. This unit is attributed to the Scisti a
Fucoidi Formation. The lower and upper boundaries
are not exposed here; the total thickness of the
formation can be estimated from other localities as
100120 m.
The disappearance of black chert in the Cretaceous
pelagic sediments has been used to define the bound-
ary between Maiolica and Scisti a Fucoidi Formations
in the Umbria-Marche Basin (Coccioniet al., 1987).
In the Gargano Promontory, the black chert occurs up
to the top of the Scisti a Fucoidi; thus, this character
is not useful here to separate the two formations.
According to Cobianchi et al. (1997), the boundarycorresponds to the major lithological change from
a limestone unit (Maiolica) to a bioturbated
(Chondrites=Fucoidi) mainly marlstone unit. On the
other hand, the chert colour is a diagenetic feature
which can vary in different areas, whereas a marked
lithological change reflects primary conditions and can
therefore be used for wider correlations.Erba (1994)
also recognized the general similarity between the
Scisti a Fucoidi of the Umbria Marche Basin and
the marly sediments above the micritic deposits of the
Biancone Formation in the Southern Alps (Cismon
section, northern Italy).
About 14 m above the base, three thin (2 cm each)
black bituminous shales were observed. Sixteen
samples were collected from this section.
The nannofossil and foraminiferal content was
examined for each sample. The main results are
discussed in the following paragraphs.
4.1. Planktonic foraminifera
Planktonic foraminifera from the Coppitella section
were analysed in both thin sections and washed
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residues. Their abundance and state of preservation
varies throughout the section (Figure 3). The samples
(mainly thin sections) yielding abundant radiolaria
generally contained the most poorly preserved fauna
and most impoverished assemblages. Calcispheres
are sometimes abundant (sample 13). Specific and
generic identification of planktonic foraminifera
was sometimes difficult, particularly in thin section,
owing to both the small size of the tests and the
recrystallization of the walls of the specimens.
The most important bioevent recognized in the
Coppitella section is the first occurrences of
Leupoldina cabri. On the basis of this event, the
Globigerinelloides blowi and the L. cabri Zones of the
standard low-latitude biostratigraphic schemes have
been identified (e.g.,Caron, 1985;Sliter, 1989,1992;
Robaszynsky & Caron, 1995).
The basal part of the section (15 m) can be
attributed to part of the Globigerinelloides blowi Zone,
which spans the interval from the FO of the zonal
marker to the FO ofL. cabri, according to the original
definition ofMoullade (1974)(Figure 2). The base of
the zone was not recognized; in fact,G. blowioccurs in
the lowermost sample examined. Besides the zonal
marker, the planktonic assemblages also contain other
Globigerinelloides (G. gottisi, G. duboisi, G. saundersi),
clavihedbergellids and hedbergellids. The last group,
together with favusellids, constitutes the bulk of
the planktonic foraminiferal faunas. The species
Leupoldina pustulanswas recorded only from sample 9.
The upper part of the zone contains the black shale;
across this level, globigerinelloids are temporarily
absent. They reappear at the base of the overlying
zone. AGlobigerinelloidescrisis was recorded at the top
of the Globigerinelloides blowi Zone by Coccioni &
Premoli Silva (1994, Spain) and called the
Globigerinelloides eclipse. An eclipse of this genus
also occurs in the Ischitella section from northern
Gargano (Cobianchi et al., 1997), and a similar
critical interval was observed below the Selli Level in
the Umbria-Marche Basin (Coccioniet al. 1992).
The black shale of the Coppitella section contains
only sporadic, very small hedbergellids (Hedbergella
kuznetsovae).
Figure 2. Stratigraphic column of the Coppitella section with sample numbers alongside, calcareous nannofossil-planktonicforaminiferal biostratigraphic data, and main changes in microfossil assemblages. The dark shading across the Selli Levelillustrates the critical interval with pronounced faunal variations.
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In the G. blowi Zone, small, rare specimens of
Guembelitria have been encountered. They are
probably attributable to two different species. They
resemble those illustrated from the Albian Vico del
Gargano section by Cobianchi et al. (1997, fig. 18,
specimens 810).
The species L. cabriappears in sample 17, indicat-
ing the base of the L. cabri Zone (total range zone,
according to the original definition of Bolli, 1959).
This zone is characterized by common clavate forms.
The top of the zone was not observed, as L. cabristill
occurs at the top of the section.
Over the whole interval, planktonic foraminifera re-
main small (mainly
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The Chiastozygus litterarius Zone has been defined
byThierstein (1973)as the stratigraphic interval fromthe FOs ofRucinolithus irregularis and/orChiastozygus
litterariusand the last occurrence (LO) of Nannoconus
colomii to the FOs of Eprolithus floralis and/or
Rhagodiscus angustus. The species R. irregularis was
recorded from the bottom of the interval upwards.
According to many authors (Mutterlose, 1992;
Braloweret al., 1993;Erba, 1994) and to our data, the
FO of E. floralis postdates that of R. angustus. The
nannofloras are characterized by frequent to abundant
species of the genusWatznaueriaand rare to common
other species, where the most abundant and consist-
ently recorded are Assipetra infracretacea, Biscutum
constans, Cyclagelosphaera margerelii, Lithraphidites
carniolensis, Manivitella pemmatoidea, Reinhardtites
fenestratus, Rhagodiscus asper, R. embergeri, R.
splendens, Rucinolithus terebrodentarius and Zygodiscus
erectus. Rare to frequent nannoconids are represented
by Nannoconus bucheri, N. globulus, N. kamptneri,
N. minutusand N. truittii.
The Parhabdolithus angustus Zone has been defined
byThierstein (1973) as the stratigraphic interval from
the FO ofEprolithus floralis to the FO ofPrediscosphaera
columnata. The top of the zone was not recorded in the
interval studied. The assemblages are characterized by
the same species as those listed above; the nannoconidsare rare and poorly diversified.
Abundance and assemblage composition fluctuate
widely throughout the succession. From the bottom of
the section to sample 14 nannoconids are rare but
morphologically diverse, while in sample 15 the nan-
noconid crisis (Erba, 1994) is recorded; above this
level an interval in which nannoconids are very scarce
is documented. From the base of the Upper Aptian
(sample 18) nannoconids reappear, but their return is
marked by a decrease in diversity and abundance.
As pointed out by previous authors (e.g., Coccioni
et al., 1992), Zygodiscus erectus is consistently present
throughout the interval studied, but abundance peaks
occur both a little below and above the black shale.
The black shale contains a small nannofossil assem-
blage of low diversity. The nannoflora is represented
by frequentWatznaueria barnesaeand rareRhagodiscus
splendens, Rucinolithus terebrodentarius and Zygodiscus
erectus.Erba (1992)pointed out that an abundance of
W. barnesae (more than 40% of the total nannoflora),
low numbers of nannofossils, low species diversity,
and no or very little micarb indicate primary
dissolution at the sediment/water interface.
Figure 4. Nannofossil species distribution and events in the Coppitella section.
260 M. Cobianchi et al.
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Finally, in the lower portion of the section (from
samples 813) and in the stratigraphic interval
immediately above the black shale (from samples 17
20), the assemblages record a considerable increase in
abundance of bothLithraphidites carniolensisand Rha-
godiscus asper. These species are regarded as indicators
of moderate fertility and warmer waters (Erba, 1992).
5. Le Batterie section
This section is located about 1 km from the Coppitella
section, south of the town of Vieste (Figure 1). It is
28.5 m thick and presents the same lithological charac-
ters as those of the Coppitella section. A thin black shale
interval (68 cm) was observed at about 10 m above the
base. The lithostratigraphical unit outcropping in this
section is entirely referable to the Scisti a Fucoidi Forma-
tion; the base and top of the formation are not exposed.
The planktonic foraminiferal and nannofossil distri-
butions are discussed below for the 18 samples analyzed.
5.1. Planktonic foraminifera
The study of planktonic foraminifera from the Le
Batterie section was carried out mainly in thin section.
Figure 5. Stratigraphic column of the Le Batterie section with sample numbers alongside, calcareous nannofossil-planktonicforaminiferal biostratigraphic data, and main changes in microfossil assemblages. The dark shading across the Selli Levelillustrates the critical intervals with pronounced faunal variations. The cross-bars indicate covered parts of the section.The occurrence of theGlobigerinelloides ferreolensisandG. algerianusZones in the upper covered part is presumed; a hiatuscannot be excluded in this stratigraphic interval.
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Planktonic faunas are in a variable state of preser-
vation and are unevenly distributed throughout the
section, with variations in percentages generally out of
phase with radiolarian abundances.
Planktonic assemblages from the lower 11.2 m of
the section were referred to the Globigerinelloides blowi
Zone. The foraminiferal faunas are similar to those of
the Coppitella section; some differences can be related
to different preservation. The black shale occurs at
the top of this zone; it contains only very rare speci-
mens of Clavihedbergella eocretacea in a fair state
of preservation. The interval in which specimens of
Globigerinelloides are virtually absent precedes and
overlies this level (Globigerinelloides eclipse). Slightly
above, the marker of the overlying zone, Leupoldina
cabri, occurs for the first time (sample 43B,Figure 6).
Planktonic foraminiferal tests are small, mainly
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algerianus to the FO of T. bejaouaensis. This zone
corresponds to the Hedbergella gorbachikae Zone of
Caron (1985) and Sliter (1989). More recently,
the same stratigraphic interval has been named the
Planomalina cheniourensis Zone by Robaszynski &
Caron (1995).
The Globigerinelloides ferreolensis and G. algerianus
Zones, located between the Leupoldina cabri and
Hedbergella trocoideaZones in the standard Cretaceous
low-latitude zonations mentioned above, are probably
included in the covered tract. A hiatus in this interval
cannot, however, be excluded.
5.2. Calcareous nannofossils
Nannofossil numbers fluctuate from scarce to com-
mon, and the state of preservation is generally fair
(Figure 7). The black shale yields a more diverse and
abundant assemblage of calcareous nannofossils than
the Coppitella section. In sample 44 Eprolithus floralis
appears for the first time; the lower 12.8 m of the
section can therefore be correlated to part of the
Chiastozygus litterarius Zone, while the upper 15.8 m
corresponds to part of the Rhagodiscus angustus Zone
of Thierstein (1973).
The most abundant species recorded in the interval
studied are Assipetra infracretacea, Cyclagelosphaera
margerelii, Flabellites oblongus, Lithraphidites carnio-
lensis, Rhagodiscus asper, R. embergeri, Watznaueria
barnesae, W. aff. manivitae and Zygodiscus erectus.
Nannoconids are common only in the lowermost part
of the section and are represented by Nannoconus
bucheri, N. globulus, N. kamptneri, N. minutus and N.
truittii. As in the Coppitella section, considerably
below the black shale (sample 42) the nannoconids
record a time-interval of crisis. They reappear in the
Upper Aptian (sample 45) where Nannoconus truittii
becomes the dominant species. This stratigraphic
interval can probably be correlated with the N. truittii
Acme Zone of Mutterlose (1991).
In this section the Z. erectus and L. carniolensis - P.
asper peaks are recorded in the same stratigraphic
intervals as those in the Coppitella section. Finally,
the calcareous nannofossil assemblage of the black
Figure 7. Nannofossil species distribution and events in the Le Batterie section.
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shale is characterized by the occurrence of common
Watznaueria barnesae and W. aff. W. manivitae, fre-
quent Cyclagelosphaera margerelii and W. biporta, and
rare Assipetra infracretacea,Cretarhabdus angustiforatus,
Rhagodiscus asper, R. embergeri, R. splendens, Rucino-
lithus irregularis, R. terebrodentarius, W. britannica, W.communis and Zygodiscus erectus.
6. Other fossil groups
Ammonites and other macrofossils are absent in the
sections studied. Among microfossil groups, ostracods
and calcispheres are rare and their record is
discontinuous.
Radiolaria are generally frequent, but are unevenly
distributed throughout the two sections. Marked
increases in abundance correspond to a reduction in
number of planktonic foraminifera, and vice versa.
This group is almost the only component of
planktonic fauna that occurs in the black shales. A
qualitative observation of Gargano radiolarian faunas
analyzed here shows that, by comparison with the rest
of the section, less diverse assemblages, apparently
dominated by spumellarians, coincide with the black
shale. Recent studies (Erbacheret al., 1996;Erbacher
& Thurow, 1997) have emphasized the relationship
between extinction and radiation events of radiolaria
in the Early Cretaceous and the oceanic anoxic
events. Specialist study and more closely spaced
sampling are, however, necessary to evaluate the
ecological and evolutionary changes in radiolarianassemblages in the Coppitella and Le Batterie
sections.
Benthonic foraminifera are generally a minor
component of the microfossil assemblages. In some
samples they apparently increase in abundance with
respect to the planktonic foraminifera, probably in
relation to the decrease of the latter group as a
result of dissolution. Genera represented are mainly
hyaline forms, such as Dentalinoides, Gavelinella,
Gyroidinoides, Lagena, Marginulina and Nodosaria,
together with agglutinating forms (Clavulinoides,
Dorothia, Marssonella). These assemblages are gener-
ally considered to be deep-water, bathyal indicators
(e.g., Sikora & Olsson, 1991; Coccioni & Galeotti
1993; Premoli Silva & Sliter, 1994). The benthonic
fauna records a drastic decrease in abundance and
change in composition through the black shale
analyzed here; only rare, small forms belonging to the
flattened, planispiral genusSpirillina were observed.
In recent years, Cretaceous benthonic micro-
foraminifera have been investigated as indicators of
aerobic, dysaerobic and anaerobic conditions. These
conditions on the sea floor, in both recent and fossil
assemblages, have been documented on the basis of
variations in size, dominance and composition of the
benthonic populations (e.g., Koutsoukos & Hart,
1990; Koutsoukos et al., 1990; Coccioni & Galeotti
1993; Lamolda & Peryt, 1995). According to these
studies, the assemblages indicating oxygenated con-ditions on the sea-floor are those containing diversi-
fied fauna, with convex epifaunal forms represented
(such asGyroidinoides). This kind of assemblage char-
acterizes the samples investigated in this study with
the exception of a critical interval across the black
shale. Low diversity, small size and a predominance of
flattened morphologies have been frequently observed
in deposits that accumulated when oxygen levels were
low, particularly in Mesozoic shales (Bernhard, 1986).
This is probably related to the higher area/volume
ratio of these forms by comparison with the inflated
specimens. In fact, flattened specimens can simul-
taneously oppose sinking within a sediment (where
less oxygen occurs), maximize oxygen uptake, and
probably also benefit from increased food supply.
Small tests can also help to maximize relative surface
area (Bernhard, 1986).
The dysaerobic conditions on the sea floor, sug-
gested by the exclusive occurrence of small Spirillina
among the benthonic fauna, span an interval of about
34 m across the black shale.
7. The Selli Level of the Gargano Promontory
Our integrated biostratigraphic analyses allow the
Scisti a Fucoidi Formation, exposed in the Coppitella
(Figure 8) and Le Batterie sections, to be assigned to
the foraminiferal Globigerinelloides blowi-Hedbergella
trocoidea Zones p.p. and nannofossil Chiastozygus
litteriariusParhabdolithus angustusZonesp.p.This cor-
relation between the nannofossil and foraminiferal
biozones confirms that reported in previous integrated
biostratigraphic schemes (e.g., Coccioni et al., 1992;
Coccioni & Galeotti, 1993; Bralower et al., 1995).
The age of the analysed sections spans the late Early
Aptian to the early Late Aptian, according to the
correlations generally in use between the biozones and
the standard stages (e.g.,Bralower et al., 1995).
The two thin black intervals (Figure 9) observed in
these sections are correlatable, and both fall within the
upper part of the G. blowi (planktonic foraminiferal)
and C. litterarius (calcareous nannofossil) Zones, thus
suggesting that they correlate with the Selli Level
in the Umbria-Marche Basin and Southern Alps
(Bersezio 1992, 1993; Erba, 1994). This level is
considered to be the sedimentary expression of
oceanic anoxic event OAE 1a ofArthur et al. (1990).
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It is well known that the Barremian-Aptian interval
is characterized by the occurrence of widespread
anoxic deposits. In particular the Selli Level, ident-
ified and named for the first time by Coccioni et al.
(1987,1989), appears to be the best and most widely
documented. It correlates as well with global event
OAE1a, although its sedimentological and geo-
chemical features and thickness vary in different
basins.
A detailed biostratigraphical and palaeoecological
study, based on quantitative analyses of planktonic
foraminifera and calcareous nannofossils, was carried
out by Coccioni et al. (1992) for the Barremian-
Aptian interval in the Umbria-Marche region of
central Italy. In this region the interval corresponding
to the Selli Level is about 2 m thick, and consists of an
alternation of black shales with radiolarian silty/sandy
layers. Even though our study is preliminary, and
hence less detailed, it indicates similar major events
across the Selli Level highlighted by these authors.
Our data confirm that a crisis of globigerinelloids
and nannoconids, documented by their temporary
absence, precedes and follows the Selli Level, thus
attesting that these forms are the most oligotrophic
indicators through this stratigraphic interval (Caron &
Homewood, 1983;Leckie, 1987,1989;Premoli Silva
et al., 1989; Coccioni et al., 1992; Erba, 1994).Moreover across the Selli Level, an increase in
numbers of hedbergellids, Zygodiscus erectus and
Biscutum constans, considered to be eutrophic groups,
was recorded. The eutrophic radiolarians are also very
abundant in this interval, but the composition and
abundance of the assemblages varies widely through-
out the sections, and is generally out of phase with
planktonic foraminifera. The same biotic variations of
the nannofossil assemblages have been documented
by Erba (1994) for the Selli Level in the Cismon
section (Southern Alps). The distribution and com-
position of microfossil faunas of the Selli Level from
the Gorgo Cerbara section induced Coccioni et al.
(1992)to interpret this level as being related to a very
high-fertility event. In the Umbria-Marche Basin,
the black shale is completely barren of benthos and
calcareous plankton, and enriched in radiolarians
and Corg.
The temporary absence of calcareous plankton,
often recorded in correspondence with black shales,
has been related to different causes, possibly in com-
bination; for example, a rise in the carbonate compen-
sation depth (CCD) for a short period of time, or
highly corrosive waters associated with the degra-
dation of large amounts of organic material on the seafloor or during early diagenesis (Braloweret al., 1994).
A further explanation for the absence of calcareous
plankton is high productivity associated with oxygen
deficiency. In present-day oceans, nannoplankton
become less competitive with respect to diatoms and
dinoflagellates in highly eutrophic conditions, while
planktonic foraminifera are intolerant of oxygen
deficiency. There is still widespread debate about the
causes of deposition of Cretaceous Corg-rich sedi-
ments. In case of organic matter of marine origin, they
are interpreted as a result of high primary productivity
or conditions favouring the preservation of the organic
material produced (e.g.,Dean et al., 1986;Schlanger
et al., 1987; Premoli Silva et al., 1989; Bralower &
Thierstein, 1984, Bralower et al., 1994). The black
shales of the OAE1a are, however, generally related to
a high productivity event.
The variations in planktonic assemblages below and
above the Selli Level in the northern Gargano sections
record critical conditions, similar to those in the
Umbria-Marche Basin, for some groups that are in-
terpreted as more specialized, oligotrophic forms. At
the same time, an increase in eutrophic forms occurs.
Figure 8. Typical marlstone-limestone couplets of theScisti a Fucoidi Formation in the Coppitella section.
Figure 9. The Selli Level in the Coppitella section.
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This suggests a high fertility episode of superficial
water. A more detailed analysis involving geochemical
data and a wider stratigraphic interval would, how-
ever, be necessary to relate more precisely eutrophy
to preservation of organic matter and quantitative
variations in fossil assemblages. In fact, levelsshowing peak abundances of radiolaria or hedbergel-
lids are not necessarily associated with the preser-
vation of large quantities of organic matter in black
shales.
Various differences exist, however, between the
Umbria-Marche and Gargano areas. In fact, the Selli
Level of the Gargano Promontory, which is notably
thinner, is not completely devoid of calcareous plank-
ton; this probably indicates slightly lower eutrophy.
Benthonic foraminifera are also present in this level,
although they record a notable change in the compos-
ition and size of the assemblages. In particular, diverse
faunas are substituted by an assemblage characterized
by the genus Spirillina. The occurrence of benthonic
fauna in the Selli Level of the Gargano Promontory
therefore indicates local dysaerobic conditions on
the sea-floor in contrast with the complete anoxia
recorded in the Umbria-Marche Basin.
The onset of critical conditions in marine environ-
ments during the late Early Aptian (OAE 1a), are
widely documented by an evident perturbation in
the biotic signal. However the crisis for calcareous
plankton was not so dramatic as that related to OAE 2
at the Cenomanian/Turonian boundary (CTBE of
Arthuret al., 1987), which led to the extinction of thedeeper-dwelling specialized rotaliporids. In fact, the
interval of crisis related to OAE 1a during the late
Early Aptian is followed by a recovery of calcareous
planktonic communities without any major extinction
events. As far as the foraminifera are concerned, this is
probably because of their lower state of specialization
at this time, when they had not yet colonized the
deepest habitats (Bralower et al., 1994).
8. Summary and conclusions
The classical Cretaceous succession of the Umbria-
Marche Basin (Maiolica, Scisti a Fucoidi and Scaglia)
can be recognized further to the south, in the Gargano
Promontory, as recently pointed out by Cobianchi
et al. (1997). The similarity between the two succes-
sion is also supported by the occurrence of anoxic
episodes of Albian age which are correlatable in the
two areas. In this study, an integrated biostratigraphic
analysis (planktonic foraminifera and calcareous
nannofossils) of two sections exposed near Vieste
(northern Gargano), suggests that the black shale
newly recognized in the lower portion of the Scisti
a Fucoidi is equivalent to the Selli Level in the
Umbria-Marche Basin.
The most important results of this study can be
summarized as follows:
(1) Our biostratigraphical data enable the Scisti a
Fucoidi Formation, outcropping in the Coppitellaand Le Batterie sections, to be assigned to the
foraminiferal Globigerinelloides blowiHedbergella
trocoidea p.p. and nannofossil Chiastozygus litterarius
Parhabdolithus angustus p.p. Zones. The correlation
between nannofossils and foraminiferal events ob-
served for the Gargano promontory confirms that of
previous integrated biostratigraphic schemes (e.g.,
Coccioni et al., 1992; Coccioni & Galeotti, 1993;
Bralower et al., 1995).
(2) The genusGuembelitria (probably represented by
two species) was found in northern Gargano in levels
of Early Aptian age (upper part of the G. blowiand C.
litterarius Zones). This datum represents the oldest
known occurrence of the genus, previously reported
from the Upper Aptian of northeastern Brazil
(Koutsoukos, 1994).
(3) The black shale recorded in the lower portion of
the Scisti a Fucoidi can be attributed to the upper part
of theG. blowiand C. litterariusZones, suggesting that
it can be correlated with the Selli Level, recognized in
the Umbria Marche Basin and the Southern Alps. It
is considered to be the sedimentary expression of
oceanic anoxic event OAE 1a ofArthuret al. (1990).
(4) The microfossil distribution patterns across the
Selli Level present some major variations. A crisis ofGlobigerinelloides and nannoconids preceded and fol-
lowed the anoxic episode, while the black shale con-
tains evidence of a marked increase in hedbergellids,
Zygodiscus erectus and Biscutum constans. Radiolaria,
which are considered to be eutrophic indicators, are
also very frequent and of low diversity in the Selli
Level, although their abundance fluctuates consider-
ably throughout the sections. Nevertheless some dif-
ferences exist between the Umbria-Marche and
Gargano areas. The Selli Level in northern Gargano,
in contrast to the black level in the Gorgo a Cerbara
section, but in common with the Cismon section
(Erba, 1994) is not completely barren of calcareous
plankton; this may well suggest slightly less fertile
surface water or a shallower environment. Moreover,
the occurrence in the Gargano Selli Level of the
benthonic genusSpirillina indicates local dysaerobic
conditions versus complete anoxia on the sea floor.
Acknowledgements
The authors are indebted to A. Bosellini and M.
Morsilli for suggesting that these sections should be
analysed, and for their help in the field. This paper is
266 M. Cobianchi et al.
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financially supported by the Italian MURST (ex 60%
grant, V. Luciani), CNR (A. Bosellini) and FAR (G.
Brambilla) grants. We thank D. J. Batten, J. Jeremiah
and an anomymous referee for critically reviewing the
manuscript and useful comments.
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Appendix
List of cited species with author attributions and dates
Planktonic foraminifera
Clavihedbergella eocretacea Neagu, 1975C. semielongata (Longoria, 1974)Globigerinelloides aptiense Longoria, 1974G. barri(Bolli, Loeblich & Tappan, 1957)G. blowi(Bolli, 1959)G. cepedai(Obregon, 1959)G. duboisi(Chevalier, 1961)
G. ferreolensis (Moullade, 1966)G. gottisi(Chevalier, 1961)G. saundersi(Bolli, 1959)Guembelitria sp. 1Guembelitria sp. 2Hedbergella aptiana Bartenstein, 1965H. aptica (Agalarova, 1951)H. delrioensis (Carsey, 1962)H. excelsa Longoria, 1974H. gorbachikae Longoria, 1974H. kuhryiLongoria, 1974H. kuznetsovae (Banner & Desai, 1988)H. malaskovae Longoria, 1974H. planispira (Tappan, 1940)H. sigaliMoullade, 1966H. similis Longoria, 1974H. simplex (Morrow, 1934)
H. trocoidea (Gandolfi, 1942)Leupoldina cabri(Sigal, 1952)L. pustulans (Bolli, 1957)Ticinella bejaouaensis transitoria Longoria, 1974
Calcareous nannofossils
Assipetra infracretacea (Thierstein, 1973) Roth, 1973Biscutum constans (Gorka, 1957) Black, 1967Braarudosphaera regularis Black, 1973Chiastozygus litterarius (Gorka, 1957) Manivit, 1971Conusphaera mexicana Trejo, 1969Cretarhabdus angustiforatus (Black, 1971) Bukry, 1973C. conicus Bramlette & Martini, 1964C. surirellus (Deflandre, 1954) Reinhardt, 1970Cyclagelosphaera margereliiNoel, 1965Diazomatolithus lehmaniiNoel, 1965
Discorhabdus rotatorius (Bukry, 1969) Thierstein, 1973Eprolithus floralis (Stradner, 1962) Stover, 1966Flabellites oblongus (Thierstein, 1973) Crux, 1982Lithraphidites alatus magnus Covington & Wise, 1987L. carniolensis Deflandre, 1963Manivitella pemmatoidea (Deflandre in Manivit, 1965) Thierstein,
1971Markalius circumradiatus (Stover, 1966) Perch-Nielsen, 1968Micrantholithus hoschulzii(Reinhardt, 1966) Thierstein, 1971Microstaurus chiastus (Worsley, 1971) Grun in Grun & Alleman,
1975Nannoconus bucheriBronnimann, 1955N. globulus Bronnimann, 1955N. kamptneriBronnimann, 1955N. minutus Bronnimann, 1955
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N. steinmanniiKamptner, 1931N. truittiiBronnimann, 1955Parhabdolithus angustus (Stradner, 1963) StradnerP. asper(Stradner, 1963) Manivit, 1971P. embergeri(Noel, 1958) Stradner, 1963P. pseudoangustus Bralower et al. in Covington & Wise, 1987P. splendens (Deflandre, 1953) Noel, 1969
Reinhardtites fenestratus (Worsley, 1971) Thierstein in Roth &Thierstein, 1972
Rucinolithus irregularis Thierstein in Roth & Thierstein, 1972
R. terebrodentarius Applegate, Bralower, Covington & Wise, 1987Vagalapilla stradneri(Rood, Hay & Barnard, 1971) Thierstein, 1973Watznaueria barnesae (Black, 1959) Perch-Nielsen, 1968W. biporta Bukry, 1969W. britannica (Stradner, 1963) Reinhardt, 1964W. communis Reinhardt, 1964W. aff. W. manivitae Bukry, 1973
W. ovata Bukry, 1969Zygodiscus diplogrammus (Deflandre in Deflandre & Fert, 1954)Z. erectus(Deflandre, 1954) Bralower, Monechi & Thierstein, 1989
Planktonic foraminifera and nannofossils from the Selli Level 269