The Selli Level of the Gargano Promontory, Apulia, southern Italy: foraminiferal and calcareous nannofossil data

8/14/2019 The Selli Level of the Gargano Promontory, Apulia, southern Italy: foraminiferal and calcareous nannofossil data

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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).

256 M. Cobianchi et al.


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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

Planktonic foraminifera and nannofossils from the Selli Level 257


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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.



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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.



12/15

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



13/15

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


Documents

The Selli Level of the Gargano Promontory, Apulia, southern Italy: foraminiferal and calcareous nannofossil data