Rendiconti online Soc. Geol. It., Vol. 2 (2008), 175-180, 4 figg., 1 tab.
The upper Albian-lower Cenomanian inner shelf carbonate succession of the Calcare di Bari Fm. from the Murge area (Apulia, southern Italy): lithostratigraphy, biostratigraphy and facies analysis LUIGI SPALLUTO(*), MAURO CAFFAU (**) & GIORGIO DE GIORGIO (*)
RIASSUNTO La successione di piattaforma interna del Calcare di Bari di età Albiano superiore-Cenomaniano inferiore affiorante nell’area delle Murge (Puglia, Italia meridionale): litostratigrafia, biostratigrafia e analisi di facies. I lavori di rilevamento geologico del F° 438 “Bari nell’ambito del Progetto CARG per la nuova carta Geologica d’Italia (scala 1: 50.000) hanno consentito di revisionare la stratigrafia della locale successione in facies di piattaforma interna appartenente alla Formazione del Calcare di Bari. I nuovi ed originali dati hanno riguardato l’intervallo stratigrafico di età Albiano superioreCenomaniano inferiore. Questa successione è risultata spessa circa 250 m e mostra caratteristiche di facies piuttosto eterogenee. In particolare, si assiste ad una graduale evoluzione, dal basso verso l’alto, da facies di laguna aperta e di shoal interno, caratterizzate dall’abbondante ricorrenza di calcari a rudiste e foraminiferi bentonici e da calcareniti bioclastiche e peloidali verso facies di ambienti peritidali, caratterizzati da calcari micritici bioturbati privi di macrofossili con un ristretto numero di foraminiferi bentonici e da calcari stromatolitici e loferitici. L’associazione micropaleontologica descritta in questa nota è documentata per la prima volta nella successione cretacea delle Murge ed è risultata essere molto significativa per attribuzioni bio e cronostratigrafiche. Nella parte inferiore di tale successione è stata descritta l’associazione a Valdanchella dercourti, Neoiraqia insolita e Paracoskinolina fleuryi, Nezzazatinella picardi e Praechrysalidina infracretacea, riferibile alla Valdanchella dercourti Taxon-Range Zone dell’Albiano superiore. Nella parte superiore si segnala la presenza dell’associazione a Sellialveolina vialli, Nezzazatinella picardi e Cuneolina pavonia che è attribuibile alla biozona a Sellialveolina vialli e Ovalveolina maccagnoe del Cenomaniano inferiore.
Key words: upper Albian-lower Cenomanian, inner shelf carbonates, Calcare di Bari Fm., Murge, Apulia, southern Italy. INTRODUCTION Most of the mid-Cretaceous sedimentary successions of the Apulia Carbonate Platform consist of mud-supported shallowwater carbonate rocks, primary micrites and dolomicrites. They
(*) Dipartimento di Geologia e Geofisica - Università degli Studi di Bari, via E. Orabona, 4, 70125 Bari (Italy). (**) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (O.G.S.), via Borgo Grotta Gigante 42/C, - 34010, Sgonico (Italy). Email:
[email protected] This work was financed by Carg Puglia founds (new Geologic map of Italy). Coordinators: P. Pieri & L. Sabato. Directors: G. Ricchetti & M. Tropeano. Surveyors: G. De Giorgio, M. Labriola, G. Lotito, L. Spalluto, C. Zoppi. Analysts: M. Caffau, M. Marino.
formed well-bedded and laterally continuous beds deposited in lagoons and tidal flats commonly compared to present day lowenergy tropical carbonate environments of Bahamas (e.g. BERNOULLI, 2001). In such environments, biotically controlled precipitates dominated carbonate factories (T factory sensu SCHLAGER, 2000). Most abundant among them are photoautotrophic organisms (mostly calcareous algae) and certain foraminifers living with photosynthetic symbiotic algae (i.e. orbitolinids). Heterotroph organisms (mollusks) are common and biotically-induced precipitates, such as microbialites, are also important contributors especially in carbonates formed in the peritidal domain (e.g. LUPERTO SINNI & MASSE, 1982). Characteristic products are also abiotic precipitates in the form of early marine cements. During work on the new geological map of Italy (Foglio “438” Bari; scale 1:50.000) the lithostratigraphy and biostratigraphy of the mid-Cretaceous shallow water carbonates cropping out on the Murge area (Apulia, southern Italy) have been deeply revised. The Upper Albian-Lower Cenomanian stratigraphic interval has been found to contain a rich benthonic foraminiferal association in which orbitolinid forms are the most important and abundant taxa. In spite of the abundance of litho and biostratigraphic studies about the Cretaceous succession of the Murge area (see LUPERTO SINNI & MASSE, 1993; LUPERTO SINNI 1996 for reviews and comments) this stratigraphic interval was not studied in detail and its peculiar microfossiliferous association was poorly known. The aim of this paper is to propose an original lithostratigraphy and biostratigraphy, based exclusively on the association of benthonic foraminiferal taxa and their chronostratigraphic position, and to perform a lithofacies analysis of shallow-water carbonates.
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Fig. 1 – A) Synthetic map of the Apulian Foreland (modified after PIERI et alii, 1997). B) Simplified geologic map of the northwestern part of the Foglio 438 “Bari”.
GEOLOGICAL SETTING The Murge area is part of the Apulian foreland formed in the Tertiary during the Apennines orogenesis (RICCHETTI et alii, 1988). The geologic structure of the Murge area consists mainly of an autochthonous carbonatic region slightly deformed by brittle structures (FESTA, 2003). The backbone of the outcropping sedimentary succession is made up of Cretaceous shelf limestones showing mainly mud-supported facies associations formed in low-energy environments. In this shallow-water carbonate succession RICCHETTI (1975) and CIARANFI et alii (1988) recognized two formations: the Calcare di Bari Fm. (Valanginian-early Turonian?), 2000 m thick, and the Calcare di Altamura Fm. (late Turonian?-Maastrichtian), 1000 m thick, separated by an unconformity, which cover the most part of the Turonian, locally marked by bauxites (e.g. CRESCENTI & VIGHI, 1964). Into the Calcare di Bari Fm., rudist layers form distinctive and laterally continuous stratigraphic concentrations informally used as reference levels (see “Livelli guida” in VALDUGA, 1965; RICCHETTI, 1975) for lithostratigraphic correlations of stratigraphic sections and, coupled with biostratigraphic data based on benthonic microfossils, for chronostratigraphic attributions. The studied sections belong to the middle part of the Calcare di Bari Fm. and were formerly attributed to the beds of the “Livello Palese” mostly made up of Apricardia sp. and undetermined
orbitolinids of the “conoidea-discoidea” group (RICCHETTI, 1975). STRATIGRAPHY AND FACIES ANALYSIS The shallow-water carbonate succession of the Calcare di Bari Fm. cropping out in the studied area (fig. 1) is 250 m in thickness (fig. 2). Facies analysis allowed us to recognize the following lithofacies (see tab. 1 for details): A) intraclastic floatstone in a residual clayey matrix formed in supratidal environment; B) fenestral spongiostrome bindstone formed in intertidal environment; C) burrowed Ostracod mudstone/wackestone and D) burrowed Miliolid-OstracodAlgal wackestone/packstone formed in restricted subtidal environments; E) peloidal laminated and non-laminated packstone/grainstone and aggregate grains grainstone showing planar and/or F) cross lamination formed in internal shoals; G) foraminiferal wackestone/packstone and H) molluscan floatstone (mostly para-autochtonous rudist layers) formed in open lagoon environments. The spatial distribution of lithofacies and depositional environments and subenvironments is showed in fig. 3. The lower part of this succession consists of m-thick beds of rudist limestones alternated to fine to medium sized calcarenites rich in bioclasts, micritic intraclasts and aggregate
THE UPPER ALBIAN-LOWER CENOMANIAN SHALLOW-WATER CARBONATE SUCCESSION OF THE CALCARE DI BARI FM.
clay layers with micritic intraclasts and black pebbles are intercalated in this part of the succession and suggest relatively longer exposure periods than ephemeral ones mostly recorded by loferitic limestones. Benthic foraminifers associations recorded in the upper part of the studied section are the following: Sellialveolina viallii (fig. 4D) and Cuneolina pavonia. This association is referred to the Sellialveolina viallii and Ovalveolina maccagnoe biozone (sensu VELIĆ & VLAHOVIĆ, 1994). CONCLUDING REMARKS
Fig. 2 – Composite stratigraphic section of the studied succession showing stratigraphic distribution of main index taxa.
grains formed in open marine domains. Stromatolites and burrowed micritic limestones formed in the peritidal domain are subordinated. Benthic foraminifers associations are rich and contain the following species: Valdanchella dercourti (fig. 4A), Neoiraqia insolita (fig. 4B), Paracoskinolina fleuryi, Nezzazatinella picardi and Praechrysalidina infracretacea (fig. 4C). This association is referred to the Valdanchella dercourti Taxon-range Zone (sensu HUSINEC et alii, 2000; HUSINEC & SOKAČ, 2006). The upper part of the succession consists of dmthick beds made up of burrowed micritic limestones cyclically alternated to stromatolitic and loferitic ones in forming shallowing-upward peritidal sequences. Few dm-thick residual
Stratigraphic and sedimentologic analysis performed on shallow-water carbonates of the Apulian Platform in Murge area clearly documented that the Upper Albian/Lower Cenomanian succession is made up of shelf deposits formed in peritidal and in open marine settings. The lower part of this succession shows lithofacies assemblages pointing to open marine conditions, as suggested by the rich content in rudists and benthic foraminifers. Most representative species of benthic foraminifer assemblages are “primitive” orbitolinids, which are documented in this study for the first time also in the Murge area. Their association, found only in open lagoonal lithofacies, allowed us to refer this part of the studied section to the Valdanchella dercourti Taxon-range Zone. The stratigraphic range of “primitive” orbitolinids, found in corresponding inner shelf deposits in central-southern Italy, is Upper Albian-basal Cenomanian (e.g. MANCINELLI & COCCIA, 2002). However, since in the studied succession these species have not been documented in association with “true” orbitolinids and other Cenomanian taxa, a late Albian age is inferred in the Murge area. This interpretation fits well with coeval carbonate shelf deposits described in Croatia, in which the same foraminiferal association is documented and the same chronostratigraphic attribution is proposed (HUSINEC & SOKAČ, 2006 and references therein). The upper part of the studied succession shows lithofacies assemblages pointing to peritidal conditions as suggested by the lacking of rudist layers and by the lower content in benthic foraminifers implying restriction in water circulation. In addition, loferitic limestones and intraclastic floatstone in a residual clayey matrix suggest periodic emersions. The benthic foraminifers association is referred to the Sellialveolina viallii and Ovalveolina Maccagnoe biozone (sensu VELIĆ & VLAHOVIĆ, 1994), pointing to an early Cenomanian age. This interpretation agrees with many coeval shelf successions cropping out in the periAdriatic region. In conclusion, the stratigraphic and sedimentologic study performed in this area allowed us to document, for the first time in shallow-water carbonates of the Apulian Platform, the stratigraphic range of benthic foraminifers associations in the upper Albian-lower Cenomanian succession. The gradual decrease in foraminiferal abundance and diversity coupled with a general shallowing-upward trend in lithofacies assemblages implies a decrease in the accommodation space on the platform
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which favoured the deposition of limestones formed in shallower and more restricted platform environments. This trend was recorded in several coeval platforms in the periAdriatic area, which in many cases resulted in karstification (e.g. MINDSZENTY et alii, 1995). Bauxites mark this regional uplifting of peri-Adriatic carbonate platforms and consequently their emersion, but, as the example of the Murge area suggests, several areas of this region did not emerge during the late
known long term emersion episode commonly related to the propagation of the tectonic intraplate deformations in Adria plate during the early phases of the alpine orogenesis (MINDSZENTY et alii, 1995). Tectonic stresses produced the Albian/early Cenomanian for a time long enough to develop bauxites.
THE UPPER ALBIAN-LOWER CENOMANIAN SHALLOW-WATER CARBONATE SUCCESSION OF THE CALCARE DI BARI FM.
Tab. 1 – Lithofacies features and depositional environments defined in the studied succession.
Fig. 3 – Spatial distribution of depositional environments interpreted in the upper Albian-lower Cenomanian succession of the Murge area.
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Fig. 4 – A) packstone with Valdanchella dercourti; B) packstone/grainstone with Neoiraqia insolita; C) packstone with Praechrysalidina infracretacea; D) packstone with Sellialveolina viallii and Cuneolina pavonia.
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THE UPPER ALBIAN-LOWER CENOMANIAN SHALLOW-WATER CARBONATE SUCCESSION OF THE CALCARE DI BARI FM.
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