Miriam Cobianchi

Tethyan Cenomanian pelagic rhythmic sedimentation and Pleistocene Mediterranean sapropels: is the biotic signal comparable?

A comparison between the biotic signal recorded in the Tethyan Cenomanian black shales and in the Mediterranean Pleistocene sapropels is proposed here. To this end the Antruiles section (northeastern Dolomites, Italy) has been investigated as a reference section for the Cenomanian black shales. The data obtained, based on calcareous nannofossils and foraminifera, have been compared to those from a core in the Mediterranean Sea containing a complete succession of Late Pleistocene–Holocene sapropels. Our study suggests that deposition of black shales, like sapropels, occurred when the water column was stratified, as implied by increased abundance of the Cenomanian deeper-dwellers Rotalipora cushmani and R. greenhornensis. The superficial/subsuperficial opportunist form Heterohelix reussi, which strongly increases in numbers within the black shales, could be analogous to the modern surface-dweller Globigerinoides ruber, which is abundant in Late Pleistocene–Holocene Mediterranean sapropels. The peaks of G. ruber can be related to decreased salinity in surface water, thus the increase of H. reussi could also be related to a freshwater superficial layer. We hypothesise also that an increase in abundance of Watznaueria barnesae reflects conditions of superficial low salinity. In black shales increases in abundance of Dicarinella algeriana and Praeglobotruncana gibba, by analogy to modern Neogloboquadrina dutertrei, could reflect availability of food restricted to subsuperficial waters. However, a Cretaceus equivalent of the Pleistocene nannoplankton form Florisphaera profunda, which proliferates within the Deep Chlorophyll Maximum, is not observed. In addition, the abundance of H. reussi indicates a well-expanded oxygen minimum zone, although less in extent than in Pleistocene sapropels, as shown by the occurrence within the Cenomanian black shales of a benthic fauna, even though reduced in size and impoverished. Spectral analysis applied to calcareous nannofossils identifies close relations with the Milankovitch precession and eccentricity cycles in both the black shales and the sapropels.

STRATIGRAPHY OF THE CENOZOIC SUBSURFACE SUCCESSION OF THEVENETIAN-FRIULIAN BASIN (NE ITALY): A REVIEW

The present paper reviews and improves the stratigraphy of the Cenozoic subsurface succession of the Venetian-Friulian Basin by means of a foraminiferal study, integrated by calcareous nannofossil analysis in some key stratigraphic intervals, of thirteen ENI wells drilled in the 60-70s and kept reserved since now.The Venetian Friulian Basin is a complex basin due to the superimposition of three overlapping foreland systems, during the Paleocene to Pleistocene time interval. These systems are related to the evolution of three collisional belts (Southern Alps, Apennines and Dinarides) which experienced structuring phases at different time intervals, with different belt orientation, vergence and topography. The peculiar location of the basin at the tectonic knot, between the Po Plain Basin and the Adriatic Basin that are the most important Italian sites of gas reservoirs, makes it as an interesting case-study both from a scientific and industrial point of view. Data of both foraminiferal and nannofossil assemblages were performed using, tentatively, the standard zonations proposed for the tropical-subtropical realm and the Mediterranean region. In the chronostratigraphic intervals where standard events were not recorded, other biohorizons from “regional” zonal schemes were applied. The data obtained allowed to check the applicability of the biohorizons used in the stated schemes to a subsurface succession analysed mainly from cuttings. Results allowed also to provide a rather precise biostratigraphic correlation of the thirteen studied wells which is a fundamental step to better describe the geometry of the sedimentary infill and to reconstruct the synoptic chronostratigraphic frame providing the timing of the sedimentary events and the main sedimentary or erosional hiatuses. Based on this chronostratigraphic scheme, the Cenozoic subsurface succession of the Venetian –Friulian Basin is subdivided into five depositional sequences ( S1-S5 ) bounded by four major unconformities ( U1-U4 ) and by their correlative conformity surfaces, regionally recognised throughout the basin.