BIOSTRATIGRAFIA A NANNOFOSSILI CALCAREI CALPIO.NELLIDI E FORAMINIFERI PLANCTONICI DELLA MAIOLICA (TITONIANO SUPERIORE-APTIANO) NELLE PREALI'I BRESCIANE (ITALIA SETTENTRIONALE)

Abstract

A derailed lithostracigraphic and niicropalcontological study was undertaken on (he Maiolica formation outcropping In the Lombardian Picalps, NW of Brescia. The Maiolica mainly consists of whitish to pale brown io grey pelagic limestones with several black shaly inter beds in i:s upper portion; then nodules and lenses occur within the limy beds whereas chert layers arc jnterbedded, the color of which varies from pinkish to grey to black. Fre-qucnt resistant bluish layers consisting of radiolarian silt occur in the upper portion of the Maiolica. Locally, at the base of the Maiolica a breccia ("Formazione di Piabionc" or "Breccia di Prcgasio") occurs: lntraformational resedimented levels and a few? slumps were recorded. Bedding thickness ranges from several decimeters in the lower portion of the unit io a few centimeters upwards bui frequent oscillations arc observed. The total thickness of the Maiolica is estimated some 290 m; this high value is related to the palco-geographic and geological setting of the studied area, which was belonging to the deep Scbino Through within the Lombardian Basin.The great depth of the depositioual environment is also pointed out on the basis of the poor preservation of microfossils: a depth below the calcite lysoclinc. approximating the CCD can be- suggested. Foui detailed sections {M. Pernice. Polavcno. S. Giovanni and Pie' del Dosso) were analyzed from both lithostratisraphic and micro pa leontological points of view. They cover the entire thickness of the Maiolica including the lower transition to the Rosso ad Aptici formation and the upper limit to the Scaglia Variegata formation. The time interval spans from Late Tithonian to Late Apiian. Calcareous nannofossils are always present even if moderately preserved; 21 biohorizonsand 8 bio-zones (Conusphdera mexicana, Nannoconus colomii, cretarhabtlu angusltforaiuf, Calcicalathina oblongata hithraphtdites bollii, Micrantholilhus hosehukii, Chiaslozygus litterarius and ParhabdoUthus angustus Zones) were recognized from 1-atc Tithonian io Late Aptian. The lowrr portion of the Maiolica (Late Tithonian-Valangwian) contains frequent calpionellids; in this time interval these microfossils allow to obtain a high resolution biostratigraphy. Five calpionellid zones I A. B, C. Dand E). 5 sub-zones (A1. A2. A3. Di, D3) and 13 biohorizons were identified. Plankionic foraminifcrs have their first occurrence at the Barremian/Aptian boundary. Three biozoncs of Early Aptian age (Hedbergella similis, Globigerineitoides gottisi/G. duboisi, G. maridalensis /G. blowi Zones) and 3 biohorizons were recognized. An informai "Radiolarian Zone" wai recorded in the - Hautcrivian - Barremian interval. The obtained integrated biostraligraphy is consisicnt with those known Iroin the Tethyan region except for minor differences around the Barremian/Aptun boundary. In the present paper this limit was defined with the first occurrence of Hedbergelh similis: thus the calcareous naimofossil events usually adopted to place the Barremian/Aptian boundary are younger. Rucinolilhus irregularis first occurs in the earliest Aptian, ilie first occurrence of Chiastzygus litterarius and the last occurrences of Sannocottus colomii and Nannoconus steinmannii were recorded in the Early Aptian.