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Milankovitch Cyclicity and Rock-Magnetic Signatures of Palaeoclimatic Change in the Early Cretaceous Biancone Formation of the Southern Alps, Italy

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Milankovitch Cyclicity and Rock-Magnetic Signatures of Palaeoclimatic Change in the Early Cretaceous Biancone Formation of the Southern Alps, Italy Cretaceous Research (1999) 20, 189–214 Article No. cres.1999.0145, available online at http://www.idealibrary.com on Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change in the Early Cretaceous Biancone Formation of the Southern Alps, Italy Helmut Mayer1 and Erwin Appel Institut fu¨r Geologie und Pala¨ontologie, Abteilung Geophysik, Eberhard-Karls-Universita¨t Tu¨bingen, Sigwartstr. 10, 72076 Tu¨bingen, Germany. 1Present address: Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309-0450, USA; email: [email protected]; also at: Geomathematik, Fachbereich VI Geographie/Geowissenschaften, Universita¨t Trier, 54286 Trier, Germany Revised manuscript accepted 21 October 1998 Detailed cyclostratigraphic analyses of the Valanginian to Hauterivian part of the Biancone Formation, a pelagic nannofossil limestone in the Southern Alps of Italy, were carried out. The Cismon section in the Belluno Trough near Feltre and the Pra da Stua section on the Trento Plateau near Avio were studied. Carbonate content, magnetic susceptibility and natural remanent magnetization were measured on densely spaced samples from Cismon. The first two properties vary in a cyclic fashion in this pelagic limestone section and are almost perfectly negatively correlated, while cyclicity in natural remanent magnetization is only vaguely indicated. Quantitative time-series analysis is critical in cyclic stratigraphy. The geostatistical method of cova functions (a generalization of the cross-variogram) which has proven to be the most versatile and robust time-series-analysis method is applied. Cova functions can be calculated from unevenly and non-correspondingly spaced time series without any preprocessing. This method also retains relatively more of the signal when noise and extreme outliers obscure the picture. The periodicities detected in the Cismon time series fall in the range of Milankovitch cycles. Cycle periods of 45 cm, 80 cm and 180 cm likely correspond to dominant precession, obliquity and eccentricity cycles. Owing to the inaccuracy of the Cretaceous time scale, periods cannot be matched exactly, but cycle ratios are extremely close to expected ratios so that Milankovitch climate cycles could be positively identified in this Early Cretaceous section. In the Pra da Stua section bedding thickness was measured and analyzed quantitatively. A cycle period of 55 cm is dominant in this data set, while periods of 115 cm and 170 cm are only vaguely indicated, although bedding in the sampled interval visually appears cyclic and even hierarchically structured. It can be expected that densely spaced measurements of sedimentary properties such as susceptibility and carbonate content will reveal the cyclicity much better. This identification of Milankovitch cyclicity in the pelagic Biancone Formation has important consequences for our understanding of the climate system in the past. These results demonstrate that orbital forcing was effective enough to create palaeoclimatic cycles even in the Cretaceous warm, equable, ice-free climate state. Magnetic susceptibility proved to be a reliable proxy for carbonate content reflecting palaeoproductivity cycles in this pelagic setting. 1999 Academic Press K W: Milankovitch cycles; rock magnetism; palaeoclimate; carbonate content; susceptibility; Biancone Formation; Valanginian; Hauterivian; Southern Alps. 1. Introduction the Mesozoic sequence of the Southern Alps offered The focus of this study is on sedimentary-parameter most favourable conditions. The Valanginian to variations through the sections studied, their analysis Hauterivian portion of the Cismon section was as stratigraphic time-series and their palaeoclimatic studied in detail. Rock-magnetic results from Cismon interpretation. The role of rock-magnetic parameters and Pra da Stua are presented. The cyclostratigraphy in this context is emphasized. The study area was of the Cismon section is investigated utilizing suscep- selected based on the following criteria: continuity of tibility and carbonate-content fluctuations. For the sedimentation, uniformity of facies, lack of tectonic Pra da Stua section bedding-thickness measurements and metamorphic overprint, low degree of diagenetic are analyzed. Quantitative time-series-analysis is alteration and quality of exposure. In all these respects applied to the evaluation of these geologic time series. 0195–6671/99/020189+26 $30.00/0 1999 Academic Press 190 H. Mayer and E. Appel fossil ooze was deposited at the basins in depths of several thousand metres (Bosellini & Winterer, 1975). The Trento Plateau also received a reduced thickness of this pelagic sediment. The resulting limestone is generally known as the Maiolica Formation or locally as the Biancone Formation. In the Belluno Trough, where the Cismon section is situated, the Biancone Formation extends stratigraphically from latest Tithonian/Berriasian to Aptian (Weissert, 1981). In general the Maiolica/Biancone is characterized by abundant slumps and similar synsedimentary defor- mation features (Weissert, 1981). Pelagic conditions Figure 1. Location map for Cismon and Pra da Stua prevailed through the Cretaceous into the Eocene sections in northern Italy. Palaeogeographic domains of when terrigenous flysch was deposited in response to the Southern Alps are shown: diagonal ruling— Lombardian Basin; horizontal ruling—Trento Plateau; the onset of Alpidic deformation. The tectonic defor- stippled pattern—Belluno Trough; vertical ruling— mation of the Southern Alps during the Alpidic orog- Friuli Shelf (boundaries after Gaetani, 1975, and eny produced gentle large-scale folds, thrust faults Weissert, 1981). and transcurrent faults (e.g., van Bemmelen, 1966; Doglioni & Bosellini, 1987). During the Neogene the Venetian Alps in particular, i.e., that part of the The palaeoclimatic significance of Milankovitch cycles Southern Alps where the Cismon section is located, and rock-magnetic parameters in the Cretaceous is have been deformed into a fold-and-thrust belt of discussed. This paper contains overview sections to coherent thrust blocks with little internal deformation provide some background information about the (cf., Doglioni, 1992). Overall, the Alpidic defor- various fields of research tied together here. mation of the Southern Alps was relatively mild compared to that of the Western and Eastern Alps. 2. Geological setting Stratigraphic framework Evolution of the Southern Alps The development of the stratigraphic sequence of the The sections studied are located in the Southern Southern Alps (Figure 2) since the Jurassic was as Alps of northern Italy (Figure 1), which represent a follows. On the Trento Plateau peritidal conditions tectonically inverted, former passive continental prevailed through the Liassic. During the Middle margin. Jurassic a thin layer of red nodular limestone (Rosso Extensional tectonism began in the Early Jurassic Ammonitico Inferiore) covered the platform reflecting with the rifting and opening of the Piemonte-Ligurian its drowning by suddenly increased subsidence. Tethys Ocean. On its southern end a block-faulted Pelagic conditions continued with the deposition of extended continental margin formed, whose palaeo- cherty aptychus limestones (Oxfordian Fonzaso relief is clearly reflected in Jurassic facies distributions Formation) and the Kimmeridgian Rosso Ammo- in the Southern Alps (Aubouin, 1963; Bernoulli & nitico Superiore (Bosellini et al., 1981). Starting with Jenkyns, 1974; Gaetani, 1975; Winterer & Bosellini, the Tithonian the white nannofossil-lime ooze of the 1981). From west to east, the alternating palaeogeo- Biancone Formation covered the area and accumu- graphic basins and swells are the Lombardian Basin, lated slowly through the Early Cretaceous and Trento Plateau, Belluno Trough and Friuli Shelf Cenomanian, when it was replaced by the Scaglia (Figure 1). Rossa, reddish pelagic limestones and marls extending Jurassic breccias and slump deposits as well as across the Cretaceous/Tertiary boundary (Premoli thickness contrasts along the boundaries between Silva & Luterbacher, 1966). Near the western margin these blocks are evidence for synsedimentary normal of the Trento Plateau the varicoloured marls and faulting (Bernoulli, 1964; Bosellini et al., 1981). Dur- marly limestones of the Scaglia Variegata are inter- ing the latest Jurassic and Early Cretaceous the entire calated between Biancone and Scaglia Rossa, similar realm deepened, and differential movements between to the situation in the Lombardian Basin (Cita, 1964). the blocks diminished so that a blanket of pelagic In the Eocene the Scaglia Rossa grades into the marls nannofossil-lime ooze draped over the pre-existing of the Scaglia Cinerea. Volcanic layers are common in relief which was gradually levelled out. This nanno- these Palaeogene beds. However, during the Eocene a Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 191 This study is restricted to the Lower Cretaceous Biancone Formation of the Trento Plateau and the Belluno Trough, which corresponds to the Maiolica of the Lombardian Basin to the west and to the Maiolica of the Umbrian-Marchean Basin in the Apennines. Sedimentologic and lithostratigraphic investigations of the South Alpine Maiolica/Biancone Formation were carried out by Weissert (1981), Barberis et al. (1992) and Bersezio (1993). Since the chronostratigraphy, biostratigraphy, magnetostratigraphy and geochronology of the Early Cretaceous stages covered are still active fields
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