Vittoria Ferreri

Cyclostratigraphy: Approaches and Case Histories

This volume is derived from an SEPM international workshop entitled Multidisciplinary Approach to Cyclostratigraphy, organized by the editors in May 2001 and held in Sorrento (Naples, Italy). In the Introduction we offer a brief history of how concepts of orbital cyclicity and its effects on the Earth evolved, an appraisal of the present state of research, and an overview of the papers in this volume. The main body of the volume consists of the contributed studies. These include a paper on conceptual and pragmatic approaches to stratification cycles by one of the pioneers of cyclostratigraphy, Walther Schwarzacher, who, in the 1940s, discovered the hierarchical expression of orbital cycles in rocks. The other contributions are specific studies of cyclic sequences, extending from the Quaternary back to the Triassic, covering the range from continental deposits to the deep sea, and employing a wide variety of techniques for extracting and processing the information.

Eustatic cycles and tectonics in the Cretaceous shallow Tethys, Central-Southern Apennines

Carbonate platforms bordering the Tethyan margins carry a distinct periodic signal that can be related to Jurassic-Cretaceous climate and eustatism. They yield a rich archive of information, including the tectonics affecting the platforms at regional (subsidence) as well as at «local» (uplift) scale, at a time scale between 104 and 105/106 years (order of magnitude). In Central and Southern Italy, we have analyzed at centimetre scale, along well exposed sections and in bore cores, textures and early diagenetic features of Cretaceous carbonate platform deposits that evidence oscillations in which a hierarchy of cycles (elementary cycles, bundles and super bundles) has been recognized. Eustatic-climatic, high-frequency changes, linked to the Earth’s orbital perturbations, have been considered at the origin of this hierarchy, where the elementary cycles record the precession and/or the obliquity periodicities, while the bundles and super bundles record the short- and long-eccentricity, respectively. These orbital cycles are, in turn, superimposed on lower-frequency cycles (Trangressive/Regressive Facies Trends, T/RFTs). Adopting a sequence stratigraphy approach, the super bundles and the T/RFTs have been interpreted in terms of depositional sequences and used for high-resolution, long-distance (regional to supraregional) correlation, as well as for assembling orbital chronostratigraphic diagrams which quantify the minimum time required for each succession to stack up. Moreover, we have observed that a number of gaps are randomly intercalated in the various sections so that the high-precision correlation of distant intervals rises the problem of explaining their local absence. To reconcile these discrepancies, we propose a tectonic mechanism, already used to explain the stratigraphic gaps related to bauxitic horizons intercalated up section in the same stratal successions. Namely we postulate that the action of transient lithospheric bulges (few meters to tens of meters in elevation, few to several tens of kilometres across), arising from distant compressional or extensional tectonics, may give an explanation for the stratigraphic architecture characterizing the cyclic organization of the lower Cretaceous shallow-water carbonates. In conclusion, we assume that while the eustatic oscillations, driven by orbital and allied climatic variations, follow high-frequency composite rhythms of few tens to few thousand years (Milankovitch periodicities), the regularity of the subsidence is locally modified by transient lithospheric bulges, that result in increase (lows) and decrease (ups) of subsidence (and hence variation of rate of sediment deposited), up to the emersion of more or less large areas evidenced by omission of strata as well as by paleokarst and bauxites.

The Barremian-Aptian shallow-marine carbonates of Monte Faito and of Santa Maria bore cores (central-southern Apennines, Italy). Regional correlation based on cyclostratigraphy

In the last twenty years, high-resolution (cm scale) studies of several Cretaceous carbonate platform successions in centralsouthern Italy have demonstrated that they carry clear evidence ofastronomically controlled eustatic oscillations refl ected on their hierarchical stacking patterns (elementary cycles, bundles and superbundles) and exhibit a sequence-stratigraphic configuration, that is well evident in their superbundles (long eccentricity cycles) as well as in the composite mode of their aggradational pattern, laterally consistent also at more than 100 km distance. Here are presented the centimetre-scale facies analysis of the S. Maria 6 and 4 Agip bore cores (east of Maiella Mountain, Abruzzi region, Early Barremian- Late Aptian in age) and of the Monte Faito section (Monti Lattari, Campania Region, Barremian – Early Aptian in age). Taking into account the cyclic stacking patterns of the lithofacies and their grouping as well as of the early diagenetic features, the environmental oscillations have been constrained in well defi ned time intervals, typical of the Milankovitch periodicities (from ≈ 20 to ≈ 400ky).Based on cyclostratigraphy and sequence stratigraphy criteria and using appropriate biostratigraphic, isotopic and paleomagnetic markers, high-resolution physical correlations (precision 100 ky) have been traced at regional scale among S. Maria composite core, Monte Faito and the previous studied Monte Raggeto (Monte Maggiore) sections. A chronostratigraphic diagram has been assembled for each of the above sections showing that bundles may be locally missing. Moreover, a Barremian-Aptian composite orbital chronostratigraphy has been assembled suggesting the minimum time duration recorded in the considered interval, so that a regional sea-level oscillations history may be restored with a precision of ≤400 ky. On these bases, a minimum duration of 5.2 my for the Barremian interval (13 superbundles) has been estimated, close to the 5 my calculated for the whole Barremian from OGG et alii (2008). These results show as cyclostratigraphy of carbonate platform sequences, integrated with other stratigraphic methodologies and high-precision long distance correlations, is a valid tool to estimate the time duration of stratigraphic intervals and to assemble a floating orbital time scale.

Travertines of central and southern Italy

This is a short presentation of central and southern Italy travertine deposits based on several case histories analyzed in the last decades. Travertines are freshwater carbonate sediments forming in miniature depositional systems and are originated by precipitation from fluvial or spring waters. They display peculiar characteristics, like the ability to modify the morphology of their substrate, and to give rise commonly to “small” sedimentary bodies. Parent water temperature is a key factor in these deposits: high values bring on elevated carbonate precipitation rates and a decrease of abundance, size, and diversity of the eukaryotic organisms colonizing the depositional sites, while the decrease of temperature has an opposite outcome. This allows to distinguish between thermal- and ambient-water travertines (the latter often referred to as calcareous tufa), the transition between these two end-members being often gradual. The space-time evolution of the above sediments tends to modify the original substrate morphology, giving origin to different sedimentary environments (slope, shallow-lake/swamp, rapid/waterfall). Owing to their areal and vertical distribution, Italian travertines build terraced bodies whose upward growth is punctuated by recurrent erosional and/or non depositional discontinuities. The systematic (cyclic?) alternation of periods of calcium carbonate precipitation and starvation intervals suggests allocyclic regulation of climatic origin, even though autocyclic control mechanisms (sometimes including volcanism and/or tectonics) have to be also taken into account locally.