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

Abstract

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.