Laura Corda

Aphotic zone carbonate production on a Miocene ramp, Central Apennines, Italy

The lower Miocene Latium–Abruzzi platform was a low-angle ramp that developed under tropical-to-subtropical conditions, but was dominated by bryomol and rhodalgal sediment associations. The Aquitanian to Serravallian sequence described here paraconformably overlies the Cretaceous limestones. It consists of a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Based on facies analysis and on the light dependence of biotic associations, the ramp is divided into three parts: an inner ramp, a middle ramp and an outer ramp. The inner ramp facies are represented by a few metres of coral framestone, rhodolith floatstone–rudstone and balanid macroids floatstone without wave-related structures. The middle ramp consists of structureless bioclastic grainstone to packstone, floatstone and rudstone with rhodoliths and larger foraminifera. The outer ramp facies—proximal sector—are composed of crudely stratified bryozoan-dominated packstone to floatstone which extend over the whole platform. The outer ramp facies—intermediate sector—are represented by wackestone, packstone and rarely grainstone with foraminifera and echinoid fragments. The final depositional profile of the ramp was strongly influenced by the main organisms producing sediment. During the lowstand, the resulting profile is a ramp type. During the transgressive phase, the rapid spreading of the outer ramp facies belt, as a consequence of the enhanced productivity of the light-independent biota, is believed to be promoted by a change from oligotrophic to eutrophic conditions. Climate and/or tectonics are presumed to have played an important role in continental runoff and then in the nutrients delivery. During the highstand phase, the system returns to rates of production uniform throughout the platform. The high rates of carbonate production occurring in the aphotic zone are quite unusual in tropical settings and represent a provocative trend in apparent contrast with the common idea about the locus of the most significant carbonate production, as derived from the ‘‘tropical model’’. The example described here shows that carbonate production and accumulation below the photic zone might be higher than in the shallow euphotic zone even in tropical conditions. D 2003 Elsevier Science B.V. All rights reserved.

Mesozoic Syn- and Postrifting Evolution of the Central Apennines, Italy: The Role of Triassic Evaporites

Jurassic–Cretaceous syn- and postrift successions from the central Apennines were backstripped to gain information on the Mesozoic evolution of the passive margin of the Adriatic Plate. Early Jurassic rifting led to the development of a horst-and-graben paleogeography (the Latium-Abruzzi Carbonate Platform and the Sabina-Umbria-Marche Pelagic Basin). Subsidence curves were built for both carbonate platform and pelagic-basin domains from original and literature stratigraphic data. The paleodepositional depths of the deepwater sediments were reconstructed from field geology data, including new paleontological data. It is proposed that after the deposition of lower Hettangian shallow-water carbonates, an abrupt increase in paleowater depth, to 600–1000 m, occurred during the late Hettangian–Sinemurian synrift stage. The postrift stage was characterized by basin filling, with decreasing paleowater depths during the Jurassic, and by a new deepening during the Cretaceous. Our backstripping curves show, for the Sabina-Umbria-Marche Basin, a short period (<5 m.yr.) of rapid tectonic subsidence at the beginning of the Jurassic, followed by very slow (likely thermally controlled) or absent tectonic subsidence until the Cretaceous. The slight increase in subsidence observed from Cenomanian time is linked to a renewal of extensional tectonics. The Latium-Abruzzi Carbonate Platform shows variable subsidence rates in both place and time. Fast subsidence occurred in the Rhaetian–Hettangian, Toarcian, Berriasian, and Cenomanian and is linked with extensional or transtensional tectonic events. After the Early Jurassic rifting, subsidence rates (on average 30–40 m/m.yr.) affecting the Latium-Abruzzi Carbonate Platform were faster than those recorded by the Sabina-Umbria-Marche Basin. Faster postrift subsidence in carbonate platform areas is a geological paradox that is here explained by the lateral flow of upper Triassic evaporites toward the deepwater domains, as a result of higher sedimentary loading in the carbonate platform areas and the onset of a pressure gradient toward the pelagic basin at the depth of the evaporites.

Indicators of paleoseismicity in the lower to middle Miocene Guadagnolo Formation, Central Apennines, Italy

Convolute bedding—pillow horizons—of likely seismic origin are identified in a bioclastic carbonate succession, the Guadagnolo Formation, in the central Apennine Mountains of Italy. These sediments, which were deposited in a carbonate-ramp environment, are from Burdigalian to Langhian (Relizian) in age. The lower part, about 500 m thick, consists of marlstones, marly limestones, and calcarenites, representing cyclic, shallow-water, coarsening-upward sequences. The second part, about 100 m thick, is dominated by prograding bodies of calcarenites. The horizons containing the pillow beds are in the topmost of the lower part, about 30 m below the base of the overlying calcarenites. They are present at the same stratigraphic position from the Prenestini to the Ruffi Mountains across a distance of about 20 km. Pillows, 20 cm to more than 1 m thick, are present in all the deformed layers and consist of marly calcarenites, which differ texturally from the enclosing matrix. They are regarded as the product of deformation ensuing from liquefaction of a denser layer overlying a lighter, silty layer that is richer in clay. These structures are interpreted to reflect liquefaction processes induced by seismic shocks, and they correlate well with coeval Miocene tectonism in this sector of the Apennines. Mariotti, G., Corda, L., Brandano, M., and Civitelli, G., 2002, Indicators of paleoseismicity in the lower to middle Miocene Guadagnolo Formation, central Apennines, Italy, in Ettensohn, F.R., Rast, N., and Brett, C.E., eds., Ancient seismites: Boulder, Colorado, Geological Society of America Special Paper 359, p. 87–98.

On the peritidal cycles and their diagenetic evolution in the Lower Jurassic carbonates of the Calcare Massiccio Formation (Central Apennines)

Abstract This paper shows the environmental changes and high-frequency cyclicity recorded by Lower Jurassic shallow-water carbonates known as the Calcare Massiccio Formation which crop out in the central Apennines of Italy. Three types of sedimentary cycle bounded by subaerial erosion have been recognized: Type I consists of a shallowing upward cycle with oncoidal floatstones to rudstones passing gradationally up into peloidal packstone alternating with cryptoalgal laminites and often bounded by desiccation cracks and pisolitic-peloidal wackestones indicating a period of subaerial exposure. Type II shows a symmetrical trend in terms of facies arrangement with peloidal packstones and cryptoalgal laminites present both at the base and in the upper portion of the cycle, separated by oncoidal floatstones to rudstones. Type III displays a shallowing upward trend with an initial erosion surface overlain by oncoidal floatstones to rudstones that, in turn, are capped by pisolitic-peloidal wackestones and desiccation sheet cracks. Sheet cracks at the top of cycles formed during the initial phase of subaerial exposure were successively enlarged by dissolution during prolonged subaerial exposure. The following sea-level fall produced dissolution cavities in subtidal facies, while the successive sea-level rise resulted in the precipitation of marine cements in dissolution cavities. Spectral analysis revealed six peaks, five of which are consistent with orbital cycles. While a tectonic control cannot be disregarded, the main signal recorded by the sedimentary succession points toward a main control related to orbital forcing. High frequency sea-level fluctuations also controlled diagenetic processes.