Marco Murru

Miocene carbonate factories in the syn-rift Sardinia Graben Subbasins (Italy)

SummaryMiocene syn-rift carbonate production areas are exposed in south-eastern Sardinia (Italy) along the eastern side of the Plio-Pleistocene Campidano Graben. Here the downfaulted margins of the Oligo-Miocene Sardinia rift-basin crop out with a complex alignment of smaller subbasins. Carbonate factories developed during the Aquitanian times in these tectonically controlled small depositional subbasins, where terrigenous input might be high due to the erosion of the uplifted pre-Neogene substrata. Terrigenous deposits generally acted as passive substratum for pioneer communities of rhodalgal-type (red algae, ostreids, bryozoans) and locally evolved into coral-dominated assemblages.Carbonate production areas occur on tilted, uplifted blocks as well as along basement margins. These have varying organic communities and facies characteristics strongly dependent on different environmental conditions. In footwall areas, devoid of (or with a very reduced) terrigenous supply, open foramol carbonate factories occur with aggradational-progradational stratigraphic geometries. The resulting pure calcareous successions are organized in sequences, bounded by tectonically driven discontinuity surfaces.In sectors close to the hinterland area, with a higher freshwater input and a consequent significant clastic input, coral-dominated assemblages grew during long periods of quiescence between flash floods. Repeated coral-rich communities developed on thick wedges of terrigenous debris.Carbonate facies composition and distribution were mainly controlled by local syn-sedimentary tectonics and eustasy as well as by climate. However, although the inception of the carbonate factories was presumably not synchronous, their growth represented the response to a relative regional sea level rise subsequent to episodes of significant clastic supply: the expression of an early syn-rift stage.During the Burdigalian (N6 zone) a hemipelagic cover uniformly sealed the neritic successions. This basin wide drowning event appears to correspond to the transgressive system tract of the TB2.1 cycle ofHaqet al. (1987) but it can also be interpreted as the expression of an evolved syn-rift stage passing to late syn-rift and quiescence stages.Beneath the hemipelagic cover in some areas, ?late Aquitanian-early Burdigalian deep erosive surfaces, terrigenous clastics and paleosoils have been recognized. These suggest a relative sea level fall and may relate to the global sea level drop at the TB1-TB2 boundary (seeHaqet al., 1987) or to diacronous uplift of different fault blocks.

Rhodalgal/bryomol assemblages in temperate-type carbonate, channelized depositional systems: the Early Miocene of the Sarcidano area (Sardinia, Italy)

Abstract During Aquitanian-Burdigalian times, temperate-type carbonate-siliciclastic successions were deposited in basins located on the graben and half-graben along the Oligo-Miocene Sardinia Rift Basin (Sardinia, Italy). In one of these basins, the Isili Basin, rhodalgal/bryomol limestone was deposited in a temperate-type carbonate depositional system in which a shallow carbonate factory, a marginal tributary belt and a main channel were identified. In this paper, the Isili Limestone is described in terms of biogenic components and taphonomic characterization. To understand the palaeoecology of the depositional settings and the related palaeoceanographic controls, the coralline algal assemblages were differentiated according to coralline growth forms, rhodolith shapes, rhodolith inner structure and taxonomic composition. In addition, a first analysis was carried out in order to discriminate among the different bryozoan and mollusc assemblages present in different localities corresponding to different palaeoecological settings. The results obtained provide fundamental support for palaeoenvironmental reconstruction that combines palaeobiological analyses (i.e. rhodolith characteristics, bryozoan growth forms, taphonomic features) with previous physical (sedimentological and geometrical) observations. This allows a more detailed reconstruction of the complex relationships between the different sectors of the sedimentary system: the carbonate factory, the smaller erosive tributary channels and the deeper main channel.

The Palaeocene–Middle Eocene deposits of Sardinia (Italy) and their palaeoclimatic significance

Abstract The Palaeocene of southern Sardinia includes a continental ferruginous sedimentation, with a high content of Al and Fe, indicative of a subhumid tropical climate. The subsequent microcodium carbonated detrital microcodium levels, containing an abundant quartzose fraction, rather suggest semiarid conditions. In SW Sardinia, the marine deposits, referred to Late Thanetian–Lower Ypresian (=Ilerdian) times, are limestones including larger foraminifers and contain significant amounts of quartz. A lower content of detrital kaolinite points a sediment source from the surrounding areas with a semiarid climate. The occurrence, at the top of this unit, of small trochospiral rotaliids and larger amount of detrital kaolinite suggests a transition to a rainy tropical climate in the adjacent areas. These limestones pass gradually to carbonate sediments characterized by large amounts of detrital kaolinite and intercalated coal layers, with pollen of tropical palms, attributed to the Late Ypresian (=Cuisian)–Early Lutetian and referred to a humid-subhumid tropical climate.

Paleocene palustrine and ephemeral alluvial facies in southern Sardinia (Italy)

Paleocene ephemeral alluvial and palustrine deposits have been recognized in several small outcrops in southern Sardinia. Three sedimentary units have been distinguished: the basal one, mainly terrigenous, made up of alluvial pedogenized deposits, the intermediate one, entirely carbonate, made up of pedogenized fresh water deposits, and the third one, terrigenous-carbonate, recording the beginning of the Thanetian marine ingression in Sardinia. The carbonate deposits are the most developed and are mainly characterized by the presence of Microcodium, both with corn-cob aggregates, in some cases forming palisades or reef-like buildups, and with disarticulated grains forming resedimented deposits; charophyte stems and gyrogonites and stromatolites are also present, in some cases frequent. Deposits rich in charophyte stems indicate the presence of perennial water ponds. Alternances of humid and dry periods favoured erosive fluctuations, as indicated by the frequence of resedimented intraclasts of fresh water sediments, biotic remains as charophytes and stromatolites and, over all, detrital disaggregated Microcodium grains. The studied small outcrops are the only remnants of a probably wide Paleocene continental sedimentary cover, and provide informations on a variety of depositional facies, which are useful proxies for depicting a landscape with wetlands and detrital Microcodium deposits, similar to the coheval continental landscape widespread in southern France and in northeastern Spain.