Turbid-water coral assemblages: a case study from the Oligocene Tertiary Piedmont Basin (N Italy)

In the geological record, but also at the present time, coral assemblages thriving in permanently or episodically turbid waters as a result of terrigenous input are much more common than what is expected, thinking to the conventional optimal conditions for coral reef growth (i.e. warm, well-illuminated, oligothrophic shallow waters of the tropics and subtropics). Despite, in fact, relatively high turbidity conditions, restricted light penetration, and the presence of often mobile terrigenous sediment substrates, active coral growth is well developed and many coral bioconstructions in the past are actually associated with marly and mixed carbonate-siliciclastic sediments. In the Tertiary Piedmont Basin (northern Italy) coral assemblages and associated bioconstructions developed under terrigenous sedimentation during the Oligocene. They occur in siliciclastic conglomerates and sandstone of fan delta and clay-rich prodelta settings. Collected data provide information basically about the composition of the coral assemblages, facies and depositional products of coral growth together with taphonomic aspects. The succession is characterized by the episodic production of abundant coral rubble of the branching corals Acropora and Stylophora. These rudstone and floatstone facies are distinguished on the base of their sedimentological and paleontological aspects, and taphonomic features such as bioerosion and encrustation provide useful information about the different effetcts of turbidity and sediment accumulation. Coral rubble deposits are often buried or occasionaly represent a hard substrate for successive coral growth. Corals in growth position also occur and consist of a suite of phaceloid and massive colonies (Caulastrea, Goniopora, Astreopora, Diploria, Favia, Antiguastrea) with growth habits and morphological characters typical of sediment-resistant corals. These corals form monospecific clusters or, especially towards the top of the succession, can co-exist together to form a small “framework”. Diversity and thickness of these coral bioconstructions, however, are quite reduced compared to many other Oligocene coral assemblages, suggesting that sediment imput, restricted light penetration, and maybe episodic high concentration of nutrients, were major controls on coral growth and carbonate production

Oligocene nearshore benthic assemblages and coral facies of the northern Gargano Promontory

Shallow water carbonate successions of Oligocene age are quite well distributed in the Apulia domain and are often associated with the development of luxuriant coral reef systems (Salento Peninsula, Maiella Mountain). The dominant biogenic components generally consist of rich associations of scleractinian corals and larger foraminifera. In the Gargano Promontory, the Oligocene has been recently reported for the first time and a new carbonate unit informally established and named as Grotta S.Michele limestone (Morsilli et al., 2005). The aim of the present study is thus to investigate the type locality of this unit in order to describe and interprete its lithologic composition and biotic content through facies analysis. The stratigraphic succession consists of grainstones-rudstones to wackestones-mudstones, reaches a maximum thickness of about 28 m and clearly shows a fining-upward trend. The fossil fauna is mainly characterized by abundant scleractinian corals, associated with benthic forams and coralline algae. Corals are represented by very few genera and basically consist of globose colonies of Goniopora near the base of the succession, followed by floatstones-rudstones rich in branched Acropora, Stylophora, Alveopora and phaceloid colonies of Caulastrea towards the top. Similarly, also the foraminiferal assemblages, dominated by miliolids and soritids, are rather poor, often with a small number of specimens and low taxonomic diversity. Some larger foraminifera such as Archaias, Austrotrillina, Nephrolepidina, and Nummulites also occur, thus indicating an Oligocene age (Chattian). Several facies types have been identified on the basis of macroscopic observations in the field and microfacies analysis. Data are provided in particular for the distribution and relative abundance of the main biotic components, including coralline algae, bivalve and gastropod fragments, together with textural features. The facies association suggests a nearshore, sheltered, shallow-water marine paleoenvironment with possible evidence of changes in hydrodynamic conditions from very low to moderate-high energy. Within this setting, the unusual low-diversified coral assemblages and the absence of a true framework can be interpreted as a response to stressed conditions probably controlled by the inherited paleotopography

Rhodalgal lithofacies of the Porto Badisco calcarenite (Upper Chattian, Apulia Italy)

There are few detailed studies on the evolution from the Oligocene to Miocene carbonate facies. This is anomalous considering the transition form the warmth of the late Oligocene to the Neogene ‘Icehouse’, with a series of major changes in climate, ice volume, ocean circulation and carbonate factories (from chlorozoan to rhodalgal dominated). In literature it is accepted that rhodalgal lithofacies became most widespread during the Neogene, with thick accumulations of rhodolith-dominated facies known mostly from the Miocene of the Tethys and Paratethys region. This study reveals the development of a rhodalgal dominated carbonate factory during late Chattian in the southern Apulia platform. The Calcarenite overlies the Castro Limestone Formation (Lower Chattian). The basal rhodolith-rich horizon is 4-m thick and 80-m wide. It consists of rhodolith rudstone with bioclastic grainstone-packstone matrix. Rhodoliths are ellipsoidal (68%) and subspherical (38%). Ellipsoidal rhodoliths major axis range from 3 to 9 cm and laminar (in some case boxwork) to columnar structure (RANGE?). The subspherical rhodoliths range 3 to 7 cm in diameter, the structure is laminar in the centre to columnar outward. Rhodoliths long axes parallel bedding (hydraulic stable orientation). Rhodolith structure is irregular, with large voids partially filled with bioclastic sediment. Nuclei generally consist of biogenic grains. Red algae association is dominated by melobesioids (Lithotamnion 1.24%, Lithothamion/Phymatolithon 48.55%), sporolithaceans (Sporolithon 32.6%) and subordinate mastophoroids (Spongites, 8.04%, Lithoporella 8.78%) and peyssonneliaceans (0.25%). The overlying 20 meters of the succession are composed by 0.6-m to 3-m thick beds of alternating larger foraminiferal floatstone-rudstone and medium to coarse-grained bioclastic packstone-grainstone. Coralline red algae debris, small benthic (porcellanaceous, small rotalids, Neorotalia) larger benthic (Nephrolepidina, Eulepidina, Miogypsinoides, Heterostegina, Operuculina, Amphistegina) and encrusting foraminifera (Acervulina, Planorbulina), as well as bivalve fragments, bryozoans (Celleporids), and echinoderm plates are the main skeletal components. These carbonates facies represent fully marine production within the photic zone. Red algae associations and foraminiferal assemblages suggest carbonate deposition took place in the oligophotic zone (lower infralittoral zone) in a tropical climate

The Messinian reef complex of the Salento Peninsula (Southern Italy): Stratigraphy, facies and paleoenvironmental interpretation

An integrated study of the early Messinian reef complex cropping out along the eastern coast of the Salento Peninsula (southern Italy), including stratigraphy, facies analysis and paleoecological aspects, is here presented. Fourteen facies types belonging to three main facies associations (back reef and shelf, shelf-edge, slope) have been recognized. They document a wide spectrum of depositional environments, reef building organisms and growth fabrics, in response to depth and other environmental factors in different parts of the reef complex. The biotic structure of the reef is also described and discussed in detail. It consists of different types of reef building organisms and of their bioconstructions (mainly Porites coral reefs, Halimeda bioherms and vermetidmicrobial trottoirs), that differ in composition and structure according to their position on the shelf edge-to-slope profile. Results indicate that the reef complex of the Salento Peninsula has strong similarities with the typical early Messinian reefs of the Mediterranean region. However, the recognition of some peculiar features, i.e. the remarkable occurrence of Halimeda bioherms and of vermetid-microbial trottoirs, gives new insights for better understanding reef patterns and development of the reef belt during the Late Miocene in the Mediterranean