Michele Piazza

Climatic and tectonic control on the distribution of the Oligocene reefs of the Tertiary Piedmont Basin (NW Italy)

The Tertiary Piedmont Basin (TPB) stretches along the Piedmont-Liguria border and is a late- to post-orogenic basin that evolved in a piggy-back position on the Monferrato thrust belt. Its depositional story is strongly controlled by tectonic and eustatic events. The basin was filled with mainly marine sediments (upper Eocene-upper Miocene), which unconformably overlie the Ligurian Alps and the Northern Apennines. The early stage of sedimentation of TPB records a transgressive phase (time-transgressive from the eastern to the western sectors, upper Eocene-upper Oligocene), characterized by the deposition of alluvial fan and fan delta siliciclastic conglomerates and sandstones, marine shallow-water coarse to fine grained siliciclastic sediments, and reef limestones. The TPB reefal buildups, which developed close to the paleoshoreline, have a lower Rupelian-middle Chattian stratigraphic distribution and are restricted to the central part of the southern margin of the TPB. The Valzemola-San Bernardino reef is the younger and the westernmost settlement, while the Val Lemme reef represents the older and easternmost one. The depicted stratigraphic and geographic distribution may be the result of a combined climatic and tectonic control. The lack of reef settlements west of Valzemola-Bric S. Bernardino may be due to climatic condition unfavourable for the reef development (temperature cooler than those of the early Rupelian-middle Chattian time interval). The absence of reefs in the area to the east of Val Lemme may be the consequence of bottom instability and high sedimentation rates (unfavourable conditions for the reef development) caused by the syn-sedimentary tectonic activity along the Scrivia fault, during the time interval spanning from the late Eocene to the early Oligocene (notwithstanding the favourable climatic conditions). Finally, the age of the older reefal complex, i.e. early Rupelian (Val Lemme), gives an obvious constraint to the age of the end of the N-vergent thrusting phase of the Voltri Unit onto the flysch units and its sedimentary cover.

THREE NEW SPECIES OF CORALLINE ALGAE (GENERA LITHOTHAMNION AND LITHOPHYLLUM ) FROM THE TERTIARY PIEDMONT BASIN

Two new species of the genus Lithothamnion and one of Lithophyllum were discovered in the burdigalian section of Bric Cardinelle (Visone Formation, Tertiary Piedmont Basin). The holotypes are present in rhodoliths coming from oncolitic rudstone or calciruditic bodies. The new species are characterised by morphological features and dimensions of cells that are clearly different from those of the other recorded species of Lithothamnion and Lithophyllum .

Quantification of the coralline carbonate from a Serravallian rhodolith bed of the Tertiary Piedmont Basin (Stazzano, Alessandria, NW Italy)

ABSTRACT Digital photographs of the surface of Serravallian rhodolith-bearing strata from Stazzano (Tertiary Piedmont Basin) Italy have been elaborated by Image-J opensource software (U.S. National Institutes of Health — NIH), to obtain a map of the algal surface in each photograph. The 33 images in the 7 m-thick outcrop, covering a total of 11.53 m2, were complemented by data on rhodolith shape, structure, composition and taphonomy. The Serravallian rhodolith body was produced in a long-lasting infralittoral sedimentary environment and then transported into deeper water by slumping. Although biological and geological definitions of rhodolith-dominated facies are inconsistent in some respects, the image analysis described here allows direct comparison of percentages of rhodolith dominance in discrete fossil rhodolith facies and their living counterparts. The procedures involved are both rapid and inexpensive, so the method appears very useful for carbonate quantification. The calculated algal cover, based only on rhodoliths > 2 cm, ranges from 11.9 to 59.7% (mean 27.3%). The carbonate production rate of the Serravallian rhodolith bed was probably in the range 55 to 136.3 g CaCO3 m-2yr-1 as calculated for a Brazilian present-day analogue.

Subterraniphyllum and free-living Neogoniolithon (coralline algae) from the Oligocene reef facies of Costa d’Ovada (Tertiary Piedmont Basin, Alessandria, NW Italy)

The present study examines the siliciclastic sediments and reef limestone of the Oligocene transgressive event of the Tertiary Piedmont Basin outcropping in the area of Costa d’Ovada (Alessandria, Southern Piedmont, NW Italy). Seven lithostratigraphic sections have been studied. On the basis of the lithostratigraphic evidence, sedimentary architectures and structures, and paleontological data a paleoenvironmental reconstruction is suggested. The reef of Costa d’Ovada rests on siliciclastic marine sediments and was buried by siliciclastic sandy and/or gravely shallow marine deposits. This reef was built in warm tropical or subtropical, clear and very shallow waters, on sandy gravely, moderately exposed bottoms. It might be interpreted as a patch reef developed close to the coastline. In the depicted paleoenvironmental frame, an unusual, for the Tertiary Piedmont Basin, Neogoniolithon-dominated coralline algae association is described and interpreted. This association is characterised by the common occurrence of freeliving thalli of Neogoniolithon raripunctatum and intergenicula of Subterraniphyllum thomasii. Finally, the paleoecological meaning of these species is discussed.

AUTOCHTHONOUS BIOFACIES IN THE PLIOCENE LORETO BASIN, BAJA CALIFORNIA SUR, MEXICO

The present paper examines the molluscan and/or echinoid assemblages recovered from two lithostratigraphic units (Piedras Rodadas Sandstone and Arroyo de Arce Norte Sandstone) outcropping in the Pliocene Loreto Basin, Baja California Sur, Mexico. Ten biofacies have been identified, i.e. Trachycardium procerum - Trachycardium senticosum Biofacies, Chione compta - Transennella modesta Biofacies, Laevicardium elenense - Chione kelletii Biofacies, Xenophora sp. 1- Strombus subgracilior Biofacies, Crassostrea californica osunai Biofacies, Myrakeena angelica Biofacies, Vermetid- Nodipecten Biofacies, Argopecten abietis abietis Biofacies, Aequipecten dallasi Biofacies and Encope Biofacies. The first four biofacies have been defined on the basis of statistical analyses (cluster analysis, MDS). The other six, which are monospecific or definitely low-diversity, were already identified during field work. The deduced paleoecological bearing of biofacies, largely relying upon the comparison to their closest modern counterparts, provides the basis for the paleoenvironmental reconstruction. The latter also considers sedimentological evidence and is framed within the tectonic and sedimentary context recently proposed by American workers. Biofacies point toward environments differing in terms of substrate texture, presence/absence of vegetal cover, energy level, variously distributed within the low tide mark-40 m bathymetric range.

Biostratigraphic, paleobiogeographic and paleoecological implications in the taxonomic review of Corallinaceae

Some fossil species of nongeniculate coralline algae are synonyms of living species. These synonimies allow us to use stratigraphic distributions ofPhymatolithon calcareum (Pallas) Adey and McKibbin 1970 (Oligocene — Recent) and of the Mediterranean endemicsLithophyllum racemus (Lamarck) Foslie 1901 (Aquitanian — Recent) andLithothamnion valens Foslie 1909 (Priabonian — Recent) to clarify their paleobiogeography. It also illuminates the bionomy and paleoenvironments of benthic Mediterranean bottoms dominated by unattached corallines. The changes in species and morphologies in the coating sequence of rhodoliths, from nucleus to periphery, document sequential environmental changes.RiassuntoAlcune specie fossili di Corallinaceae non genicolate sono sinonimi di specie viventi. Queste sinonimie permettono di utilizzare la distribuzione stratigrafica diPhymatolithon calcareum (Pallas) Adey and McKibbin 1970 (Oligocene-Recente), come anche diLithophyllum racemus (Lamarck) Foslie 1901 (Aquitaniano-Recente) eLithothamnion valens Foslie 1909 (Priaboniano-Recente), due specie endemiche del Mediterraneo, per chiarirne l’evoluzione paleobiogeografica. Queste sinonimie tra specie fossili ed attuali e quindi il significato attribuibile ai fossili nell’ambito della bionomia bentonica mediterranea (almeno dal tardo Miocene) consentono di ricostruire con maggior dettaglio alcuni paleoambienti di fondo mobile dominati da queste corallinacee. In presenza di rodoliti multispecifiche, è possibile tracciare l’evoluzione del paleoambiente attraverso la sequenza di accrescimento delle rodoliti stesse.

Evidence of syn-sedimentary tectonic activity in the «flysch di Ventimiglia» (Ligurian Alps foredeep basin)

In the study area, located between Argentina Valley and Roya Valley, south of the village of Triora, the Meso-Cenozoic cover of the Argentera-Mercantour Crystalline Massif consists of marls and marly-clays, Campanian in age, that are overlain by the Paleogene unit named «Nummulitique». This unit is composed of a transgressive upper Lutetian complex evolving into a Bartonian-Priabonian siliciclastic turbidite complex («Ventimiglia flysch»). The lower complex includes coarse grained siliciclastic, mixed and carbonate lithofacies of shallow marine environments (Capo Mortola calcarenite) grading upwards to deep marine marls (Olivetta San Michele silty marl). In the area, the nummulitic limestone is commonly made up of larger foraminifera-bearing calcarenite and calcirudite that, from South to North, laterally grade to oncolithic limestone (Loreto calcarenite member), mainly composed of rhodoliths and Acervulina macroids. The Ventimiglia flysch can be regarded as a lateral equivalent of the «Gres d’Annot». The sedimentation of the lower and the upper complex is controlled by the tectonic events of the Ligurian Alps. The former deposits took place during the early evolution stage of the Ligurian Alps foredeep basin, whereas the turbidite deposits of Ventimiglia flysch record the basin filling phase. Evidences of this tectonic control are provided by the occurrence in the Ventimiglia flysch of olistoliths and olistostromes supplied by the chain front, and of slumping and synsedimentary folding.

Stratigraphic vs structural contacts in a late orogenic basin: the case of the Tertiary Piedmont Basin in the Sassello area (Ligurian Alps, Italy)

This geological map (1:10.000 scale) of the ‘Sassello Basin’ remnant covers an area of about 33.4 km2 of Liguria (NW Italy); it highlights the occurrence of two main types of contacts between the sediments of the Tertiary Piedmont Basin and the metamorphic substratum (Voltri Unit): (i) stratigraphic and (ii) structural (thrust or steeply dipping faults). (i) Stratigraphic contacts are represented by the main transgressive surface and the nonconformity between the metamorphic rocks of the substratum and the subaerial deposits. They are locally folded and occur along the steeply dipping short limbs of asymmetric folds related to the late-alpine/apennine tectonics. (ii) Structural contacts are related to the late-alpine/apennine tectonics (thrust faults) or (mostly) to Plio-Quaternary extensional/transtensional faulting.

LITHOPHYLLUM IPPOLITOI . A NEW SPECIES OF CORALLINE ALGAE FROM TERTIARY PIEDMONT BASIN

A new species of the genus Lithophyllum ( Lithophyllum ippolitoi ) was discovered in the Oligocene section of Toleto (Molare Formation, Tertiary Piedmont Basin). The holotype overgrows a coral colony, and is characterised by morphological features and dimensions of cells that are clearly different from those of the other recorded species of Lithophyllum .