Mauro Aldinucci

Geological map of the Pliocene succession of the Northern Siena Basin (Tuscany, Italy)

Abstract Please click here to download the map associated with this article. The late Miocene-Pliocene sedimentary fill of the Siena Basin (Tuscany, Italy) consists dominantly of clastics and has internal architecture that reflects the interplay of tectonics, relative sea-level changes and climate variations. Pliocene sediments are extensively exposed and overlay both late Miocene deposits and pre-Neogene bedrock. Specifically, Pliocene basin margin sediments consist largely of sand with gravel and mud intercalations, deposited mainly in nearshore settings with minor fluvial depositional episodes. They grade basinward to dominantly offshore fines with intervening turbiditic sand bodies. New fieldwork revealed that basin margin deposits, notwithstanding lithologically rather homogeneous, are made of a variety of sedimentary facies and bear several unconformities. They have been traditionally described and mapped using lithostratigraphic criteria, that have proven to be unfit to represent such complex stratigraphic architectures. The aim of this paper is to describe the allostratigraphic architecture of the Pliocene deposits exposed in a marginal key-area (45 km) of the northern Siena Basin by means of a 1:10,000 scale geological map. The recognized succession of allostratigraphic units and their bounding discontinuities, along with new biostratigraphic data from calcareous plankton, provides new insights into the geological history of the Siena Basin and represents valuable constraints for long-distance correlation.

Detection of detached forced-regressive nearshore wedges: a case study from the central-southern Siena Basin (Northern Apennines, Italy)

The detection of detached nearshore wedges formed in response to relative sea-level drops is considered one of the hottest topics in sequence stratigraphic analysis due to their importance as reservoir analogues. In fact, they usually constitute sandy and porous bodies generally encased in impermeable clay, thus presenting a good potential as traps for fluids. This paper focuses on the sequence stratigraphic analysis of the Pliocene deposits cropping out in the central-southern sector of the Siena Basin (Tuscany, Italy), a post-collisional basin of the Northern Apennines. The exposed sedimentary succession was investigated through a detailed sedimentological and stratigraphic approach, integrated by biostratigraphic analyses, aimed at a better characterization of the infilling history of this sector of the basin. Specifically, this study revealed the occurrence of repeated facies shifts that allowed the identification of two depositional sequences. In detail, a thick sand-rich body far from the basin margins, and previously considered as a turbiditic lobe, has been reinterpreted as formed in a nearshore setting during a fall in relative sea level. This body is totally encased in offshore clay, and due to the lack of physical connection with the related HST deposits, it has to be considered as a detached forced-regressive wedge. The present work led to the recognition of some sedimentological and stratigraphic features typical of falling stage systems tract deposits (e.g. presence of intrabasinal recycled materials, sedimentological evidence of a pre-existing fluvial network subsequently eroded) that can provide useful clues for the identification of detached forced-regressive nearshore wedges in core studies and poorly exposed settings.

Latest evidence of Palaeoamasia (Mammalia, Embrithopoda) in Turkish Anatolia

ABSTRACT Geological investigation of the Boyabat Basin in north-central Anatolia, Turkey, yielded palaeoamasid (Embrithopoda, Mammalia) gnathodental fossil specimens from two localities dated to the Eocene—Oligocene transition. These specimens include an upper incisor (BOY-1) and maxillary fragment preserving M2–M3 (BOY-2), and are geologically the youngest embrithopods known from Eurasia. The maxillary specimen is taxonomically more useful and, among embrithopods preserving the same molars, is most similar to the late Paleocene—middle Eocene Turkish palaeoamasid Palaeoamasia kansui. However, the new specimens are cautiously classified as Palaeoamasia. sp. nov., based on distinctions between BOY-2 and P. kansui in M3 morphology. Features distinguishing the M3 in BOY-2 are intermediate between those of P. kansui and the more derived Afro-Arabian arsinoitheriid embrithopods Arsinoitherium spp. and Namatherium blackerowense, although the new Turkish specimens are stratigraphically too young to be ancestral to these middle Eocene—late Oligocene arsinoitheriids. Salient differences in dental and gnathic morphology between contemporaneous Eurasian and Afro-Arabian embrithopods indicate long, separate phylogenetic trajectories for these taxa, supporting the view that they are divisible at the family level into Palaeoamasidae and Arsinoitheriidae, respectively. Improved documentation of the lengthy paleogeographic separation of palaeoamasids and arsinoitheriids confirms that Afro-Arabia was sufficiently isolated from Eurasia during the Paleogene to limit embrithopods to rare sweepstakes dispersal between these landmasses.

Geological map of Pliocene-Pleistocene deposits of the Ambra and Ombrone valleys (Northern Siena Basin, Tuscany, Italy)

The study area is located across the Chianti Ridge (Tuscany, Italy), between the Upper Valdarno Basin and the Siena Basin. This area covers about 25 km2, forming a narrow belt oriented N–S and drained by the Ambra and Ombrone creeks, which flow northward and southward, respectively. Field mapping was carried out at 1:10,000 scale through an allostratigraphic-sedimentological approach. The study area deposits represent the infill of a SW-draining paleovalley, cut both in pre-Neogene bedrock and marine Pliocene deposits of the Siena Basin. The valley-fill succession consists of two main allounits (V1 and V2), which are conformably stacked in the northern sector of the study area, and unconformably offset in the southern sector. V1 comprises gravelly to sandy fluvial deposits, whereas V2 deposits show noticeable downvalley variability. V2 consists of poorly drained floodplain deposits in the northern sector of the paleovalley, whereas gravel and sand-bed river deposits fill its southern part. Alluvial-fan and palustrine deposits are also associated with V2 fluvial facies. A normal fault trending NW–SE is the main structural feature of the area. This fault cuts the V2 unit lowering the upstream reach and is thought to have promoted the marked facies changes observed in the fluvial deposits of unit V2.

SEDIMENTATION AND BASIN-FILL HISTORY OF THE PLIOCENE SUCCESSION EXPOSED IN THE NORTHERN SIENA-RADICOFANI BASIN (TUSCANY, ITALY): A SEQUENCE-STRATIGRAPHIC APPROACH

Basin-margin paralic deposits are sensitive indicators of relative sea-level changes and typically show complex stratigraphic architectures that only a facies-based sequence-stratigraphic approach, supported by detailed biostratigraphic data, can help unravel, thus providing constraints for the tectono-stratigraphic reconstructions of ancient basins. This paper presents a detailed facies analysis of Pliocene strata exposed in a marginal key-area of the northern Siena-Radicofani Basin (Tuscany, Italy), which is used as a ground for a new sequence-stratigraphic scheme of the studied area. The study reveals a more complex sedimentary history than that inferred from the recent geological maps produced as part of the Regional Cartographic Project (CARG), which are based on lithostratigraphic principles. Specifically, four sequences (S1 to S4, in upward stratigraphic order) have been recognised, each bounded by erosional unconformities and deposited within the Zanclean-early Gelasian time span. Each sequence typically comprises fluvial to open marine facies, with deposits of different sequences that show striking lithological similarities.The architecture and internal variability shown by the studied depositional sequences are typical of low-accommodation basin-margin settings, that shows: i) a poorly-developed to missing record of the falling-stage systems tract; ii) a lowstand system tract predominantly made of fluvio-deltaic deposits; iii) a highstand system tract with substantial thickness variation between different sequences due to erosional processes associated with the overlying unconformity; iv) a highly variable transgressive system tract, ranging from elementary to parasequential organization.