H. Martyn Pedley

Sedimentology and palaeoenvironment of the southeast Sicilian Tertiary platform carbonates

Abstract A thick succession of shelf and shelf-slope carbonates is developed in southeastern Sicily. Sedimentological aspects of the Oligocene to Tortonian sequence, including distribution and palaeoenvironmental factors, are considered. A new formation and four new members are proposed in the upper part of the succession. The region comprises a western half, typically exhibiting the principal deeper-water planktonic facies including marls, and an eastern area displaying typically inner-shelf carbonates and an attenuated succession. Coralline algal limestones and calcarenites are more typical of this latter region but there are also some volcanic intercalations. An attempt is made to relate the Hyblean succession to that of the Maltese Islands which lie at the southern end of this Tertiary carbonate platform.

The sedimentology and development of Terravecchia Formation carbonates (Upper Miocene) of North Central Sicily: Possible eustatic influence on facies development

Abstract On at least two occasions during the Late Tortonian, siliciclastic deposition ceased and carbonates accumulated at the northern margin of the “Caltanissetta Basin” in north-central Sicily. Bioclastics dominated in the shallower proximal areas, while coral reef facies developed where shallow-water sand and cobble substrates existed or where sea-floor highs has created local shallows. The genetic model proposed is that siliciclastic deposition was repeatedly terminated during the Late Tortonian by rises in sea level which transgressed fluviatile and deltaic environments. Carbonate deposition was widespread, but, later erosion and reworking removed much of the sediments except within abandoned delta distributary channels and minor sea-floor depressions. Reef developments, associated with channel mouths and the shelf slope break, have been preserved in situ, whereas channels in more proximal locations are filled with siliciclastic, bioclastic and intraclastic rubble from elsewhere on the delta top. The wide lateral extent of the truncated tops of the carbonate beds argues for eustatic, rather than tectonic controls of erosion and subsequent reworking. These events may be closely related in time to other recorded Late Tortonian eustatic events in the foreland zones, such as the Hyblean Plateau and the Maltese Islands further south. Collectively they give some support to a model involving multiple eustatic fluctuations associated with the onset of the Messinian desiccation of the Mediterranean.

Miocene syntectonic sedimentation along the western margins of the Hyblean-Malta platform: A guide to plate margin processes in the central Mediterranean

Abstract The central Mediterranean Hyblean-Maltese Platform is part of the Pelagian Block of the African Plate and occupies a critical site between two neotectonically active structural zones. To the south in the Sicily Channel lies the Malta Graben, part of the larger Pantelleria Rift System. To the north lies a broad, foreland verging thrust-belt generated by Africa-Europe plate convergence during the Tertiary. Miocene sedimentation patterns within the NW Hyblean syntectonic successions record the initiation of tectonic deformation associated with African intraplate tectonic processes. Initially, in the Early Miocene, local NE-SW orientated highs developed on the seafloor but later, in the Middle Miocene, these irregularities were levelled by prolonged marl sedimentation. These structures became active again in the Late Tortonian, in association with volcanicity. By Early Messinian times the sea floor irregularities had developed into NE-SW oriented grabens associated with evaporite deposition. A similar, time related syntectonic sedimentary history can be deduced for the Maltese region to the south. The common tectonic behaviour pattern of the two regions results from both being controlled by a common N-S to the NE-SW orientated transform fault zone, the full effects of which are documented for the first time. This dextral wrench system became active during the Late Miocene in order to accomodate continuing intraplate divergence (especially associated with the Malta Graben) and differential rates of African Foreland underthrusting.

Radiocarbon-dated Holocene pollen and ostracod sequences from barrage tufa- dammed fluvial systems in the White Peak, Derbyshire, UK

Sedimentary evidence, in the form of pollen, ostracod and radiocarbon-age data from three barrage tufa-dammed systems in the White Peak area of Derbyshire, is used to reconstruct the regions environmental history from 10000 to 4000BP. The data highlight the useful role that such fluvial systems can play in environmental reconstructions, through their ability to act as sumps for a wide range of proxy indicators, in a region where environmental change during the Holocene period has been relatively poorly documented. Pollen data suggest that the period of climatic amelioration following the last (Devensian) glaciation facilitated the spread of thermophilous woodland taxa on the White Peak, although there are some indications that the first immigrant trees were not established until a relatively late date. Forest clearance commencing during the mid-Holocene, and possibly as early as the late Mesolithic, was most probably to provide land for grazing, and was virtually complete by 4000BP. Ostracod data reveal fluctuations in discharge of the River Lathkill draining the White Peak, particularly lower flow rates after 7400 BP, which are difficult to explain simply in terms of climate change.

In vitro investigations of the impact of different temperature and flow velocity conditions on tufa microfabric

Abstract A series of experiments on freshwater carbonates (tufas) involving biofilm colonization in both fast-flow and slow-flow mesocosms was carried out in order to assess the changing nature of biofilm and associated precipitates under contrasting conditions. A thin biofilm developed over 14 weeks during the ‘summer’ experimental run contained a basal calcite layer overlain by small calcite crystals suspended within the Extracellular Polymeric Substances (EPS). The ‘autumn’ biofilm, however, showed the development of multi-laminated calcite precipitates within the EPS despite constant environmental conditions throughout the run. The experiments also showed that the largest volume of calcite precipitate developed in the fast-flow flumes regardless of temperature control. Development of an extensive calcite layer at the base of EPS in conditions of complete darkness within the sump was also observed. This study provides increased weight for the concepts: (1) that fresh- and saltwater stromatolites appear to be highly comparable multi-laminated systems with precipitation strongly influenced by both phototrophic and heterotrophic microbes; and (2) that microbial precipitation may be more common within aphotic (including cave, lake bottom and soil) systems than has previously been considered.

Did changes in late Last Glacial and early Holocene atmospheric CO2 concentrations control rates of tufa precipitation

Gases trapped within Arctic, Antarctic and Greenland ice-cores document a dramatic increase in atmospheric CO2 levels (by almost 100%) in the period between the last glacial maximum and the late Holocene. The authors note an apparent correlation between increases in levels of atmospheric CO2 during this period and an episode of mass deposition of freshwater carbonate tufas and travertines. As changes in atmospheric CO2 levels are likely to affect carbonate deposition (Tucker and Wright, 1990), we propose the hypothesis that a relationship exists between increasing atmospheric CO2 levels and tufa deposition.

Introduction to tufas and speleothems

Ambient temperature freshwater carbonates precipitate as surface deposits within karstic stream, lake and swamp environments (tufas) and in subterranean situations (speleothems), where they line vadose caves and fracture systems. Although physico-chemical mineral precipitation contributes significantly to both kinds of deposit, there is a clear spatial association between the development of tufa deposits and the occurrence of microbial biofilms. This fact, and the recent discovery that the occurrence of certain heterotrophic bacteria promote precipitation onto the surface of stalactites (Cacchio et al. 2004), strongly implicates a degree of microbial influence in the calcite precipitation process, regardless of the environmental context. To add to the inherent complexity of these systems, there is considerable interplay between biological and physical processes to consider. Water velocity and turbulence will strongly affect biofilm colonization and may damage the community, thereby affecting carbonate precipitation rates, in addition to regulating important kinetic limitations on precipitation via modifications of the calcium ion delivery rate. Exchange of CO2 gas at the air–water interface is an important conditioner for precipitation in vadose systems but will also occur within surface systems as a consequence of photosynthesis. It is only by considering karst hydrological systems holistically that these processes can be untangled. Tufas and speleothems share the same soilderived meteoric water supply, represent zones of deposition of calcium ions chemically eroded from the same geological sources and produce laminated deposits which are superficially similar. In passing from cave environments via resurgences (Fig. 1) into surface waterways, individual packages of water pass down an interconnected hydrological system at any point in which the conditions necessary for calcite precipitation may be achieved. Within the deposits that this precipitation creates, it is apparent that there is a progressive gradation from massive, laminated speleothems fabrics into stromatolitic, biofilm dominated tufas fabrics. In fact, speleothems and tufa represent two end members within a continuum of freshwater carbonate reflecting different balances of physicochemically and biologically controlled precipitation. On a regional scale the occurrences of tufas and speleothems are both controlled by water table fluctuations. Typically, tufa deposition is associated with predominantly high water tables and although tufas enjoy global distributions from the tropics to polar regions, they are most effective as bioconstructors where spring fed streams are not subjected to spate conditions. Similarly, tufa developments are severely impaired by fluctuating water tables associated with increasingly arid climatic cycles. Limitation on surface carbonate precipitation is consequently derived from the necessity for biofilm development combined with the equal necessity of adequate supply of dissolved calcium and carbon, which must be present at least in part as carbonate. The latter requirement of sufficient Ca(aq) and CO3 22 (aq) ionic activity demands that these ions are not lost from solution before resurgence, making it likely that tufas will develop best where caves are flooded, thereby minimizing the distribution of the subterranean vadose environment where speleothems develop most abundantly. Curiously, these elevated tables are frequently encouraged by the tufa growth itself as a consequence of the valley bottom ponding and back flooding caused by barrage development. Conversely, as lower water tables become established and the subterranean vadose environment becomes more important, speleothems will become established. Conceptually, the occurrence of abundant tufa or speleothem deposition simply reflects the position of a hydrochemical ‘knick-point’, which occurs when sufficient CO2 has been lost from solution for carbonate ions to become abundant, for example when the thermodynamic gradient promoting precipitation (Gibbs Free Energy) exceeds the barrier presented by the activation energy. This knick-point may occur either above or below ground depending on the height of the water table. As part of the same hydrochemical system, tufas and speleothems offer an inseparable duo when exploring the climatic archive, and will reflect the same processes within the catchment. Much palaeo-environmental information in tufas and speleothems can be extracted from geochemical time series created from these deposits. However, one of the greatest obstacles to collective use of these materials in ‘climate’ reconstruction is the

Geological map of the Corvillo and Mandre Basins (Caltanissetta Basin, Central Sicily): Explanatory notes

In Sicily, the imbrication of the Apenninic thrust belt above the African foreland determined the progressive southward migration of marine basins that were shortened from the Middle Miocene until the Pleistocene. The area studied in detail is located in the Caltanissetta Basin, which was in fact, during Neogene times, a series of thrust-top basins (e.g. Corvillo and Mandre). The relationships between Neogene sedimentation and the synchronous development of fold and thrust structures are recorded by unconformities, lap relationships of strata, hiatuses. The stratigraphy of the substratum is dominated by varicoloured clays and siliciclastic deposits of Cretaceous-Early Miocene age. The thrust-top basin stratigraphy is represented by Tortonian siliciclastic deposits and evaporitic deposits, which are the product of the Messinian lowstand. The Early Pliocene flooding was responsible of deposition of chalks and marls and a later regional uplift produced a regression, during Late Pliocene times, with deposition of marls and sandstones. Compressional deformation acted continuously from Middle Miocene until, at least, Middle Pliocene, amplifying the pre-existing structures and creating differing styles of deformation depending on the lithologies involved.