RICERCA ITALIANA     

Origin, timing and facies distribution of the Messinian Salt deposits in the basins of the central Mediterranean area (Sicily, Calabria and Tuscany) and their larger-scale implications for the Messinian salinity crisis

Università degli Studi di Parma

Abstract

More than thirty years after the formulation of the deep-desiccated basin model (Hsu et al., 1973), the origin, timing and depositional setting of the giant Messinian Salt bodies buried below the deep Western and Eastern Mediterranean basins is still unknown and many uncertainties persist concerning their genetic and stratigraphic relationships with peripheral basins successions. This led to the production of several working models of the Messinian salinity crisis which appear to be variants of the deep-desiccated basin model. The scientific community is waiting for the drilling of a borehole able to reach the base of deep evaporites and to give the appropriate answers to many unsolved problems. Nonetheless, such a project will still probably take a relatively long time to be carried out; moreover, halokinetic processes could have masked their primary depositional features of salt, thus possibly making it more difficult to achieve some of the expected results.
For this reason we believe that it would be important to deepen the knowledge of those salt deposits formed in different geodynamic contexts on the floor of the Mediterranean basins, uplifted after their deposition and now buried at shallow depths that allow their direct study in mine galleries and cores. This is the case of the salt units preserved in the basins of the Apenninic-Maghrebid thrust belt mainly in Sicily, Calabria and Tuscany. In particular, the central Sicilian basins have always been of great interest because of their key position within the Mediterranean basin, a deep sill athwart the Western and Eastern abyssal plains. The Sicilian succession has long been considered equivalent to the deep Mediterranean one; although this view is no longer accepted, it remains the best outcrop example to unravel the genesis of deep-water salt.
This project would contribute to the resolution of such problems by providing the reconstruction of a depositional model for Messinian salt deposits of Sicily, Calabria and Tuscany basins and the comparison with the other other deep and peripheral Mediterranean basins. A particular attention will be paid to their position within the regional-scale geologic evolution, a key element for subsequent evaluation of their significance at a Mediterranean-scale. Summarizing our aims, the project wishes to define: 1) the nature and age of the upper and lower boundaries of salt bodies; 2) their depositional settings; 3) the internal stratigraphy and depositional trends as a function of both tectonic and climatic cyclical forcing, and 4) the genetic and stratigraphical relationships with adjacent units. This objective will be carried out through a regional-scale, multidisciplinary study of both outcropping and buried successions in selected areas, based on the quality and accessibility of both outcrop and subsurface data.
One of the most innovative features of this project will be the drilling of a continuously cored borehole, up to 200 m deep, in the salt mine of Realmente (Agrigento, Sicily); this location is chosen because of the primary, unaltered nature of salt; the borehole should drill the lower part and the base of the salt body allowing the study of its primary depositional features, and reach the underlying deposits that would allow to carry out chronostratigraphic and palaeoenvironmental determinations. The project, beside the main objective, would address other relevant topics strictly related to it and object of specific tasks of the research programme such as:
- the regional-scale assessment of structural-stratigraphic setting of Messinian basins in the study areas;
- facies characterization and distribution of limestones (primary and diagenetic) and gypsum (primary and/or clastic resedimented) deposits often found in salt basins;
- origin and periodicity of the high-frequency cyclicity of salt deposits;
- role of salt deposits in the post-Messinian structural evolution of the orogenic wedge. <<<

Principal Investigator

Marco Roveri Università degli Studi di PARMA

Research Objectives

This project is designed to improve the general knowledge concerning one specific and fundamental problem of the Messinian salinity crisis: the chronology and modalities of deposition of the giant salt bodies occurring in several basins from the different geotectonic contexts within the Mediterranean area. These deposits, only known through seismic, borehole and mine gallery observations, are still poorly understood in terms of 1) depositional processes and settings (marine/non marine, deep-water/shallow-water), 2) chronology (onset of deposition, total time elapsed, relationships with cyclical forcing), 3) genetic and stratigraphic relationships with adjacent Messinian evaporitic (primary and/or clastic) and terrigenous units.
This question remains a strong obstacle to the full comprehension of the complex of palaeoenvironmental changes related to the Messinian salinity crisis, especially to the efforts for palaeohydrologic and palaeoclimatic modelling.
As a matter of fact, the stratigraphic uncertainties weaken all the attempts to reconstruct the Messinian palaeogeographic setting of the Mediterranean, and, as a consequence, the formulation of reliable and accepted evolutionary models of the MSC.
The project aims to contribute to such problems through the study of the salt bodies of some of the Messinian basins of Sicily, Calabria and Tuscany. For many of such basins, the integration of core data, direct mine gallery observations, seismic and surface geology makes it possible the attempt to define with accuracy the stratigraphic position of salt unit and their meaning within the regional geological evolution. Data and interpretations will be compared with the other buried and outcropping Mediterranean Messinian successions (Spain, Crete, Northern Apennines, deep Mediterranean basins), to evaluate their implications at a wider scale about the general evolutionary models of the salinity crisis.
This proposal has a great importance in order to provide a more comprehensive picture of the Messinian salinity crisis through the comparison and integration with future data of the ultradeep drilling project of deep Mediterranean basins. This project, currently being submitted by an international research group within the IODP frame, will shed new light on some of the fundamental questions still waiting an answers since many years. The data will be of fundamental importance and will significantly improve the researches in several fields, particularly in the palaeoceanographic modelling. We believe that their importance will be further strenghtened by the possibility of integration with similar data from other Mediterranean areas. According to this view, the study proposed in this research project concerns key areas to establish the relationships between the MSC evolution in western and eastern Mediterranean basins.
The principal objectives of the project (task 1) will be the definition of the depositional processes and settings and the stratigraphy of Messinian salt-bearing units (halite and potash salts) within the orogenic basins of Sicily, Calabria and Tuscany. In particular, the fundamental points to achieve the principal objective concern the definition of: 1) nature and age of lower and upper boundaries of salt bodies; 2) depositional settings and processes of salt deposits; 3) detailed stratigraphy and depositional trends of salt bodies and their relationships with regional-scale tectonics and cyclical climatic forcing; 4) genetic and stratigraphic relationships with adjacent Messinian units. This objective will be achieved through facies and stratigraphic analysis of outcropping successions integrated with subsurface data, both already available for study (salt mines, boreholes, seismic) and to be acquired with this project (a new continuously cored borehole, 200 m depth, in the Realmonte mine). To do that, all the scattered data and information from salt and sulphur mines and/or hydrosolution plants of the study areas will be organized in a web database.
The achievement of the main aim of this project has tight feed-back relationships with several topics that have been indicated as the objectives of specific sub-projects (tasks); they will be carried out through an interdisciplinary approach by specific working groups with scientific personnel from all the Research Units:
task 2 - regional-scale structural-stratigraphic setting of Messinian basins during the pre-, syn- and post-evaporitic stages (Late Tortonian-Early Pliocene);
task 3 - facies characterization, time and areal distribution of the large family of primary and diagenetic limestone rocks usually grouped under the generic term "Calcare di base" and of primary/resedimented gypsum deposits;
task 4 - palaeoclimatic and palaeoenvironmental conditions during pre- and evaporitic stages;
task 5 - origin and periodicities of the high and ultra-high frequency cyclicity of salt deposits;
task 6 - role played by salt deposits during the post-Messinian tectonic evolution. <<<

Timescale

24 months

National and international background

More than thirty years after the formulation of a unifying theory for the Messinian salinity crisis (MSC), the so called "deep-basin desiccation model" (Hsu et al., 1973), several aspects are still not fully understood (Hardie and Lowenstein, 2004).
A crucial point is the nature and age of the giant salt deposits buried under the deepest Mediterranean basins, imaged by seismic data but never drilled by boreholes, except for their uppermost part (Hsu et al., 1973).
These deposits have been classically subdivided into three seismostratigraphic units that from the bottom are: Lower Evaporites, Messinian Salt and Upper Evaporites; only the latter have been reached by DSDP and ODP boreholes. The deep-desiccation theory considers that both the Lower Evaporites and Messinian Salt (total thickness &gt; 3000 m) formed through direct precipitation from very shallow waters during the desiccation stage of the Mediterranean basin. This requires a sea-level fall in excess of 1500 metres, leading to the widespread development along the continental margins of a subaerial erosional surface (MES) with deeply incised canyons in front of the main river systems (Ryan &amp; Cita, 1978; Clauzon, 1982).
Due to the lack of direct observation data from the lower evaporitic complex and the underlying deposits, their deep vs shallow-water nature as well as the chronology of Messinian events are object of speculations.
A very high degree of attention and interest is paid by the international scientific community to this problem, so that an ambitious project for an ultradeep continuously cored borehole (up to 7 km deep) in the Gulf of Lions has been proposed.

One of the main problems is the correlation of the deep basinal succession with the outcropping ones, many of those also accumulated in shallow peripheral basins. In Spain (Betic basins), Italy (Apennine foredeep, Sicily) and Eastern Mediterranean (Crete, Cyprus), these successions are characterized by a lower evaporitic unit made up of cyclically arranged selenitic gypsum (up to 16 cycles recording periodic, precession-related, variations of marine waters concentration) and cut at the top by a subaerial erosional surface (MES, intra-Messinian unconformity) overlain by hyposaline deposits (Lagomare facies) which locally contains thin evaporitic horizons precipitated from waters with a strong meteoric apport (Upper Evaporites).
Below the Lower Evaporites, deep-water euxinic deposits are found, which reflect the progressive restriction of mairne water exchange with the Atlantic Ocean and, as a consequence, of the deep water circulation of the entire Mediterranean basin. These deposits are characterized by a well-developed lithologic cyclicity that allowed the reconstruction of a very detailed, astronomically calibrated chronology of the pre-evaporitc stage (Hilgen &amp; Krijgsman, 1999; Negri &amp; Villa, 2000; Sierro et al., 2001) and the onset of evaporitic stage is placed at 5.96 Ma (Krijgsman et al., 1999). Based on cyclostratigraphic considerations the time span encompassed by Lower Evaporites is evaluated in about 350 ka (Vai, 1997); as a consequence, a minimum age of 5.6 Ma for the MES has been calculated. A further time constraint is given by the sudden reactivation of the connection with the Atlantic Ocean at the base of the Pliocene, placed at 5.33 Ma (Van Couvering et al., 2000; Iaccarino et al., 1999). In order to fully understand the MSC, the relationships between the different Messinian units should be defined in detail within this time framework.

One key for correlating deep and peripheral basin successions is to trace basinward the MES to its correlative conformity. On this base Clauzon et al. (1996) refined the desiccation model by proposing a two step development of the MSC (Fig. 1): the first one with primary evaporite precipitation only in peripheral basins; the second one, following a sea-level fall of more than 1500 m, characterized by shallow-water evaporite deposition (mainly halite) in the deepest Mediterranean depressions. In this model the MES is traced at the base of the deep Lower evaporites or, at least, of the Messinian Salt (Lofi et al., 2005).
The threefold stratigraphic subdivision of Messinian evaporites can be recognized elsewhere in Mediterranean basins. However, where deep-water Messinian successions have been subsequently uplifted and can now be observed in outcrop a quite different picture appears from what the general model predicts. In the case of the Apennine foredeep, the MES can be traced into a correlative conformity at the base of a clastic evaporite complex emplaced through gravity flows in fully subaqueous and relatively deep depositonal settings (Roveri et al., 2001, 2003, 2004; Manzi et al., 2005). This hypothesis, which put into discussion at a larger scale the true nature of basinal evaporites, has been actually recalled by Lofi et al. (2005) for the origin of the Lower Evaporites of the Gulf of Lions; based on seismic data it is suggested that, during the initial sea-level fall, fully subaqueous processes could have transferred to the basin huge volumes of mixed, siliciclastic and evaporitic, sediments to the basin.
However, a non-clastic, shallow-water precipitated nature of most deep Lower Evaporites is the mostly accepted hypothesis within the scientific community; these deposits are generally considered to postdate marginal evaporites. However, a post-desiccation age for the latter has been also proposed, thus implying the permanence of marine connections with the Atlantic throughout the MSC (Braga et al., in press; Riding et al., 1998; Fig. 1). Only Krijgsman et al. (1999) proposed the synchronous character of the onset of all the Mediterranean evaporites, thus implying the possibly deep-water nature of basinal successions.


Fig. 1 - The different MSC scenarios (from Rouchy &amp; Caruso, in press)

One of the more crucial points that is generally overlooked is the role of tectonics in controlling the overall Messinian stratigraphy and the event related to the MES. The peri-Mediterranean area can be considered a single deformation complex related to the convergent boundary between the African and Eurasian plates; tectonostratigraphic studies document the occurrence of discrete phases of accelerated deformational processes in different geodynamic settings; the MSC occurs exactly in one of them, starting in the Late Tortonian (Meulenkamp &amp; Sissingh, 2003).
The tectonic activity is considered responsible for the progressive closure and subsequent reopening of the Atlantic gateways and, as a consequence, of the MSC. The earlier hypothesis of a glacio-eustatic trigger for the MSC, related to the two sea-level falls associated to isotopic stages (Kastens, 1992; Shackleton et al., 1995; Clauzon et al., 1996) TG20 e TG22 has been recently ruled out by the astronomic calibration of the age of the base of the Lower evaporites, placed at 5.96 Ma, and of the two glacial peaks, placed respectively at 5.70 and 5.75 Ma (Krijgsman et al., 1999, 2004).
A more striking tectonic signature on Messinian stratigraphy within the MSC is on the contrary ruled out by most authors, unless at a local scale.

Notwithstanding a growing amount of evidence for angular discordance associated with the MES in the Apennines, Sicily, Spain and, more recently, even in the Gulf of Lions (Lofi et al., 2004), its development is usually related to the exceptional sea-level fall associated with the desiccation of deep Mediterranean basins. Besides the Apennine foredeep, a few evolutionary models for the MSC do take into account the regional geologic framework for the development of evaporitic successions; one of these instances concerns Sicilian basins.
According to the sequence-stratigraphic model by Butler et al. (1995), evaporitic successions of Sicily predate those of the deep Mediterranean and formed diachronously in a series of small basins developed at increasing depths above the actively deforming the Maghrebian orogenic wedge during the overall eustatic fall culminated with the desiccation of deepest basins.

Recent interdisciplinary studies suggest that the MSC was not triggered by climatic changes and that no dramatic palaeoclimatic changes occurred during its different stages. These studies documented that, despite what was earlier thought, the Mediterranean area was not characterized during the Messinian by a homogenous warm-dry climate (Suc &amp; Bessais 1990; Bertini et al. 1998), but by a much more complex scenario also with evidence of both warm-wet and cool-dry conditions.
The different temperature and humidity values documented by palynological studies allow suggest the development of latitudinal climate gradients (Bertini, 1994; Fauquette et al. in press) that are able to explain the apparently contradictory data distribution. However, it should be remembered that these reconstructions strongly depend on the accurate chronostratigraphic calibration of the successions from which the data derive; in this sense, many uncertainties persist, as far as, for example, the indications coming from salt deposits are concerned.

The origin of the giant salt bodies and their stratigraphic relationships with the other evaporitic deposits are the subject of palaeohydrologic modelling which is necessarily based on approximate reconstructions of the palaeogeographic and palaeoclimatic settings of the peri-Mediterranean area. The most recent models suggest a diachronous precipitation of salt with respect to gypsum, and also of the salt precipitated in Western and Eastern basins (Blanc, 2000, 2002).

As a consequence, the more accurate definiton of the palaeogeographic setting of the Mediterranean area and its modifications induced by the regional tectonic evolution are fundamental points. Waiting for the ultradeep borehole in the Western Mediterranean, a contribute to the solution of these problems might derive from a better knowledge of the salt deposits characterizing the Messinian succession formed in different geotectonic contexts: the orogenic basins of Sicily (a key area that during the Messinian was a sill between the two deepest Mediterranean basins), Tuscany and Calabria.

Notwithstanding the good outcrop availability and the large amount of subsurface date, a detailed stratigraphic synthesis of salt deposits in these basins does not exist.
In Tuscany, Messinian deposits occur in extensional basins characterized by a articulated topography controlling the local facies distribution. Evaporite deposits only occur in western basins and salt is present in the subsurface of the Volterra basin (Testa &amp; Lugli, 2000); eastern basin are characterized by terrigenous successions (Velona basin Ghetti et al, 2002).
In Calabria, huge salt bodies occur in the subsurface of the Crotone Basin (Roda, 1964; Roveri et al., 1992), formed in a compressive regime within the broader context of the Calabrian Arc subduction complex. Their bases never crop out, while at the top interbedded gypsarenites, marls and sandstones with hyposaline faunal assemblages occur, similar to the Upper Evaporites of Sicily. The stratigraphic relationships between the evaporitic deposits of the lower cycles are not fully understood.
The evaporitic succession of Sicily is commonly referred to as an equivalent of the Mediterranean ones (found on the deep-basin floor), uplifted in post-Messinian times following the evolution of the Maghrebian thrust belt.
Salt bodies, intensely exploited by mine industry since the 50's, are here distributed in a series of subbasins aligned in a broad belt within the Caltanissetta basin and attain maximum thicknesses of about 1000 metres. They are in primary condition on only mildly deformed.
These salt deposits are usually considered part of the lower evaporitic cycle but their true stratigraphic position is not clear. According to some Authors (Decima &amp; Wezel, 1971; Decima, 1976), they lie above the Cattolica Gypsum (massive selenites). Garcia-Veigas et al. (1995) suggested instead lateral transitions between salt, selenitic gypsum and limestones (the sulphur-bearing Calcare) that would imply synchronous deposition of different facies in basinal, marginal and sills settings. Subsurface and outcrop data point indeed to complex genetic and stratigraphic relationships between salt bodies, limestones (often brecciated and sulphur-rich) and gypsum, both clastic and selenitic (Decima et al., 1988; Pedley &amp; Grasso, 1993).
More recent studies (Butler et al., 1995) suggest a strong tectonic control on areal distribution of such deposits; within the larger-scale context of basins developed above an active orogenic wedge, the Calcare di Base limestones would be deposited on top of structural highs, while interbedded limestones and gypsum would be found on their flanks and salt in the deepest depressions.
No clear information exist about the lower boundary of the Lower Evaporites, as well as the nature and age of underlying units, at least as far as the deepest basinal areas are concerned. Borehole data suggest the presence at the base of salt units of a thin horizon of anhydrite and shale breccia, lying above mudstone deposits generically attributed to the Upper Tortonian-Lower Messinian. In outcrop successions the evaporites usually start with the Calcare di Base lying indifferently above basinal diatomites (Tripoli Fm.) or deltaic deposits (Terravecchia Fm.) (Butler &amp; Grasso, 1993).
The upper boundary of the Lower Evaporites is given by the intra-Messinian unconformity here clearly related to an important deformational pulse of the Maghrebian chain. This surface is overlain by Upper Evaporites deposits, consisting of the Pasquasia Gypsum, made up of interbedded marls and selenitic gypsum, and the Arenazzolo; the latter unit is characterized by the typical hyposaline faunal assemblages with paraTethyan affinity occurring in the latest phase of the MSC (Lagomare; Bonaduce e Sgarrella, 1999). Above the Arenazzolo there is a sharp transition to deep marine deposits (Trubi Fm.) which represents the base of the Pliocene (Zanclean GSSP, Van Couvering et al., 2000; Iaccarino et al., 1999).
The detailed stratigraphy as well as the depositional features of salt deposits are only locally recognized; according to Decima &amp; Wezel (1971) and Decima (1976) a regional-scale subdivision of salt deposits into four units characterized by different composition and halite content (among them one containing at least 6 kainite levels) with possible separation by shale and/or breccia layers, thus implying a synchronous deposition in the different sub-basins.
In the Realmonte mine two units deposited at different water depths have been recognized; the lower one formed in relatively deep waters and shows a shallowing-upward trend culminating in the development of an erosional surface with clear evidences of subaerial exposure (desiccation cracks; Lugli et al., 1999; Fig. 2); the upper one was deposited in very shallow waters. A similar surface has been recently discovered also in the Racalmuto mine (Lugli, 2006, pers. comm.).


Fig. 2 - Erosional surface with desiccation cracks in the Realmonte mine.

The salt succession is characterized by a pervasive small-scale lithologic cyclicity given by the rhythmic alternation of 10-20 cm thick halite-shale couplets. An annual or interannual periodicity for these cycles has been proposed by Bertini et al. (1998) and this led to the suggestion that Sicilian salt formed in a very short time span (a few thousand years). In the Racalmuto mine a horizon made up of interbedded halite and thick shale layers provided rich and diversified assemblages with planktic and benthic Foraminifera and calcareous nannoplankton, indicating the influx of normal salinity waters into the evaporating basins. However, due to the lack of further investigations, no information exists about the true meaning and regional-scale extension of such horizon. <<<