The Petrology and Palaeoenvironment of the Sortino Group (Miocene) of SE Sicily: Evidence for Periodic Emergence

J. geol. Soc. London, Vol. 140, 1983, pp. 335-350, 10 figs. Printed in Northern Ireland.

The petrology and palaeoenvironment of the Sortino Group (Miocene) of SE Sicily: evidence for periodic emergence

H. M. Pedley

SUMMARY: The Sortino Group of carbonates consists of three formations which developed ona shallowmarine shelf during Tortonian andlower Messinian times. The twolower formations contain three bioherms and are intercalated with thick volcaniclastic sequences. The youngestformation is dominated by shoalwater, lagoonal and peri-tidal carbonates. Early submarine lithification is well-developed within the bioherms and consists of construction by bothalgae and invertebrates, the development of peloidalmicrites and isopachous fringe growth. Superimposed upon these are a series of both meteoric phreatic and vadose textures related to episodic emergence of the bioherms. These features include bladed cements showing cathodoluminescence,pendent cements, brecciation, alveolar texture andpalaeosols. The Sicilian strata may be directly correlated with the Maltese Islands sequence, and collectively exhibit the effects of fluctuating- Mediterranean sea levels at an early stage in the late Miocene salinity crisis.

TheSortino Group occupies much of theeastern rated by Grasso et al. (1982), who described the upper Hybleanregion of SE Sicily (Fig. 1) andoutcrops twoformations and ascribed them to the Sortino extendfrom Ognina (WA 2392)in the S tothe Group. This is a major unit whichalso includes the Agnone area (WB 0929) to the N. Further exposures Monti Climiti Formation of Pedley (1981). occur on the Monte Tauro (WB 2322) and Maddalena (WA 2995) peninsulas. Techniques The strata of interest comprise the bioherm at the top of the Monte Climiti Formation (basal bioherm of Extensive field sampling of all carbonate strata was carried Grasso et al. 1982); the thick volcaniclastic succession out. Large blocks from the patch reefs of the bioherm levels of the overlying Carlentini Formation, especially the wereslabbed and polished in orderto revealinterrela- two bioherm levels intercalated within the volcaniclas- tionshipsbetween frameworks and space fillings. Scanning tic suite(intermediate and upper bioherm levels of electronmicroscopy was used on reef samples to resolve problems relating to the nature of micrite morphologies. All Grasso et al. 1982); and the oolitic shoal, lagoonal and thin sections were stained for carbonate recognition using the peri-tidalcarbonates of theyoungest strata of the method of Dickson(1965). Recourse was alsomade to group, the Monte Carrubba Formation. ultra-thinsections (10-15 pm)since these reveal greater TheSortino Group is thickestin the vicinity of details of peloid morphology. Cathodoluminescence micros- Sortino(WB 025125), Carlentini (WB 015255), and copy was carried out on many samples containing cements. Melilli (WBllllSO), all close to well exposedlate The luminoscope at the Department of Geology, Cambridge Miocene vents from which the Carlentini volcaniclas- University was used, the operating parameters for which are tics were erupted (Fig. 1). These rubbly sediments and outlined in Fairchild (1980). local lavas weather rapidly and lead in consequence to poorexposure. Only the intercalated bioherms are Stratigraphy traceable intermittently across country (see Grasso et al. 1982, fig. 2). The volcaniclasticsuite thins rapidly Carlentini Formation awayfrom thevents and the three bioherm levels unite in the eastern peninsula to form a single thick, Carbonates of thisformation are restricted to two reefoidalunit (Fig. 2) whichweathers into vertical bioherm-richlevels sandwiched between volcaniclas- cliffs (e.g. Murro di Porco. WA 299955). The Monte tics (seeFig. 3,Monte Carrubba section). The CarrubaFormation is alsowell-exposed incoastal bioherm levels, together with the bioherm at the base sectionsand can readily be traced inland. Post- (Grasso et a2. 1982; Pedley 1981)belonging tothe Pliocenenormal faulting has cut the region into a underlying Siracusa Limestone Member, Monti Climiti series of NE-SW trending slices, although the Sortino Formation, may be considered collectively as they all Group is still sub-horizontal. reflect similar depositional environments. Fuchs(1874) first drewattention to these strata Thesecarbonates were all depositedon ashallow which hefound to contain restricted 'Sarmatian'-like submarineplatform which extended as a continuous faunas.Grasso et al. (1979)produced a general rise southwards as far as the present Maltese Islands. lithological description of the succession, later elabo- Fig. 4 illustrates a reconstructed E-W cross-section of 0016-764918310500-0335$02.00 0 1983 The Geological Society

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FIG. 1. Regional distribution of the Tortonian and Messinian strata of the N Hyblean region.

theplatform for late Tortonian times which is an to 2-3 m (Maddalena area, WA 2996). Progressively, amplification of fig. 4 inGrasso et al. (1982). The in an easterly direction, the change from mudstones to deeper platform areas to theW (Monte Santa Venere) packstones suggests increasing turbulence. Rhodolitic weresites of micritemudstone build-up associated coralline algae became the dominant growth form in with ‘crustose algal pavement’ (cf. Bosence & Pedley thoseareas and were associated with Clypeaster, 1979). Eastwards water depths decreased from c. 50 m foraminiferasuch as miliolids and Borelis melo, and

WES T WEST €AS T MALTESE ISLANDS

FIG.2. Stratigraphy and probable correlation of the Maltese and Sicilian strata. Volcaniclastic units are stippled. Omission surfaces are indicated by a crenulated line.

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MONTE FARO Ver tied scde CARRUEBA SANTACROCE [:metres rm/ Cord Ipor,tes und Tarbellustreul U Grey and creurn m/crite

B Grey rnar/y clay Volcon/clasfics ifuffs,lithic bombs . ... and luvus locull~i m Terrurossa M Cross-strotihcot/on ldorn/nunf/y N.€. facing 1 m Pulrnono te gus tropods Pec tinids

FIG. 3. Local correlation within the Carlentini and Monte Carrubba formations of the N Hyblean region

pectinid bivalves (Fig. SF). Small isolated colonies of slope is associated with the amalgamated bioherms at Porites and Tarbellastraea in the W became commoner FaroSanta Croce. Similar-facing, well-bedded, rho- eastwards and formed extensive patch-reefs. There, all dolitic algal packstones form a thick sequence at Scala three bioherms were united into a single thicker reef Greca(WB 249068) (Siracusaarea) and are also unit(Murro di Porco, WA 299955 andFaro Santa believed tobe eastward-facing fore-reef apron de- Croce, Fig. 3). These patch reefs became sites for the posits.Cheilostome bryozoa are common in these entrapment of micritic sediments in the extreme E and deeperwater fore-reef deposits. The fieldevidence wereassociated with adiverse fauna of nestling, suggests that deeper water lay immediately E of the encrusting and burrowing molluscs (Fig. 5C). Encrust- presentcoastal outcrops, probably being related to ing coralline algae and Halimeda were typical of these penecontemporaneous fault movement associated with reefs.A poorly developed eastwards-facing fore-reef the Malta Scarp (see Fig. 10).

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A. MONTE CARRUBBA FORMATION WE5 T EAST

.. . .-Scrobicu/ariidae . . . . - .. . . - -.. - - .. . - - - - - OOLlTE NO OTHER FAUNA SHOAL MlCRlTE MUDSTONES WACKESTONES

MlCRlTE BOUNDSTONE

PACKSTONES OF FORE REEF SCOPE PACKSTONES L PATCH REEFS L CORAL PATCHES MICRITE MUDSTONES

FIG. 4. A, Fauna1 associations and distributions related to the lithologies of the Monte Carrubba Formation. B, Coral thicket and patch-reef development related to lithologies of the Carlentini Formation.

Volcaniclasticsediments predominate within the Perecontate vent (WB 105171) immediately N of CarlentiniFormation, and are typicallytuffs, ashesMelilli, Bouma cyclesincluding slumping occur in one and agglomerates, frequently with lithic bombs but are of the proximal cone deposits. usually massive to parallel-bedded. Close to the vents crudely-gradedand-chaotic debris Rows are associated Monte Formation withinterpenetrative channels. These show a poor radial arrahgement around the vents and suggest both This is theyoungest formation of theSortino subaqueousand rare subaerial sediment movement Group, and is often truncated abruptly at its top by downthe slopes of theextrusive cones. At the Pliocenelavas. It may be subdivided into a south-

FIG.S. A, Cross-stratified oolitic grainstones, typical of eastern outcrops of the Monte Carrubba Formation, Faro Santa Croce; scale bar 1 m. B, Thinclay palaeosol containing clasts (2) derivedfrom breakdown of theunderlying marine carbonate. Unfossiliferous thinly-bedded micrites above contain dessication polygons (1). Monte Carrubba Formation, Capo Santa Croce (WB 225218); scale bar 1 m. C, Typical patch-reef sediments from eastern outcrops. The inter-framework micrites are rich in molluscs, some still in life position. Lower bioherm, Siracusa Limestone Member, Monti Climiti (WB 120090); lens cap 500mm diam. D, Scanning electron photomicrograph of the compact crust framework to the bioconstructed areas of patch reef; note the ultra fine-grained structureless micrite areas and the fenestral areas which are now filled by space filling sparite. Amalgamated bioherm. Carlentini Formation. Murro di Porco; X3SO. E, Brecciated upper surface to the lower bioherm, Siracusa Limestone Member. The clasts remain unrotated but with some interstitial packstone. Road cutting at WB 153160 on Siracusa road N of Priolo; lens cap 500 mm diam. F, Photomicrograph of off-reef carbonates typical of the Siracusa Limestone member and Carlentini Formation. A large foraminifer, Borelis melo, top centre, is closely surrounded by echinoid, coralline algal and pectinid allochems. Miliolid foraminifera are also present. Lower bioherm, Siracusa Limestone Formation, Sorgenti Paradiso; scale bar 500 pm. G, Geopetal cavity filling of peloidalmicrite. Note the grain support aspect to the peloidal filling andtheir entombment in later space filling sparite. Amalgamated bioherm, Carlentini Formation, Murro di Porco; scale bar S00 pm. H, Well-developed pendent cement (1) extending into a cavity now occupied by typical rhizocretion fabric showing alveolar texture. Monte Carrubba Formation, Faro Santa Croce; scale bar 500 pm.

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/3/335/4888371/gsjgs.140.3.0335.pdf by guest on 29 September 2021 340 H. M. Pedley western facies, typically thin, oolitic, parallel stratified Euxinicardium sp.,Abra-like bivalves andhydrobid beds which become cross-stratified only in the eastern gastropods occurred here. peninsula(Fig. SA, Fig. 3) and a northernperi-tidal In theextreme N of the regionthere is a and emergent facies. The latter iswell developedin thinly-beddedmicrite with bedded chalcedony and the Agnone and Cozzo Telegrafo (WB 118272) areas. opalassociation. Broad, shallow channels are de- Fig. 6 illustrates the distributionof sediment types and velopedin the strata. Thin dolomicrite crusts com- biota in the formation. The marine platform at that monly occur in the upper part of the sequence, many time was much shallower than during the deposition of beingdisrupted, with resulting allochthonous frag- theCarlentini Formation. Fully marine faunas are mentsdistributed as flake lags. Biota is restrictedto only developed in the Monte Santa Venere area to the rare verticalstacked stromatolite colonies and scat- W wherethey occur in structureless cream wacke- tered swamp turtle bones. stones. A broadbelt of ooliticgrainstone occupies Collectively thestrata represent peri-tidal and much of the area from Sorgenti Paradiso (WB971174) sabkha deposition marginal to an island of low relief. and Monte Carrubba (WB 020185) in the W, to Faro Fig. 3 illustratesthegeneral inter- and intra- Santa Croce in the E. Clearly this oolitic shoal zone formational aspects of the Sortino Group, and Fig. 4 effectivelybarred access to a northern lagoonal area the distributions of the major fauna1 elements. for all fully marine faunas except occasional incursions of freeswimming pectinids. The shallow lagoonal Carbonate diagenesis zone, behind the tidal oolite shoal barrier, was an area of wide salinity fluctuation and possible temperature Collectively the upper Tortonian to lower Messinian variation.Very IOW diversityfaunas dominated by carbonates of the Sortino Group (Grass0 et al. 1982)

Mollusc debris

Final void fillibioclasts and coated grains)

Void fill (second epkodeJ malnlymlcrlte

Branching coralline algae am' micrite reef matrtx Early void fill (mainly micrite into borings prior to amgonite dissolution Porites

Vermetus

Crustose coralline algae and compact crust

FIG. 6. Sketch of a cut and polished slab of reef rock from Capo Murro di Porco to show the constructional fabric and cavity filling network.

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FIG. 7. Hypotheticalreconstruction to account for the development of rubblehorizons. A, Thickpyroclastic accumulationcausing local emergence. B, Development of frettedpavement and protosoil under subaerial conditions. C, Submergence and patch reef development in a carbonate dominated environment. D, Eustatic fall of sea level causes emergence and rubble profile development under subaerial conditions.

exhibit a wide range of diagenetic features. It can be Biological binding established that at several correlateable horizons (Fig. 3) rapid episodes of submarine lithification occurred, Thepatch reefs of theCarlentini Formation and followedshortly after by deposition of phreatic Monti Climiti Formation alone exhibit features in this andvadose meteoric cement fabrics. Many of these category,which become progressively better de- fabricsand associated structures have been recorded velopedeastwards towards the presumed main reef fromRecent and ancient carbonates but rarely are front.These patch-reefs grew in well-oxygenated, such complete associations found. The Sortino Group turbulent and shallow sea water and were the sites of providesan example illustrating criteria of value in rapid fauna1 and floral productivity. establishingepisodes of emergence or submergence Coralswere subordinate framebuilders in these within lithified carbonate strata. patch-reefs,the principal construction coming from algae.The highest turbulence zones of the well illuminated, outer surfaces of the reefs were the sites of prolific crustose algal growth, represented by both Early submarine lithification Lithophyllum and Mesophyllum (Fig. 8A, 4). These Earlysubmarine lithification is widespreadin Re- bound together bioclasts and provided a hard surface centand Holocene carbonate strata, in the form of forfurther encrustation by foraminiferaand other peloidalmicrite (e.g. Alexandersson 1969; Macintyre algae.Second in importance in this location and 1977);isopachous fringe cements (e.g. James et al. constantlyin competition with thecorallines was an 1976),and biological binding (Garrett et al. 1971). indeterminate floral contributor, probably a sub-tidal These features arc poorly known from older strata but stromatolite (Fig. 8A, 2). Its constructions consist of are described from the Triassic (Zankle 1971), Jurassic massive to laminated 0.5-2mm thick sheets of dense (Marshal1 & Ashton1980), Eocene (Purser 1969), micritewith fenestral porosity (Fig. 5D). Theyare Miocene(Pedley 1979) and Pleistocene (Shinn 1969; embedded into the irregular reef surfaces but clearly Schroeder 1973). The Sortino Group of Sicily exhibits grew in free, sunlit locations on the outer-reef face. manyfeatures of submarinelithification typical of This is the ‘compact crust’ seen in Maltese patch-reefs modern day reef environments. (Pedley1979) and is probablyidentical tothe

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microstromatolite-like mats of Land & Goreau (1970) to be surrounded by spar but, towards the base of the from Recent Jamaican reefs. cavity,grade down into structureless micrite. The Almostequally important in some areas of the peloids are opaque, the micrite from which they are Sicilian reefs are homatrematinae, probably Humotre- constructedbeing most comparable to the micrite ma, which arefrequently sandwiched between algal isopachousfringe cements lining the cavities. These layers(Fig. 8A, 2).This encrusting foraminifer features suggest that they are not marine invertebrate becamethe dominant encruster in gloomy cavity faecal pellets (Moore 1977), nor are theylikely to be of locations.Bryozoans are rare in the Sicilian patch- terrestrialinvertebrate origin (Wright 1980)since reefs,possibly due tothe destructively turbulent marine bioclasts are frequently associated with them. conditions.Serpulid worms, however, and vermetid Furthermore,the relatively sharp marginsbetween gastropodcolonies are locally abundantand may surrounding spar and peloid margins (Fig. 8C, contact provide a secondary framework. between 2 and 3) argues against them being neomorphic relict textures (James 1972). Structureless to peloidal biomicrites The size and distribution of the peloids suggests that they arethe low-Mgcalcite successors tothe high- Within the larger patch-reefs in the eastern area are Mgcalcite peloids (cf. Recent reefs e.g. Land & large areas of micrite. Frequently they are unusually Moore 1980; Marshall &iDavies 1981). MacIntyre rich in mollusc shells, many still articulated and a few (1977, 1978) suggested that many reef peloids are the stillin life position(Fig. 5C). Thesesediments result of repeatednucleation of magnesiumcalcite typically exhibitwackestone textures. Clearly the precipitateswithin the reef body. Alexandersson micritewas entrapped within pockets on the patch- (1978), on the other hand, considered them to develop reefsurface and provided auseful niche for filter as a continuousgrowth phase from pre-existing reef feeding bivalves. micrites.It seems unlikely that the Sicilianpeloids Atscattered intervals throughout these micrite developed by Alexandersson’s mechanism, since many pockets, Lithuphaga in life position proves that rapid show a grain support texture within geopetal cavities. submarinecementation of thedetrital micrite was The Sicilianreef peloidsprobably developed by a common. No additionalcement accompanies this sequence of events similar to that described by Land& lithification. Moore (1980). It is concluded that the peloids and the Oftenthe interrelationships between the biocon- micriticcrystalline isopachous fringe cements de- structedareas and the micritic internal cements are velopedconcurrently from calcium carbonate satu- very complex (Fig. 6). This sketch of a hand specimen rated interstitial sea water. Peloids probably seededon clearlyshows the multiple filling andcolonization themicrite fringe cements later became free, to be episodes. pushed around in cavities by the pumping tidal action. In thin sections the micriteis frequently opaque, but Finallythey came to rest in blind conduit systems, a clotted or even peloidal texture becomes visible in withincoral pore canals and at inter-allochem sites ultra-thinsections (

FIG. 8. Scale bar is 500,um. A, Constructional fabricsassociated with theCarlentini Formation. 1. encrusting Homotrematinae. 2. massive to poorly laminated compact crust. 3, peloidal micrites filling a depression within the growing reef front. 4, Crustose coralline algae. Much of the primary patch-reef framework is composed of elements 1, 2 and 4. withcavities filled by the peloidalmicrites. Intermediatebioherm. Carlentini Formation, Sorgenti Paradiso. B, Early submarine diagenetic fabrics. An early subsea isopachous fringe cement sequence (2) consists of a first cycle of clear spar followed by a second cycle of mini-micrite. In some parts (left) fringe cement development was terminated by influx of marine biomicrite (1) but in other areas (right) remaining void space was filled by a final cycle of acicular clear spar. Basal bioherm level, WB 115093, Monti Climiti. C, Later vadose and phreatic fabrics affecting a dissolution cavity within a Porites colony. After an early dissolution episode trabecular voids (4) and cavity base (3) were filled by marine peloids and peloidal biomicrite. A single cycle of bladed spar cement (2) lines the remaining void space and covers the peloids. Final void space is filled by bedded crystal silt (1). Amalgamated bioherm, Murro di Porco.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/3/335/4888371/gsjgs.140.3.0335.pdf by guest on 29 September 2021 344 H. M. Pedley Early submarine fringecements Descriptions of Recentisopachous fringe cements from both the E coast of America (James et al. 1976) and from Australia (Marshall & Davies 1981) indicate that fringe cements may be both aragonitic and calcitic and be either bladed spar or micritic in morphology. The Sicilianisopachous fringe cements are mostly comparableto magnesium calcite fringes and are of twotypes. The earliest frequently encircle former aragonitic allochems and coral pore canals and consist of dense micrite fringes (3-10 p thick) (Fig. 8C, and onthe right of Fig.8B). This fringe type may be repeated as a third cycleof isopachous cement in many samples. The second cycle of fringe cement consists of 7-5Opm(usually c. 10-2Opm) of clearbladed spar, which grew normal to the cavity surface. In Fig. 8B an A unusually thick development of the second cycle fringe cement is seen and a fourth cycle, also consisting of bladedspar, finally plugs the remaining cavity at 3. Clearly the final number of isopachous fringe cement episodes in any reef area was controlled by the timing of peloidal micrite emplacement (Fig. 8B). Significant- ly there appears to be a definitesequence of fringe cement morphologies within modern reefs comment- ingabove aragonitic material with high-Mg micrite fringes followed by high-Mg calcite spar (Marshall & Davies 1981; MacIntyre 1977; James et al. 1976). The morphologies are remarkably similar to those occur- ring in the Sortino Group reefs. The underlying cause of thisremains obscure, although the similarities suggest thatthe Siciliancarbonate was originally of high-Mg calcite. B

FIG. 9. Scale bar is 500pm. A, Photomicrograph Early phreatic and vadose meteoric fabrics under plane polarized light of the internal fabric to Several well developed cements and deposits within patcha reef. Carlentini Formation, Murro di the Sortino Formation indicate that both phreatic and Porco. Note the echinoidplate (centre) and vadose bonding episodes occurred. associated syntaxial overgrowth. B, The same area, taken froma colour photomicrograph under luminescentconditions. The Phreatic cementation syntaxial overgrowth (c) to the echinoid allochem (d) containswell defined luminous bands. These Simpleone cycle fringes of bladedscalenohedral are continuouswith the luminousbands in the calcite (50-100 pm thick) and syntaxial overgrowths to fringe cements lining the cavitywhich was later echinoid grains (Fig. 8B, 2; Fig. 9A) occur within the filled by vadose micrite(a). Other components are: patch-reeffabrics of theeastern Hyblean region. b, marineframework infilling micrite; X, various bioclasts,principally benthic foraminifera (top Initially, the syntaxial overgrowths might be thought centre), surpulid tubes and coralline algae. toillustrate neomorphic aggradation of aprimary micrite (Bathurst 1971), but this is not so. They are intimately related to the scalenohedral fringe cements and merely represent enhanced growth of the fringe theorange-yellow range (see Fig. 9B,overgrowth cementat well-developed lattice sites. The fringe labelledc). The presence of distinctlyluminescent cements post-date peloid accumulation and submarine calcite suggests that Mn in excessof 1000 ppm (Meyers fringecements, but they pre-dated crystal silt and 1974) to 100ppm (Pierson 1981) is present. Michard vadose micrite emplacement. Under the luminoscope (1968) stated that 10-50 ppm Mn occurs in normal sea the scalenohedral fringe cements and related syntaxial water,therefore these luminous cements could not overgrowths show well developed luminous banding in havearisen from normal sea water, but are more

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Broad, outmarine phreatic evenor marine phreatic- irregular,non-luminous fringes of clearsparite hang freshwater phreatic mixing zones, it is considered that fromthe lower surfaces of allochems(Fig. SH, 1). non-oxygenatedconditions are most unlikely in an Individual calcite crystals radiate away from the under evolving reef framework due to the floral associations surfaces of allochems and increase in width away from andgeneral turbulence. If this is acceptedthen a thesesurfaces. The textures are very similar to meteoricphreatic origin must be attributed to the examplesdescribed by Purser(1969), and are consi- luminousovergrowths and single cycle bladedspar dered to represent vadose meteoric cements. cements. Inseveral thin sections, (including Fig. 9), the luminous fringe cements are seen to have developed Macroscopic evidence for emergence after the termination of submarine isopachous fringe Further supportive evidence for episodic uplift and cementation and peloidal generation. The absence of emergence is widespread in the Sortino Group, and is luminosity from the calcite of the replaced aragonitic frequently closelyassociated with erosion surfaces. coral skeletons of Fig. 8 argues against development of Many of these truncated surfaces are planar, though thatcalcite concurrently with the luminous fringe several undulose horizons also occur. episode.Calcite replacement of aragoniteprobably occurredlater under vadose conditions when non- Sedimentary dykes luminousequant spar growth invaded all remaining pore spaces in both reef and off-reef sites. Sedimentarydykes are present throughout the group. Typically they consist of mm to cm wide joints, Crystal silt filled withpale cream micrite whichoccasionally containsmarine bioclasts. The joints are variously Crystal silt is only common in the patch-reefs of the orientated,may penetrate up to a metreinto the Murro diPorco exposed at Maddalena. The general truncatedunderlying strata and are themselves trun- featuresare as described by Durham (1969) forthe cated by youngerbeds. The ubiquity of these Townsend Mound of New Mexico. Fig. SC illustrates structuresclearly indicates fracturing of rapidly the material, which consists of shards of comminuted lithified strata. Synsedimentary faulting also occurs in calcitecrystals (4-10 pm).Grading is frequentand the Monte Carrubba section (Fig. 3). leads to well developed banding being imparted to the voidinfillings. Parallellaminated and cross-stratified Brecciation silts occur,suggesting considerable flushing andre- filling of theremaining voids with this material. No At all three bioherm levels the lithified carbonates fossils have been found in the crystal silt in any thin suffered local but intense fracturing (Fig. SE) down to sections. a depth of 1 m or more. The resultingclasts show little Inmany thin sections from other patch-reef loca- or no evidence of rotation. Their brecciated appear- tions, unfossiliferous micrites comprise large areas of ence compares favourably with descriptions of similar fabric(e.g. Fig. 9A) andclearly post-date the lumi- occurrences(James 1972; Harrison & Steiner 1978) nous, phreatic. bladed calcite episode. These micrites fromCarboniferous, Pleistocene and Recent strata. are considered to be of a similar origin to the crystal Walls et al. (1975) considered this typeof brecciation to silt. Durham (1969)suggested that crystal silt is be the product of subaerial weathering and diagenesis. derivedfrom break up of bladedcement linings to Several of the Sicilianbreccias havebeen subse- cavities and conduits. They have suffered subsequent quentlycemented by sparite and iron oxide. Others transportation deep into the reef structure by vadose containeither green clay or biomicriticinterstitial watermovement. This isalso anappropriate inter- matricesand, when followed across country, brecci- pretationfor the crystal silts of theSortino Group. ated levels pass laterally into unfractured strata. In the Although individual crystal shards within the silt are area of the Melilli SportsCamp (WB 1051499) the luminous, most of the materialis not. Much of the silt, intermediatebioherm is absent,probably due to therefore,probably originated as non-luminous cal- subaerialemergence during that time. A terra rosa cite,perhaps derived from disintegration of earlier soil is developed in its place (Fig. 3). In other areas, submarine fringe cements, though more probably from e.g.around the Pericontate and Sortino vents, total youngervadose cements. The absence of marine bioherm destruction occurred, either due to explosive bioclastsargues against a marinedepositional en- eruption or subaerial emergence (Grasso etal. 1982) vironment. and scattered reef blocks occur in the volcaniclastics.

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Palaeosols weathering of an associatedvesicular basalt in the Monte Carrubba section lends further support to this The Melilli SportsCamp palaeosolisthe best interpretation. example of thisphenomenon within the Sortino Group. It occurs above a fretted volcaniclastic surface Palaeoenvironment in thelower part of theCarlentini Formation and consists of 1.3 m of grey clay with a well developed Fig. 7 illustrates the likely sequence of events which nodular calcrete, followedby 0.2 m of terra rossa. This led to the developmentof all the fabrics and structures is closelyoverlain by leaf-bearing pale grey micrites described. which also contain pulmonate gastropods (M. Grasso, A, Active eruption from submarine vents built out pers.comm.) A possiblesecond pale grey palaeosol thickvolcaniclastic sequences into the shallow plat- closelyoverlies the first. This is the onlydefinite form areas until the deposits became emergent. palaeosol locally recognized in the fieldwithin the B, During quiescent periods a soil profile developed CarlentiniFormation, although a thinnerpale grey onthe lithifiedvolcaniclastic pile asresulta of clay horizon containing limestone pebbles within the weathering and colonization. Monte Carrubba Formation at Faro Santa Croce (Fig. C, A marine transgression inundated the land area, 5B) may be cited. terminating soil development. Most of the soil profile The pendent cements of Faro Santa Croce (Fig. 5H, wasflushed out of theold land surface and shallow 1) are noteworthy. These reticulate micrite wall fabrics marinecarbonates with patch-reefs developed. Early infilled with equant clear spar are remarkably similar submarine lithification occurred throughout the patch- to the figured examples of Esteban (1974) and Adams reefs. (1980). D, A marineregression caused emergence of the The preservation of these paleosols is haphazard at lithified carbonates and a weathering profile developed bestbecause they were readily stripped off during inthe upper strata causingbrecciation of therock. subsequentmarine transgressions. In such situations Further colonization by plants occurred. thebrecciated bedrock alone might survive or, Theclimate during the deposition of theSortino perhapsan underlying dissolution surface or fretted Group is difficult to ascertain. Terra rossa and nodular pavement. calcrete palaeosols in the Carlentini Formationsuggest by modernanalogue (Goudie 1973)a warmto hot semi-arid to arid climate with a prolonged dry season. Pavements Themarine biota of theCarlentini Formation also indicatesub-tropical conditions, as do oolites and The tops to both the lower and middlevolcaniclastic submarinecements. Conditions during the develop- units of theCarlentini Formation are sharply trun- ment of theMonte Carrubba Formation are less catedinmany outcrops in the vicinity of their certain. The low fauna1 diversities there were almost depocentre(an elliptical area marked by aring of certainly due to salinity fluctuations within a lagoonal volcanic vents extending from Sortino via Carlentini, environment;however, alveolar textures point to Cozzo Telegrafo and Melilli, Fig. S). some vegetational activity. Thetruncation may take two forms. Firstly, as at MonteCarruba (WB 023189)it mayconsist of an Correlation with the Maltese irregularlyincised surface with open clefts, cen- timetres wide, which extends down 3G90cm into the Islands underlyingvolcaniclastics. These clefts may giverise Felix (1973) argued that the youngest Maltese Tertiary laterally tonarrower branching networks, now all strataare entirely Tortonian in age. On the other entirely infilled by marine carbonates continuous with hand,Giannelli & Salvatorini(1975) suggested that the overlying bioherm material. Secondly, the trunca- theUpper Coralline Limestone Formation (Pedley tionmay take the form of an 15-10 cmamplitude 1978) is entirely of Messinian age. DiGeronemo et al. undulose upper surface to the volcaniclastics. This is (1981) fixed theage of the closelyunderlying Blue well seen in the volcaniclastics below the top bioherm Mar1 horizons of SE Sicily and the Maltese Islands as level Sorgentiat Paradiso. This blind potholed late Tortonian. appearance to the surface is well developed in several The Sortino Group closely overlies these Tortonian exposures of thebasal bioherm (top of Siracusa marls in western areas of SE Sicily. Precise age dating Limestone member of Monte Climiti Formation) espe- is based on the occurrenceof the late TortonianPecten cially to the E of Monte Carrubba (WB 025188). vigolenensis within the basalCarrubba Formation, These features are considered to have been caused together with the correlation of the Carlentini Forma- by subaerialweathering. They may originally have tionvolcaniclastics with pre-Messiniana volcanic beencovered by soilswhich wereremoved by horizonnear Licodia Eubea (Grasso et al. 1982). subsequent marine transgressions. General ‘rotting’ of Erosionhorizons within the Sortino Group (see the upper surfaceof the volcaniclastics, and spheroidal above)and at comparable levels within theUpper

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I , IOOkm, Nt SeaTyrrhenian ...

'-2oom...,(Present day) ? +--./ ...... %IT...... Cent. Sicilian Basin U...... , (Messinian evaporites) '.

FIG. 10. Distribution of late Tortonian and Messinian patch reefs around the central Sicilian basin andMaltese Islands. Arrows indicate net water movement towards the eastern Tethys basin.

Coralline Limestone Formation, Tal Pitkal and Gebel to the N and deeper water mark and evaporites S. to the Imbark Members (Pedley 1979) adds further support Grasso et al. (1982),and this paper, record patch for contemporaneitydespite the distance separating reefs in the Hyblean region of SE Sicily, and Pedley the islands (125 km). (1978,1979) described patch reefs of thesame age withinthe Maltese Islands. These two areas of reef developmentare not typical. They occupy unusually Palaeogeography broadzones and are not directly associated with a terrestrial hinterland. The Hyblean and Maltese reefs Though far apart, the SE Sicilian and Maltese regions lieon a threshold between the central Sicilianbasin bothlie on a shallow shelf, the Malta-Ragusa Rise, andthe newly formingIonian Abyssal Plain of the which probably existed as a shallow platform through- eastern Mediterranean. A Neogene extension to this outNeogene times. The present day platform is threshold, now destroyed due to later block faulting boundedapproximately by the200m isobath (Fig. (Illies 1969, 1981) directly linked the Malta-Sicily area 10). with North Africa. This shallow marine ridge received Messinianevaporites banked up against the Malta carbonatesedimentation, whereas deeper basinal Scarp (Grandjacquet & Mascle 1978) suggest that the markaccumulated to the W in thestraits of Sicily steepfault-bound eastern boundary to the platform (Borsetti et al. 1974) and to the E beyond the Malta wasalready developing during the late Tortonian- Scarp. Messinian interval; here the platform rapidly descends Theeastern Tunisian late Miocene coastline lay into the Ionian Abyssal Plain. To the W the platform some distance further E than at present and mountain descends gradually into the central Sicilian basin, an rangesextended continuously through much of the area of evaporiteprecipitation during the Messinian Italian peninsula and N Sicily. By late Tortonian times (boundaryto main central Sicilian evaporite basin the only direct route for sea water exchange between modifiedfrom Colantoni 1975 andBorsetti et al. theeastern and western Mediterranean basins was 1974). Dataon the suggestedland areas are mainly acrossthe comparatively narrow and extremely shal- fromColacicchi (1963), Pedley (1978) andCatalano low threshold. A net eastward sea water flow (arrows, (1979). Fig. 10) is strongly implied by Pedley (1978, 1981) and Patch reef distributions in western and central Sicily Grasso et al. (1982).This could only occur if the arefrom Catalano (1979)and M. Grasso (pers. enclosedeastern basin was evaporatingrapidly. Sea comm.). They occur in an E-W belt lying between the waterwould be drawn across the Hyblean-Maltese terrestrial and marginal marine Terravechia Formation threshold, the rate of movement varying according to

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/3/335/4888371/gsjgs.140.3.0335.pdf by guest on 29 September 2021 348 H. M. Pedley drawdown in the eastern basin. Atlantic water, drawn Messiniansediments in the Hyblean and Maltese in via the Betic Straits of Spain and the Rifs region of regions is significant.It is not necessarilycorrect to Morocco would replenish the loss to the system. These acceptthe existence of asingle terminalcarbonate conditionswould lead to a considerablevolume of complex in the Messinian (cf. Catalano 1979; Esteban waterexchange across the threshold and asufficient 1980). Indeed, terminal carbonate complexes consist- nutrient supply to permit the development of a wider ing of ooliticstrata, stromatolites and mollusc-rich patch reef beltthan might otherwise be the case in levels can be expected to develop whenever there is a the fringing reef locations. markedeustatic sea level drop. In the case of the During periods of drawdown, when Tethys became Maltese and Sicilian examples these complexes (Gebel landlocked,the Hyblean-Maltese threshold would ImbarkMember and Monte Carrubba Formation rapidly become emergent and this would be recorded respectively) arealmost certainly not younger than in the succession by erosion surfaces and depositional lower Messinian in age and are the time equivalents to breaks. Vadose and phreatic meteoric textures would the‘Calcare di Base’ of Italianauthors. Unlike the be associated with these surfaces. In the case of the Spanish Messinian succession there does not appear to Hyblean carbonates of the Carlentini Formation this havebeen a returnto marine conditions after the interpretation is complicated by the thick volcaniclas- major drawdown episode associated with the ‘Lower ticintercalations. Although mainly accumulated in Evaporate Unit’. Perhaps this is due to the elevated submarinelocations some areas became emergent location of theHyblean-Maltese threshold, which locally andmay be responsible for the subaerial mightbe expected to be covered by seawater only features seen at the upper contactsof the volcaniclastic during relatively high stands of sea level. beds. No sucheffects of emergencedue to igneous Anattempt to correlate these erosion surfaces activitycan be attributed to those subaerial erosion associatedwith the Messiniansea levelfluctuations surfaces associated with brecciation of the bioherms. might be more fruitful if viewed in conjunction with Indeed, the planar upper surfaces to these carbonates reef and evaporite episodes. Such features as breccia- suggestsdegreea of marineplanation after the tion,palaeosols and emergent petrological fabrics subaerialepisode and before the onset of thenext havebeen described from thelate Tortonian- volcaniclasticepisode. These brecciated horizons Messinianstrata of Spain(Esteban & Giner 1980). must, therefore, represent true episodesof eustatic sea Assuming that they are all caused by the same process level fall. There are three and they correspond to the of sea level drawdown during landlocked episodes in tops of each of the three bioherm levels in SE Sicily thewestern Mediterranean then they should be (Fig. 2). Significantly three similarlyplaced erosion extremely useful in correlation. Unfortunately tectonic levelsoccur in theTal Pitkal Member in western movements affect the strataof most of the well studied Malta. A fourtherosion horizon at the base of the circum-Mediterraneanlocations and this complicates Monte Carrubba Formation (Sicily) and Gebel Imbark the issues greatly. A tectonically undisturbed area in Member(Malta) represents a furthercorrelation the western Mediterranean must be located for study betweenthe two regions. Clearly such widespread before widespread correlation on the basis of erosion occurrences of erosion surfaces ona common platform surfacesbecomes viable. This would then act asa must be caused by processes other than local volcanic- standardagainst whichall otherstudies could be ity, and are probably produced by oscillations of sea gauged. levelassociated with the commencement of the ‘Messian Salinity Crisis.’ ACKNOWLEDGMENTS.I would like to thank the Royal Society for a generous scientific investigations grant (1980) to carry Conclusions out 6 weeks fieldwork in Sicily, and Professor F. Lentini and Dr M. Grasso, University of Catania, for their assistance in Theapparent absence of bothmiddle and upper the field and much valuable advice.

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Received 10 June 1982; revised typescript received 18 October 1982. H. M. PEDLEY, Departmentof Geography & Geology, The Polytechnic of North London, The Marborough Building, 383 Holloway Road, London N7.

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