The Petrology and Palaeoenvironment of the Sortino Group (Miocene) of SE Sicily: Evidence for Periodic Emergence
Total Page：16
File Type：pdf, Size：1020Kb
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 volcaniclastic suite thins rapidly Carlentini Formation awayfrom thevents and the three bioherm levels unite in the eastern peninsula to form a single thick, Carbonates of this formation 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 he found 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/3/335/4888371/gsjgs.140.3.0335.pdf by guest on 29 September 2021 336 H. M. Pedley 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/3/335/4888371/gsjgs.140.3.0335.pdf by guest on 29 September 2021 Sortino Group (Miocene), SE Sicily 337 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). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/3/335/4888371/gsjgs.140.3.0335.pdf by guest on 29 September 2021 338 H. M. Pedley 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