Mem. Soc. Geol. /t., 20 (1979), 405-423. 9 ff., 4 tabb. PALEOECOLOGICAL REMARKS ON TUE "ORTHOCERAS LIMESTONE" OF SOUTHWESTERN SARDINIA (MIDDLE-UPPER SILURIAN) ( * ) Memoria di MAURIZIO GNOLI (**), GrAN CLEMENTE PAREA (***). FRANCO Russo(**) & ENRICO SERPAGLI (**) (presentata a Siena, nella Seduta /ematica del /8-/9 maggio 1979) CO NTE NT ricco di fauna esclusivamente pelagica nelle parti p age superiori e decisamente tossico verso il fondo. lNTRODUCTION 405 Analisi statistiche ed esperimenti di laborato­ l"Ìo hanno portato gli scriventi a considerare il GEOLOGICAL REMARKS 406 particolare tipo di orientamento presentato dai SEDIMENTOLOGICAL REMARKS 407 fossili come un effetto del moto ondoso. I gusci ed i loro frammenti, sparsi su un fondo di fango Geometry of the "Orthoceras lenses" 407 fine, verrebbero disposti dalle onde a bande suc­ The problem of Nautiloid orientation 410 cessive che rappresenterebbero l'equivalente mec­ Laboratory experiments 411 canico delle increspature di fondo tipiche dei fon­ Orientation analysis of the orthoconic di sabbiosi. Potrebbe trattarsi di un mare epicontinentale nautiloids 413 di profondità limitata tale da permettere al moto GENERAL FEATURES OF THE FAUNA 417 ondoso di agire sul fondo. Nektic organisms associated with the grap- tolites 417 Organisms possibly attached to floating ABSTRACT plants 417 A study of the fauna and of the sedimento­ ENVIRONMENTAL CONCLUSIONS 420 logica! characteristics of the "Orthoceras limestone" REFERENCES 422 has shown that these sediments were deposited in a normally oxygenated sea rich in an exclusively pelagic fauna in the upper parts but definitely KEY WORDS : Shelf environment, death as­ toxic towards the bottom. semblage, sedimentary structures, depth Statistica! analyses along with laboratory ex­ periments have led. the writers to consider that indicators, Silurian. the particular type · of orientation found in the fossils is the result of ware motion. The shells and their fragments, distributed on the fine mud of the bottom, must have been positioned by the RIAS SUNTO waves in successive belts which represent the mechanical equivalent of ripple marks, typical of Dallo studro della fauna e delle caratteristiche sandy bottoms. Possibly this was an epicontinental sedimentologiche dei calcari a « Orthoceras » di sea of Iimited depth so that the ware motion Fluminimaggiore risulta che questi sedimenti si could affect the sea bottom. depositarono in un mare normalmente ossigenato INTRODUCTION (*) Work supported by the Consiglio Naz. delle Ricerche, grants: 78.01881.66, IGCP Project n. 53: Ecostratigraphy and 78.00308.05. The objective of this study was to try (**) Istituto di Paleontologia dell'Università di to define the environmental conditions that Modena, Italy. controlled the deposition of the "Orthoceras (***) Istituto di Geologia dell'Università di Mo­ dena, Italy. limestone" in Southwestern Sardinia. 406 MAURIZIO GNOLI ET Alli Both the fauna content and the litholog­ less flattened prismatic blocks surrounded ical and sedimentological characteristics of by non calcareous pelites. These limestone these limestone are taken into consideration blocks are generally not longer than a meter in this report. From the paleontological and their edges are slightly rounded. The point of view we have tned to determine pelites clearly have flown around the hard the environmental significance of the indi­ blocks which are often folded in various viduai taxa as well as that of the entire ways, sometimes very tightly. association. The sedimentological aspects On the basis of what can be observed of the study were directed towards evalua­ in the field, the breaking up of the calcare­ ting the energetic characteristics of the en­ ous layers and the incorporation of the vironment by means of the sedimentary fragments in the silt-clay matrix have been structures. In particular we based our con­ caused by intense tectonic activity. No signs clusions on the èistribution and orientation of deformation of the blocks in the plastic of the orthoconical shells of nautiloids. state were observed not even at the edges. Further information was obtained from The occasionally observable rounding of the lithological characteristics of the rocks. the edges of the limestone blocks, which is not pronounced however, was caused solely by dissolution and disintegration as­ GEOLOGICAL REMARKS sociated with surface weathering. The "Orthoceras limestones" in the A brief summary of the Silurian rocks is studied region are made,up of sparite or given here in order to piace in perspective microsparite having widely variable crystal the "Orthoceras limestone" dealt with in this sizes, even within the same thin section, paper. (generai between 30 and 300 microns). These Silurian marine sediments, which are sparites must be derived from the recrystal­ exposed in two main areas (lglesiente at lization of a very fine calcareous mud in SW an d Gerrei at SE), are represented chief­ which fragments and shells of orthoceratids ly by black shales rich in graptolites. Well - were scattered with a few bivalves and bedded black limestone with cephalopods some gastropods as described in the para­ ("Orthoceras"), Cardiola, graptolites, gastro­ graph regarding the fauna. pods, brachiopods, ostracods and conodonts The orthoceratid shells have varying occurs- in the upper part of the sequence. sizes from a few millimeters to around 30 On the basis of conodonts, the "Orthoceras centimeters in length for whole specimens. Iimestone" was dated by SERPAGLI (1967, Some fragments of living chambers reach 1971) as Uppermost Wenlockian-Ludlovian a diameter of about 20 centimeters, hence, in age. considering an angle of expansion of approxi­ A complete Silurian stratigraphic record mately 2°, these chambers must have been is unknown in Sardinia since it is impossible part of a 2 meter long shell. to find continuous sequences of fossiliferous In the microspar surrounding the nauti­ marine strata. Furthermore, it is virtually loids, shell fragments of ali dimensions down impossibile to trace individuai beds over to a few microns are present. The overall long distances. Latera! facies changes and aspect is that of a completely random as­ intrusions of magmatic rocks, to which sociation of shells and shell fragments. Con­ mineralized veins are connected, greatly centrati o n of small shells in the "shadows" complicate the geologica! mapping of this of those having greater dimensions were regio n. never observed. Because of the intense tectonic activity Some chemical analyses gave values of and poor outcrops, it is impossible to Ca C03 close to 96-98%, which can certainly measure the thickness of the "Orth.oceras be considered a little in excess given the limestone" with any precision; however, the impossibility of eliminating with certainty overall thickness should be of the order of every spot or veinlet of sparry calcite. X-ray 30 meters. diffraction analyses of the small amount of The "Orthoceras limestones" in the Flu­ insoluble residue, after removal of the or­ minimaggiore area are found as more or ganic fraction, showed the presence of " " PALEOECOLOGICAL REMARKS ON THE ORTHOCERAS LIMESTONE OF SARiliNIA 407 quartz, mica, chlorite and/or montmoril­ cumulation of particular organisms, or sedi­ lonite and feldspar (l). ments derived from them, on certain areas Often the calcareous mud partialiy pen­ of the bottom particularly favorable to etrated the shelis, occupying only the low­ the organisms concerned. In a situation er part and decreasing in thickness towards of this type, there should exist a tran­ the apical portion (fig. l). The remaining sition belt in which the favorable con­ upper parts of the shelis are filled by ditions progressively disappear outwards blocky calcite cement. until they no longer exist. In this way lenticular accumulation would be produced whose dimensions are greater by one or SEDIMENTOLOGICAL REMARKS more orders of magnitude to those of the constituent elements. Various other ways of GEOMETRY OF THE "0RTHOCERAS LENSES" accumulation of Ienticular deposits can be hypothesized, ali of which, however, imply The term "Orthoceras lens", used by the the existence of sedimentary structures or first authors that delt with them {TARICCO, characteristics which change from the center 1921-1922, p. 13; GORTANI, 1922, p. 365; No­ to the margins because of the variation in VARESE & TARICCO, 1922, p. 318-319) and genetic conditions. SERPAGLI recently adopted also by (1970) On the contrary, the most evident aspect and SERPAGLI & GNOLI (1977), is purposely in ali of the blocks of "Orthoceras lime­ not used in this artide since the lenticular stone" examined is the uniformity of the form in any sedimentary body implies the characteristics aver the entire external sur­ existence of different environmental condi­ face and in the observable sections. In ad­ tions "between the thicker center and the dition, the observable blocks have sizes of ends. The difference in environmental con­ the same arder of magnitude of the shells ditions, far example, could be a different they contain and in certain cases the di­ distribution of the action of the current mensions of the blocks are less than those between the bottom and the sides of a of the enclosed shells. channel of which the kns represents the ·From these considerations it foliows that filling. lt is important to note here that if the Orthoceras formations must have origi­ "Orthoceras the so called lenses" represent naliy consisted of an alternation of calcare­