Microbiofacies Analysis of Cambrian Offshore Carbonates from Sardinia (Italy): Environment Reconstruction and Development of a Drowning Carbonate Platform
Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01)
Microbiofacies analysis of Cambrian offshore carbonates from Sardinia (Italy): environment reconstruction and development of a drowning carbonate platform
Olaf ELICKI1
Abstract: The Campo Pisano Formation of southwestern Sardinia is represented by an offshore carbonate succession spanning the latest Early to late Middle Cambrian. Paleogeographically, the fauna is characteristic of western Perigondwana, and indicates faunal relations to France, Spain, and Turkey. Microfaunal paleoecology reflects drowning of an isolated carbonate platform at tropical latitudes. Sessile epibenthic filter feeders dominate at the base, succeeded upward by sessile, epibenthic, suspension feeders. Upsection, a shift in the ratio of faunal groups indicates increasing replacement by mobile epibenthos. Autochthonous faunal elements decline near the top where allochthonous taxa become important. In the basal portion of the formation the faunal succession indicates relatively shallow neritic habitats with a moderate influx of suspended sediment, followed by a period of slightly deeper neritic conditions. Probably a shallow bathyal environment was established at the transition to the overlying siliciclastic Cabitza Formation. The bathymetric and ecofacies changes in the Campo Pisano Formation are interpreted as having been caused by a discontinuous rise in eustatic sea-level, probably accompanied by subsidence due to tensional tectonics. The depositional environment was that of a distal open-marine shelf or ramp without strong relief. This interpretation is supported by the lack of significant lateral changes in the fossil assemblages, and an upward deepening of facies. The applied micropaleoecological approach is proven a powerful tool in regional stratigraphy and in the reconstruction of sedimentary realms in the Mediterranean Cambrian, particularly when high-resolution biostratigraphic data and diagnostic sedimentary characteristics are sparse. Key Words: Cambrian, Italy, Sardinia, micropaleontology, microfacies, paleoecology
Citation: ELICKI O. (2006).- Microbiofacies analysis of Cambrian offshore carbonates from Sardinia (Italy): environment reconstruction and development of a drowning carbonate platform.- Carnets de Géologie / Notebooks on Geology, Brest, Article 2006/01 (CG2006_A01) Résumé : Analyse microbiofaciologique de séries carbonatées marines du Cambrien de Sardaigne (Italie) : reconstitution paléoenvironnementale et développement d'une plate- forme carbonatée ennoyée.- La Formation de Campo Pisano du SW de la Sardaigne est constituée par une série carbonatée - marine - couvrant un intervalle de temps qui s'étend du Cambrien inférieur terminal à Cambrien moyen élevé. Du point de vue de la paléogéographie, la faune y est de type périgondwanien et présente des affinités occidentales certaines (France, Espagne et Turquie). L'évolution de la paléoécologie des microfaunes retranscrit l'ennoiement de cette plate-forme carbonatée isolée en climat tropical. À la base, les organismes sessiles épibenthiques filtreurs dominent. En montant dans la série, leur succèdent des formes sessiles épibenthiques suspensivores. Enfin, vers le sommet, le glissement dans les proportions au sein des associations fauniques traduit le remplacement croissant des faunes autochtones par un épibenthos vagile, c'est-à-dire par des faunes allochtones. Dans la partie basale de la formation, la succession des faunes suggère qu'à une période initiale marquée par des environnements néritiques relativement peu profonds, avec de faibles apports détritiques en suspension, a succédé une période caractérisée par des environnements néritiques légèrement plus profonds. Le stade bathyal supérieur a vraisemblablement été atteint au toit de la série, avant le dépôt de la formation silicoclastique de Cabitza. Les variations écologiques et bathymétriques de la Formation de Campo Pisano semblent résulter d'une remontée irrégulière du niveau marin, probablement couplée à une subsidence d'origine tectonique. L'environnement de dépôt était comparable à celui de la partie distale d'une plate-forme ouverte ou d'une rampe avec des reliefs peu exprimés comme en atteste l'absence d'évolution latérale significative des associations fauniques reconnues. L'analyse micropaléoécologique constitue un outil puissant pour les études stratigraphiques régionales ainsi que pour les reconstitutions des ceintures de faciès du Cambrien méditerranéen, en particulier quand figures sédimentaires caractéristiques et données biostratigraphiques de haute résolution sont rares ou absentes. Mots-Clefs : Cambrien, Italie, Sardaigne, micropaléontologie, microfaciès, paléoécologie Introduction East; Fig. 1). But sedimentary records of these In Early- to Middle Cambrian times environments are commonly incomplete, due to carbonate and mixed siliciclastic–carbonate variations in bed thickness and shelf environments were widely distributed on the morphology, together with widespread erosion European Shelf of the low-latitude during the Late Cambrian to Early Ordovician, Perigondwana realm (Morocco, Spain, France, as in Sardinia (Sardic Phase movements), and Sardinia, Germany, and the Near– and Middle possibly also in Germany. Thus, a full
1 Geological Institute, Freiberg University, Bernhard von Cotta street 2, 09599 Freiberg (Germany) [email protected]
Manuscript online since February 14, 2006
1 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01) reconstruction of early Paleozoic development The main reason for the limited investigation in this region is still problematic for of this critical interval in Sardinia may lie in the biostratigraphic, paleogeographic and lithologic character of the transition, postdepositional geological processes are not represented in general by rather monotonous, yet completely defined. condensed, often nodular limestones, affected Earliest Cambrian sediments on the by a strong diagenetic overprint (COCOZZA & Gondwana European Shelf are generally not GANDIN, 1990; ELICKI, 2001). Nevertheless, the exposed, either because they were not carbonates are commonly rich in shelly deposited or were removed through tectonism microfossils that usually are an important and subsequent erosion. However, from the means of obtaining additional information on later Early Cambrian to the late Middle depositional conditions. However, here this Cambrian sedimentary successions up to 2000 microfossil wealth is frequently hard to bring to m or more are well developed (see BENDER, light. It is an extremely intricate, time- 1974; PILLOLA, 1991; GEYER & LANDING, 1995; consuming and almost impossible task to ELICKI, 1997; LIÑAN et alii, 2002; ÁLVARO et alii, extract these fossils from the host rocks if they 2003). These deposits show an overall trend are not diagenetically silicified or preserved as upward from siliciclastic to relatively shallow steinkerns. This problem exists for nearly all carbonate and mixed carbonate–siliciclastic the important fossil groups other than environments, followed by a siliciclastic cover organophosphatic brachiopods and problematic denoting deepening. Younger Cambrian small shelly fossils that can be extracted by deposits are only sporadically developed in acidic dissolution. Furthermore, organic biota, some areas of the European Shelf. Although the such as palynomorphs are usually not paleontology and sedimentary facies of the preserved in the mostly light- and reddish- carbonate successions (mainly Early Cambrian) colored carbonates. These hindrances have led and the siliciclastic deposits (Early and Middle to a paleontological focus on archaeocyaths in Cambrian) are well known, the transition from carbonate to siliciclastic deposition, reflecting Early Cambrian carbonate rocks and on the drowning of the carbonate platforms and trilobites in Early and Middle Cambrian consequently the reorganization of the siliciclastic beds. Therefore, the Middle Perigondwana realm, has been investigated Cambrian drowning stage of the Sardinian only minimally in some areas. platform has not been properly investigated paleontologically and geological processes near the Early to Middle Cambrian boundary are not well known. In southwestern Sardinia the drowning stage of the Cambrian carbonate platform is represented by the Campo Pisano Formation. A genetic interpretation of the depositional environment of this succession is problematic because of its limited sedimentological characteristics. Previous environmental interpretations ranged from shallow water, low- energy areas (COCOZZA, 1979) to lagoonal and partly euxinic tracts (GANDIN, 1979), to ammonitico rosso-type deposits (the typical Tethyan red-colored, nodular, deep water facies; SCHLEDDING, 1985; GANDIN, 1987b; BECHSTÄDT et alii, 1988), that are interpreted as being on tectonically unstable shelves with highs and troughs (COCOZZA & GANDIN, 1990). The fauna of the Campo Pisano Formation is mainly trilobites, echinoderms, poriferans, brachiopods, and chancelloriids. Nevertheless, this fauna has been investigated very scantily because of its extremely poor accessibility. Only some trilobites, a few foraminiferans, poriferan spicules, and hadimopanellids (dermal plates of palaeoscolecid worms) have been the subject of sporadic work (CHERCHI & SCHROEDER, 1985;
MOSTLER, 1985; PILLOLA, 1991). Systematic Figure 1: Paleogeographic map of Gondwana during studies on mollusks, trilobites, brachiopods and late Early to early Middle Cambrian times with the some small shelly fossils were done recently by tentative location of the Mediterranean province in ELICKI (2002), ELICKI et alii (2003), ELICKI & dark gray (modified after COURJAULT-RADÉ et alii, PILLOLA (2004), and MERGL & ELICKI (2004). 1992). T – Turkey, CE – Central Europe, F – France, Sa – Sardinia, Sp – Spain.
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This paper presents for the first time a with the study of about 300 thin-sections, some regionally-scaled detailed micropaleontological chemical preparations, and sedimentologic and and paleoecological analysis of the Campo diagenetic observations, a paleoecologic Pisano Formation. Based on extensive field reconstruction of the drowning of the Sardinian work that involved the measurement and Perigondwanan platform is feasible and offered sampling of 19 stratigraphic sections, together here.
Figure 2: Location and simplified geological map of the work area in southwestern Sardinia. The two pictures at the bottom show the same segment and well correlation of main geological structures and those in satellite view (map modified after Bechstädt et alii, 1988; satellite photographs from NASA World Wind). 1 – Precambrian, 2 – Nebida Group (Early Cambrian), 3 – Gonnesa Group (Early Cambrian), 4 – Iglesias Group (late Early Cambrian to Early Ordovician), 5 – Early Ordovician to Devonian, 6 – allochthonous Paleozoic, 7 – Variscan granites, 8 – Mesozoic, 9 – Cenozoic.
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now been done on sedimentology, litho- and biofacies, paleogeography and economic aspects (e.g., BORNEMANN, 1886, 1891; DEBRENNE, 1964; RASETTI, 1972; GANDIN, 1979; CHERCHI & SCHROEDER, 1984; MOSTLER, 1985; GANDIN, 1987a, 1987b; COCOZZA & GANDIN, 1990; PILLOLA, 1991; BECHSTÄDT & BONI, 1994; ELICKI, 2002; PILLOLA et alii, 2002; ELICKI et alii, 2003; ELICKI & PILLOLA, 2004; MERGL & ELICKI, 2004). When the deposition of deposits of Cambrian age began in Sardinia is not precisely known: the only area where underlying rocks (Precambrian gneisses) are exposed is the southern part of SW Sardinia, but here the transition from gneisses to Cambrian sediments is represented by a nonconformity (JUNKER & SCHNEIDER, 1983). From bottom to top, the Cambrian succession is divided into the Nebida, Gonnesa, and Iglesias groups (Fig. 3; PILLOLA, 1991). The middle to late Early Cambrian Nebida Group consists of clay-, silt- and sandstone in the lower part (Matoppa Formation), and a mixed carbonate–siliciclastic unit in the upper part (Punta Manna Formation). In southwesternmost Sardinia a discrete siliciclastic unit, the so-called "Bithia Formation", is described below the Nebida Group by JUNKER & SCHNEIDER (1983) and GANDIN et alii (1987). This placement was doubted by later authors who consider it as slightly heteropic facies of the lower Nebida Group (BECHSTÄDT & BONI, 1989). The Nebida Group is widely accepted as indicating a shallowing upward from a deep-water siliciclastic facies to a shallow mixed carbonate–siliciclastic ramp (e.g., BECHSTÄDT et alii, 1985; REUTER, 1987; BECHSTÄDT & BONI, 1994. Alternative interpretations initiate this evolution with a prodelta environment COCOZZA, 1979, GANDIN, 1979, and GANDIN et alii, 1987). The overlying Gonnesa Group (late Early Cambrian) is characterized almost entirely by carbonate Figure 3: Stratigraphic succession of the Cambrian of deposits with no siliciclastic influx; thus this SW Sardinia related to Spanish substages. 1 – group represents an isolated carbonate platform claysone/siltstone, 2 – sandstone, 3 – limestone, 4 – (sensu READ, 1985; e.g., BECHSTÄDT & BONI, microbial dolostone, 5 – nodular limestone, CPF – 1994, and cited literature). Low in this group, Campo Pisano Formation, L–C: Leonian and Caesaraugustan stages. higher salinity is denoted by the restricted, dolomitic, very shallow subtidal to intertidal Geological framework laminites of the Santa Barbara Formation, The Lower Paleozoic strata of southwestern overlain by shallow subtidal, open-marine Sardinia represent an autochthonous unit limestones of a discrete and different facies bordered to the east and north by Hercynian (San Giovanni Formation). The youngest traps (Fig. 2). Prominent Cenozoic grabens Cambrian unit is the Iglesias Group (latest Early separate this part of Sardinia from the Cambrian to Early Ordovician). At the base of remainder of the island and divide the area this group are the limestones of the Campo internally into several geographic regions. Pisano Formation (at times nodular) deposited Locally, Hercynian granitoids have during platform drowning. It is succeeded by metamorphosed the Lower Paleozoic rocks. the deeper-water siliciclastics of the Cabitza Formation (Fig. 3). For detailed description of Cambrian sediments are widely exposed in the origin and development of the early southwestern Sardinia (Fig. 2) and reach a platform stages (Nebida and Gonnesa groups), thickness of 1500–2000 m (PILLOLA, 1991). see the cited literature or the extended Initial investigations were focused mainly on summary by BECHSTÄDT & BONI (1994), and the paleontology and biostratigraphy, but work has references cited therein.
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Figure 4: Plan of southwestern Sardinia showing the location of the measured sections of the Campo Pisano Formation, their thickness (A); and the relative amounts of the four carbonate lithotypes comprising them (B). The area occupied by suspected weak sea-floor high is in blue (see text for explanation). On the postulated high sections are thicker and the percentage of autochthonous carbonates greater, so the high is presumed to have favoured carbonate production.
The upper boundary of the Cambrian Terraseo, Fig. 4) and continues farther East and succession lies within the siliciclastic Cabitza South (Sulcis area), SCHLEDDING (1985). The Formation which continues to late Tremadocian most prominent features of the Campo Pisano times. The Cabitza Formation is truncated by a Formation are: (1) the generally rich fossil prominent erosional discordance caused by content and (2) millimeter- to centimeter-thick mid-Ordovician Sardic tectonic movements on a bands of insoluble material, generated by regional scale (BECHSTÄDT et alii, 1994; LEONE et diagenetic processes (GANDIN, 1987b; late alii, 2002). diagenetic pressure dissolution: ELICKI, 2001). Lithology GANDIN (1979) and SCHLEDDING (1985) reported The contact between the Campo Pisano thin clay-rich and silty layers from the Campo Formation and the underlying massive Pisano Formation. The degree of diagenetic limestones of the San Giovanni formation is nodularity of the limestone varies both laterally abrupt, sharply defined (Fig. 3), and in many and vertically. There is no regional or localities is marked by a breccia consisting stratigraphic trend in the occurrence or mainly of San Giovanni clasts. But fragments intensity of nodularity, nor any correlation with from the basal Campo Pisano occur locally. a particular microfacies (see below). (e.g., at San Benedetto, Mt. Marganai area, Mt. The limestone of the Campo Pisano Tasua, Acquacadda, Terraseo, Fig. 4) (BONI et Formation is generally micrite-dominated. The alii, 1981; SCHLEDDING, 1985; GANDIN, 1987a, most prominent lithofacies is light grey, 1987b). The Campo Pisano Formation is mainly yellowish, or reddish bioclastic wackestone wavy-bedded to nodular limestone with a (Figs. 4B, 5 & 6) with qualitatively and variable clay content and a few quartz grains, quantitatively distinct fossil content (see next although the siliciclastic content is greater section). Bioclastic floatstone is another locally (e.g., sections at Su Banatzu, Mt. Tasua,
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W Figure 5: The Campo Pisano Formation ranges in 1 – Field view of the San Benedetto section. Massive thickness from about 30 to100 m, but most of light limestones on the left represent the San the mapped sections are 40–60 m thick. From Giovanni Fm (isolated platform, Gonnesa Group); with the "San Benedetto–Su Banatzu line" the sharp transition (red arrow), the Campo Pisano Fm thickness of the sections tends to decrease to follows (yellowish/reddish nodular banks of the the west–northwest as well as to the east– drowning platform, thickness here about 45 m); to southeast (Fig. 4A). The occurrence of breccias the right (yellow arrow) the Cabitza shales (deep siliciclastic cover of the platform; overgrown by at the base of the Campo Pisano Formation is vegetation) follow. reported from several sites by BONI et alii 2 – Internal breccia at Arcu Sa Cruxi locality near San (1982), GANDIN (1987a, 1987b), and BECHSTÄDT Benedetto. Note the big clasts imbedded in reddish et alii (1988). The distribution of these coarse matrix. Camera lid for scale. clastics shows no distinct regional trend. 3 – Macroscopic view of a typical Campo Pisano SCHLEDDING (1985) and BECHSTÄDT & BONI (1989) nodular limestone (Campo Pisano type section). Note described slumping and mass flow deposits the colour changes parallel to the bedding plane and from different locations in the area. the change in the degree of nodularity vertically. Hammer for scale. Fossils of the Campo Pisano 4 & 5 – Nodular texture of the Campo Pisano Fm in mesoscopic view. If the carbonate is removed by Formation erosion, only siliciclastic bands accumulated by Unlike the very pure limestone of the diagenetic pressure solution remain (4, Gutturu Pala underlying San Giovanni Formation (isolated section); on fresh surfaces the carbonate nodules and carbonate platform stage sensu READ, 1985) stages of origin of pseudo-components are visible (5, and the overlying siliciclastic Cabitza Formation Gutturu Pala section). Camera lid for scale. (cover of the drowned platform), the deposits of 6 – More or less isolated carbonate nodules enclosed the Campo Pisano Formation are characterized in residual siliciclastic bands are rather rare, but, by a rich fauna of shelly fossils. indicate the great role played by diagenetic pressure solution (Masua section). Hammer for scale. The most numerous are trilobites, 7 – Wavy bedded to slightly nodular textures in the brachiopods and echinoderms, with poriferans Campo Pisano Fm with nests of residual siliciclastics and chancelloriids locally common (Figs. 6-5, 6- instead of bands (Gutturu Pala section). Scale bar is 10 cm. 6, 6-7, 6-8, 6-9, 6-12 & 7). The proportion of 8 – Massive texture of the Campo Pisano Fm indicated molluscs (pelagiellid gastropods), hyoliths (Fig. by a limestone bank (white arrows) within the wavy 6-13) and endolithic filamentous microbes is to slightly nodular succession at Monte Scorra section. distinctly lower. Phosphatic small shelly fossils Hammer for scale. (palaeoscolecids, cambroclaves; Figs. 6-10 & 6- 9 – "Shaly" texture of the Campo Pisano Fm at Su 11) are present only sporadically in some beds Banatzu section. Hammer for scale. of several sections. important lithofacies that occurs as The trilobite content is generally high but intercalations at nearly all the sites investigated varies within sections (Fig. 7). The size of the and in some sections occurs in significant identified trilobite remains ranges from a few quantities (e.g., Su Corovau, Mt. Serrau; Fig. millimeters up to one centimeter. Although 4). Fossil-bearing mudstone is intercalated in most of them were recognized in thin sections varying amounts (Fig. 4B). Bioclastic packstone (Fig. 6-7), some rocks yield silicified remains, was found as a small layer in only one section so that dissolution to expose three-dimensional (Monte Scorra; Fig. 4B). More packstone pseudomorphs made taxonomic investigation occurrences were mentioned by SCHLEDDING possible (Fig. 6-9) (ELICKI & PILLOLA, 2004). The (1985), GANDIN (1987b), BECHSTÄDT et alii chemical digestion procedure typically yields (1988) and BECHSTÄDT & BONI (1994), but these trilobite remains consisting mainly of authors did not report grainstone intercalations disarticulated cephala, pygidia, and a range of in the Campo Pisano Formation. Generally, thoracic elements, but in some cases enrolled wackestone and floatstone are more common in specimens and early ontogenetic protaspis and the western to northwestern areas ranging from meraspis stages were obtained (ELICKI et alii, the San Benedetto district to Su Corovau and 2003; ELICKI & PILLOLA, 2004). Generally, Campo Pisano via Carbonia up to Su Banatzu, agnostid and polymeran species are present whereas mudstone is more abundant to the together. Among agnostids, Dawsonia bohemica east and southeast (Fig. 4). Within the interval (Fig. 6-7) and Condylopyge antiqua are very of transition to the overlying siliciclastic Cabitza common, whereas Peronopsis is rather rare. Formation, limestones commonly alternate with Polymerans are represented mainly by thin greenish claystones. Corynexochus and less commonly by Clavigellus, Dorypyge, Agraulos and Protolenus Distinct (planar) bedding is not developed in cf. pisidianus. Taxonomic descriptions and the Campo Pisano Formation. Significant early discussions of this trilobite fauna are given in compaction of the sediment was not observed. ELICKI & PILLOLA (2004). The fossil content is often random within the sediment (Figs. 6-1, 6-2 & 6-7), most probably, In large part brachiopods too are preserved the result of shallow bioturbation (Fig. 6-7), as silicified pseudomorphs, but at one locality though recognizable burrows were not seen.
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W Figure 6: Frequent annulate-segmented sclerites may 1 – Origin of nodular texture by pressure solution represent brachiole or ambulacral parts. (photomicrograph, Campo Pisano Fm, Monte Serrau MOSTLER (1985) stated that most of the Campo section). Note the snug fit of the pseudo-components. Pisano Formation echinoderm remains Scale bar is 1 mm. represent eocrinoids, but a minor content of 2 – Photomicrograph of a siliciclastic band showing at cincta (carpoideans) may be present too. the left a continuous lateral transition into limestone Unfortunately, most were recognized in thin- (the siliciclastic band seems to vanish. So, the band is not a primary sedimentary layer, but, a pressure sections so taxonomy is rendered difficult. But, solution suture. Nevertheless, the band can include epispires (typical in eocrinoids, cincta and some carbonate relicts (pseudo-clasts). Masua section. edrioasteriods) and the estimated ratios of Scale bar is 1 mm. sclerite morphologies: ([1] dominance of small 3 – Photomicrograph of platy texture in the Campo probable brachiole/ambulacral elements, [2] Pisano Fm described by previous authors as many thecal plates, [3] rare stem segments, "calcareous slates" (Terraseo-West section). Scale bar [4] a lack of typical stylocone segments sensu is 1 mm. CLAUSEN & SMITH, 2005, and of [5] cinctan 4 – Photomicrograph of a rapid alternation of limestone and thin siliciclastic laminae typical of the marginal plates and [6] axis channels sensu transition from Campo Pisano Fm to the Cabitza Fm ZIEGLER, 1998),do indeed point to a probable (Gutturu Pala section). Scale bar is 1 mm. dominance of eocrinoids. However, such a 5 – Massive layers of sponge spicules (spiculite differentiation is not of key importance for basic wackestone) common at the base of the Campo paleoecological interpretations. Pisano Fm (biofacies unit 1). Photomicrograph, Masua section. Scale bar is 1 mm. Disarticulated monaxons, tetractines, and 6 – Echinoderm wackestone is characteristic for pentactines are the most frequent sponge biofacies unit 2. Stereome preservation in the form of remains (Fig. 6-8). But, only the monaxons are individual calcite crystals can be seen. occasionally abundant in discrete layers (Fig. 6- Photomicrograph, San Benedetto section. Scale bar is 5). MOSTLER (1985) reported polyactinellid 1 mm. heteractinides from the Pisano Formation 7 – Trilobites predominate in the upper portion of the sections (biofacies unit 3), sometimes they indicate (Phobetractina polymorpha). Another bioturbation (see orientation of fossils). Trilobite heteractinide, Eiffelia araniformis, and the wackestone. Photomicrograph, Gutturu Pala section. possible calcarean Dodecaactinella triradiata Scale bar is 1 mm. were identified. 8 – Sponge spicule from the Masua section (biofacies unit 1). SEM-micrograph. Scale bar is 0.1 mm. Chancelloriids, a systematically problematic 9 – Silicified trilobite cephalon (Dawsonia bohemica) 'sponge-like'group, are lumped as from biofacies unit 3 (Gutturu Pala section). SEM- coeloscleritophorans together with halkieriids, micrograph. Scale bar is 1 mm. wiwaxiids, and siphonogonuchitids by BENGTSON 10 – Phosphatic palaeoscolecid Hadimopanella sp. & MISSARZHEVSKI (1981), but considered as a ("small shelly fossil") from biofacies unit 2 (Su polyphyletic, non-poriferan group of unknown Corovau section). SEM-micrograph. Scale bar is 0.03 systematic affinity by MEHL-JANUSSEN (1999) and mm. 11 – Phosphatic cambroclave ("small shelly fossil") JANUSSEN et alii (2002). In the Sardinian from biofacies unit 2 (Su Corovau section). SEM- samples most of the generally poorly preserved micrograph. Scale bar is 0.1 mm. chancelloriids are represented by three- or four- 12 – Phosphatically preserved echinoderm ossicle rayed (Allonnia-type) and rosette-like from biofacies unit 2 (Su Corovau section). Note the (Chancelloria-type) sclerites. Many of the stereome structure and the distinct lumen. SEM- sclerites are single disarticulated rays. micrograph. Scale bar is 0.1 mm. Identified forms include Allonnia tripodophora, 13 – Hyolith Microcornus sp. from Su Corovau section Chancelloria sardinica, and Archiasterella (biofacies unit 2). SEM-micrograph. Scale bar is 0.1 mm. pentactina. MOSTLER (1985) mentions Chancelloria maroccana and Archiasterella sp. (Gutturu Pala) there are phosphatic shells. Two as occurring in the Campo Pisano Formation. groups of linguliformeans are recorded in great Hyolithelminths (probable polychaetes), are quantity: lingulides (Lingulella, Schmidtites) nearly straight phosphatic open tubes with a and acrotretides (Opisthotreta, Vandalotreta, more-or-less coiled proximal part and a round Linnarssonella). cross-section. The few specimens, up to 3 mm Echinoderm (Fig. 6-6) remains occur long, collected from near the base of the Campo generally as disarticulated plates that often Pisano Formation, are identified as Hyolithellus preserve the original netlike stereome cf. filiformis (ELICKI et alii, 2003). A few structure. Phosphatized plates, segments and specimens of the first known Sardinian barrel-shaped elements are obtained by acid bradoriid were found at the same stratigraphic digestion (Fig. 6-12). The rarity of barrel- level. Their subtriangular shape, phosphatic shaped elements is interpreted to indicate (1) a mineralogy, and other surface morphologic rather small proportion of stemmed forms and features identify them as Hipponicharion (2) that the stems were probably rather short. ichnusum (ELICKI & PILLOLA, 2004). Much more widely distributed are polygonal Molluscs are represented by pelagiellid and thecal plates, commonly with sutural pores hyolith steinkerns with partially preserved (epispires) for respiration (SPRINKLE, 1992).
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Figure 7: Fossil content (from thin sections) of stratigraphic sections of the Campo Pisano Formation arranged from West (Masua section) to East (Santa Barbara). Some sections shown on Fig. 4 are not included because of a stronger metamorphic overprint and/or local tectonics. Some tectonic events cause an interruption in the continuity of the fossil record. The units so affected are designated. pseudomorph shells. Though the systematic hadimopanellids (considered to be affinity of pelagiellids is debated (GUBANOV & palaeoscolecids; HINZ et alii, 1990; MÜLLER & PEEL, 2000; PARKHAEV, 2001; LANDING et alii, HINZ-SCHALLREUTER, 1993) are represented by 2002), the Sardinian specimens all belong to very well preserved specimens of Pelagiella subangulata (for detailed description Hadimopanella (Fig. 6-10), abundant in discrete see ELICKI, 2002). Both hyolithomorphs and horizons of the lower portion of the Su Corovau orthothecimorphs are present in the Campo section (Fig. 4). In addition, some poorly Pisano Formation, but the only genus identified preserved sclerites are known from the Campo thus far is Microcornus (Fig. 6-13). Pisano Formation type section (Hadimopanella cf. oezgueli; see CHERCHI & SCHROEDER, 1984). So called "small shelly fossils" (phosphatic MOSTLER (1985) also mention the occurrence of microproblematica) are represented by hadimopanellids in the Gutturu Pala section hadimopanellids and cambroclaves. The (Fig. 4), but gives no detailed information or
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figures. Cambroclaves (Fig. 6-11) are taphonomic and diagenetic phenomena. represented by minute sclerites of probable wormlike animals. They came from the same Biofacies and paleoecology locality and levels as the hadimopanellids Taphonomic remarks (ELICKI & WOTTE, 2003). The microfauna of the Campo Pisano Microbes occur in the Campo Pisano Formation is chaotically oriented (Fig. 6-7). Formation too. The endolithic cyanobacterium There is no parallel "bedding", distinct layering, Endoconchia angusta was observed between or size gradation. Even though no clear the steinkerns and shell pseudomorphs of indication of burrows exists, the chaotic molluscs, and some single specimens of the orientation of the biota in this succession, which possible oscillatoriacean Girvanella problematica was deposited at a low rate of sedimentation are reported from the Campo Pisano Formation (LOI et alii, 1995), might be interpreted to be a by CHERCHI & SCHROEDER (1984). The lack of result of bioturbation. Nevertheless, because of other microbial fossils may be to the result of the lack of infaunal burrows or other deep traces and the absence of fecal pellets, a rather
11 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01) shallow bioturbation near the sediment surface pelagiellids, and (rare) hyoliths. Of particular can be assumed. The lack of surface traces of paleoecologic and biostratigraphic interest is bioturbation may have been caused by the restriction in the occurrence of phosphatic intensive bioerosion due to low sedimentation small shelly fossils to this biofacies and rates, or by late diagenetic pressure dissolution stratigraphic level (Figs. 6-10 & 6-11). (ELICKI, 2001). Most of the fossils are preserved Transition into 'biofacies unit 3' (bu3; Figs. 7 as calcite pseudomorphs. & 8) is gradual. Bu3 is characterized by about Mechanical abrasion of the fossil remains the same fossil groups as bu2, but trilobites was not seen. Even very fine and fragile dominate the fauna (Fig. 6-7). Echinoderms are structures on cambroclaves and also important constituents, and brachiopods, microbrachiopods are perfectly preserved (Figs. poriferans, hyoliths, and chancelloriids are 6-10 & 6-11). That portion of the fauna that occasionally present. This faunal composition had scleritomes covering or embedded in soft continues upward to the base of the Cabitza tissue is represented exclusively by Formation, but fewer and smaller fossils are disarticulated sclerites (poriferans, present in the last few decimeters of the Campo chancelloriids, echinoderms, etc.). Postmortem Pisano Formation, designated here as 'biofacies microbial decomposition of organic matter may unit 4' (bu4; Figs. 7 & 8). have led to disarticulation in situ rather than as All sections of the Campo Pisano Formation an accumulation caused by currents or slides. show more-or-less the same general trends of Too, trilobites in thin sections commonly show biofacies described above, albeit with local articulations (if the orientation of the section minor variations in fossil content and permits their recognition). These observations, distribution (Fig. 7). In several regions, e.g., Su together with the absence of lithoclasts and the Banatzu, the Terraseo region, the Mt. Scorra– near-total absence of coarse siliciclastics Masua region, and at Mt. Serrau the content of suggest that taphocoenosis was autochthonous. fossils increases in the upper levels of bu3 (Fig. Cone-shaped fossils such as hyoliths 7). At Gutturu Pala, sponge spicules are occasionally show cone-in-cone stacking, thus prominent and reach a second maximum at indicating some bottom currents with low to these same levels (Fig. 7). Throughout the moderate energy. Generally, bioclasts become entire succession at Masua, poriferans are much fewer and smaller immediately below the more abundant than they are at other localities. transition to the overlying Cabitza Formation.
Paleoecological implications Biofacies units The fossil groups of the Campo Pisano There is no clear correlation between the Formation occupied a variety of habitats and occurrence of certain fossil groups and display a number of feeding strategies. carbonate textural types (mudstone, floatstone, Completely infaunal forms are represented by wackestone, packstone). Sections in the hadimopanellids (Fig. 6-10), usually interpreted southeastern part of the study area tend to as palaeoscolecids, for which a deposit-feeding have fewer fossils than sections in the strategy is assumed (e.g., WRONA & HAMDI, northwest. Generally, the various fossil groups 2001). Whether or not some of the agnostid show a similar pattern of stratigraphic trilobites were also deposit feeders cannot be distribution everywhere: at most localities fossil determined with certainty. The lack of eyes and content is highest at the base of the Campo small size alone do not validate such an Pisano Formation. In nearly every section, the interpretation. Unquestionable infaunal trace Campo Pisano Formation begins with a fossils have not been found in the formation. poriferan spiculite (locally as mass Semi-infaunal forms, represented by hylolihs accumulations; e.g., at Su Banatzu, Terraseo with a rounded cross-section are found only West, Santa Barbara, Monte Scorra, Su rarely in the Campo Pisano Formation (ELICKI, Corovau, Monte Serrau; Figs. 4 & 7), whereas 2000). other fossil groups are clearly under– represented (rare brachiopods, small trilobites Most of the fauna in the Campo Pisano or echinoderms and single hyolithelminths and Formation is epibenthic. Sessile and semi- bradoriids), or totally absent. This remarkable sessile epibenthic forms are represented by initial biofacies is referred to here as 'biofacies poriferans, chancelloriids, most of the unit 1' (bu1; Figs. 6-5, 7 & 8). echinoderms, and brachiopods (Figs. 6-8 & 6- 12). For hyolithelminths a secondary epibenthic The overlying 'biofacies unit 2' (bu2; Figs. 7 mode of life is probable. Whereas the filter- & 8) is characterized by an abrupt onset of feeding sponges ingested mainly very small echinoderm remains (probably mostly food particles and bacteria, the other sessile eocrinoids, MOSTLER, 1985; Figs. 6-6 & 7) which epibenthos lived on suspended detritus and are dominant here. Trilobites and brachiopods microorganisms. The infaunal and sessile- are conspicuous accompanying faunal elements epifaunal organisms noted above all suggest a while sponge spicules are of minor importance. soft firm- to mud ground habitat. There is no New (in comparison to bu1), but minor evidence for hardgrounds. elements of the biota are chancelloriids,
12 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01)
Mobile epibenthic biota are also common feeding may be provided by the cambroclaves, (most of the trilobites, some hyoliths, of which the skeletal elements have been pelagiellids, bradoriids and some small shelly interpreted as the protective external armor of fossils such as cambroclaves; Figs. 6-9, 6-11 & a mobile epibenthic animal (CONWAY MORRIS & 6-13). Some of the echinoderms too may have CHEN, 1991). Exclusively nektic elements of the had only limited movement (cinctans, perhaps Campo Pisano Formation fauna are unknown stylophorans, some edrioasteroids; SPRINKLE, (even though some animals may have had such 1992; FRIEDRICH, 1993; ZIEGLER, 1998; behavior during their early ontogeny). Whether GUENSBURG & SPRINKLE, 2001; LEVEBVRE & FATKA, or not some of the biota were scavengers is not 2003; CLAUSEN & SMITH, 2005). A few of the apparent. So far, the only representative of the epibenthos could have adopted a sporadic or primary producers identified is Girvanella nektobenthic life-style (some trilobites and (oscillatoriacean cyanobacterium; CHERCHI & bradoriids). All these groups are commonly SCHROEDER, 1984). viewed as low-level suspension feeders, but The geographic (lateral) and stratigraphic deposit feeding (e.g., some trilobites and (vertical) distribution of the biofacies units (bu1 possibly some carpoid echinoderms) or grazing to bu4) and the trophic and habitat (some molluscs) might have been feasible. characteristics of the fauna are fundamental to Direct indication of predation was not found. an interpretation of the depositional However, indirect evidence for this mode of environment of the Campo Pisano Formation.
Figure 8: Bathymetry and thickness of the Sardinian Cambrian deposits and paleoecological succession (character of habitats and the dominant feeding strategy) in the Campo Pisano Formation that together indicate an intermittent drowning process.
To summarize: in the 19 sections Upsection, a low-level suspension-feeder fauna investigated the basal unit of the Campo Pisano becomes important (echinoderm biofacies, bu2; Formation is overwhelmingly dominated by Figs. 7 & 8). The dominant fossils of bu2 are sessile epibenthic filter feeders (spiculite sessile forms (most echinoderms, brachiopods), biofacies, bu1; Figs. 7 & 8). Sessile and mobile but mobile elements are also common suspension feeders are rarely present in bu1. (trilobites). The proportion of poriferans
13 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01) decreases significantly. In the accompanying consistent with the trilobite biostratigraphy. fauna, distinctive elements include a rare Lingulid and acrotretid brachiopods from the infaunal biota (hadimopanellids) and other upper Campo Pisano Formation (especially fossils, which suggest adaptations for protection Linnarssonella sp.) imply a late Middle against predators (cambroclaves). The upper Cambrian age (MERGL & ELICKI, 2004). Campo Pisano Formation (bu3; Figs. 7 & 8) is Poriferans, chancelloriids, hyoliths characterized by mobile epibenthic suspension (Microcornus), pelagiellids, and small shelly feeders and by an increase of probable nektic or fossils (e.g., Hipponicharion) are in accordance planktic elements (greater proportion of with a late Early to Middle Cambrian age agnostids). The proportion of sessile organisms (BENGTSON et alii, 1990; PARKHAEV, 2001; ELICKI, decreases significantly. In the uppermost 2002; GOZALO & HINZ-SCHALLREUTER, 2002; ELICKI Campo Pisano Formation (bu4) an increase of et alii, 2003). transported elements (indicated by small Precise correlation of sections in the study broken bioclasts; Figs. 7 & 8) is interpreted area is hindered by a lack of trilobite material. here as indicative of a more pelagic origin. However, on a regional scale this problem can Figure 8 depicts this paleoecological succession be somewhat mitigated through the use of of biofacies along with habitat characteristics, ecostratigraphic data. For example, in the feeding strategies and bathymetry. Campo Pisano type section, the abrupt Biostratigraphy appearance of 'biofacies unit 2' (bu2) is the The age of the Campo Pisano Formation was equivalent of the top of the Protolenus determined primarily by trilobites, but, because (Protolenus) cf. pisidianus level. This distinct of their scarcity, the stratigraphic resolution change is interpreted as a regional obtained is low. environmental reorganization. For the study area, this means that the base of bu2 coincides The assignment of the trilobites from the with the base of the Middle Cambrian (in the Campo Pisano Formation to the Middle Mediterranean sense) and that bu1 is latest Cambrian (RASETTI, 1972) was refined by PILLOLA Early Cambrian in age. So the bu1–bu2 (1991) and LOI et alii (1995) who identified two transition represents an ecostratigraphic level stratigraphically sequential assemblages. The permitting recognition of the Early–Middle oldest trilobite fauna of the Campo Pisano Cambrian boundary in southwestern Sardinia. Formation (CP1 fauna of LOI et alii, 1995), Using this model, a correlation of the basal located several meters above the base of the portions of most of the sections is possible formation, includes Protolenus (Protolenus) cf. although trilobites are absent. The validity of pisidianus that indicates the Early to Middle this technique has been confirmed recently by Cambrian boundary interval (for discussion see finding the Protolenus (Protolenus) cf. LOI et alii, 1995). The base of the Campo Pisano pisidianus index taxon in the Su Corovau Formation therefore is in the uppermost Early section (PILLOLA, pers. comm., 2004; Fig. 4) at Cambrian in the Mediterranean sense (for precisely the level predicted by ecostratigraphy. discussion of the problematic interregional correlation of the Early–Middle Cambrian Paleobiogeography boundary see GEYER & LANDING, 2004), whereas The Campo Pisano trilobites have a western the immediately overlying portion of the Gondwanan affinity (Fig. 1). Geographical formation (early CP2 fauna of LOI et alii, 1995) relationships exist with southern France correlates with the Paradoxides mureroensis (Montagne Noire), northern Spain and southern Zone (Leonian, early Middle Cambrian; early to Turkey, i.e. the Mediterranean region of the so- middle Agdzian of the western Gondwana scale called "Atlantic Faunal Province" sensu SDZUY of GEYER & LANDING, 2004; Fig. 3). The middle (1972) and PILLOLA (1994). Associations with portion of the Campo Pisano Formation (middle the northern region of the "Atlantic Faunal CP2 fauna of LOI et alii, 1995) yields Calodiscus Province" seem to be more distant. foveolatus, some oryctocepalids, dorypygids Biogeographic relations with the Far East as and others and corresponds to the Badulesia interpreted by SDZUY (1972), PILLOLA (1991) and Substage of the mid-Middle Cambrian (early ÁLVARO et alii (2003) for some Early Cambrian Caesaraugustan; LOI et alii, 1995; Fig. 3). The faunas have not yet been found among the upper part of the Campo Pisano Formation (late trilobites of the Campo Pisano Formation. But, CP2 fauna of LOI et alii, 1995) contains, among some small shelly fossils (cambroclaves) may others trilobites, Pardailhania hispida and indicate biogeographic relationships with the Ptychagnostus sp., relating to the Pardailhania Asiatic realm. Until recently, cambroclaves were to basal Solenopleuropsis Substages (middle known only from Kazakhstan, China, and Caesaraugustan) of the mid- Middle Cambrian. Australia. But, the latest finds from the late The transition from the Campo Pisano Early Cambrian of Germany (ELICKI, 1994) and Formation to the Cabitza Formation should thus early Middle Cambrian of Sardinia (ELICKI & be in the late Caesaraugustan (late WOTTE, 2003), both on the western Gondwana Solenopleuropsis Substage of the late Middle shelf, represent the oldest and youngest Cambrian; Fig. 3). representatives of this group known thus far and may have some paleogeographic Non-trilobite age determinations are
14 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01) significance. The mollusc Pelagiella subangulata western Gondwanan late Early and early Middle shows a similar paleobiogeographic distribution Cambrian beds of Germany, Spain, Morocco, along this "Palaeotethyan belt" (BRASIER, 1989; and Sardinia (ELICKI, 1994; GOZALO & HINZ- PARCHAEV, 2001; ELICKI, 2002). Small shelly SCHALLREUTER, 2002). In summary, a good fossils offer further support for close faunal exchange among and connections connections between the Mediterranean between the western Gondwanan Cambrian areas: e.g., the bradoriid paleogeographic regions is clearly established. Hipponicharion is a good index taxon for the
Figure 9: Interpretation of development stages at different time slices: During late Gonnesa time, the carbonate platform was already segmented by tensional tectonics (origin of internal breccias) resulting in a significant bottom topography; at steep flanks mass flows occurred. Early in Campo Pisano times the sea level rose and tensional- related faults were still active locally. This led to the continued development of internal breccias and bottom topography was only moderately accidented. Early in Cabitza times the rise in sea level continued and deeper marine clastics covered the drowned carbonate series. 1 – San Giovanni Formation, 2 – Campo Pisano Formation, 3 – Cabitza Formation, 4 – oolites, 5 – internal breccia, 6 – external breccia, 7 – mass flow, 8 – fault.
Discussion Benedetto to Su Banatzu line (Figs. 4 & 7). Thickness decreases northwest of this line. Distribution of facies patterns Because deposition was almost entirely Microfacies analysis of the Campo Pisano autochthonous (see above), this region is Formation demonstrates that many macroscopic assumed to be a carbonate depocenter – a features, e.g., color and nodularity, are of little weakly elevated, low-energy area – where value in such studies. After extensive field and conditions for carbonate precipitation were microfacies investigations, the following favorable (Fig. 4). The range in number and characteristics were found to be essential for quantity of the autochthonous biota shows a environmental interpretation: (1) microfacies, similar distribution (Fig. 7), suggesting that this (2) geographic and vertical distribution of biota, area was favorable to life and hence a preferred (3) taxonomic variety, (4) non-biogenous habitat. This view agrees well with the inference components, (5) ichnology, (6) regional that it was a weak high. For a part of the Early variation in stratigraphic thickness, and (7) the Cambrian, a similar, but high-energy shallow characteristics of underlying and overlying area was proposed by BECHSTÄDT et alii (1985), strata and of the formational limits. but it was sited some 5–10 km to the west, coinciding approximately with the present Within the study area, the thickness of the southwestern Sardinian coastline. The weak rise Campo Pisano Formation increases slightly from during the Middle Cambrian may represent a southeast to northwest as far as the San
15 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01) later stage of this topographic elevation, stratigraphic succession, from bu2 to bu3 (Figs. relocated by a rise in relative sea-level (see 7 & 8), took place without a conspicuous below). change in lithology; trilobites (especially agnostids) became more numerous, while the The abrupt arrival of the silty Campo Pisano sessile epibenthos declined (Fig. 8). This Formation over the shallow, isolated carbonate change is best attributed to an ongoing, steady platform of the Gonnesa Group (Figs. 3 & 5-1) rise in relative sea-level. is indicated by its initial spiculite biofacies (bu1; Figs. 6-5 & 7) in nearly every section, Near the lithological transition to the sometimes as a gradual transition, in other overlying siliciclastic Cabitza Formation (Fig. 7), places associated with breccias (see GANDIN, the environment changed again. At this level 1987b; and the discussion of these breccias the fossil content as a whole decreases below). The sudden influx of siliciclastic significantly and among the trilobites and material into the depositional area with the brachiopods, only small, mostly broken parts arrival of the Campo Pisano Formation is best remain (bu4); this fragmentation is assumed to interpreted as the aftermath of the completion have been caused by the action of currents that of the fill of a sediment-trapping basin between transported the fossils. The accompanying an erosional area and the isolated carbonate lithological change is interpreted as evidence of platform, This is the interpretation of BECHSTÄDT a second distinct eustatic pulse that caused the et alii (1985) ("Eastern Sulcis basin"), that depositional environment to deepen (Fig. 8). accounts for fine siliciclastic sediment reaching However, it is improbable that the depth of a formerly isolated depositional area. water exceeded that of an outer shelf because Suspension currents were presumably the of the general persistence of similar lithofacies primary agent for the transport of the very fine- in overlying strata. The existence of deeper grained siliciclastics of the Campo Pisano conditions may not be taken as clearly Formation. Infrequent storms may have expressed lithologically until a fully siliciclastic occurred, but the hummocky cross-bedding, regime was established in the Cabitza allochthonous shell layers, reworked and Formation. LOI et alii (1995) have demonstrated transported fauna, and extraclasts commonly that younger portions of the Cabitza Formation produced by storms were not observed. These were deposited in relatively shallow waters near facts support the suggestion by previous storm wave base. This supports the view that authors that extensional tectonics combined the uppermost Campo Pisano and the lower with a relative rise in sea-level affected the Cabitza Formations were not much deeper than area and led to the "drowning" of the platform the overlying strata, because extreme (GANDIN, 1987a, 1987b; BECHSTÄDT et alii, 1988; fluctuations in bathymetry in such a short time BECHSTÄDT & BONI, 1989; COCOZZA & GANDIN, are unrealistic. Hence, the rather rapid 1990). The brief dominance of filter feeders lithological change from the carbonates of the (bu1; Figs. 7 & 8) at the base of the Campo Campo Pisano Formation to the siliciclastic Pisano Formation reflects the successful sediment of the Cabitza Formation probably adaptation of one group of organisms to the does not indicate a dramatic increase in water newly increased depth and to the sudden rise in depth, but rather reflects a drift to a higher the quantity of suspended material along with latitude (cooling and increased precipitation). more influential water currents, and in addition Morphology and bathymetry of the the advantages gained by a rapid adaptation to depositional area new ecological niches. The interpretation of the depositional The transition from bu1 (spiculite biofacies) environments of the Campo Pisano Formation to bu2 (echinoderm biofacies) is a conspicuous used in previous models was based on facies change of milieu in the lower Campo Pisano and sedimentological studies (GANDIN, 1979, Formation affecting the entire depositional area 1987a, 1987b; BECHSTÄDT et alii, 1988; COCOZZA (Figs. 7 & 8). It is interpreted as reflecting an & GANDIN, 1990; and others). The resulting increase in water depth. The persistence and model for the Campo Pisano limestone was a strong dominance of sessile epibenthic type of nodular deep water facies ("ammonitico suspension feeders in this biofacies (Fig. 8) rosso" lithofacies) deposited on pelagic suggest a permanent increase in the influx of topographic highs on a broken, drowned suspended nutrients. Reasons for this change platform, while troughs between these highs might include a reorganization in current would serve as areas of deposition for patterns or a wetter climate that would intensify calcareous clays and silt. Nevertheless, judging chemical erosion and sediment transport in the from the microfacies and paleoecological results hinterland and thus an increase in the input of presented here, this model must be modified. siliciclastic sediment. These changes may have In the 19 studied sections (Fig. 4A), nodular, been caused by a shift of this region to a higher wavy-bedded, silty limestones, and massive paleogeographic latitude (COURJAULT-RADÉ et alii, carbonate banks alternate repeatedly and 1992; MCKERROW et alii, 1992; SESLAVINSKY & irregularly (Figs. 5-3, 5-4, 5-5, 5-6, 5-7, 5-8 & MAIDANSKAYA, 2001). 5-9). Closely adjacent sections do not show the The next change in the biofacies of the same patterns of vertical distribution for these
16 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01) lithotypes, so that their sequence cannot be differentiation does not exist. Biotic used for correlation. Furthermore, biofacies do assemblages occur at the same stratigraphic not correlate with these lithofacies. In the levels over the whole area, and there are no discrete more or less widely separated sections marked lateral differences in their composition. although there is no difference in the sequential The characteristics of the breccias mentioned position of the zones established by the above (clast content and shape, matrix, autochthonous fossils, this fossil zonation has thickness, stratigraphic position) are easily no relationship to the lithology of the explained as typical of fault-related internal approximately corresponding stratigraphic brecciation (Fig. 9). Breccias of this origin are levels: that is whether or not the same fossils typically monomictic (or nearly monomictic) (qualitatively and quantitatively) are enclosed and part of their marl matrix may be derived in nodular, wavy-bedded, silty or massive from overlying strata, so can contain fossils limestones. The origin of these textural types is from overlying beds injected with the marls into related to diagenetic processes (GANDIN, 1987b; the still unconsolidated internal breccias below ELICKI, 2001) that have accentuated minor (FÜCHTBAUER, 1988). Breccias of this kind differences in the lithology of the primary originate beneath a sedimentary cover that is sediment (e.g., siliciclastic content, under a tensional tectonic regime (FÜCHTBAUER & permeability), and early cementation; the RICHTER, 1983; FÜCHTBAUER, 1988). ARRIBAS et alii direction and velocity of minor currents may (1990) observed transitions from the Sardinian also be involved. The relative uniformity of breccias in the upper levels of the Gonnesa biofacies over the entire are of study platform into unbrecciated rocks below, a demonstrates that during deposition of the phenomenon characteristic of such an origin Campo Pisano Formation bottom conditions that FÜCHTBAUER, 1988). ARRIBAS et alii (1990) were could have caused ecological differences were the first to interpret the breccias at the almost completely absent. However, a slight Gonnesa–Campo Pisano transition as flexure- difference in bottom topography may have led related rocks caused by tension. A to small regional differences in siliciclastic reinterpretation of lithological features, taking content (an increase to the East) as reported by into consideration regional tectonics (e.g., BECHSTÄDT et alii (1988). Semi-quantitative GANDIN, 1987b; BECHSTÄDT & BONI, 1989; analysis of sections from several localities found COCOZZA & GANDIN, 1990), and paleoecologic quartz content to be below 2 % that HAMANN data presented here indicate that when these (2003) regarded as probably aeolian. breccias formed the morphologic gradient was Nevertheless, this weak eastward increase in rather low (in contrast to the gradient inferred siliciclastic content in the Campo Pisano for other types of breccias and debris flows that limestones may support the interpretation of developed on the platform slopes during older BECHSTÄDT et alii (1988), that a hinterland lay to Gonnesa time, BECHSTÄDT et alii, 1988). If the the East, in sharp contrast to the views of gradient was low, a horst-and-graben GANDIN et alii (1987), who postulates a basin environment for the breccias of Campo Pisano that direction. time cannot be sustained (Fig. 9). Breccias There are two opinions regarding the depth Breccias exist in the beds marking the of water under which the Campo Pisano changeover from the Gonnesa platform to the Formation was deposited. The fossil content has Campo Pisano offshore carbonates (BONI et alii, been interpreted as a neritic shelf fauna, but 1981; GANDIN, 1987a, 1987b; BECHSTÄDT et alii, the nodular texture of the carbonates has been 1988). These breccias (Fig. 5-2) have usually thought to indicate depth up to several been interpreted as due to tensional tectonics hundreds of meters (e.g., MOSTLER, 1985; and/or karstification. GANDIN (1987b) described SCHLEDDING, 1985). Shallower conditions were two discrete types: (a) breccias consisting of assumed by COCOZZA (1979, "shallow water"). well sorted clasts (up to 20 cm), and (b) The Campo Pisano carbonates are typical unsorted megabreccias. The first type (a – Mt. Cambrian offshore sediments, deposited in the Agruxiau type) GANDIN interpreted as karst- photic zone below storm wave base (Figs. 8 & related with a red hematitic and/or marly 9). No indicators of wave and tidal activity, matrix sometimes yielding Campo Pisano erosion, transport of clasts from shallower fossils. The second (b – Mt. San Giovanni type) areas, or gradation, or hummocky bedding was was called a collapse breccia with a marly observed. Rarely, thin allochthonous shell matrix, and lacking Campo Pisano Formation layers occur (ÁLVARO, GANDIN, pers. comm.); clasts. A karstic origin for these breccias was possibly they are indicative of storms. All these doubted by BECHSTÄDT et alii (1988). For some features are typical in distal ramp or shelf authors these breccias indicate a profound relief environments, which, in the Cambrian are on the sea floor (CARANNANTE et alii, 1984; usually characterized paleoecologically by a SCHLEDDING, 1985; GANDIN, 1987b; BECHSTÄDT et dominance of echinoderms and sponges and by alii, 1988; COCOZZA & GANDIN, 1990). However, intensive bio-retexturing (READING, 2002). The significant relief with highs and lows would source of carbonate mud in such environments promote distinct differentiation in the biofacies could be (1) adjacent shallow platforms (if of the autochthonous fauna. But such allochthonous), or (2) biotic precipitation by
17 Carnets de Géologie / Notebooks on Geology - Article 2006/01 (CG2006_A01) planktic microbial organisms (if autochthonous) dominated biofacies in which the mobile or (3) a mixture of both (HANDFORD, 1986; epibenthos becomes increasingly important AURELL et alii, 1995; READING, 2002). An upward as the depth of water increases (Figs. 7 autochthonous microbial origin is favoured for & 8). Near the boundary with the deeper water the Campo Pisano carbonates. No shallow Cabitza Formation an increase in the strength of platform remnants "able to deliver carbonate currents is suggested by the fact that the fossils ooze are known from that time" BECHSTÄDT & are all fragmentary and have been transported. BONI, 1989). Furthermore, the high mud This trend to less proximal and deeper-water content points to a rather low slope on the sea sedimentation is apparent over the entire floor, a conclusion compatible with the working area for it is expressed in both interpretation from fossils of a shelf/ramp stratigraphy and paleoecology. Regional environment. paleoecologic trends (Figs. 4 & 7) are areally extensive and not pronounced, so they are Although "neritic" conditions are strongly thought to have been caused by a rise in sea indicated by the fauna, by the lithology (LOI et level rather than by tensional block-tectonics alii, 1995), and by the context of syntectonic and fracturing on the sea floor. The data are evolution (BECHSTÄDT & BONI, 1989; GANDIN et interpreted as indicating the existence of a alii, 1987), absolute water depths are hard to weak rise (low-energy swell) trending SE–NW calculate because of limited knowledge that served as a carbonate depocenter and is concerning rates of subsidence, sediment thought to have been a preferred habitat for the budgets and the range of rises in sea-level. Campo Pisano biota (Fig. 4B). There is no Nevertheless, because of (1) the shallow evidence of a high-energy shallow area subtidal to intertidal conditions indicated for the anywhere. The breccias are classed as underlying upper Gonnesa Group (BECHSTÄDT et "internal" that is formed below a cover of alii, 1985; GANDIN et alii, 1987; FRÖHLER, 1994), sediments while tensional tectonics were active (2) the conclusions of COCOZZA & GANDIN (1990) (Fig. 9). and LOI et alii (1995), that the overlying lower Cabitza Formation indicates a deeper shelf, and The biofacies and the sedimentary criteria: (3) the two regional well-defined paleoecologic thickness, mud-, fossil-, and clastic content changes in the Campo Pisano Formation indicate that the area gradually drowned as one indicate a two-step deepening of the whole area unit; it was undifferentiated bathymetrically during Campo Pisano time (Fig. (Figs. 8 & 9). It is classed as a neritic open- 8).Consequently, it is with considerable marine shelf or ramp. An intermittent rise of confidence that the siliciclastic lower Cabitza relative sea-level, accompanied by a tensional Formation is assigned a depth below the photic tectonic regime, along with a move into higher zone and the Campo Pisano Formation itself is paleogeographic latitudes, led to a regional considered to have recorded an intermittent uniformity in facies. The sudden rise of relative deepening of the sea within the photic zone sea-level at the transition from the Gonnesa (Figs. 8 & 9). platform to the Campo Pisano offshore carbonates caused the entire area to be under Conclusions about the same depth of water (Fig. 8). Great The late Early to late Middle Cambrian morphological differences within the area, Campo Pisano Formation of southwestern caused by tectonism during the isolated Sardinia is represented by a succession of platform stage (Gonnesa Group), are nodular, wavy-bedded, silty, and massive interpreted here as having been leveled by offshore limestones (Fig. 5) deposited during Campo Pisano times through relief-filling the drowning of the isolated Sardinian Early redeposition during late Gonnesa times and by Cambrian carbonate platform. After a distinct offshore sedimentation during early Campo rise in relative sea-level that caused the Pisano times (Fig. 9). So the model presented isolated carbonate platform conditions of the here resolves seeming contradictions between underlying Gonnesa Group to end (Fig. 9), but sedimentologic and paleontologic criteria. with water-depth still relatively shallow, the initial Campo Pisano fauna developed with The techniques of micropaleoecological epibenthic filter feeders (sponges) dominant analysis presented here are a very useful tool and with a moderate amount of suspended for modeling the configuration and evolution of input from the hinterland (Fig. 7). This fossil Perigondwanan depositional areas during the community predominated in the lower beds of Cambrian. The method can be successfully the Campo Pisano Formation in nearly all of the applied to the reconstruction of regional 19 sections mapped. geological processes for that time, in particular where biostratigraphic data and sedimentary Higher in the succession poriferans persist manifestations are fragmentary or unavailable. as an important element of the fauna only in some westernmost areas. Upsection, echinoderms come to dominate the biofacies (mainly sessile epibenthic eocrinoids and other low-level echinoderm suspension feeders) and they in turn are succeeded by a trilobite-
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Acknowledgements platform.– Facies, Erlangen, v. 12, p. 113– I express my warmest thanks to Thilo 140. BECHSTÄDT (Heidelberg, Germany), Maria BONI BECHSTÄDT T., SCHLEDDING T. & SELG M. (1988).– (Naples, Italy), and Anna GANDIN (Siena, Italy) Rise and fall of an isolated, unstable for encouragement, discussions, and joint field carbonate platform: the Cambrian of trips. Thanks also to Gian Luigi PILLOLA (Cagliari, Southwestern Sardinia.– Geologische Sardinia) and Michal MERGL (Plzeň, Czech Rundschau, Berlin–Heidelberg, v. 77, p. Republic) for collaboration on special fossil 389–416. groups and to Petra MÜNZBERGER (Regensburg, BECHSTÄDT T., BONI, M. & LASKE R. (1994).– The Germany) and Yvonne HAMANN (Leipzig, Sardic unconformity. In: BECHSTÄDT T. & BONI Germany) for analytic work. I am very grateful M. (eds.), Sedimentological, stratigraphical to Pierre KRUSE (Darwin, Australia), Bosiljka and ore deposits field guide of the GLUMAC (Northampton, Massachusetts), James autochthonous Cambro–Ordovician of F. MILLER (Springfield, Missouri), Paul COPPER southwestern Sardinia.– Memoire descrittive (Sudbury, Canada), and Nestor SANDER della carta geologica d'Italia.– Servizio (Modesto, California) for important remarks on Geologico Nazionale, Roma, v. XLVIII, p. the manuscript and for linguistic help. Jan 109–114. FISCHER and Michael EATON (both Freiberg) are BENDER F. (1974).– Geology of Jordan.– gratefully thanked for technical help. The Gebrüder Borntraeger, Stuttgart, 196 p. project was generously supported by the BENGTSON S. & MISSARZHEVSKI V.V. (1981).– German Research Foundation (project EL 144/1 Coeloscleritophora – a major group of 'Comparative studies of facies and paleoecology enigmatic Cambrian metazoans. In: TAYLOR of the final stage of a Cambrian platform M.E. 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