A Unique Fossil Record from Neptunian Sills: the World's Most Extreme Example of Stratigraphic Condensation (Jurassic, Western
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Acta Geologica Polonica, Vol. 67 (2017), No. 2, pp. 163–199 DOI: 10.1515/agp-2017-0015 A unique fossil record from neptunian sills: the world’s most extreme example of stratigraphic condensation (Jurassic, western Sicily) JOBST WENDT Institut für Geologie und Paläontologie der Universität, Sigwartstrasse 10, D-72076 Tübingen, Germany E-mail: [email protected] ABSTRACT: Wendt, J. 2017. A unique fossil record from neptunian sills: the world’s most extreme example of stratigraphic condensation (Jurassic, western Sicily). Acta Geologica Polonica, 67 (2), 163–199, Warszawa. Neptunian sills at Rocca Busambra, a fragment of the Trapanese/Saccense Domain in western Sicily, host the most abundant ammonite and gastropod fauna which has ever been recorded from the Jurassic of the western Tethys. The fauna is dominated by parautochthonous organisms which were swept into the sills by gentle transport. Ammonites are characterized by perfect preservation and small size, a feature which is due to the predominance of microconchs but also of stunting. The most complete sill is 0.7 m thick and could be separated into 17 levels which range in age from the early Toarcian into the late Kimmeridgian, thus representing the most extreme case of palaeontologically and depositionally documented stratigraphic condensation in Earth history. The unique feature of the Rocca Busambra sills is due to the interaction of three processes: extreme stratigraphic condensation on the sea floor, weak tectonic fracturing of the host rock and repeated reopening on top of already existing sills. Contrasting percentages of gastropods in individual levels reflect sea-level oscillations which correspond to long known low- and highstands during the Jurassic of the western Tethys. Comparisons with other ammonite-bearing sill faunas reveal several similarities, but represent only short-timed phases of tectonic pulses and deposition. Key words: Sicily, Jurassic; Biostratigraphy; Synsedimentary tectonics; Neptunian sills; Condensation; Ammonite fauna; Sea level. INTRODUCTION ments are a particular type of “Fossil-Lagerstätten” (Seilacher 1970) which may preserve a unique fossil The knowledge of neptunian dykes and sills goes record which is not found, or only poorly represented, back into the late 19th century. Many examples, the in normal deposits on the sea floor. Generally, dykes majority from the Mediterranean Jurassic, have been were apparently filled by a single rapid sedimentary described from the northern and southern Calcareous process that rarely trapped skeletal remains sufficient Alps, the Klippen Belt of Slovakia, the Dinarides, the for an exact dating of the time of infilling. A much Iberian chains and elsewhere (references in Flügel more complete biostratigraphic history, however, can 2010: 216–223), a few also from the boreal realm be revealed from neptunian sills, which were formed (Jenkyns and Senior 1991; Wall and Jenkyns 2004). close to the sea floor. In these bedding-parallel voids, But only since the early 1960’s has it become evi- shells were not only accumulated but, due to the ab- dent that some of these unusual depositional environ- sence of currents and reworking, are commonly pre- 164 JOBST WENDT served in unusual perfection. Neptunian sills may PREVIOUS WORK represent a short time-span only but, in the case of condensed or totally missing sedimentation (omis- Gemmellaro (1877) was the first to describe and sion) on the sea floor, they may contain a unique figure a few ammonoids and brachiopods from the stratigraphic record that has no counterpart in the so-called “Zona con Posidonia alpina” (Dogger) normal sequence. from Rocca Busambra which, distinguished by their Many profound thoughts and careful consider- small size, excellent preservation and fine-grained ations have been produced about the aspects and red matrix are clearly derived from neptunian sills. processes of stratigraphic condensation (Heim 1934; A more complete overview of the ammonite fauna Jenkyns 1971a; Fürsich 1978; Kidwell 1986; Loutit et from the same area, at that time labelled as Tithonian, al. 1988; Martire 1992a; and others; additional refer- was published by De Gregorio (1922), unfortunately ences in Flügel 2010: 211–216). The contribution to illustrated with rather distorted and barely distin- this subject by Gomez and Fernández-López (1994) guishable figures. Personal efforts to trace the orig- appears relatively academic. They distinguish three inal specimens in the ancient palace of the family different processes of condensation (stratigraphic, in Palermo, now in the possession of Antonio de sedimentary and taphonomic), as if not all of them Gregorio’s grandson, remained unsuccessful, so that interact to produce one and the same end-member. this important fauna must be regarded as lost forever. The quintessence of all these reflections about strati- Renz (1924) collected a new ammonite fauna from graphic condensation is rather simple: Minimum net Rocca Busambra which remained unknown however, sedimentation leads to an accumulation of skeletal until Christ (1958, 1960) incorporated it into his de- remains whose different age cannot be distinguished scription of Upper Jurassic ammonite faunas from any more in a straightforward vertical sequence. A western Sicily. Unfortunately he overlooked the fact critical point is reached, when condensation turns that some genera, attributed to the Oxfordian, were into a hiatus (omission) and finally into the negative in fact Toarcian and Middle Jurassic in age (Wendt process of submarine erosion. The latter is an im- 1962). During subsequent years, the exact strati- portant secondary effect already during the process graphic range of the extremely rich fauna and its of condensation proper and is generally accompanied long misunderstood provenance from neptunian sills by submarine solution and mineral encrustations. was unraveled by Wendt (1963, 1965, 1969b, 1976, The result of stratigraphic condensation, i.e. the jux- 1971a). The depositional and tectonic evolution of taposition of index-fossils of different age in one and the area was described by Giunta and Liguori (1976), the same level, has a natural lower limit of preser- Basilone (2009, 2011), Barraca and Maesano (2012) vation, because deposition takes place on an open, and Lentini and Carbone (2014, pp. 109–116). current-swept sea floor. A random, but nevertheless representative compilation of examples from the Cretaceous, Jurassic, Triassic and Devonian shows MATERIAL AND METHODS that this lower limit is reached at an average sedimen- tation rate of about 0.1 m/m.y. (Table 1). In neptunian A total of 56 closely spaced sections through the sills, however, this value may be surpassed by one or a few centimetres up to a maximum of about one even two decimal points. metre-thick sequence of the sills were measured and In this study, such an unusual and unique record sampled in utmost detail. Samples were crushed into is presented from Rocca Busambra, a fragment of a 1–5 cm3 big pieces in order to obtain a most com- drowned and tectonically dismembered pelagic pla- plete and representative survey of the entire fauna. teau in the Jurassic of western Sicily. Repeated tec- By this time-consuming manual work it was guar- tonic fracturing created an intricate pattern of neptu- anteed also that tiny specimens and rare taxa, which nian sills and intervening dykes in the topmost levels are generally overlooked or ignored, were also saved. of the platform. This phenomenon was accompanied Unfortunately several boxes with hundreds of per- by extreme stratigraphic condensation during a long fectly preserved ammonites were stolen during one time span (late Early to middle Late Jurassic). By of my numerous trips to Sicily on the way back to these two processes a unique depositional environ- Germany near Catanzarro in Calabria. In spite of ment was established in which the most numerous, meticulous investigations, supported by the local diverse and exquisite ammonoid and gastropod fauna Carabinieri, they have never been recovered and re- ever discovered in the Jurassic of the western Tethys main probably buried in a nearby road trench or the is preserved. adjacent macchia. EXTREME STRATIGRAPHIC CONDENSATION IN NEPTUNIAN SILLS 165 Thickness of Average condensed sedimenta- System/Stage Zone Locality Sedimentology Reference sequence tion rate (m) (m/MA) L. Cretaceous tardefurcata to Subalpine Chains, 0.25–0.75 0.2–0.4 O, R, phosphorite Gebhard 1983 Albian lautus Zone S-France L. Cretaceous tardefurcata to Vorarlberg, Austria 0.6–0.8 0.15–0.2 O, R, phosphorite Gebhard 1983 Albian dispar Zone U. Jurassic/ virgatus to Eastern Russian Zorina et al. 1.6–2.4 0.5 ? L. Cretaceous subditus Zone Platform 2009 U. Jurassic tenuilobatus to O, F, nodular Tata Horst, Hungary 2.0 0.4 Fülöp 1976 Kimmeridgian beckeri Zone limestone U. Jurassic dentatum to Melchtal Alps, 0.1–0.5 0.1–0.15 O, R, F Rod 1946 Kimmeridgian beckeri Zone Switzerland U. Jurassic transversarium Cima Campo, Tren- Martire et al. Oxfordian/ 0.9 0.2 O, F to divisum Zone to-Platform, N-Italy 1991 Kimmeridgian U. Jurassic cordatum to Kachch Basin, India, Alberti et al. 0.5 0.35 O, R, S, B Oxfordian transversarium Zone Dhosa Conglomerate 2013 M./U. Jurassic parkinsoni to Asiago, Trento-Plat- Bajocian/ 15 1.15 O, F, cherts Martire 1992 platynota Zone form, N-Italy Kimmeridgian M./U. Jurassic zigzag to l. Bathonian– Rocca Busambra, Sicily 0.8–1.0 0.07 O, R, F, S Wendt 1971a acanthicum Zone l. Kimmeridgian M. Jurassic zigzag to Monte Inici, W-Sicily 0.2–0.5 0.15 O, R, F, S Wendt 1964 Bathonian-Callovian calloviense Zone M./U. Jurassic Subbetic Plateau, Fels and Sey- l. Bathonian/ ? 1.0 0.1 O, R, F E-Spain fried 1993 l. Oxfordian M. Jurassic subfurcatum to Bayeux, France 0.25–0.45 0.3 O, R, F Fürsich 1971 Bajocian parkinsoni Zone Aalenian/ opalinum to Prérif, N-Morocco 0.1–0.3 0.1 O, phosphorite Lelièvre 1960 l.