” Posidonia Alp1na ” Beds of the Venetian Alps
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CARLO ST URANI AMMONITES AND STRATIGRAPHY OF THE ” POSIDONIA ALP1NA ” BEDS OF THE VENETIAN ALPS (MIDDLE JURASSIC, MAINLY BAJOCIAN) (With 46 text-figures and 16 plates) PADOV A Societa Cooperativa Tipograiica 19 7 1 Memorie degli Istituti di Geologia e Mineralogia dell’ Universita di Padova Vol. XXVIII INTRODUCTION AND SUMMARY The Jurassic of the Venetian Alps has been long renowned for its rich ammonite faunas : one need only mention those from the Aalenian of Cape San Vigilio, Monte Grappa and the Feltrine Alps, or those from the « Rosso ammonitico Veronese », ranging in age from the topmost Bajocian up to the Tithonian. Among the richest and most interesting, but yet incompletely known, are those from the « Posidonia alpina beds » of the Sette Comuni and a few other localities. The first account of this formation (variously known as « Strati a P . alpina », « Strati a P. alpina del Veneto », « Lumachella a P. alpina », etc.) is to be found in Oppel’s classic paper « Ueber das Vorkommen von jurassischen Posidonomyen-Gesteinen in den Alpena (1863), where the fine section near Brentonico was described and compared to the Klaus beds of the Austrian Alps, and a first list of fossils — including several ammonites — was given. Further details of the « Siidalpinen Posidonomyen-Gesteinen » outcropping in the Venetian Alps (Brentonico, Ponte di Tierno, Madonna del Monte by Rovereto, Torri al Benaco and Nomi), with additional faunal lists, were published by B e n e c k e (1865). Both authors regarded the P. alpina beds of this region as a perfect equivalent of the Klaus beds in the Austrian A lps; the latter were stated to fall within the « obersten Horizonten des Unteroolith » and to include the « alpine equivalent of the extraalpine Bathonian » as well. In the case of the Brentonico section, we shall see that this chronological attribution is still valid. B e n e c k e ’s paleoecological analysis is also worth noting, for the time when it was written. According to him, the great accumulations of disarticulated crinoid ossicles which occur locally in the P. alpina beds, could be taken as evidence that the sea bottom had been uplifted, so that the crinoid forests, which, by comparison with living forms, must have covered it in calm, deep waters, had been disrupted and their remains washed up along the sea shore by wave action; the brachiopods and the Posidonomyas ( = Bositra), on the other hand, must have lived gregariously in the (( Vertiefungen und Fiorden » sheltered from wave agitation, so that their occurrence in areally separated clusters and patchy accumulations could also be explained. The first — and still one of the best available accounts of the geology of the Sette Comuni was published by V a c e k a few years later (1877, 1878); he gave an excellent description of the P. alpina beds, also listing a few fossils, among which are Oppelia fusca, Sphaeroceras brongniarti, Phylloceratids and various brachiopods. V a c e k ’ s description of the P . alpina beds in the Sette Comuni was confirmed by T a r a m e l l i (1880), who listed a few other fossils from Camporovere. These were monographed in the same year by P a r o n a (1880); as to their age, after reviewing O p p e l ’s and B e n e c k e ’s opinions, P a r o n a did not commit himself to a more precise evaluation. In 1886 A. de G r e g o r io established a new sub-stage — the Ghelpinian — as a substitute for the « Posidonia alpina or Terebratula curviconcha zone » of previous authors, and monographed the fossils from the type locality (the Ghelpach valley in the Sette Comuni). The palaeontological part of de G r e g o r i o ’s monograph will be discussed in detail further on (p. 61); as to the Ghelpinian substage, it was regarded by de G r e g o r io as the upper part of the Alpinian stage, which he defined as follows : « L ’etage Alpinien comprend les faunes citees par les differents auteurs sous le titre vague de Juralias et de Dogger. II correspond particulierement a l’etage Bajocien d’ORB., comprenant en partie le Bathonien d’ORB.. II s’etend a peu pres a travers toutes les couches alpha-epsilon du Jura brun de Quenstedt et a travers les couches epsilon-zeta du Lias superieur du meme auteur. C’est a dire il s’etend des lambeaux superieur de la zone a Harpoceras falcifer jusqu’a la zone a Harp. sowerbyi Mill. ». It is not surprising that de G r e g o r i o ’s names never came into use. The main contribution, in de G r e g o r i o ’s monograph, is a general account of the P. alpina beds of the Venetian Alps written by E. N i c o l i s , who mentioned the discovery of a new fossil locality at Acque Fredde on the Garda lake, whence a rich fauna of pygmy ammonites and gastropods had been collected. These fossils were described and figured by P a r o n a several years later ( 1 8 9 4 ) and stated to be Callovian in age. In 1896 Parona published a third monograph, to illustrate the much richer assemblages from the P. alpina beds of Monte Meletta, Monte Longara, Ponte sul Ghelpach and Camporovere, all in the Sette Comuni. In this too he assigned the faunas to the Callovian. P a r o n a ’s identifications have subsequently been criticized by E. H a u g (1910, p. 1029), S. S. B u c k m a n (1922, pp. 418-419) and W. J. A r k e l l (1956, pp. 175- 176); all three pointed out that the assemblages monographed by P a r o n a included several undoubtedly Bajocian species, while there was little or no evidence for a Callovian age. Similar criticisms had already been made by G. D a l P ia z in his classic work on the Feltrine Alps (1907); in that region G. D a l P ia z had been able to demonstrate that the coquina facies, yielding Bositra buchi ( = P. alpina auctt.) and several characteristic brachiopods, graded laterally into micritic limestones with abundant Stephanoceratids and other ammonites of the Sauzei - Humphriesianum zones. In 1964 (S t u r a n i 1964a), I published an essay on the succession of the ammonite faunas within the Middle Jurassic of the Western Prealpi Venete ; on that occasion I reviewed the previous literature (including several papers not mentioned here, which deal more or less directly with the P . alpina beds), revised most of the sections and fossil localities mentioned by previous authors and gave a description of the P. alpina beds in the Sette Comuni area, including a preliminary account of their sedimentary features and palaeoecology. The latter two subjects have subsequently been developed in a short paper ( S t u r a n i 1967b) on the palaeogeographic significance of the coquina facies (« lumachella a P. alpina » of Italian authors) in the Middle Jurassic of the Mediterranean region, also taking into account other areas beside the Venetian Alps. Since the publication of this last paper, in the light of more modern literature on carbonate shelf sedimentology (A. G. F i s c h e r 1964; B. D’A r g e n io 1966, 1967 ; etc.), I have done more field work and have been able to discover — or to reinterpret more correctly — several sedimentary features of the P. alpina beds all over the Trento ridge. This led to a partially renewed and more complete interpretation of their sedimentary features and palaeogeographic significance, as will he discussed in detail in the first part of this memoir. Since 1964 I also went on collecting more fossils in the P. alpina beds, both at previously known and at newly discovered localities ; over five thousand ammonites have been thus collected and prepared for study. At the same time I was able to gather in Turin for revision all the ammonites monographed by P a r o n a in his different papers (de G r e g o r i o ’s specimens, unfortunately, turned out to be probably lost and could not be restudied). The main results of this study can he summarized as follows. The palaeogeographic evolution of the Trento ridge, now corresponding to the Western Venetian Prealps, begins in the Lower Lias with the establishment of a wide shelf where rapid subsidence was counterbalanced by an active carbonate sedimentation of Bahamian type. During this period, the Lower to Middle Liassic a Noriglio grey limestones » and the Upper Liassic « Cape San Vigilio oolite » were deposited. By the end of the Aalenian (locally earlier) subsidence and sedimentation rates had come to an abrupt halt, while the central part of the shelf, now corresponding to the Sette Comuni area, was an emergent area, in the form of a low, flat island, from the beginning of the Toarcian. Short before the end of the Bajocian the Trento ridge sank and was transformed into a pelagic, intrageosynclinal swell, on which the « Ammonitico rosso Veronese » started to form. Deposition of the P. alpina beds was sporadic both in time and in space and took place in the period of extremely reduced subsidence intervened between the end of the Bahamian-type, massive carbonate deposition and the sinking of the shelf; i.e., mainly during the Lower and early Upper Bajocian. In the Sette Comuni area the P.