H Giurassico Di Monte Nerone (Appennino Marchigiano, Italia Centrale): Biostratigrafia, Litostratigrafia Ed Evoluzione Paleogeografica
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Atti II Conv. Int. Pallini pp. 63-139 Il Giurassico di M. Nerone F.E.A. Pergola, 87 et alii cur. 6 tavv., 62 figg. H Giurassico di Monte Nerone (Appennino marchigiano, Italia Centrale): biostratigrafia, litostratigrafia ed evoluzione paleogeografica FABRIZIO CECCA* STEFANO CRESTA* GIOVANNI PALLINI** & MASSIMO SANTANTONIO* * Servizio Geologico d'Italia, Largo di S. Susanna 13, 00187 Roma - ** Dipartimento di Scienze della Terra, Università "La Sapienza" di Roma, P. A.Moro 5, 00100 Roma. Dedichiamo questo lavoro a Don Domenico Rinaldini, Curato di Piobbico, per averci mostrato durante tutto il lungo lavoro di ter reno come si può amare, conoscere e rispettare il Monte Nerone. RIASSUNTO tation. From the Middle Carixian onwards also the resedimented deposits of the Bosso Valley sequence became pelagic, following the drowning Lo studio di 15 sezioni nei sedimenti giurassici di Monte Nerone, event which affects the Monte Nerone area causing the neritic carbonate compiuto effettuando osservazioni sedimentologiche e raccolte di am production to be halted. In the Toarcian and Aalenian the widespread moniti strato per strato, é alla base dì questo lavoro. Dal Sinemuriano slumped beds testify a period of instability in the basin, possibly relat superiore al Titonico è stata riconosciuta la maggior parte delle zone ed to minor adjustments in the fault-block system. The greater part of ad ammoniti utilizzate nell'area mediterranea, rendendo possibile l'in the Toarcian (namely the Rosso Ammonitico umbro-marchigiano fm.) dividuazione di una notevole lacuna di sedimentazione in gran parte displays sharply lowered sedimentation rates (few mm's/1000 years). del monte, compresa tra il Bajociano superiore ed il Kimmeridgiano A hardground marks the base of the unit in both platform and basinal inferiore. La datazione, a livello di zona e talvolta di sottozona, dei sedimenti giurassici, permette di calcolare le velocità di sedimentazio areas, followed by more clayey and marly lithotypes. From the Middle ne: per i sedimenti che precedono la lacuna tale velocità è di circa tre Jurassic onwards the sedimentation reflects more stable conditions at volte maggiore di quelli che la seguono. All'interno delle formazioni the sea-bottom, being mainly made up of current-laminated or diluted costituenti la serie giurassica di Monte Nerone e di alcune aree limitro turbidity current deposits. The Late Bajocian-Earliest Kimmeridgian fe, abbiamo distinto alcune litofacies precisandone i limiti nello spa sediments contain a high proportion of secondary cherts and represent zio e nel tempo. Si sono evidenziati così, ove possibile a livello di zo a prolonged period of crisis in the production of pelagic carbonate (and na ad ammoniti, i rapporti stratigrafici tra le litofacies delle zone rico in its preservation at the sea bottom) as well as in the overall diversity nosciute come alti strutturali con quelle delle zone più profonde. Su of carbonate-shelled organisms. Ammonites are totally absent, thin- questa base stratigrafica assumono particolare rilievo sia la presenza di shelled "Posidonids" are dominant up to somewhere in the Callovian; diversi fossili di facies (coralli, bivalvi, gasteropodi, brachiopodi, echi- then they almost totally disappear and radiolarian tests are virtually nidi e crinoidi) che i fenomeni di risedimentazione riconosciuti a più livelli della serie. L'area di studio si è così delineata come un sistema the only sediment being deposited and preserved in Callovian fp.pj di piattaforma pelagica a partire dal Carixiano medio (M. Nerone) - to earliest Kimmeridgian times. In this interval the ages could only be margine (Pieia) - bacino (Bosso) di cui è stata chiarita nel dettaglio l'e detected by means of radiolarian stratigraphy. Then pelagic carbonates voluzione tettonico-sedimentaria nel corso del Giurassico. follow again and in the Late Tithonian the passage to the white calpionellid-rich Maiolica Fm. can be seen. I dati ottenuti, nella loro proiezione regionale, consentono qualche considerazione sul regime dinamico di questo settore dell'Appennino Pieia-Fondarca (platform margin/slope). This area remained in shal durante una parte dell'Era Mesozoica. low water during the first recognizable, Sinemurian extensional event leading to the creation of the Bosso basin: it was part of the Monte Nerone platform, where Calcare Massiccio-type sedimentation continued ABSTRACT until the base of the Pliensbachian. Thin pelagic, ammonitiferous and bioclastic micrites where then deposited, as a result of widespread drown A detailed (litho- and bio-) stratigraphie work has been carried out ing of the platform, which turned into a pelagic carbonate platform in the Jurassic pelagic sequences exposed in the Monte Nerone, Pieia- at the end of the Early Carixian. Soon after, however, the Pieia-Fondarca Fondarca and Bosso valley areas. These Jurassic outcrops represent a area collapsed, following another tectonic phase which restricted the beautifully preserved platform-platform margin/slope-basin system. platform area and created a stepped margin towards the Bosso basin Fifteen key localities were sampled bed by bed and thousands of am (SE). This younger fault can be traced in the field as an approximately monites were collected along with the additional information being NNE-trending line. The remainder of the Pliensbachian sedimentation obtained from the remaining outcrop areas. The complicated pattern of vertical and lateral fades changes was resolved at the ammonite zone is made up of soft pebble conglomerates with occasional fallen blocks level. The sedimentological and biostratigraphic data were thereafter of "Calcare Massiccio", reflecting instability of this intermediate step, utilized as forceful constraints for our palinspastic reconstructions and at the foot of the new, fault-related, bypass margin. Toarcian and Aale the final remarks concerning the tectonic and sedimentary history of nian sediments seem to be missing in this area. Bajocian to Early Kim this fragment of the so called Umbria-Marche geologic region. meridgian deposits are remarkably similar to those of the basinal Bos Bosso (Basin): highly distinctive fades and thickness patterns testify so section, being composed by well bedded and laminated "Posidonia"- the tectonics-induced birth and rapid collapse of the Bosso Basin in rich, then Radiolarian-rich limestones, cherty limestones and cherts Late Sinemurian times (Tumeri Zone). While the Mt. Nerone area was reaching some 40 metres in thickness. The following Late Kimmeridgian- remaining in shallow water conditions as a carbonate-producing, small Early Tithonian sediments display, on the other hand, rather unique isolated platform, impressive volumes of carbonate sands and larger lithic characters. They make up a prism of variable thickness (thinning towards fragments were transported downwards, via a faulted bypass margin the plaform as well as towards the Bosso area) which is mainly con- (not exposed at present), into the newly formed basin. Here these were stitued by red calcareous turbidites, slumped beds and associated debris deposited as graded turbidite beds and slumped and associated debris flow deposits, interbedded with nodular, ammonitiferous limestones flow deposits. Slumped turbidite beds document the highly unstable setting at the foot of the inferred fault scarp, while micritic, sometimes and marls, non-turbiditic sands with abundant aptychi, echinoids and ammonite-bearing levels represent the "background" basinal sedimen brachiopods and hummocky cross-bedded storm deposits. The rese- 64 FOSSILI, EVOLUZIONE, AMBIENTE - F. CECCA, S. CRESTA, G. PALLINI, M. SANTANTONIO dimented beds are clearly slope deposits; they appear to be interbedded with one from the Bosso basin had to be in the order of 600- 700 metres. The pelagic platform deposits. In our interpretation this means that the relief Bosso was an asymmetric basin, due to block rotation towards NW and between the Mt. Nerone and Pieia-Fondarca areas was progressively attenu it was connected, via a gentle slope, to another submarine high area lying ated. A slope was thus created, following the burial of the submarine es SE (Monte Acuto). The other, Pliensbachian, tectonic phase, responsible carpment, and the platform margin was switched from bypass to accretion- for the backstepping of the Mt. Nerone platform, created offsets in the ord ary mode. The nodular beds, the storm deposits and the levels rich in well er of about 100 metres. For the rest of the Jurassic clues for relevant sin- preserved benthic faunas testify a phase of progradation of the pelagic car sedimentary faulting are missing in our study area, though evidence for bonate platform. Noteworthy some of this beds are well dated (Semiforme marked slope instability can be detected in the Aalenian and in the Kim Zone - Early Tithonian) and they can be exactly correlated to a maximum meridgian and Lower Tithonian. Local rejuvenations of liassic faults have in hermatypic coral development on Mt. Nerone (see below). This phase been reported in our region usually as a cause for explaining rockfall and was apparently disturbed by seismic activity occurring on the structural step mass flow deposits. We do not know at present to wich extent could these causing pronounced slope instability. inferred movements have modified the gross morpho-structural pattern of Monte Nerone (platform). The platform deposits differ from the above the affected areas. As already noticed by other Authors, most of