DEPOSITIONAL TRENDS IN THE VALDORBIA SECTION (CENTRAL ITALY) DURING THE EARLY JURASSIC, AS REVEALED BY MICROPALEONTOLOGY, SEDIMENTOLOGY ANDGEOCHEMISTRY

In the Umbria-Marche basin open marine Jurassic sediments are well exp osed in the Valdorbia section (ENE of Gubbio, Central Italy). The time interval considered here is from the Carixian to the Early Aalenian. The depositional units. already dated by means of ammonites and calcar eous nannofossils, are: limestones and cherty limestones, Pliensbachia n in age (''Corniola'' = COR); marls of Early - Middle Toarcian age (' 'Marne del Monte Serrone'' = MS), including black shales in the Tenuic ostatum Zone; reddish nodular calcareous marls and limestones, Middle Toarcian to Early Aalenian in age, which constitute the ''Rosso Ammoni tico Umbro-Marchigiano'' ( = RAUM): and bivalve-bearing cherty limesto nes, Aalenian in age (''Calcari a Posidonia'' = CP). Micropaleontologi cal, sedimentological, trace fossil and geochemical-mineralogical anal yses have been carried out. The microfossil study has revealed changes in the microforaminiferal assemblages, corresponding to changes in bo th oxygen conditions and depth of the sea floor: Miliolina. Textularii na and Lagenina are common in the Carixian: Textulariina and Lagenina in the Domerian and Lagenina, Spirillinina in the Toarcian/Aalenian. O pportunistic small species bloom in the most anoxic levels of the blac k shales. The sedimentological study reveals two peaks in the detrital sedimentation. The first - probably connected with local tectonics (w ithout excluding sea-level changes) - is found in the interval from th e Carixian to the lower part of the Lower Toarcian. Metre-scale cycles of fine-grained calcareous turbidites, due to low-density flows, evol ve gradually into coarse-grained, metre-thick turbidites often amalgam ated and containing reworked skeletal grains of a carbonate platform e nvironment, and gravity flow deposits. The second peak occurs in the M iddle-Upper Toarcian. Fine-grained turbidites are overlain by hummocky cross-stratified (HCS) deposits and winnowed beds (WB), with large an d pervasive bioturbation. The vertical transition from turbidites to s harp-based HCS deposits and WB is probably indicative of a general reg ressive trend and of a depositional environment above major storm wave base. This trend has also been indicated from the microfossil study. Geochemical analysis of the Lower Toarcian (Tenuicostatum Zone) has re vealed strong positive anomalies in Ba, V. Cr, Ni. Co, Cu, Zn, As, Sb and Pb, elements which are characteristic of black shale episodes. Wea ker positive anomalies occur in similar sediments of the lower part of MS Formation, while such positive anomalies are absent in the largely bioturbated sediments deposited below (COR) and above (RAUM) the MS. Depositional trends related to tectonic-eustatic variations in the dep ositional environment are suggested on the basis of information provid ed by the study of the Valdorbia Section and of other Umbria-Marche se ctions.