Carbonate Cementation in Sandstones Edited by Sadoon Morad © 1998 The International Association of Sedimentologists ISBN: 978-0-632-04777-2 Spec. Pubis int. Ass. Sediment. (1998) 26, 241-260 Diagenetic evolution of synorogenic hybrid and lithic arenites (Miocene), northern Apennines, Italy E. SPADAFORA*1, L.F. DE ROSt2, G.G. ZUFFA*, S. MORADt and I.S. AL-AASM: *Dipartimento di Scienze della Terra e Geologico-Ambientali, University ofBologna, via Zamboni, 67, 40127, Bologna, Italy, e-mail zufa@geomin.unibo.it; tSedimentary Geology Research Group, Institute of Earth Sciences. Uppsala University, S-752 36 Uppsala. Sweden. e-mail sadoon.morad@geo.uu.se; and :Department of Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada, e-mail alaasm@delta.uwindsor.ca ABSTRACT The Bismantova-Term ina (Miocene) succession was deposited in satellite basins generated within the collisional orogenic framt of the northern Apennines. The succession is divided into four major sequences separated by regional unconformities. Sequences S l and S2 are composed of hybrid arenites rich in carbonate bioclasts and deposited in a shallow-marine shelf environment. Sequence S3 contains outer shelf/slope arkosic turbidites interbedded with marls, and sequence S4 is composed of turbiditic arenites rich in carbonate rock fragments, shales and marls deposited in slope and basin settings. Calcite cementation in the shelf arenites started with marine rims and syntaxial overgrowths on echinoderms, and proceeded towards blocky pore-flling cements. The loose packing of the arenites and the isotopic values of these cements (8180p08 from -3.6oo to Ooo and 813Cp08 from -4.5oo to +0.5oo) indicate precipitation at shallow depth below the sea floor from marine pore waters influenced by bioclast dissolution. Similar isotopic values in the arkosic slope arenites suggest potential additional derivation of ions for carbonate cementation from the interbedded marls. Small amounts of dolomite, heulandite. chlorite and K-feldspar are related to the· early alteration of volcanic rock fragments, heavy minerals and detrital dolomite grains. The isotopic values of calcite cement (8180p08 from -5.8oo to -1.7oo; 813Cp08 from -2.8oo to +0.1oo) and the tighter packing in the S4 turbiditic arenites indicate cementation under progressive burial, related mostly to the pervasive pressure dissolution of extrabasinal carbonate rock fragments. Maximum burial depth is, however, estimated to be less than 1 km. INTRODUCTION Sandstones with abundant carbonate grains consti tute an important petrofacies in many sedimentary sequences (Zufa, 1987). One class of such sand stones has abundant contemporaneous intrabasinal carbonate particles which include bioclasts, ooids, peloids and intraclasts (allochems, sensu Folk, 1 Present address: AGIP Servizi ELSI, Via Fabiani 1, Ctr Studi S.D. Milanese, 20097 Milano, Italy. 2Present address: Universidade Federal do Rio Grande do Sui, lnstituto de Geociencias, Departamento de Mineralogia e Petrologia, Av. Bento Goncalves, 9500, CEP 91501-970 Porto Alegre, RS, Brazil, e-mail lfderos@if. ufrgs. br. 241 1968), mixed with terrigenous quartz, feldspars and rock fragments. These sandstones are more pro perly termed hybrid arenites (sensu Zufa, 1980). A voluminous literature has been published during the past decade on the deposition and provenance of hybrid arenites (Mount, 1984; Doyle & Roberts, 1988; Fontana et a!., 1989; Budd & Harris, 1990; Loman do & Harris, 1991; Critelli & Le Pera, 1994; Zufa et a!., 1995). Hybrid carbonate sand-sized sediments are normally deposited: (i) in low latitude shelf environments by, for example, punc tuated mixing by storms or in situ allochem generation (e.g. carbonate fauna in siliciclastic set-