Basin, Calabria, Southern Ltaly

Chapter 4 lsolated Olistoliths from the Longobucco Basin, Calabria, Southern ltaly C. Tarquin Teale and Jeremy R. Young, Geology Departmelt lmperial College of Science and Technology, London SW7 2BP ABSTRACT The Longobucco Group is a 1500-m thick sequence of sediments which records the formation of The Longobucco Group is a cover sequence of Liass- a deep extensional basin on Hercynian crystalline ic sediments recording rifting of Hercynian crys- basement in the Early Jurassic (Young et al., 1986; talline basement. Within turbidites of the upper part Santantonio and Teale, this volume). The olistoliths of this sequence are numerous olistoliths, up to are colossal blocks, up to 250 m long, of shallow 250 m long. These are not parts of olistostromes, or marine limestones and other lithologies from the other mass movement deposits, but appear to have base of the succession. They occur, however, in the travelled into the basin independently; hence the upper part of the sequence, within basin-plain turbi term 'isolated olistolith' seems appropriate for them. dites. Recognition of the blocks as isolated olistoliths Soft-sediment deformation structures around the helped considerably in the development of our margins of one well-exposed olistolith suggest an understanding of the history of the basin. Addi- analogy with'outrunner-blocks' recently described tionally, finely preserved soft-sediment deformation from a modern submarine slope-failure complex by features found around some of the olistoliths provide Prior et al. (1982, 1984). In both cases the blocks new evidence for the mechanism of emplacement of appear to have travelled considerable distances such blocks. across very low slopes without disturbing the under- After describing the Longobucco olistoliths and lying sediments to any great extent. Transport prob- their setting, we briefly review analogous cases ably occurred relatively rapidly with the aid of described from other areas. [n particular, compari- ) overpressuring of the underlying sediments. sons are made between the Longobucco olistoliths and some blocks emplaced during a recent submarine slope failure, described from British Colum- INTRODUCTION bia by Prior et al. (1982, 1984). This comparison leads us to propose a possible mechanism of Isolated olistoliths may be defined as massive blocks emplacement for isolated olistoliths. of rock occurring out of place, but with sedimentary contact relationships, within unrelated sediments. Termlnology These relationships, which suggest that the blocks were emplaced as single masses into the basins in which they occur, are conceptually improbable, and Some comment on terminology is needed, since vir- only rarely observed. In this paper we describe exam- tually every author has used different nomenclature ples found while mapping the Longobucco Group in when describing isolated olistoliths (see Table 1). The Calabria. S. Italv. most common terms are: olistolith; slide block; and Mafine Clastic Sedimentology O J. K. Leggett & G. G. Zuffa (Graham and Trotman, 1987) pp. 75-88 MARINE CLASTIC SEDIMENTOLOGY: CONCEPTS AND CASE STUDTES glide block. Others include: exotic block; allochthon- They probably extend farther beneath the late Ter- ous block; allochthonous exotic block; sedimentary tiary deposits of Calabria, and may occur offshore. klippe; olisthothrymma; olisthoplaka; and local Associated with the group are condensed pelagic names such as Cipit block. The use of the terms sediments of Liassic to Early Cretaceous age, the 'exotic' and'allochthonous' seems inappropriate, Caloveto Group (Santantonio and Teale, 1985; and and potentially confusing, since they have strongly in preparation). Together these two groups constitute tectonic overtones, whereas the relations are clearly the Mesozoic cover sequence to Hercynian gran- sedimentary. The combination 'allochthonous itoids and metamorphites of the Longobucco Unit. exotic' is pointless since the two words have essen- This unit forms a part of the Calabro-Peloritani Arc, tially the same meaning in this context. 'Sedimentary which is generally interpreted as an isolated fragment klippe' evocatively describes a common outcrop of the Eo-Alpine chain thrust over the Apenninic car- mode but is completely misleading in terms bonate sequences (Scandone 197 9, 1982). Palinspastic of geological relationships. The neologisms reconstructions suggest that the Longobucco Unit 'olisthothrymma' and'olisthoplaka' were proposed occupied a position between the Apenninic units to by Richter and Mariolakis (1973) and Richter (1973) the west and the main Calabrian Alpine units to the for, respectively, slide masses occurring independent- east, on the African margin of the Tethys (De Rosa et ly, and ones occurring as isolated fragments of a/., 1980). gravity nappes. These seem unnecessary and The main subsidence and basin formation event unwieldy names, and they have not been widely used. recorded by the Longobucco Group occurred during 'Slide block' and 'glide block' are more attractive the Late Pleinsbachian-Early Toarcian and so was terms, and have been increasingly used of late. In synchronous with the well-documented late Liassic current usage (Nardin et al., 1979) slides are mass break-up of Triassic carbonate platforms throughout movements with limited internal deformation. They the Tethyan area (Bernoulli and Jenkyns, 1974). This include both slumps, in which rotation is important, suggests that it too developed by rifting, although in and glides, in which only linear motion occurs. Thus, this case of a crystalline basement. The facies devel- 'slide block'and 'glide block' have nearly the same oped in the Longobucco Basin were, however, quite meaning and can both legitimately be used. Of them different from the typical Tethyan carbonates, due to 'slide block' seems preferable as its meaning is the influx of siliciclastic detritus. Santantonio and clearer. and since some blocks are rotated. Teale (this volume) discuss further aspects of the The term is, however, explicitly genetic and so an setting of the group. Important studies of its sedi- alternative is needed for cases where the mechanism mentology and stratigraphy include those of Magri of emplacement is uncertain, and for field use. (1963-65), Zuffa et al. (1980), and Young et al. (1986\. Although olistolith (Greek-slide stone) does have The succession commences with up to 70 m of genetic roots they are not explicit, so it has been used 'red beds', mixed continental clastics, the Torrente much more in a descriptive sense. The main problem Duno Formation (Fig. 2). These lack age-diagnostic with its use is that as originally defined, by Flores fossils but are probably Hettangian. The base is com- (1955), it only covered matrix-supported clasts. monly faulted, but in places the formation can be However, this definition, and that of olistostromes. seen to rest directly on crystalline basement rocks. was clearly emended by Jacobacci (1965) and Abbate Upwards, it passes gradationally into variable et al. (1970), so that blocks occurring independently shallow marine carbonates, commonly sandy up to of olistostromes were included. We favour this usage 60 m thick. These have Sinemurian shelly faunas and with the addition of the qualifier 'isolated' where constitute the Bocchigliero Formation. The suc- appropriate. ceeding Petrone Formation is up to 200 m thick and records passage into deeper water with bioturbated marls the dominant lithology; it has yielded quite THE LONGOBUCCO OLISTOLITHS good Pliensbachian ammonite faunas. The bulk of the succession is formed of turbidites Background-The Longobucco Group of the Fiume Trionto Formation, in which the olistoliths occur. Up to 1200 m of the turbidites are pre- The olistoliths discussed in this paper occur within served, and presumably they were originally thicker, the Liassic sedimentary rocks of the Longobucco as the top of the formation is everywhere either trun- Group. These outcrop over some 170 km2 as a tec- cated by erosion, or cut-out by thrust faults. Near the tonically shortened, but well preserved, sequence on preserved top of the sequence ammonites of the ten- the north-east margin of Calabria, S. Italy (Fig. 1). uicostatum Zone, earliest Toarcian, have been found ISOLATED OLISTOLITHS FROM CALABRIA, SOUTHERN ITALY 77 Xl;,ll;;llill:;;t-:'.': t :'.':l +1*:j:lt*"..:"'":g+*:::'":":. *1"ffi + + + + + @ + + + + + + $$$$ffi + + + + + + + + + + + + + +i*;rlit+ri*iiiiffi + + + + + + + €*id:ii*:n'....' + + + i +\nsJz+ Li.i:.l!o : oo + + + + + + /'"-."3o. +,--^l + GalOvCtO-." J- + + + \ i.: + + + t ******N: + c ol?34sKm |...r lo : ool b",-."1 Pliocene - Recenf Poludi Formotion (U.Cretoceous - M. Eocene) - Coloveto Group ffi (U. Liossic - L.Cretoceous) Longobucco Group (Liossic) Bosemenl I +l (Hercynion) Low ongle lhrust Locolilies discussed Steep foult A Olistolith, cf fi9. 3 Olistoliths B Mojor debris flow complex c Dropslones ond thin debris flows Flg. l. Sketch map of Longobucco Basin showing extent of the Longobucco and Caloveto Groups and distribution of the olistoliths. Inset, location map. 78 MARINE CLASTIC SEDIMENTOLOGY: CONCEPTS AND CASE STUDTES EI rHl E.Toorcion @ lHl E 3lE u gl llf,s tl p Fl-'rtr @ @@ 3i rfr $r lz bl EI FIUME TRIONTO FM. I U' o!gU' .L.Phimbochion 5i gl PETRONE $g FM. ,EiE FldlHl 72a Ei=' NIO BOCCHIGLIERO gi ro< FM. 1"'*"" oG-4 a Ei si ui ;i TORRENTE DI'NO U FM. 2z ildlSiEi=i OE HERCYNIAN z< BASEMENT = 5 oti3fotiths Trocc fossil occurcnccs within olistotiths O IOO zOOm @ 4 @ Ammonites 8e Bivolve - brochiopod

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