Sedimentation of the late geosynclinal stage

Sedimentary Geology - Elsevier Publishing C o m p a n y , A m s t e r d a m - Printed in T h e N e t h e r l a n d s

S E D I M E N T A T I O N OF T H E LATE G E O S Y N C L I N A L STAGE G. S E S T I N I

Department of Earth Sciences, University of Leeds, Leeds (Great Britain) (Received M a y 8, 1969)

SUMMARY

The sediments of the Upper Eocene to Middle Miocene Ranzano-Bismantova sequence of the Padan side of the Northern Apennines are considered late geosynclinal, because they were formed after the Middle Eocene folding stage in the Ligurian domain and were later involved in the main westward movement of the AIlochthon. The sediments were essentially clastic. During the Oligocene they were deposited in a deep-water marine environment, in part by turbidity currents and slumping (e.g., Ranzano Sandstone); later, the environment tended to become shallower (e.g., Bismantova Formation) and deposition was by turbidity and normal currents. Other sediments in a clearly similar structural situation occur in eastern Tuscany and in the Romagna Apennines; they are of Middle to Late Miocene age and were formed in a shallow marine environment. This paper summarizes published information on the lithology, stratigraphy and structural relations of the late geosynclinal rocks, and it includes a summary of paleogeographic evolution. It is also pointed out that in the Northern Apennines the late geosynclinal stage is defined essentially on the basis of structural relations. The term molasse, as understood in the Swiss Alps, applies only to parts of the sequences and, in general, it cannot be equalled with late geosynclinal sediments (AuBoUIN, 1965). INTRODUCTION

The concept of a late geosynclinal period in the development of the Alpine geosynclines was introduced by AUBOUIN (1965, pp.96--97). This period is represented by sediments that formed after the early folding of parts of the geosyncline and were later deformed, though less intensely than the underlying rocks. The sediments are predominantly clastic and of great thickness; they are according to Aubouin the late geosynclinal molasse. The basins of deposition are backdeeps, foredeeps, or intradeeps, according to their position relative to the already folded portion of the geosyncline (AuBOUIN, 1965). A late geosynclinal period can be recognized also in the Northern Apennines. On the Padan side of the chain there are several Late Eocene-Oligocene to Middle Sediment. Geol., 4 (1970) 445-479

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Fig.l. Distribution of the late geosynclinal and equivalent formations in the Northern -\pennines, The allochthonous eugeosynclinal rocks are indicated by 4[/. ,\utochlhon: ~'/,e Macigno ,~Oligocene): Mu Marnoso-arcnace~i ~Middte Miocene,.

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SEDIMENTATION OF THE LATE GEOSYNCL1NAL STAGE

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Miocene (Tortonian) sequences that rest on the deformed Allochthon Complex of Ligurian derivation, but bear evidence that they were moved horizontally and gently tilted during the Late Miocene general folding of the Apennines. This situation is often referred to as semi-allochthony by Italian geologists (MERLA, 1951). There are essentially two groups of late geosynclinal sequences; one on the northern slope of the Apennines with the Late Eocene to Middle Miocene Ranzano Bismantova and Loiano sequences; and a group including the Lower to Middle Miocene Verna-Fumaiolo-San Marino sequence of eastern Tuscany and the Romagna Apennines, and the Ponsano, Manciano and Sgrilla sandstones of western and southern Tuscany. The geographical distribution of these rocks is shown in Fig.1. Most of them are included in the late geosynclinal classification, because (A~BATE et al., 1970) they were moved horizontally after deposition, along with their substrate of already deformed eugeosynclinal rocks. Sedimentological and structural difficulties arise in trying to apply Aubouin's definition literally to all the rocks of Fig. l. Some of the sediments (e.g., the Loiano and Ranzano sandstones) could be termed molasse, in the sense of sediments related to an orogenesis (EARDLEY and WHITE, 1947; AUBOU1N, 1965): they include much detritus from uplifted eugeosynclinal sequences (internal zones of both the Apenninic and the Alpine geosynclines). In the Northern Apennines there are however other "molassic" sequences, the Pliocene and Pleistocene thick sediments. which accumulated after the Late Miocene folding of the miogeosyncline. Another difficulty lies in the fact that on the Padan slope of the Apennines, not all the Oligocene and younger rocks are semi-allochthonous; for instance the Oligocene to Miocene rocks of the Voghera Apennines are probably autochthonous or have moved very little (ABBATE and SACRI, 1967), and to the west they are continuous with formations of the same age in the Piemonte Tertiary Basin, which are autochthonous. On the other hand, the northeastward trend of deformation of the Apennines had led to the deformation and some horizontal movement of post-Tortonian sediments. Borings by A G I P between Parma and Bologna have shown tongues of allochthon interposed between Upper Miocene and even Lower Pliocene sediments (LuCCHETTI et al., 1962). Other borings have revealed that buried structures under the plain in Emilia involve folded and faulted Late Miocene to Lower Pliocene rocks. The overlying later Pliocene and Pleistocene rocks are not folded. This paper is only concerned with sedimentary units that moved horizontally before the Messinian. An exception is the Messinian and Lower Pliocene of the Marecchia Valley (Rimini) semi-allochthon sequence. The Oligocene and later rocks of the Voghera Apennines are not semi-allochthonous but are included here for the convenience of a uniform paleogeographic description. The Lower and Middle Miocene sandstones of western and southern Tuscany form few and small outcrops (Fig.l). They rest on formations of the Allochthon Setfiment. Geol., 4 (1970) 445 479

448

G. SESX~Y}

Complex, but there is no clear field evidence that they moved horizontally after deposition. They are considered to be late geosynclinal in the light of general arguments (MERLA, 1951: PELLEGRINL 1967; BALDACCI et al., 1967). The Verna~ Monte Fumaiolo and the San Marino sequences, however, clearly rest on the allochthon, and this in turn on Macigno of Aquitanian-Langhian age and the Marnoso-arenacea of Hetvetian-Tortonian age. THE RANZANO-BISMANTOVA AND LOIANO SEQUENCES

The Ranzano-Bismantova sequence of the Padan side of the Apennines includes six formations (Table 1) in upward succession: Monte Piano marls. Ranzano Sandstone, Antognola marls and their lateral variation the Contignaco Tripoli, Bismantova Formation and Termina marls. The first systematic description of the sequence was published by AGIP geologists (PIERI, 1961 ); substantial later information on parts of the outcrop area, or on specific units, has been contributed by PASSERIN! (1962), BRAGA (1962, 1963, 1965), MUTT~ (1963, 1964), RADRIZZANI (1963), FAZZ1NI and TACOL1 (1963), GHELARDONI et al. (1965). MUTTI et al. (1965), ROVER1 (1966), MONTEFORTI and RAGGI (1967), LOSAC(O (1967), MEDIOLi et al. (1967), and BON1 (1967). Sedimentological studies of some extent are available only on the Ranzano sandstones (MUTT1, 1963; M U T n et al,. 1965; MUTTI and DE ROSA, 1968), on the Bismantova Formation (VEzzAM and PASSEGA,1963; SPtNELLI, 1963; ANGELUCCI et al., 1967), and on equivalent formations of the Piemonte Basin (GNACCOLINI, 1968). The Loiano sequence is exposed in the Bologna Apennines between the Panaro, Reno and Idice valleys (Fig. 1). In the Loiano area it rests unconformably on the overturned slab of the Upper Cretaceous Monghidoro Sandstones, itself embedded in the Chaotic Allochthon (MERLA, 1951; AZZAROLi, 1953; MAXWEI,L. 1959; Hsi2, 1967; L1PPARINI, 1966). The sequence starts with Late Eocene marls under the Loiano Sandstone, and is depositionatly continuous up to Tortonian marls; these are overlain by Messinian sediments with a slight unconlbrmity and the latter by the Pliocene sequence. According to MERLA (1951) the Loiano "molasse" was deposited conformably over a moving sheet of older allochthonous rocks. The sequence has been little studied sedimentologically. Except tor the Loiano Sandstone, it is rather similar to the Ranzano-Bismantova sequence. Monte Piano marls (Eocene-Oligocene)

The Monte Piano marls (Marne di Monte Piano), after Monte Piano in the Taro Valley near Monte Barigazzo {PlEat, 1961), outcrop on the Padan foothills of the Apennines from the upper Borbera and Curone valleys (Voghera Apennines) to the Secchia Valley. In many areas the formation can be divided into a lower part. characterized by red, splintery marlstones and ctaystones (the former "red beds"} with a low CaCO 3 content; and an upper part, which is more calcareous and has a Sediment. Geol., 4 ~1970) 445-479

~D

Rigoroso marls Zuccaro marls Monastero Formation Savignone Conglomerate

Variano Formation

Loiano Sandstone

Monte Piano marls marls

Ranzano Sandstone

grey clays red clays

Anconella Sandstone

marls

"Schlier" and Helvetian sands

grey marls

Bologna Apennines (Loiano)

San Marino Limestone

Fumaiolo Sandstone

Montebello clays

"Messinian"

East Tuscany and Romagna

Scisti Policromi

Macigno

Cervarola Sandstone

Allochthon Complex 1

~ MancianoSs /

J

~ I Sgrilla ) Sandstone

Ponsano Sandstone

Central and southern Tuscany

The relations between the Allochthon Complex and the Cervarola Sandstone are tectonic, rather than stratigraphic.

Eocene

Oligocene

Aquitanian

Contignaco Tripoli

Bismantova Formation

Termina marls

Cremolino Formation Montada Formation Antognola marls

Cessole marls

Serravalle Scrivia Sandstone

Helvetian

Langhian

St. Agata Fossili marls

Tortonian

Miocene: Messinian

Southeast Piemonte Piacenza-Modena Basin Apennines

CORRELATION OF THE LATE GEOSYNCLINALFORMATIONSAND EQUIVALENTUNITS OF THE NORTHERN APENNINES

TABLE l

ScagliaCinerea

Bisciaro

Marnoso-arenacea Formation

Gessoso-solfifera

Umbrian sequence

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greenish-bluish or grey colour (in the Tidone and Nure valleys it is also more silty and sandy: Mu'rT~, 1963). Intercalations of sandstones, some of which are clearly turbiditic, vary in abundance; generally (PIERI, 1961) there are only a few thin beds of very finegrained sandstone and siltstone. In the Modena sub-Apennines (LOSACCO, 1967~ intercalations of greenish-grey, fine-grained sandstone and of siltstones are common, and there are a few beds of micritic limestone. Septarian concretions occur in the upper part of the tbrmation. The Monte Piano marls increase considerably in thickness from southeast to northwest (PIERI, 1961; GHELARDONI et al., 1965): 80-300 m in the Modena and Parma Apennines (Losacco, 1967; MEOIOLl et al., 1967), to 7(!0 m at Cost~ Merlassino (upper Borbera Valley, SE6NtN~, 1961). The age is reported to be Late Eocene (BONL 1961; ROVERL 1966), Late Eocene locally to Oligocene (PIER1, 1961 ~. or entirely Oligocene (GHELARDONi et al., 1965; Losacco, 1967). The relations of the Monte Piano marls with their substrate (the Cretaceous to Paleocene gesima Group or the Paleocene-Eocene Viano clays are variable. In the northwest, at Costa Merlassino, the contact with the Albirola Formation is conformable (BONI, 1961 ; PIER1,1961 ; PASSER1NI,1962; TEDESCH1and COC:OCCE~rrA. 1961; AaBATE and SAGRI, 1965); it may be a paraconformity (ABBAlt~ and SAt.JRI, 1970). In the Parma to Modena Apennines the outcrops in an external, i.e~, northwestern, position show a downward stratigraphic continuity with transition to the Viano clays (e.g., Viano section, Modena: PIERI, 1961) and possibly to the G o m b o l a sandstones (LosAcco, 1967), an equivalent of the Monghidoro sandstones (Paleocene). In the external outcrops of the Piacenza Apennines (Tidone to Nure valleys) the lower contacts with the Eocene Farini d'Olmo limestones and Luretta Formation are conformable (BONI, 1961: BRAGA. 1962, 1965: MUT31, 1963). All the outcrops in a more internal position, e,g., Monte Piatello, Monte Roccone, Monte Barigazzo and Enza to Secchia area (Fig.l), show Monte Piano marls unconformable over the Cretaceous to Paleocene flysch of the Cassio Viano sequence, or the Chaotic Atlochthon. At Monte Barigazzo the flysch is overturned. The Monte Piano marls are overlain by the Ranzano Sandstone and the contact is generally gradational. In the Tidone and Nure valleys the two formations are locally heteropic (MuTr~, 1963). East of the Secchia Valley the Ranzano sandstones are missing and there is a transition from Monte Piano to Antognola marls. In the area between Pavullo and the Panaro, the Eocene Rio Giordano sequence may be a lateral equivalent of the Monte Piano marls (Losacco, 1967); the sequence consists of 100-150 m of dark grey clays of Eocene age (SERPAGLL 1962: DIEO, 1966) overlain by 700 m of thick-bedded, coarse, poorly cemented feldspathic sandstones (Rio Giordano "molasse").

Sediment. Geol., 4 l 1970) 445 474)

SEDIMENTATIONOF THE LATE GEOSYNCLINALSTAGE

451

Ranzano Sandstone (Oligocene) The Ranzano Sandstone (Arenarie di Ranzano), once referred to as "Tongriano", forms more or less isolated sedimentary bodies in the underlying Monte Piano marls (MuTTI and DE ROSA, 1968). Exposures are scattered in the Emilian Apennines from the Secchia to the Tidone valleys. Further to the west and southwest Oligocene clastics similar to the Ranzano Sandstone are extensively exposed in the Voghera Apennines. Sedimentology. The Ranzano Sandstone is characterized by a considerable variability of facies, especially bedding, grain size and sedimentary structures. It includes at least a flysch facies and a conglomerate facies, and olistostromes and lenses of marls. These facies often alternate irregularly. On a regional basis there is increase in the proportion of shales and marls to the southeast (MuTTI and DE ROSA, 1968). The formation is 2,000 m thick at Costa Merlassino, 770 m at Monte Barigazzo and about 100-200 m in the lower Piacenza, Parma and Modena Apennines. Typical Ranzano sections like that at Monte Barigazzo (RADRIZZANI, 1963; GHELARDON1 et al., 1965)are an irregular alternation of medium to coarse, grey sandstone (flysch facies) with conglomerates and micaceous dark grey marls, or greenish-grey marly siltstones. The ratio of conglomerate and sandstone to shale varies between 0.5 and 8, and it locally exceeds 8. The sandstone beds of the flysch facies are 0.5 to 1-2 m thick in parts of the exposure, but in general are more variable in thickness (few centimetres to 3 4 m) and also laterally. The pelitic intervals are 5-50 cm thick. The sandstones tend to be poorly cemented, and often are conglomeratic at the base; they display many turbidite features (MUTTI, 1964; MUTTI et al., 1965). They are characterized by an abundance of rock fragments, micritic limestones or serpentinite together with phyllite and quartzite (PIERI, 1961 ; FAZZIN! and TACOLI, 1965). More information on the sandstones will be given later. Conglomerates also occur in ungraded thick beds and lenses alternating with the sandstones, or forming units (200-250 m thick at Ranzano, 100 m at Monte Barigazzo, over 1,000 m at Costa Merlassino) in which 5-6 m conglomerate beds alternate with siltstone, thin sandstone or shale units, and marlstone. The conglomerates are concentrated in the lower part of the formation except at Costa Merlassino where they form the upper two thirds of the section (TEDESCm and CO¢O¢¢ETTA, 1961), although they are also common in the lower third (PASSERINI, 1962). Both the conglomerates and the size of their component pebbles tend to decrease southeastward, from large cobbles and even boulders at Costa Merlassino (PASSERINI,1962) to small pebbles (few centimetres to 5 cm) in the Parma and Modena Apennines. The conglomerates include a large variety of lithologic types. In general ophiolites and limestones like those of the Antola and Cassio flysch predominate; Sediment. Geol., 4 0970) 445 479

452

G. SEs;r~N~

there are also calcirudites, shaly sandstones, marlstone nodules, quartz and chert. Metamorphic rocks include micaschists, gneiss, amphibolite, quartzite and phyllite; igneous rocks such as white and pink granites and porphyries are present. No information is available on the relative percentages of these rock types, nor on their areal variations. lntraformational slumps and chaotic beds (boulder clays, the paraconglomerates of MUTTI et al., 1965) are common in some sections (Monte Barigazzo, Nure and Tidone valleys). Upward gradations from paraconglomerates to turbidites are frequently seen, as well as lateral transitions from conglomerate to turbidite in the current direction. These features were attributed by M v x l l et ai. (1965) to a gradual dilution of the slump mass by the surrounding water. One or two larger-scale chaotic zones (olistostromes) occur at the base ov higher up in the formation in the upper Taro and Enza valleys and in the Cent~ Valley. Their thickness is laterally variable, with a maximum of 500 m at Ranzano. The olistostrome of the Barigazzo and Ranzano areas (RADRIZZAN1, 1963: GHELARDONI et al., 1965) are boulder claystones with 10 c m - I m fairly rounded, clasts. These include Allochthon Complex rocks ("alberese" and "palombino "~ limestones, silty marls, ophiolites, chert, coarse nummulite-bearing sandstone) and of the Ranzano sequence itself (Monte Piano-type marls, fragments of Ranzano sandstone beds). Larger clasts are stratified and belong to the allochthonous Eocene flysch, the Monte Piano marls and the Ranzano Sandstone. The latte~ are locally tightly folded and contorted (MEDIOU et al., 1967).

Stratigraphic relations. Equivalents of the Ranzano Sandstone to the south and east are the autochthonous Macigno in the Apennines and the "southern psephitic lithofacies" of the Po Valley subsurface ( A G I P MINERAR1A, 1959), which is said to be similar to Macigno. Not far to the north are the thick Gonfolite conglomerates of the Prealps margin between Lake Maggiore and Lake Como, a typical molasse deposit according to CITA (1957). To the west, Oligocene clastic formations are extensively developed in the Piemonte Tertiary Basin (LORENZ. 1962: BONI, 1962b; GELAlt, 1967). In the Allochthon Complex (eugeosynclinal sequences. cf., ABBATE and SAGRI, 1970) there are sandstones of likely Oligocene age in the Canetolo Complex (i.e., the Monte Senario and Petrignacola sandstones).

Environment of deposition. The Ranzano sandstones and the Monte Piano marls are considered marine and fairly deep-water. According to MUTT1 (1964) and MUTT1 and DE ROSA (1968) the marls accumulated on a fairly steep slope dipping northeastward and related to the uplift of the Cretaceous-Eocene flysch of the Genoa region. The Ranzano sandstones are slope and small basin turbidites and represent the filling of depressions in the Monte Piano marls, erosion channels or even true submarine canyons (MUTTJ, 1963; MUTTI and DE ROSA 1968). This interpretation is based on the combination of proximal turbidite characters Sediment. Geol.. 4 (1970) 445 47~

SEDIMENTATIONOF THE LATE GEOSYNCLINALSTAGE

453

(such as variability of bed thickness and bedding plane shape, predominance of sand-flow deposits, frequent groove and load casts), with conglomerates, and slump and mudflow deposits. Provenance. The source of the Ranzano clastics in the Voghera Apennines seems to have been Alpine and Apenninic; mainly a southwestern source, in the Emilian exposures. In the Borbera and Curone valleys, PASSERINI(1962) found flute casts in the sandstones indicating a source to the northwest (Alps) and the southwest (Voltri Massif). The upper thick conglomerates also came from the Alps and the Voltri region. Further west, in the southern part of the Piemonte Basin, a southern and southwestern source of the conglomerates was predominant (AzzAgOLI and CITA, 1967). The environment of deposition of the Oligocene sandstones west of the Voghera Apennines was more littoral and deltaic than the typical Ranzano (PIERI, 1961; BONI, 1962a; ELTER et al., 1966). Terrestrial episodes occurred. Such a change in environment appears to have affected the upper part of the Costa Merlassino section, where the turbidites are overlain, through a gradual transition (PAssERINI, 1962) by about 200 m of grey marls which contain a 50 cm bed of biosparite with a shallow-water faunal assemblage (TEOESC•I and COCOCCETTA, 1961). In the Piacenza Apennines (Tidone Valley to Monte Roccone outcrops) flute casts indicate northward and northeastward currents (MuTTI, 1963; MUTTI and DE ROSA, 1968). In the Taro and Ceno valleys there are also southwestward directed flute casts (MUTTI et al., 1965). Loiano Sandstone (Oligocene) The Loiano Sandstone is a thick formation (1,400-1,500 m) consisting of massive beds of coarse-grained, poorly cemented feldspathic sandstone with channels at the base. There is a basal conglomerate (Ln'PXmYl, 1966) made of rocks from the Allochthon Complex (including the underlying Monghidoro Sandstone), and irregular conglomerate beds at various horizons. A few sandstone beds are clearly graded with lamination in the upper part. There are a few greenish-grey to dark grey shale intercalations with abundant carbonized plant fragments. Deposition may have been in part by normal currents, in part by slumping and some turbidity currents. MERLA (1951) and GAZZI (1963) suggested a shallowwater, deltaic environment. The Loiano Sandstone contains Lepidocyefina and is therefore Oligocene (LIPPARINI, 1966), but in the lower part of the formation there are also reworked nummulites (AzzAROLI, 1953). It is thus equivalent to the Ranzano and Macigno sandstones but differs from them in composition. Petrography of the Ranzano and Loiano sandstones and comparison with the Macigno sandstones A reconnaissance study of 43 thin-sections, 18 of Ranzano, 10 of Loiano Sediment. Geol., 4 (1970) 445-479

4~

4-~

(Mt. Roccone (Mt. Roccone (Mt. Roccone (Ranzano) (Ranzano) (Barigazzo) (Barigazzo)

(Barigazzo) (Roccone) (Ranzanol (Roccone)

RZ-15 RZ-13 RZ-16 RZ-3 RZ-I RZ-7 RZ-4

RZ-14 RZ-I I RZ-2 RZ-10

21.4 21.3l 14.65 10.61

39.42 39.33 33.15 28.64 31.50 27.74 22.3

65.54 50.81 53.0 55.5 47.8 32.39 49.47

0.27 0.65 7.13

1.0 2.81 0.62 5.58 1.88

-

0.43

1.18 1.31 3.71 0.85 1.13 1.69 0.53

10.84 8.52 16.31 25.71

8.02 11.84 12.70 17.33 14.43 14.42 30.04

13.3l 17.98 11.67 13.96 18.6 27.89 19.62

13.55 14.42 32.38 I 1.43

8.21 * 6.63 12.43" 6.33 10.92 43.61" 23*

4.14 12.71 7.68* 3.95 8.69 14.08 6.65

12.74 1.97 29.03 34.28

1.73 6.16 1.93 20.02 24.87 0.36 --

5.62 6.14 7.42 7.35 5.51 4.79 6.12

I 1.77 29.83 2.6 5.71

29.51 30.56 29.22 12.70 8.09 7.20 12.91

3.25 3.95 6.63 10.4 7.20 2.25 17.55

1.62 3.6

0.73 1.42 0.83 1.05 1.44 -0.70

0.21 .... 0.25 0.81 0.32 -1.33

2.44 0.65 0.94 0.81

1.43 1.18 -1.64 1.64 0.72 1.54

0.90 0.88 2.48 0.97 4.25 11.54 1.06

Percent of grains: out of total grains ( = 100). lntergranular material: out of total Ibm-section, *Includes igneous rock fragments below I" .except RZ-t (2.47",~. RZ-7 (234" ~

(Barigazzo) (Barigazzo) (Barigazzo) (Ranzano) (Ranzano) (Barigazzo) (Mt. Roccone

RZ-9 RZ-5 RZ-6 RZ-I 7 RZ- 18 RZ-8 RZ-12

Rock fi'agments

P E T R O G R A P H I C C O M P O S I T I O N OF T H E R A N Z A N O S A N D S T O N E S

TABLE II

0.47 0.81

2.17

0.47

3,57 1.44

--

0.30 -0.74 0.25 0.75 0,85

9.48 14.1 1.18 t0.02

9.32 1.9 6.35 4.91 1.64 0.18 5.87

6.21 2.63 6.10 4.43 4.17 3.66 2.65

0.54 4.91 0.24 0.40

1.08 0.95 1.65 0.43 0.72 0.18 1.17

3.25 3.51 0.35 1.53 1.54 0.84 0.53

Allochems

] 9.84 25.15 19.37 19.13

14.3 24.92 30.93 12.59 12.76 8.82 17.44

30.74 20.02 25.43 27.4 24.31 18.15 24.06

26.85 3.14 I 1.59 2.8

6.18 6.45 5.38 0.30 3.32

6,35

4.52 0.82 5.75 4.15 3.66 2.68 3,78

Cement and matrix

6.02 7.25 9.43 15.56

6.04 9.34 2.34 13.77 7.23 6.39 8.47

3.61 8.75 6.74 1.7 8.00 19.46 0.95

.~.

SEDIMENTATIONOF THE LATE GEOSYNCLINALSTAGE

455

and 20 of Macigno sandstones 1, was undertaken to establish the main similarities and differences of microscopic features of the Oligocene sandstones. Macigno has been studied extensively by CIPRIANI (1958, 1961), CIPRIANI and MALESANI, (1963), but only limited qualitative information was available on the Ranzano sandstones (PIERI, 1961; FAZZ1N1 and TACOLI, 1965), and none on the Loiano sandstones, with the exception of heavy minerals (PJERI, 196l, GAZZI, 1963). The Ranzano sandstones can be divided into at least two main types, according to the proportions of terrigenous grains (Table II, and Fig.2, left, l and IGNEOUS ROCK FRA GMENTS

QTZ+CHT q~,tz~reo,ee

..... &.. [o/eno

JLq

"~%

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i/

FSP

RX

SEDIMENTARY

>, METAMORPHIC

Fig.2. Composition of the Ranzano and Loiano sandstones. Left diagram: main detrital components according to McBRIDE(1963). Right diagram: distribution of rock fragments in the Ranzano sandstones (circles: sandstones of group I). 2). The first includes feldspathic sublitharenites and lithic arkoses (McBRIDE's classification, 1963) with 40-65 ~ quartz and 15-24 ~o rock fragments (percentages relative to grains only). The rock fragments are either predominantly metamorphic (quartzite and phyllite) or metamorphic, micritic limestone and serpentinite in about equal proportions. In the second type there is less quartz (10-40 ~ , litharenites and feldspathic litharenites) and the rock fragments are abundant to very abundant. In several sections there are rounded fragments of a micritic limestone with a microfacies very like that of the Antola limestones, in which Radiolaria are associated with sponge spicules (BoRTOEOTTI, 1964). In these sections serpentinite is very low and the percentage of metamorphic rock fragments is variable. In other sections serpentinite, and some spilite, are predominant either in association with limestone or with slate and quartzite. t Ranzano Sandstone: Monte Roccone (7 samples), Monte Barigazzo (6), Ranzano (5). Loiano Sandstone: Eoiano (10). Macigno: Bobbio (6), road Pontremoli-Zeri (2), Abetone (6), Fiesole (2), Calafuria, Livorno (2), Campiglia Hills (2). Sediment. Geol., 4 (1970) 445-479

6LV ~ l r

(OL61 ) ~" "lOaD "luaua.pa~

,

,

9 9 9 9 9 9

quartz o

E

chert

z ,q

i

q~

feMspar

o

© quartzite and slate

e~

R

z

©

felsite and "'granite"

©

limestones

micax

z

opaques and others ©

detrital calcite

~r .fossiL~"

sparry calcite

micrite and clay

"

mieuceous matrix

~

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INl~ S~S "0

9Si7

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

457

The sandstones of the first type are found only in the graded beds of the ftysch facies, which are devoid of slump features and are not conglomeratic. Of the second type, samples 1, 2, 3, 10, 14, which contain much serpentinite and/or limestone, come from the irregular conglomeratic sandstone layers. As these beds and the flysch facies are interbedded in the same sections, the two sandstone types are not geographically distinctive. They rather indicate a different source of the clastics (cf., MUTTI et al., 1965). In all the thin-sections quartz and feldspar are equally angular to subrounded. Feldspar tends to be in the same size range as quartz; in some sections it is even larger. This suggests a primary feldspar rapidly eroded from a gneissic or granitic terrain and rapidly transported (FOLK, 1965). Most of the single grains of quartz have an undulose extinction and the polycrystalline quartz appears to be mainly of metamorphic origin. The percentage of feldspar is variable; in most thin-sections examined it averages 13.5 ~ of the grains; in the others it amounts to no less than 8~o and reaches 30~o. Plagioclase is abundant; cataclastic textures are frequently seen. Microcline is common. All the thin-sections contain detrital calcite, monocrystalline calcite grains with rounded margins, some of which clearly of organic origin (e.g., echinoid fragments). In some cases there is as much as 1 4 ~ of the grains (Table ll). Fossils on the other hand (mainly micro-foraminifera, occasional algal fragments) occur in small amounts. The sandstones have a sparry calcite cement variably associated with micrite and/or a chloritic micaceous matrix. A notable amount of calcite is secondary, exhibiting features of recrystallization texture (irregular size and sinuous interlocked margins and no definite centripetal size increase in the intergranular cavities), and it replaces feldspar grains extensively. Calcite recrystallization affects an original micrite matrix as well as the limestone fragments and microfossils. The heavy-mineral assemblage is metamorphic (PIERI, 1961; FAZZ1NI and TACOLI, 1965; GHELARDONIet al., 1965), with zoisite-epidote predominant, followed by glaucophane, garnet, and amphiboles. Ultrastable minerals (zircon, futile, and tourmaline) amount only to a few percent. The Loiano sandstones (Table III) are characterized by a high feldspar content, with a predominance of orthoclase (at least in the coarser samples), abundance of microcline and microperthite; by the low An (8 ~ ) of the plagioclase; and by a very moderate amount of rock fragments (2-11 ~o of the grains, the variable percentage being related to grain size). Quartzite and granitic rock fragments are the most common; limestone is scarce. The heavy-mineral assemblage is dominated by staurolite, kyanite and garnet; anatase and monazite are characteristic, as they are absent in both the Macigno and Ranzano sandstones (GAzzl, 1963). The cement is sparry calcite in moderate to considerable amounts. Much of it is pore filling; the recrystallization of micrite and replacement of feldspar Sediment. Geol., 4 (1970) 445-479

458

c;. SESTIN l

TABLE IV COMPARISON OF MICROSCOPIC

FEATURES OF THE RANZANO,

J~lacigt#o Sorting Packing

Quartz "" ,,, Feldspar "/,, Quartz type Feldspar type

Quartz-feldspar size and roundness relations Metamorphics RX

'" Felsite" Serpentinite Micritic limestones Sparite limestones Chert Mica Microfossils Echinoid and algal fragments Dolomite (detrital)

LOIANO AND MACIGNO SANDSTONES

Ranzano

Loiano

poor considerable

good to moderate moderate to poor low in well-sorted low samples, moderate in the others variable according to grain size and to '),~, of feldspar/rock fragments locally high (arkosic sandstones) mainly metamorphic probably igneous and probably igneous and metamorphic metamorphic mainly plagioclase mainly plagioclase mainly K-feldspar 4.7"~i An microcline common microcline common plagioclase 8",~i An same size same size or feldsame size feldspar better spar slightly larger feldspar better rounded same roundness rounded abundant abundant to quite low, but RXm-:: RXi RX,,, . R X ~ subordinate RX,,~ ~>RXi present absent present absent present to abundant absent absent generally abundant absent present present absent absent present throughout present low to abundant low (muscovite and low to abtmdant biotite) (muscovite) absent present present absent present present absent

Matrix

abundant

Sparry calcite Heavy minerals 1

generally low garnet epidote-zoisite titanite tourmaline rutile zircon ortite cbloritoid

present in some samples present but less than sparry calcite abundant zoisite-epidote glaucophane garnet amphibole zircon titanite tourmaline rutile

absent almost absent abundanl garnet staurolite kyanite titanite tourmaline zircon anatase monazite

i Italicized heavy minerals form more than 700,/, a r e n o t so e v i d e n t as in t h e R a n z a n o s a n d s t o n e s . T h e a m o u n t o f m a t r i x is v e r y low e s p e c i a l l y in t h e c o a r s e r - g r a i n e d s a n d s t o n e s : s o m e clay r e l a t e d to t h e w e a t h e r i n g o f f e l d s p a r s , s o m e m i c r i t e a n d a b i t o f finely s h r e d d e d m i c a c e o u s m a t e r i a l . Table IV summarizes the main petrographic features of the three Oligocene Sediment. Geol., 4 (1970) 445--479

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

459

sandstones. The major similarity is the variable composition, which ranges from that of feldspathic litharenites to subarkose and feldspathic sublitharenites (MCBRIDE'S classification, 1963). Metamorphic rock fragments and quartz of probable metamorphic origin are common in all three. With regard to texture, the Ranzano and Loiano sandstones differ from Macigno for their better sorting, a normal packing (predominant point contacts) and the absence, or a generally small amount of micaceous matrix. These features suggest at least shallower burial, in addition to an original environment more capable of sorting the sands. A shallower-water environment, or at least greater proximity to one, can be probably inferred from the fossils in the Loiano and Ranzano sandstones. On the whole the composition of the sandstones indicates source areas including metamorphic and granite rocks. In addition there must have been differing local influences. The distinctive sedimentary-ophiolitic rock fragments of the Ranzano Sandstone, and the abundance of K-feldspar and igneous rock fragments in the Loiano Sandstone (granite is very common in the gravel according to WIEDENMAYER, 1950), are good arguments in favour of distinct source and depositional areas. In the Ranzano sandstones one feature may be of importance: although a large part of the single and polycrystalline quartz grains are probably of metamorphic origin, the rock fragments are mixed, in the subquartzose sandstones of type one and in the litharenites (Fig.2, right). In particular, the slump and conglomeratic beds contain metamorphic fragments in addition to the micritic limestone and the serpentinite. Antognola marls (Upper Oligocene-Aquitanian) The Antognola marls (Marne di Antognola) occur discontinuously above the Ranzano Sandstone or the Monte Piano marls, and below the Bismantova Formation, from the Piemonte Basin to the Modena sub-Apennines. In general (PIERI, 1961 ) they are bluish- grey to greenish-grey marlstones (35-45 ~o CaCO3) with a splintery fracture. There are very rare interbeds of very fine-grained sandstones (3-4 cm) which are turbiditic (FAZZINI and TACOH, 1965), or occasional lenses of marly sandstone (Vetto syncline; GHELARDONIet al., 1965). In the Parma foothills (MEDIOH et al., 1967) the marls are thin bedded and sandy. Olistostromes are found in the Enza Valley; they are made of Chaotic Allochthon material, and reach a thickness of 300 m (PAPANI, 1964; RovEm, 1966). Diatoms and Radiolaria occur in thin layers in the upper part of the formation (LOSAC¢O, 1967); this association of microfossils is widespread and lies at about the Oligocene-Miocene junction (PIERI, 1961). In the Modena region, west of the Panaro Valley the formation is split into two members (FAZZlNI and TACOLI, 1965; LOSAC¢O, 1967): (a) a lower member, 300-700 m thick, consists of light greenish marls with frequent interbeds Sediment. Geol., 4 (1970) 445 479

460

G. S~SHN~

(10-15 cm to I m) of turbiditic fine-grained sandstones, and grades downward into the Ranzano Sandstone; (b) a thinner (20-150 m) upper member of light greenish marls with occasional thin-graded sandstone beds. East of the Panaro. however, the entire formation resembles the upper member; a thin trait of black chert is present in the upper part. Further east in the Bologna Apennines, thc equivalents of the Antognola marls are the Oligocene "red clay" and "grey clay" that overlie the Loiano Sandstone, and above these the Aquitanian Anconella "molasse", well-sorted fine-grained sands (LIPPARINI,1966). In the southeastern part of the Piemonte Basin there are a number of essentially marly formations of Late Oligocene to Aquitanian age (Table l, based on SELLk 1967): lurbidites are included in some o1"them (GNACCOL~NI, 1968). The thickness of the Antognola marls is variable, from 370 m in the type section (Antognola, near Vetto, Parma) to 700-800 m in the Secchia Valley and the Modena foothills. The Oligocene-Early Miocene (Aquitanian) age makes them an equivalent (Table I) of the Macigno and of the Cervarola sandstones pro parte (cf., ABBATEand SAGRI, 1970). This correlation is reinforced by the occurrence of black chert bands in the Cervarola sandstones and in the Bisciaro of the Umbrian sequence.

Contignaco Tripoli (Upper Oligocene-Aquitanian) This unit is a lateral variation of the upper part of the Antognola marls. It occurs discontinuously from the Monferrato Hills (Turin), along the Apennine> margin as far as Bologna and Rimini. It is best developed in the Parma and Reggio areas, where it has a thickness of 35-100 m and lies between the Antognola marls and the Bismantova Formation. It is a white, compact to soft diatomaceous marl (PIERI, 1961; GHEt,ARDONI et al., 1965; ROVERI, 1966; MEDIOLI et al., 1967), locally containing abundant volcanic material, mainly rhyolitic-dacitic glass fragments (SELLI, 1948; FAZZINI and OLIVIERI, 1961; MEZZETTI and OLIVlERt, 1964; LOSACCO, 1967). Beds are thick and massive; there are some layers of marly clay and shale. At places (Vetto syncline) the formation consists of cherty marls, interbedded with clays and fine-grained sandstones. The fauna of the Contignaco Tripoli includes diatoms, radiolarians and planktonic foraminifers, as well as fish teeth. It has been pointed out (FAZZINIand OLIVIERI, 1961; ROVERI, 1966) that this siliceous unit is very extensive in outcrop (Piemonte to northern Marche), it corresponds to the cherty beds of the Cervarola and Bisciaro formations of the Tuscan and Umbrian sequences and also to a characteristic radiolarian zone at the base of the subsurface Langhian sediments in the Po Valley subsurface (I¥I)ESC~I, 1959). FAZZINI and OLIVIERI (1961) have suggested that the volcanic material may have been related to the Tertiary volcanism of the Euganei Hills (Venice).

Sediment. Geol., 4 (19711)445 47~

SEDIMENTATIONOF THE LATE GEOSYNCLINALSTAGE

461

Bismantova Formation 1 (Lower Miocene to Helvetian) Abrupt facies variations are typical of this formation, which outcrops from the Piemonte Basin where it is autochthonous, to Bologna. Three main types have been recognised in the Emilian Apennines (PIERI, 1961 ; GHELARDONIet al., 1965; ROVERI, 1966; LOSAC¢O, 1967): a calcareous detrital facies, usually in the lower part, and either a marly or a flysch facies in the upper part. The calcareous detritalfacies varies from grey, medium to coarse calcareous sandstones to sandy calcarenites that are well bedded (few centimetres to 2 m) and alternate with green clays or marls. There are abundant fossils (pelecypods, gastropods, corals, sponges, radiolaria and echinoid spines) and a sparry calcite cement. Large-scale cross bedding has been reported by VEZZANI and PASSEGA (1963). Locally, in the lower part, there is a thin band of volcanic sand (MEDIOLI et al., 1967; LOSACCO, 1967). In the Bologna sub-Apennines this facies is represented by bluish-grey sandy marls interbedded with sandy calcarenites that have the characteristic Amphistegina-Lithothamnium-bryozoa association of the San M arino Limestone (LIPPARINI, 1966). The marly facies consists of hard nodular marls. In the eastern part of the Parma Apennines there are some sandstone intercalations; they increase in number westward passing into a "molassic" facies, ranging from very fine-grained quartz sandstones to coarse thick-bedded, poorly cemented sands. The thickness of the marly facies increases eastward, from 200 m in the Parma Apennines to 300-350 m in the Modena area. In the Bologna Apennines the marls are glauconitic and there are layers containing volcanic debris (LIPPAR1NI, 1966). The facies had also been called "schlier" both in the Bologna area and in the northern Marche region (SECt.I, 1951 ). The turbiditefacies (PIERI, 1961; SPlNELLI, 1963; GHELARDONI et al., 1965: LOSACCO, 1967; MEDIOLI et al., 1967). This is a marly arenaceous flysch, medium to thick bedded (0.2-1 m), typically exposed in the Vetto syncline, where it is 500 m thick. The sandstones and sandy calcarenites weather yellowish, are poorly to well cemented, are graded and bear sole markings. Flute casts point to the east and southeast (PAREA,in ANGELUCCI et al., 1967; VEZZANI and PASSEGA, 1963). Shales and marlstones are interbedded; some of the latter contain pteropods and pelagic gastropods and have been interpreted by Parea as non-turbiditic deposits. Conglomerate layers are frequent; they contain well-rounded pebbles up to 10-15 cm diametre mainly of limestone, but also ophiolitic. Coquinoid beds (LosAcco, 1967) are present. The sandstones are poorly sorted and the grains are angular, in contrast to the quartz in the calcareous-detrital facies, which is much better rounded. The composition (PIERI, 1961; SPINELLI, 1963; PAREA.in ANGELUCCI et al., 1967) is 1 Formation is here preferred to "sandstones", the commonly used definition (Arenaria di Bismantova), because of the diverse lithofacies present.

Sediment. Geol., 4 (1970) 445-479

462

G. SES'rV~

mainly of quartz and carbonate rock fragments, marly and sandy limestones. siliceous micrite. There are also some felsitic igneous rock fragments and the heavy mineral association is mainly metamorphic (P~ERL 1961: SPINI~LLI, 1963: VEZZAN1 and PASSEGA, 1963). There is an abundant sparry calcite cement. The fossils of the Bismatova Formation are alike in the three facies (GHELARDONI et al., 1965) and indicate an Helvetian age, possibly ranging to Early Miocene locally. Equivalents of the Bismantova Formation (Table 1) are the Serravalle Scrivia Sandstone and the Cassinasco Formation, in the southern and eastern Piemonte Basin, the Marnoso-arenacea flysch to the southeast, while in the Po Valley AGIP wells (TEDESC,I, 1959) have shown the existence of a predominantly sandy facies in eastern Piemonte, southern Lombardy and Emilia. and a marly argillaceous facies under the rest of the valley, including eastern Emilia. 1n Tuscany and in the Romagna Apennines the equivalents of the Bismantova Formation are the late geosynclinal Manciano and Sgrilla sandstones and the San Marino Limestone. VEZZAYl and PASSEGA(1963) have reconstructed the paleogeography of the Middle Miocene sediments of the Northern Apennines margin and the southern part of the Po Valley on the basis of several sedimentary features (current structures, grain size distribution, and areal variations of heavy minerals and average grain size). They postulate the existence of a shallow-water (50-60 m)northwest southeast shelf, over which the sediments of the present outcrops between San Polo d'Enza, Pavullo and Loiano were transported by traction currents (Fig.3). The shelf was flanked by deeper water; on the slope to the northeast the marly facies was deposited at a depth of supposedly 70-100 m; further away turbidites like those of the Marnoso-arenacea came to rest. The flysch of the Vetto syncline formed in a depression to the southwest. This bathymetric pattern was probably of syndepositional-tectonic origin, and it is partly preserved in the Vetto syncline and the Salsomaggiore anticline (analogies are found in the Lower Pliocene sedimentation of the Emilian margin of the Po Valley: SESTIYl, 1970). Termina marls (Tortonian)

The Bismantova Formation is overlain by marly and marly-sandy sediments (Marne del Termina), except in parts of the Reggio to Bologna Apennines, where it is unconformably cut on by Messinian or Pliocene deposits. The Termina marls of San Polo d'Enza (Reggio Emilia) are light grey marls and calcareous clays, sandy in the lower part becoming silty upwards (THIEME, 1961). They are 580 m thick, decreasing to 150 m in the Modena area, and are interpreted as a pelagic to neritic deposit. The contact with the Bismantova Formation is gradational; above. they are generally transgressed by Pliocene sediments. Equivalents in the Pavia foothills (BoM, 1967) are the Helvetian to Tortonian Montu Beccaria Formation and the overlying Sant'Agata Fossili marls of the Tortona Hills, the type area of the Tortonian stage. Sediment. Geol., 4 (I 970) 445 -47 ~)

463

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

,,~........ Tidone

~I

.... •

3 2

Nure

/

~,!~.

..,v

I

/Bar,gazzo

¢3~-'~'~' /"

Lo,ano

°o°~ / / •

La

/ ~.~~"'

Verna

rm

./

/

Corsica i ~ Fig.3. Scheme to s h o w the hypothetical direction and entity of the horizontal m o v e m e n t s of the late geosynclinal rocks. 1 = E o c e n e - O l i g o c e n e rocks: 2 - . Lower and Middle Miocene rocks: 3 =~ Upper Miocene and Lower Pliocene rocks.

In the Modena area there are yellow sands alternating with bluish-grey marls (Montegibbio beds, LOSACCO, 1967). Further to the southeast several local marly formations of Tortonian age occur above the Marnoso-arenacea (Argille di Tossignano, RlzzIN1 and PASSEGA, 1964; an equivalent of member 5 of the Marnosoarenacea, RICO LUCCHI 1967; the Montebello clays of the San Marino sequence, RUGGIERI 1958; the Poggio delle Pulci Formation, of the upper Tiber Valley, NoccHI and CATINELLI, 1964; and the Formazione di Letto and part of the "schlier'" of the northern Marche, SELLI, 1951 ). During the Tortonian there was therefore an extended area of predominantly pelitic deposition which followed the clastic accumulation of the Bismantova Formation and the Marnosa Arenacea, probably a period of quiet before the major tectonic movements which in Late Tortonian to Early Pliocene times culminated with the emergence of large parts of the Apennines. THE LOWER AND MIDDLE MIOCENE SANDSTONES OF TUSCANY

The Burdigalian Manciano Sandstone, the Middle Miocene Sgrilla SandSediment. Geol., 4 (1970) 445-479

464

G. sEs'rtr~

stone and the Helvetian (or Tortonian) Ponsano Sandstone were originally interpreted as the first deposits of the transgressive neo-autochthon cycle (GIANNINI and TONGIORGI, 1959), because of their position over the ailochthon, theretbre dating its arrival in southern Tuscany as Early Miocene (or Late Oligocene). The sandstones are gently tilted and are only cut by tensional faults; there is no clea, field evidence (GIANNINI and TONGIORGI, 1959; PELLEGRINI, 1967) to either prove or disprove horizontal movements after deposition. MERLA (1951) considered them to be semi-allochthonous on general grounds. Their restricted outcrops, disconformable relations with the underlying allochthon, and their chronological and facies relations to the clearly semi-allochthonous Verna, Monte Fumaiolo anti San Marino slabs (Table I), favour their inclusion with the deposits of the late geosynclinal period (BALDACCIet al., 1967). Manciano Sandstone (Burdigalian) Five small exposures of sandstone near Capalbio and Manciano (Grosseto) in southern Tuscany were [bund to rest unconformably over rocks of the Allochthon Complex and to contain a rich Burdigalian oyster and echinoid fauna (DEsSAU et al., 1950: GIANNINI, 1957). The total thickness is about 200 m (PEt.LEGRINI~ 1967). In the basal part there is a conglomerate with rounded large pebbles oi' allochthonous rocks ("alberese" and "palombino" limestones, "pietraforte'" sandstones). The sandstones are pinkish-brown to greenish-grey, coarse to mediumgrained and fairly well sorted: they are thick bedded (1-6 m). There are rapid lateral variations in grain size and locally lenses of fine gravel. Cross-bedding is common and it indicates currents from the south and southwest. In thin-section the sandstones appear to be well sorted, with a certain negative skewness, and 20-25~o of sparry calcite cement; they range from calcarenitic sandstones to very sandy biosparites. Micrite pellets and skeletal fragments (whole and fragmented foraminifers; rounded echinoid and pelecypod fragments: some algal and bryozoan debris) are abundant. The terrigenous grains, largely quartz with some feldspar and chert, make 40-70 ~ of the sandstone. They are largely in point contact, which indicates little compaction and a shallow burial. The sandstones were probably deposited in a littoral to very shallow marine environment. Sgrilla Sandstone Very small outcrops west of Manciano, mentioned by LOSACCO (1958), were considered to be Middle Miocene in age by MALESANI and MANETTt (1967). The outcrops include moderately sorted, medium-to fine-grained sandstones, characterized by abundant fragments of extrusive igneous rocks (dacite and andesite) and of Eocene micritic limestone.

Sediment. Geol., 4 (1970) 445-479

465

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

Ponsano Sandstone (Helvetian or Tortonian) The type area of this formation (Arenaria di Ponsano) is near Ponsano, between Volterra and Colle Valdelsa, in central-western Tuscany (Fig.l). It is 300 m thick and consists of well-bedded fine-grained sandstones poorly to moderately sorted. Locally the sands are very fossiliferous, with gastropods, pelecypods, echinoids, corals, microfossils, fishes, cetacea and even Pinus cones. The macrofauna suggests, according to GIANNINI and TONGIORGI (1959), an Helvetian age, but BARTOLINI (1966) concluded on the basis of micro-foraminifera, that the formation is Tortonian. The sandstones contain mainly quartz and a fair amount of rock fragments, especially micritic limestone, also chert and serpentinite. The matrix consists of limonitic clay with chlorite and sericite (GIANNINI and TONGIORGI, 1959; MALESANI and MANETTI, 1967). The contact between the Ponsano Sandstone and the underlying allochthon is faulted where exposed; GIANNIN1 and TONGIORGI (1959)' suggested that it may be unconformable. The formation is overlain, in part unconformably, in part through an erosional contact, by the red conglomerates of the Messinian lacustrine sequence. THE VERNA~ MONTE FUMAIOLO AND SAN MARINO SEQUENCES

In a large area around San Marino in the Marecchia Valley, the allochthon is overlain by a characteristic Neogene sequence, part of which is found also at the top of Mt. La Verna and on Fumaiolo. It includes the following units in upward succession (RuGGIERI, 1958; RICCI Lucern, 1964): (1) the San Marino Formation; (2) a Helvetian transitional unit; (3) the Monte Fumaiolo Sandstone; (4) a MessinJan sequence. San Marino Formation (Langhian to Helvetian) The lower part of this formation is made up of massive biostromal limestones and biocalcirudites, very rich in bryozoans, molluscs, calcareous algae and microfossils (species of Miogypsina and Amphistegina). At the base there are pebbles of a variety of allochthon rocks, an indication that during the Langhian the Allochthon Complex was exposed in the area of deposition of the limestones. Upwards the limestones become bedded and are medium to coarse biocalcarenites with abundant Corallinacea. Tabular and trough cross-bedding is common. RICCI LUCCHI (1964, and in SELLI, 1967) studied the petrology of the limestones in detail and concluded that the environment was epineritic to littoral, variations in depth and currents being responsible for several minor facies variants. Transitional unit This unit is composed of sandy biocalcarenites alternating with biocalcarenites, sandstones and marls that are somewhat glauconitic. The age is Helvetian. Sediment. Geol., 4 (1970) 445479

466

(;. SEST~N~

The unit is not present everywhere and generally rests on the San Marino Limestone either with a sharp or a gradational contact. Monte Fumaiolo Sandstone (Helvetian)

The formation consists of greenish sandstones, glauconitic silty marls and glauconitic sandy calcarenites, medium- to thick-bedded. Cross-bedding is quite common. The sandy facies is best developed (200 m) at Fumaiolo (LoTTI, 1916: SIGNORIN1, 1946). In parts of the San Marino area the thickness is reduced and much of the interval between the San Marino Limestone and the Messinian deposits occupied by the sandy and glauconitic, very fossiliferous, Montebello clays, which attain a thickness over 500 m ( R U G G I E R I , 1958). The Montebello clays are of Tortonian age in the upper part (RIccl LICCHI, in SELLL 1967). The Monte Fumaiolo sandstones (MALESAM and MANETTL 19671 ar~: texturally immature with matrix and a calcite cement, about 30 ~ quartz, mainly with a straight extinction, and about 14}~ feldspar. Serpentinite fragments are common and the principal accessory minerals are glauconite and garnet. Fossils are common.

" Messinian'" sequence

This sequence (RuGGIERI, 1958) rests unconformably over the Montebelh~ clays or the Fumaiolo sandstones. It includes, upwards: (a) a basal conglomerate with pebbles of San Marino Limestone, and poorly cemented sandstones; (b) sandy. lignite-bearing clays with brackish and marine faunas; (c) sandstones and conglomerates with Ancilla gland(formis and Cardita jouanneti and oligotypic microfaunas; (d) clays with marine microfaunas indicating an almost normal salinit.~ (Sahelian facies); (e) selenitic and alabastrine gypsum. PETROGRAPHIC

CHARACTERS OF THE MIDDLE MIOCENE SANDSTONES OF TUSCANY

ROMAGNA

The Middle Miocene semi-allochthonous sandstones of Tuscany and Romagna have a number of petrographic characters in common, as shown by a recent study by MALESAN!and MANETTI (1967). (1) They are all very calcareous, because of fossil fragments and an abundant calcite cement. Depth of burial must have been limited because packing is normal (point contacts). (2) Quartz occurs mainly as single grains with straight extinction, which correlates with the absence of metamorphic rock fragments. (3) Orthoclase and microcline are more common than plagioclase. The composition of plagioclase is between 8 and 10~o An in the Fumaiolo sandstones. between 5 and 12% in the Tuscan sandstones; the higher value is probably due to feldspar of volcanic origin. Sediment. Geol., 4 ( t970~ 445 479

467

S E D I M E N T A T I O N OF T H E LATE G E O S Y N C L I N A L S T A G E

(4) Some authigenic feldspar is present in all the sandstones studied. (5) The correlations median/sorting and median/quartz content are negative, unlike those of the turbidite flysch sandstones such as the Macigno, Marnosoarenacea and Monghidoro sandstones. The negative correlation is probably due to the sorting of the Middle Miocene sands in a shallow marine environment and to the addition of a detrital organic fraction. (6) Some of the rocks (e.g., the Sgrilla, Manciano and Fumaiolo sandstones) occasionally contain authigenic dolomite. The Middle Miocene sandstones appear to have been derived from sedimentary rocks and ophiolites in the Allochthon Complex and from a granitic source, with a dispersal probably from the west. It is worth pointing out that both the Fumaiolo and Ponsano sandstones contain serpentinite fragments, a further indication of their close paleogeographic relations. Distinctive characters include the presence of alkaline amphiboles in the Ponsano Sandstone, which have been related to a Corsican source (MALESANI and MANETTI, 1967), the lack of rock fragments, especially ophiolites, in the Manciano Sandstone, and the acid extrusive fragments in the Sgrilla Sandstone. T E C T O N I C A N D S E D I M E N T A R Y H I S T O R Y OF T H E LATE G E O S Y N C L I N A L SEQUENCES

The interpretation of the development of the late geosynclinal deposits depends on conclusions regarding: (1) the different stratigraphic relations between the sequences and their substrate and in some cases the allochthony of the latter; (2) the amount of horizontal movement affecting the sequences; (3) the sedimentary and structural relations of the sequence with the autochthonous formations of the same age. Stratigraphic and structural relations Three types of stratigraphic relations of the Ranzano-Bismantova sequence to its substrate have been described. (1) North of Genoa, the sequence is at places conformable on the AntolaAlbirola sequence. The latter is most likely autochthonous or has been affected only by minor horizontal movements (ABBATEand SAGRI, 1965). (2) In the Piacenza Apennines, the external outcrops of Monte Piano marls and Ranzano Sandstone grade conformably downwards into the calcareous flysch of the Baganza Group (Monte Dosso Limestone) or of the Eocene part of the Trebbia Supergroup (Farini d'Olmo limestones, Luretta Formation). The Upper Cretaceous and Eocene calcareous flysch is generally considered to be allochthonous, except by REUTTER and SAMES (1964), who thought it was deposited in an independent, external Emilian geosyncline. (3) The internal outcrops of the Emilia Apennines (Piatello, Roccone, Barigazzo, Specchio, Ranzano) rest unconformably, directly or with intervening Sediment. Geol., 4 (1970) 445-479

468

G. SESTIN]

patches of Monte Piano marls (MUTTI, 1964), above the Antola-Albirola sequence, or the Chaotic Allochthon. The substrate is considered to be allochthonous and to have originated in the median-external part of the Ligurian eugeosyncline (ABBATE and SAGRi, 1970, fig.41). The Ranzano-Bismantova rocks of the internal outcrops form irregular synclines cut by normal faults of modest throw and tight folding is rare (GHELARDOM et al., 1965). The underlying sequences either have a chaotic structure or are highly folded, thrust-faulted and even overturned (e.g., Monte Barigazzo). According to GHELARDONIet al. (1965) a structural disharmony exists also in the external outcrops in spite of their depositional continuity with the Eocene flysch: it is explained by assuming that deformation of the Antola-Albirola sequences occurred in a submarine environment. In conclusion, the structural relations indicate that movement of the Ranzano-Bismantova sequence took place after the folding of the substrate. On the other hand, the autochthonist interpretation of the Cassio-Viano sequence (REUTTER and SAMES, 1964) lead IBBEKEN and REUTTER (1967) to the conclusion that the Ranzano Sandstone was originally deposited northeast ot Macigno, in an external Emilian geosyncline. This is in disagreement (ABBAH"and SAGRI, 1967, 1970) with the fairly uniform general pattern of clastic deposition in the Northern Apennines, where the sedimentary troughs migrated from the southwest to the northeast, and tectonic deformation had, in general, the same trend. Amount o f movement

The estimated amount of lateral movement involved in the allochthony of the late geosynclinat sequences is based on indirect arguments. MUTTI (1963) favoured a tectonic provenance of the Ranzano sandstones of the Nure and Tidone valleys from a position south of the Trebbia Valley Macigno, a distance at least of 35-40 km. With regard to the Bismantova Formation, the amount assumed by VEZZANI and PASSEGA(1963) was about 10-15 km for the western outcrops (Vetto to San Polo d'Enza), and 45 km for the eastern ones (Loiano to Bologna), with movement to the east and east-northeast (Fig.3). MERLA (1951) had already suggested 50 km for the Helvetian sandstones of the Loiano sequence. VEZZAN~ and PASSEGA (1963) expressed the view that the Oligocene sandstones of the Ranzano-Bismantova sequence were essentially deposited in the same place as the Middle Miocene ones. The Otigocene Loiano Sandstone rests on the Allochthon and the latter on the Oligocene Macigno as far as western Tuscany, so that the extent of movement must have been considerably greater. Furthermore, the Loiano sandstones rest unconformably over overturned Monghidoro sandstones; the folding and erosion of the latter could have hardly happened belbre the Oligocene in the area of the present Apennine (Hsu, 1967). MERLA (1951) hypothetically suggested a minimum travel distance of 120 km. A considerable distance of travel is also implied in the semi-allochthonous Sediment. Geol., 4 (1970) 445 47~

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

469

sequence of San Marino. The Helvetian and Tortonian sequence is essentially similar to that of La Verna, 50 km to the southwest, which is also semi-allochthonous. At San Marino the unconformable Messinian deposits contain lignite and are therefore more similar to the Tuscan Messinian sequence than to that of Romagna (RUGGIERI, 1958). Lignite-bearing autochthonous Messinian occurs in Tuscany at Casino, north of Siena. The Chaotic Allochthon on which rest the Middle Miocene limestones, overlies autochthonous Middle Miocene sediments with a very different facies (the Marnoso-arenacea flysch) as far as the Apennines divide, 40 km southwest of San Marino. In conclusion, if one takes into account only the present distance, and not the pre-folding distance, the San Marino limestones must have been deposited in the area that now lies between Siena and La Verna, and the movement may have been between 75 and 110 km (Fig.3). Greater difficulties in interpreting the direction and amount of semiallochthony are experienced for the Ligurian-Emilian Apennines. They arise from: (a) the gradual stratigraphic connection of the Ranzano-Bismantova sequence with the autochthon of the Piemonte Basin and the Padan margin; (b) the apparent uniformity of stratigraphic sequence over a long distance, Voghera Apennines to Bologna (ROVERI, 1966, rightly emphasizes the continuity of the silica-rich, often volcanic layers, at the top of the Aquitanian Antognola marls and of equivalent formations); (c) the structural relations between the allochthonous Ranzano sandstones of the Nure and Tidone valleys and the practically autochthonous Ranzano of the Voghera Apennines on one hand; and the presumed autochthonous Macigno of the Trebbia Valley, on the other hand; finally, (d) the possibility of movement from northwest to southeast of the Cassio-Viano sequences (ABBATE and SAGRI, 1967).

Outline of geologic history An attempt will be made at this stage to outline the general structural and depositional history of the late geosynclinal sequences, taking as a basis for the Apennines between Genoa, Piacenza and Parma, the work and hypotheses of PIERI (1961), MUTTI (1964), GHELARDONI et al. (1965) and MUTTI et al. (1965), BALDACCI et al. (1967), ELTER et al. (1966), and MUTTI and DE ROSA (1968). First stage (Middle Eocene). Emergence of the Upper Cretaceous-Paleocene Antola-Albirola flysch and tectonic advancement to the northeast over the Eocene flysch (Nure Group, Viano clays) basin. The latter probably developed by subsidence contemporaneous with the uplift of the Antola rocks. The movement was in part submarine. At the southwest margin of the Eocene flysch basin the slope was unstable and much slumping of the Cretaceous flysch occurred (the first conglomerates in the upper part of the Monte Piano marls at Monte Roccone contain essentially limestone elements, BRAGA, 1962). Further northeast or east, the Monte Piano marls were deposited in continuity with the Eocene flysch. Sediment. Geol.. 4 (1970) 445479

470

(;. SES'nM

MUTTI and DE ROSA (1968) have pointed out that the slope of the Eocene flysch basin was uniform and inclined to the southeast, as shown by the regularity of current mark directions. In the Oligocene, the slope was generally to the north and northeast; the topography was more irregular and the currents were more dispersed. Such changed conditions are attributed to the uplift of the southwest margin of the Eocene flysch basin in relation to the northeastward movement of the Ligurian tectonic units. A tectonic phase of comparable importance took placc also in the Western and Central Alps (DEBELMAS, 1964; ELTER et al., t966). it was probably during this Middle Eocene tectonic stage that the Monghidoro sandstones were overturned and eroded, and later the Loiano sandstones were deposited on them, in a shallow-marine to littoral environment, when the allochthon was still in the Ligurian area (cf. Hsu, 1967). The likely greater amount of semi-allochthony for the Loiano sandstones, and their different composition with respect to the Ranzano Sandstone, favour an origin in the southern part of the Ligurian area. Tectonic activity, or at least instability of the Ranzano basin margin, continued during the Upper Eocene and the Oligocene; some olistostromes are found in the Monte Piano marls, large ones occur in the Ranzano sandstones, made not only of clasts of Antola rocks, but also of Monte Piano and Ranzano sediments, Erosion and slumping must have been concentrated in the area of the eastern Ligurian Apennines, judging from the larger volume of Oligocene coarse clastics and the greater size of conglomerate elements in the Ranzano from the Voghera to the Parma Apennines. The Ranzano sandstones and conglomerates appear to have been derived both from the erosion of a "'paleo-Appennine ridge" (or ridges) near Genoa and from crystalline areas either to the north and northwest (internal zones of the Alps) or to the southwest. An Alpine source is indicated by flute casts in the sandstones of the Voghera Apennines (PASSEmNI, 1962) and a southwestern source by the flute casts of the Ranzano sandstones between the Tidone and Taro valleys. MUTT1 (1963) suggested that the Ranzano was originally deposited south of the Bobbio Macigno (Trebbia Valley) and that the source was possibly in the same Corsica-Provence source area of crystalline rocks that formed the Oligocene sandstones (Gres d'Annot, Taveyannaz Sandstone) of the Western Alps (Mtrrtl el at., 1965; STANLEY and MUTH, 1968). Such a southwestern source (or south-southwestern) if one accepts that an anti-clockwise rotation of Corsica took place during the folding of the eastern Ligurian Apennines: NMRN and WESTPnAL, 1968) certainly cannot be excluded as it had had an important role in the formation of some of the Late Cretaceous eugeosynclinal sandstones (PAGE& 1965; A~BATE and SAGm, 1970). One has to take into account, however, that a ridge, or ridges, of Antola calcareous flysch and of ophiolites (persisting Bracco Ridge?, A88ATE and SAGRI, 1970. fig.41) was emergent after the Middle Eocene folding and must have prevented the eastward Sediment. Geol., 4 (t970) 445 4 7 9

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

471

movement of sediment, or else there was a complex dispersal pattern. ELTER et al. (1966) reject a southwestern source, as well as a direct Alpine source, and rather postulate the existence of an emergent ridge of Alpine rocks separating the Ranzano from the Macigno areas of deposition. Such a ridge, a southern continuation of the Insubric-Lombard realm, separating the Ligurian and Tuscan basins, had already existed according to ELTER et al. (1966) in the Middle-Upper Cretaceous, as shown by the lithology of the Salti del Diavolo conglomerates (cf. SAMES, 1967) and of the Albian-Cenomanian conglomerates of the Monferrato region (ELTER et al., 1966). The view of ELTER and collaborators may be supported by the association of metamorphic rock fragments with Antola-type limestone and serpentinite in most Ranzano sandstones (Table I, and Fig.2, right), and especially in those associated with slumping and conglomerates.

Second stage (Late Oligocene and Early Miocene). A further westward movement of the substrate carried the Monte Piano and Ranzano sediments with it. The sequence may have emerged in the internal areas, since post-Ranzano sediments are missing at Monte Barigazzo, Monte Roccone and the Specchio syncline (GHELARDONI et al., 1965). Further slumping ofallochthonous rocks took place, as indicated by the olistostromes of chaotic argille scagliose in the Antognola marls, and the unconformable relation of some outcrops of Antognola marls and Bismantova Formation with the Chaotic Allochthon. The supply of sand, however, decreased, gradually at first (transition Ranzano sandstones-Antognola marls west of the Panaro); during the Lower Miocene there were only occasional finegrained turbidites. It is probable that depth in general decreased as well, later leading to the establishment of a shelf on which the Bismantova Formation was deposited. The decrease in depth may also have been related to the progressive advancement of the thick sheet of allochthonous rocks. The paleogeography during Bismantova time can be envisaged (Fig.4) as dominated by an elongated land mass in eastern Liguria, flanked on the northeast and southeast by a shelf with irregular topography on which terrigenous sands and locally organic and organic-detrital sediments were deposited. Turbidites accumulated in a deeper trough further east and southeast (Marnoso-arenacea). The sands of the Bismantova Formation were derived probably in part from the Apennines and part from the Alps, those of the Marnosoarenacea essentially from the Alps; those of the Tuscany-Romagna sandstone group from the west (Tyrrhenian area). Third stage (Late Tortonian). The sequence moved again on the Padan margin. Movements involving the Tortonian marls are assumed to have been 10-15 km (MERLA, 1951; LUCCHETTIet al., 1962). Further to the southeast, however, in the Tuscany-Romagna Apennines, slope instability produced conspicuous slumping Sediment. Geol., 4 (1970) 445-479

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in the upper (marly) part of Marnoso-arenacea (intraformational slumps and olistostromes of allochthonous material) and exotics of shallow water, Tortonian Lucina marlstones (RIccl L u c c m and D'ONoFmo, 1967). The San Marino mass was probably still in a much more western position.

Fourth stage (Late Miocene and Pliocene). In the Late Miocene and again at the end of the Lower Pliocene, movements affected the Messinian and Lower Pliocene sediments of the Po Valley margin between Piacenza and Forli. Along this line, tongues o f allochthon covered by Lower Pliocene sediments, are underlain by Lower Pliocene, in some cases even Middle Pliocene sediments (LucCHETTI et al.. 1962). The amount of movement involved in these situations is at least 10-15 km (SELLI, 1967, p.90). In the San Marino area, however, the total movement must have been a good deal greater, as the allochthon covers the Lower Pliocene (east side of the Savio Valley) and contains exotics of Lower Pliocene rocks with a facies different from those of the autochthonous ones (RuGGIER1, 1948). The huge Val Marecchia "slide", with the Middle Miocene and the lignite-bearing Messinian Sediment.

Geol.,

4 (1970) 445-479

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

473

over it, arrived at the end of the Lower Pliocene (RuGGIERI, 1958; GIANNIN1 and TONGIORGI, 1962). Relations between deposition and tectonic movements

Some workers (VEZZANI and PASSECA, 1963; BALDACCI et al., 1967) have supported the view that the Ranzano-Bismantova and the Loiano sequences were deposited, more or less in one area and that their main northeastward horizontal movements, along with the substrate of allochthonous eugeosynclinal rocks, took place entirely during the latter part of the Tortonian. However, the eastward progression of the San Marino "slide", as well as the relations between semi-allochthonous and autochthonous Late Miocene and Pliocene sediments of the Po Valley margin, suggest that deposition was continuous with tectonic movement. This is assumed also for the Ranzano-Bismantova sequence of the Parma-Modena Apennines (GHELARDONI et al., 1965), for the Loiano sequence (MERLA, 1951), and the Monte Piano-Ranzano part of the sequence in the Piacenza and Parma Apennines (MUTTI, 1964). The sedimentary characters of the late geosynclinal rocks east of the Panaro (Fig.l) strongly suggest, in fact, at least two different areas of deposition, before and after the Middle Miocene. East of the Panaro there are thick Loiano-like sandstones both in the Eocene (Rio Giordano "molasse") and the Aquitanian (Anconella "molasse", and the Rocca Malatina "molasse": LOSACCO, 1967). They are in definite contrast with the largely pelitic Monte Piano and Antognola marls of the Ranzano sequence. The Langhian sediments are much more similar (cf., siliceous zones). It can be assumed that the Bismantova Formation was deposited in a continuous area, entirely on what is now the Padan slope of the Apennines. CONCLUSIONS

In the foregoing text the term late geosynclinal has been applied to Late Eocene to Middle Miocene formations of the Po Valley side of the Apennines (the Ranzano-Bismantova and Loiano sequences) and to Middle Miocene terrains of the Tuscany-Romagna Apennines (the Verna-San Marino sequences). They are mildly folded, but lie on strongly deformed allochthonous eugeosynclinal rocks, and bear evidence of considerable horizontal displacement to the east and northeast. Some movement to the southeast is possible in the Piacenza Apennines, but the main evidence is for horizontal movements to the east and northeast of the order of 30-50 km for the Ranzano-Bismantova sequence in the Parma and Modena Apennines; and of 150 km or more for the Loiano Sandstone of the Bologna Apennines. The Loiano Sandstone was deposited in the southwest (northern Tyrrhenian area) and its source was not related to that of the Ranzano. Similar distances of tectonic displacement are likely for the La Verna-San Marino sequences Sediment. Geol., 4 (1970) 445-479

474

G. SES~N~

of the Romagna Apennines. It is likely that deposition of the sequences proceeded simultaneously with tectonic movement, displacement being greater for the lower part (e.g., Ranzano) than the upper part of the sequence (e.g., Termina marls). The Monte Piano (Upper Eocene) and Ranzano (Oligocene) formations are genetically related to the Middle Eocene uplift of the Antola-Albirola sequence (Upper Cretaceous-Lower Eocene) of the Genoa and eastern Liguria Apennines. This is shown by unconformable relations and by the occurrence in the Monte Piano and Ranzano of conglomerates with serpentinite and Antola limestone elements. The relations of the Loiano Sandstone to a folding stage in the eugeosyncline are less evident, although they rest on overturned Monghidoro Sandstone. which may have been folded in the eugeosynclinal domain during the Eocene (Hsu, 1967). In the southern part of the geosyncline (Tuscany) there are no Oligocene late geosynclinal rocks. The facies of the late geosynctinal sediments is diverse. The general deposition from the Late Eocene to the Lower Miocene was pelitic and deep water with turbidite episodes varying in importance from area to area. The thickest turbidites, the Ranzano sandstones of the Ligurian-Emilian Apennines, have on the whole a proximal flysch facies with abundant conglomerates and olistostromes. The Loiano Sandstone has some deltaic characters. During the Middle Miocene (Bismantova Formation) deposition was bioclastic and sandy, mainly by normal currents in shallow water, but some turbidites were formed. The equivalent sediments ol Tuscany (Manciano and Ponsano sandstones) and those of the La Verna-San Marino sequence, are littoral and relatively thin. These sandstones were all mainly derived from the Allochthon Complex and from a granitic area in the west. The southern and southeastern equivalents of the late geosynclinal sediments are the thick turbidites of the Macigno and Marnoso-arenacea formations, attributed by ABBATE et al. (1970) to the geosynclinal stage. The Oligocene situation, in which the Ranzano and Loiano sandstones were deposited on a shett (or a slope) and the Macigno sandstones in a subsiding trough to the southeast, was therefore repeated with the Bismantova and Marnoso-arenacea formations. This similarity between geosynclinal and late geosynclinal flysch was caused by the northeastern progression of uplift and subsidence in the Northern Apennines: the two types differ, however, in the generally shallower and more irregular environment of the late geosynclinal flysch. In the northwest the autochthonous Oligocene deposits of the Piemonte Basin have a deltaic and even a terrestrial facies; those of the Miocene, mainly a turbidite facies. Their source was essentially in the Western Alps. From this summary it appears that the term late geosynclinal ot" AuBov~y (1965) can be applied mainly with reference to the structural situation: rocks formed at a late stage in the evolution of the geosyncline but affected by the main folding. Although the style of deformation was not specified by Aubouin, it should be pointed out that in the Apennines and in the Piemonte Basin, the structure Sediment. Geol., 4 (1970) 445 47~.~

SEDIMENTATION OF THE LATE GEOSYNCLINAL STAGE

475

of the Oligocene and later rocks is not similar to that of the late geosynclinal sediments of the Hellenides (molasse of the meso-Hellenic intradeep) or of the Western Alps, where the Swiss molasse is folded and over-ridden by the calcareous nappes. With regard to facies, I feel that the application of the term molasse, following AUBOUIN (1965, p.96), to the late geosynclinal rocks of the Apennines can be misleading. There are two aspects to the term molasse. First, it indicates a tectosedimentary facies: late syn-tectonic and epi-tectonic deposits related to the main folding and uplift of the geosyncline (TR~MPY, 1960). Secondly, it indicates a sedimentary facies characterized (EARDLEY and WHITE, 1947; SEILACHER, 1958; TR~MPY, 1960) by: (a) deltaic and fluviatile conglomerates, sandstones with argillaceous and silty sandstones, bituminous shales, some coal beds and freshwater limestones; (b) sandstones with thick and massive bedding rapidly wedging out; (c) common washouts, channels, ripple marks and mega-ripples; (d) a mainly continental environment with temporary shallow marine ingressions. In the Apennines, molasse in this second sense can be applied only to the Oligocene rocks of the Piemonte Basin (AuBOUIN,1964) and to some parts of the Loiano and Bismantova sandstones. Most of the Ranzano-Bismantova sequence, however, is not molasse. On the other hand, in terms of facies, the Pliocene and Pleistocene clastics of the Po Valley and of Tuscany are molasse; yet most of them are post-geosynclinal in terms of their structure (SEsTINI, 1970). Furthermore, if one considers as molasse all the late geosynclinal sediments of the Apennines, flysch and molasse could be contemporaneous (e.g., Macigno and Ranzano sandstones). A distinction is desirable in so far as flysch and molasse are useful terms to indicate different sedimentary stages in the evolution of the geosyncline. Nevertheless, it has been pointed out (ABBATE et al., 1970a) that in the Northern Apennines the polarity of uplift and subsidence led to the overlap of sedimentary and tectonic stages. A similar problem of overlap exists with geosynclinal subdivision, e.g., whether the area of deposition of the RanzanoBismantova sequence constituted a foredeep, or an intradeep (cf. the "intermontane molasse" of |BBEKEN and REUTTER, 1967). If one takes only the Apennines into account, the term foredeep is probably appropriate. In relation to the Alps, AUBOUIN (1964) considered the Oligocene deposits of southern Lombardy, the Piemonte Basin and the Voghera Apennines, as formed in a Piemonte Liguria backdeep. But if part of the detritus of Ranzano, and also the Bismantova sandstones, was of Alpine derivation, the term intradeep would be appropriate in the larger context of the geosynclines of the Northern Apennines and Western Alps (although AUBOUIN, 1965, p.97, used intradeep for an area within one geosyncline). A discussion of this problem is however premature, since the spatial and structural relations of the Alps and Apennines geosynclines during the Eocene and Oligocene are not yet clear. Sediment. Geol., 4 (1970) 445-479

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G. SESTIN!

ACKNOWLEDGEMENTS

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