Siliciclastic-carbonate sequences of Miocene grabens of northern Sardinia, western Mediterranean sea

Sedimentary Geology, 76 (1992) 63-78

63

Elsevier Science Publishers B.V., Amsterdam

Siliciclastic-carbonate sequences of Miocene grabens of northern Sardinia, western Mediterranean Sea I.P. Martini

a,

G. Oggiano u and R. Mazzei b

a Department of Land Resource Science, University of Guelph, Guelph, Ont. N I G 2WI, Canada b Istituto Policattedra di Scienze Geologico-Mineralogiche, Universitd di Sassari, Corso Angioj 10, Sassari 07100, Italy

(Received August 29, 1990; revised version accepted September 17, 1991)

ABSTRACT Martini, I.P., Oggiano, G. and Mazzei, R., 1992. Siliciclastic-carbonate sequences of Miocene grabens of northern Sardinia, western Mediterranean Sea. Sediment. Geol., 76: 63-78. The small crustal block of Sardinia Island (western Mediterranean) underwent major rifting events during OligoceneEarly Miocene (Aquitanian), Early Burdigalian, and Plio-Pleistocene times. Sedimentation in the resulting extensional and transtensional basins was affected by both local tectonic and worldwide sea-level changes. The mixed siliciclastic-carbonate sequences which developed, are readily comparable with those predicted by sequence stratigraphy. This paper focuses on the post-volcanic, Uppermost Burdigalian to Lower Messinian (?) sedimentation at the confluence between a main N-S trending basin and a secondary SW-NE trending graben, in northern Sardinia. Silty marlstones of Late Burdigalian-Early Langhian age characterize the central part of the main basin, whereas variable siliciclastic and carbonate sequences affected by up to three major transgression and regression cycles developed at its margins and at the confluence with the smaller graben. Differential movements along the main boundary fault led to rotation of blocks which localized narrow, elongated offshore algal carbonate platforms. Low sea-level stand led to deep erosion (type 2 sequence boundary) of these carbonates and to the formation of siliciclastic sandy lowstand wedges. High sea-level stand led to re-establishment of algal platforms at the shelf margin which were bounded farther inland by siliciclastic-carbonate mixed parasequences. Although it is not possible to date directly every stratigraphic unit, local sequences are well structured and can be tentatively correlated with reported worldwide transgressive-regressive events.

Introduction M u c h o f S a r d i n i a has a b a s e m e n t o f m e t a m o r p h i c H e r c y n i c rocks i n t r u d e d by U p p e r C a r b o n i f e r o u s granites; in p o s t - H e r c y n i c times, t h a t a r e a has e x p e r i e n c e d several p e r i o d s o f distensive a n d strike-slip faulting l e a d i n g to f o r m a t i o n o f n u m e r ous rifted basins (Fig. 1). T h r e e m a j o r rifting events o c c u r r e d . (1) In t h e L a t e O l i g o c e n e - E a r l y M i o c e n e ( A q u i t a n i a n ) , rifting d e v e l o p e d w h e n S a r d i n i a was still a t t a c h e d to t h e s o u t h e a s t e r n m a r g i n o f t h e E u r o p e a n p l a t e just off P r o v e n c e (Figs. 1, 2; C o c o z z a a n d Sch/ifer, 1973; C h e r c h i a n d M o n t a d e r t , 1982). S e d i m e n t s a n d calc-alk a l i n e volcanics p a r t i a l l y filled t h e large b a s i n ( " F o s s a S a r d a " , V a r b a s s o , 1962) which crosses t h e island f r o m n o r t h to s o u t h (in its p r e s e n t d a y

o r i e n t a t i o n ) . (2) D u r i n g E a r l y B u r d i g a l i a n times, a d d i t i o n a l rifting o c c u r r e d in t h e n o r t h e r n m o s t p a r t o f t h e island a n d in Corsica, d u r i n g the t r a n s l a t i o n a n d r o t a t i o n o f S a r d i n i a to its p r e s e n t p o s i t i o n ( L e t o u z e y et al., 1982; C h e r c h i a n d Tr6moli~res, 1984; T h o m a s a n d G e n n e s s e a u x , 1986) a n d the coeval o p e n i n g o f a small o c e a n i c basin in the w e s t e r n M e d i t e r r a n e a n a r e a (Balearic Basin; B o c c a l e t t i a n d G u a z z o n e , 1972; D u r a n d D e l g a , 1980; C h e r c h i a n d M o n t a d e r t , 1982). Rifting c o n t i n u e d f r o m t i m e to t i m e d u r i n g the L a t e B u r d i g a l i a n to T o r t o n i a n - M e s s i n i a n ( R e h a u l t et al., 1985). A t that time, s m a l l e r S W - N E t r e n d i n g g r a b e n s w e r e f o r m e d on t h e island along strike-slip faults ( C h a b r i e r a n d Chorowicz, 1981), a n d w e r e filled with t e r r e s t r i a l a n d coastal sands, shelf c a r b o n a t e s a n d marls. T o s o u t h e a s t o f the

0037-0738/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

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Fig. 1. Generalized geological map of Sardinia showing traces of various grabens (after Cherchi and Montadert, 1982). Insert shows Sardinia (S) attached to the southern part of the European plate before rotation. Ba = Balearic area.

island, a second oceanic basin (the Tyrrhenian Sea) started developing at the end of the Miocene (Selli, 1985). (3) A final major rifting occurred in southern Sardinia during Plio-Pleistocene times with the formation of the large N W - S E oriented Campidano graben (Cherchi, 1987). This last rifting event was accompanied by intense alkaline volcanism throughout the island (Figs. 1, 2). This study focuses on the post-volcanic sequences of the Late Burdigalian to possibly Early Messinian grabens of northwestern Sardinia, between Sassari and Mores, and evaluates structural and eustatic influences on sedimentation near the junction of two intersecting grabens: a major N - S trending one and a secondary S W - N E trending one (Figs. 1-3). Methods In the area studied, the most continuous sections have been preserved either under thick car-

bonates, or under 5 to 30 m thick, tabular, PlioPleistocene lava (augitic-olivine basalt) flows which now cap the hills (Fig. 4). This forms an inverted Q u a t e r n a r y erosional topography, whereby the lava originally emplaced in valleys now protects the mountain tops. Some difficulties in correlating between rock exposures exist because of the following: (1)rapid lateral and vertical facies changes occur; (2) post-depositional vertical structural deformations were followed by differential erosion prior to and after emplacement of the Plio-Pleistocene lava; (3) it is possible to date accurately only marlstone units which formed toward the centre of the main basin. However, the layercake disposition of the strata, just locally tilted and not disturbed by folding, allows the visual correlation of several units across the basins; hence the overall architecture and paleosetting of the sediments can be reasonably well reconstructed. Facies analysis Six major sedimentary lithogroups can be recognized. They are underlain by volcaniclastics. Limestone. It consists mostly of shallow marine algal limestone (Ca) containing a variety of body fossils. The limestone can be subdivided into platform top deposits with rhodoliths in channels and few cross-bedded bars (Figs. 5A, 5B), and platform slope deposits with large syndepositional slumps (Cat, Figs. 4, 5C, 5D). Locally, limestone forms elongated, semi-isolated, thick (up to 60 m; Cam, Fig. 4) platform deposits, such as in the Florinas area. Sandy carbonates (Cs) mark the transition to siliciclastic units (Fig. 4). The carbonates are characterized by abundant Lithothammium, Lithophyllum, and macroforaminifera (Amphistegina and Heterostegina) which form a typical Middle Miocene faunal association (Bourrouilh-Lejan and Hottinger, 1988) similar to those widely distributed in the Paratethys and Tethys areas (Studencki, 1988). Other common fossils include annelids, bivalves (mainly Chlamys, Cardium, Ostrea, Venus), gastropods (such as Turritella), bryozoa, fragments of echinoid and isolated corals. Abundant colonial corals (Cr), some in living position, are present in

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SEQUENCES

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goids, pectinids and less common ostreae and gastropods. Abundant planktonic foraminifera and calcareous nannofossils indicate, respectively, presence of an N7 (upper part)-N8 interval of Blow (1969) and an NN4 (upper part)-NN5 (basal part) interval of Martini (1971); thus the marlstone unit (Mf) is of latest Burdigalian-Early Langhian age (Fig. 2; Mazzei and Oggiano, 1990). The benthonic foraminifera indicate a water depth of about 50-100 m, near the transition between the internal neritic and external neritic zones (Wright, 1978). Marlstone occurs primarily in the deeper central part of the main graben, but a sandier (Sm) variety is also present on parts of the basin-edge

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Conus); upper, more calcareous, horizons contain abundant calcareous algae (primarily Lithothammium moreti and minor Lithophyllum), spatan-

patches only in a restricted part of the basin (M.S. Elia (= M. Santo), M. Ruiu (= M. Ruju), and M. Pelao) (Figs. 4, 6). None of these fossils can provide accurate dates. Marly limestone. It consists of an argillaceous, fine crystalline, fossiliferous (mainly mollusca), algal limestone (Cm, Fig. 4), deposited on a muddy shell Marlstone. It consists mainly of a calcareous siltstone (marlstone) (Mf), showing regular alternation of thin- to medium-bedded strata with variable fossil content and bioturbation (Fig. 7). Lower stratigraphic horizons contain spatangoids (such as Schizaster), pectinids (prevalently Chlamys), and ostreae; middle and upper horizons have pectinids (mainly Amusium), crustaceans, annelids (frequent Ditrupa), shark teeth, isolated coral fragments, and gastropods (such as

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Fig. 2. Stratigraphic information and sea-level changes in the Miocene of Sardinia compared with Vail's curve (after Cherchi and Montadert, 1982; Haq et al, 1987).

06

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area (Fig. 4). This facies is the product of basinal deposition of suspended materials and gravity flows (Stanley, 1985) which were capable of reworking shallow-water fossils into deeper parts of the basin. The marly sands (Sin) may represent shelf turbid flow deposits associated with storm "set-ups". Black mudstone. It consists of dark grey, fossiliferous (bivalves) mudstone with disseminated

coarse quartz grains. It is found only at Florinas in a thin (1 m), local lens above the quartzo-feldspathic sand (Sqd). Although poorly represented, this is a characteristic transgressive facies, possibly developed in a restricted lagoon with quartz grains introduced by winds a n d / o r storm washover events. Sand. Siliciclastic deposits vary from coarse, conglomeratic in places, unfossiliferous, massive

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SILICICLASTIC-CARBONATE

SEQUENCES

OF MIOCENE

GRABENS

to locally cross-bedded lithic sands (S1) (Fig. 8), to purer quartzo-feldspathic sands (Sqd), generally massive and unfossiliferous, reaching 100 m in thickness at Florinas (Figs. 3, 4). Locally the sands become calcareous (Sqc), fossiliferous (Sf) and occasionally develop large-scale foresets, such as at Ploaghe and Florinas (Fig. 4). Fossiliferous, massive, bioturbated units occur repeatedly in the section alternating with calcareous sand (Fig. 4). They contain mostly Ostrea, Chlamys, some calcareous algae, in places Amphiope hollandi in living position with few Balanus sp., in others abundant Clypeaster intermedius and Scutella sardica. Only at M.S. Elia (Figs. 3, 4), sand and silty sands were found containing abundant interlaminations of plant materials (So) and sideritic nodules. The environments represented by these sands range from braided fluvial deposits (S1), to organic rich (So) lowland paralic settings, to fluviolacustrine and fluvio-marine Gilbert-like deltas (large cross-bedded units), to barred shores containing highly fossiliferous banks (Sf).

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Gravel. Gravel has two types of occurrences. (1) One type (G1) consists of a reddish, sandy, metamorphic and granitoid pebble to small boulder conglomerate (up to 20 m thick), well cemented, locally with well developed apposition fabric, massive with few cuts and fills and poorly developed cross-beds (in sandier interlayers). Paleocurrents vary from 180 ° in imbricated pebbles toward the base, to 360 ° in cross-beds toward the top. It occurs at the base of the Mores and S. Giovanni sections, underlain by volcanic tuff (Figs. 3, 4). (2) The second type (Gs), such as at M.S. Elia (Fig. 4), consists of sandy, poorly cemented, fairly well sorted, fine to medium quartzitic and granitoid pebble conglomerate, plane and crossbedded. In both cases, the gravels represent braided-stream deposits. The sedimentary sequence analyzed in this paper is unconformably underlain by volcaniclastics (tuff and cinder) interlayered with freshwater limestone and black chert. They show rhythmic laminated bedding with local small cuts and fills. In areas adjacent to the one studied, the volcani-

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SILICICLASTIC-CARBONATESEQUENCES OF MIOCENE GRABENS OF NORTHERN SARDINIA, WESTERN MEDITERRANEAN SEA

69

Fig. 6. Coralline limestone at M.S. Elia (Cr facies).

clastic bearing units contain small silicified wood and mammifer remains of Middle-Late Aquitanian to Early Burdigalian age (Bruijn and Rumke, 1974). These are lacustrine deposits developed in sags between stratovolcanoes. Transgressive and regressive sequences

Several major transgressive-regressive cycles can be established in the study area (Fig. 4): (1) a

major transgression (stages I, II) in the lower part of the sequence, from continental sands to marine marly and algal limestone units; (2) a regression (stage III) responsible for deep erosion and the formation of thick quartzo-feldspathic sand at Florinas (type 1 sequence boundary and lowstand wedge; Posamentier and Vail, 1988; Sarg, 1988); (3) a subsequent major transgression (stage IV) whereby such sand grades upward into a carbonate platform formed at the steep shelf edge of the

Fig. 7. Stratification of the marlstone (Mr) enhanced by differential weathering due to different intensity of cementation, fossil content and bioturbation.

7(]

main graben; (4) several regressive-transgressive cycles (parasequences) occur in upper levels at M.S. Elia and M. Pelao in siliciclastic-carbonate mixed shelf environments (stage V); (5) a third final major transgression occurs with deposition of an uppermost algal limestone unit over a type 2 sequence boundary (Posamentier and Vail, 1988; Sarg, 1988) (stage VI). This latter unit is preserved only in the upper slopes of M.S. Elia. The regional and vertical distribution of the facies defines sedimentation sequences characteristic of four major basin settings: (1) lower continental to marine siliciclastic-predominant sequences of the basin-margin facies belt (lower parts of Mores, S. Giovanni and Ploaghe sections), which grade vertically and laterally into: (2) complex units, representing paralic to shallow-marine siliciclastic-carbonate mixed environments at the basin margins (middle-upper parts of M.S. Elia (= M. Santo), M. Ruiu ( = M. Ruju), and M. Pelao sections); laterally into (3) well developed algal carbonate platforms at the steep shelf edge (Florinas, Ossi and Sassari sections); and (4) apparently monotonous marine

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marlstones of the centre of the main basin (Figs. 3, 4). The basal sequences 02t" the basin-margin facies belt

The lower transgressive event is well exposed at Mores, S. Giovanni and Ploaghe, that is, along a basin-margin facies belt and in the secondary confluent graben of Mores-Chilivani (Fig. 3). It is characterized by the vertical and lateral transitions of gravel to sandy continental facies grading up into marine fossiliferous sands and sandy carbonate (stage I, Figs. 4, 10). At S. Giovanni and Ploaghe, the basal lithic sand (SI) shows well developed, large foresets representing small, braided, coarse-grained deltas prograding into deeper pools or lakes (Fig. 8). The regional similarity of these sands (SI) over the whole study area is limited to their textures and sedimentary structures. They were derived from different sources (granite or schist and gneiss), have various paleocurrent directions (Fig. 9), and may have been formed at slightly different times in different localities.

Fig. 8. Lower sandy sequence at S. Giovanni: (a) large cross-beds in unfossiliferous coarse sand (S1 facies); (b) horizontal beds with some pebbly layers, bioturbated toward the top (Sqc facies); (c) fossiliferous (pelecypoda and echinoid) coarse gravelly sand (Sf facies).

S I L I C I C L A S T 1 C - C A R B O N A T E S E Q U E N C E S O F M I O C E N E G R A B E N S OF N O R T H E R N SARDINIA, W E S T E R N M E D I T E R R A N E A N SEA

The shelf-margin sequences of the Ploaghe-Florinas a rea

The lower continental facies were gradually drowned by a transgressive sea, and their topset materials were reworked into bars and sand plains which were progressively colonized by burrowing organisms, echinoids, and oysters in some places still found in living position. Finally, a carbonate environment (sandy and marly limestone: Cs and Cm) was established. Facies variations which occur in the lower transgressive sequence reflect local topographic relief due to intrabasinal structural deformations and pre-existent volcanic buildups.

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(1) The basal transgressive event is present as a condensed sequence at Florinas and adjacent areas along the shelf break of the main graben (Figs. 1, 3). The basal sand (stage I, Figs. 4, 10), here massive to plane bedded, is sharply overlain by sandy algal carbonate (Cs) which is in turn overlain by a thick algal limestone (Cam) of a platform present only in the Florinas-Ossi area. The algal limestone (Cam) is bounded basinward

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by marlstones (Mf) (Late Burdigalian-Early Langhian in age), and shoreward by marly limestones (Cm) (Fig. 3; stage II, Figs. 4, 10). This transgressive sequence and the carbonate platform of Florinas developed on a paleohigh at the shelf-margin of the main graben. This high was located on the seaward edge of a rotated boundary fault block (Figs. 3, 10). (2) Following the basal transgression and deposition of the limestone (Cam) at Florinas, a major rapid regression occurred in the area. Carbonate (Cm, Cam) and marlstone (Mf) deposits were eroded, and local deep (order of 100 m) and narrow (order of kilometres) valleys were formed. Siliciclastic sands prograded seaward over the shelf (at Ploaghe) and shelf margin (at Florinas) onto the basinal marlstone (stage III, Figs. 4, 10). Locally these sands developed large cross-bedded Gilbert-type bodies, such as at Ploaghe, closer to the margin of the basin, or formed very thick, massive, valley-fill bodies (lowstand wedges), such

as at Florinas at and below the platform edge. The absence of fossils or rarity of fossil fragments indicates that the receiving basins had fresh to brackish waters. This suggests that a major drop in sea level had occurred, sufficient to obstruct free saltwater intrusions into this part of the graben system. (3) A subsequent, gradual transgression led to the formation of more thick carbonates at the edge of the main graben (stage IV, Figs. 4, 10). Underlying these upper carbonates in the Florihas area, shelf-slope transgressive units consist of steeply inclined beds of fossiliferous sand which indicate mixing of siliciclastic sand (Sqd) with calcareous material from the shelf that was being drowned. Farther inland on the Florinas high, the transition between the unfossiliferous siliciclastic sand and overlying platform carbonates occurs by alternations in horizontal beds. Some of these transitional sandy units contain calcareous nannofossils of the upper part of the Spenolithus heteromorphus zone indicating a Late Langhianearliest Serravallian age (Mazzei and Oggiano, 1990). The effect of the shelf-margin slope, and perhaps rejuvenated fault movements, are recognizable in large synsedimentary slumps (Cat) of the lower part of the developing carbonate platform. Eventually, the carbonate sedimentation was established in the Florinas-Sassari area in a high energy, shallow sea, with deposits (Ca) characterized by few cross-bedded bars and numerous channels filled with rhodoliths. Slight sea-level fluctuations led to local brecciation and subdivision of the platform into several limestone units whose lateral continuity, however, is difficult to map. This platform has features frequently encountered in similar Mesozoic buildups in the Mediterranean area. For instance, they contain slumps and folds which characterize platform slopes, and rhodoliths found both in well defined channels and in occasional cross-bedded bars of platform tops (Fig. 5B). Some of the rhodoliths are very large, indicating relatively shallow-water conditions (Studencki, 1979; Bosence, 1983) where the algal ball could be readily moved and grow. In correlative Miocenic units of Sardinia

S I L I C I C L A S T I C - C A R B O N A T E S E Q U E N C E S O F M I O C E N E GRAB E N S OF N O R T H E R N SARDINIA, W E S T E R N M E D I T E R R A N E A N SEA

developed in other parts of the main graben, such as at Sedini, northeast of Sassari (Fig. 3), rhodoliths form very large bar foresets (Quesney-Forest and Quesney-Forest, 1984). Similar large foresets are also present in the slightly older (latest Oligocene-Aquitanian) limestone unit (Calcare d'Isili) of central-south Sardinia, where they have been interpreted as large sandwaves (Cherchi and Montadert, 1982; Cherchi, 1987). In that same area, Cherchi (1987) has also demonstrated that during the opening of the principal N-S trending Sardinian rifted basin the marginal carbonate platform (Calcare d'Isili) was laterally equivalent to basin-margin quartzose sand on one side and basinal marlstone on the other (Marne d'Ales). A similar situation occurs in the slightly younger deposits of the PloagheFlorinas area (as illustrated in this paper) and in the Sedini area (Quesney-Forest and QuesneyForest, 1984), indicating that formation and dismemberment of carbonate platforms continued through most of the Miocene everywhere along the graben margin, as they were influenced by a series of eustatic sea-level fluctuations coupled with rejuvenated fault movements.

Mixed siliciclastic-carbonate sequence of the basin margin (M.S. Elia area) The thick sections of M.S. Elia and M. Pelao (respectively 900 m and 750 m) contain rocks which have been preserved preferentially under resistant Quaternary-Pliocene lava caps (Fig. 4). They both developed at the confluence between two grabens. The section of M.S. Elia is dominated by coarse- to medium-grained quartzose sandy deposits interlayered with and capped by calcareous units. A similar terrigenous dominance exists at M. Pelao which is however located closer to the central axis of the main graben, and it contains also thick marlstone and sandy marlstone interlayered with fine sandstone and calcareous units (Pomesano Cherchi, 1971; Solinas, 1986). The sands have multiple sources as indicated by paleocurrents and composition (Fig. 9). For instance, the fine micaceous sands of M. Pelao differ significantly from others in the basin as they are

73

derived from metamorphic rocks which were, and still are, outcropping from below early postHercynic volcanics (Fig. 3). The environments of sedimentation vary as well. At M. Pelao there is a predominance of marly sandy shelf. At M.S. Ella, there is a change from coastal sandy plains rich in vegetative organic matter at lower levels, upward into fluvial sands and gravels at intermediate levels, to mollusc- and echinoid-rich shelf sand interstratified with and capped by marly algal limestones (Fig. 4). Unique to the M.S. Elia-M. Ruiu area is a development of coralline limestone unit (Cr), which represents local biohermal development on deltaic shallow-marine siliciclastic sand in the middle part of the section. The sections of M.S. Ella and adjacent areas formed for the most part in mixed siliciclasticcarbonate environments. Although located in a tectonically different setting, the modern mixed siliciclastic-carbonate, non-rimmed platform of western Florida may constitute a model for these Miocene deposits. The shore-parallel facies in Florida consist of inner shelf-quartz/molluscan calcarenite, shelf peloid, coralline algae, lithoclasts, and local reefal material, and continentalslope planktonic foraminiferal/pteropod ooze (Ginsburg and James, 1974). The overall character of these facies changes from the siliciclasticdominated northern area, more proximal to the source of sand, to the mollusc-rich and corallinealgae-rich sand to the south, to a carbonatedominant area toward the southern end of Florida. A similar siliciclastic-sand-dominant facies occurs in the M.S. Elia area indicating proximity to various clastic sources. The lower and middle parts of the sequence of M.S. Elia record prevalence of terrigenous influx from the lateral secondary graben of Mores-Chilivani, whereas the correlative Ploaghe-Florinas-Sassari system indicates graben-marginal conditions relatively unaffected by coarse clastics except during low water stands. The topmost carbonate unit of M.S. Elia was protected from erosion under a QuaternaryPliocene lava cap. It is likely to be younger than those of other, more deeply eroded sections of the study area, and probably represents the culmination of another major (third) transgression.

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Pomesano Cherchi (1971) tentatively assigned the uppermost algal carbonate layers of M.S. Elia (stage V1, Fig. 4) to the Tortonian by comparing it with dated similar units in central-south Sardinia. More recent dating of such southern units suggests a possible Early Messinian age (Cherchi and Tr6moli~res, 1984). Thus, the whole sequence studied in northwestern Sardinia could range in age from the latest Burdigalian well established in the lower basinal marlstone (Mf), to the tentative Early Messinian of the uppermost beds of M.S. Ella.

Marlstone sequence of the central part of the main basin The central part of the basin is highly dissected, and exposes an apparently monotonous thick sequence of marlstone, in beds of various thicknesses and induration due to the variable content of fossils and bioturbation. These basinal deposits have not yet been studied in detail to try and determine whether any record of sea fluctuations exists in them.

Synthesis Two major Miocenic tectono-sedimentary systems can be recognized in northern Sardinia, which bear strong resemblance to published "sequence stratigraphy" models (Wilgus et al., 1988), except that here we are dealing with siliciclasticcarbonate mixed environments (Fig. 10). (1) The first system tract is well developed at the margins of the major north-south trending graben (Figs. 3, 4, 10). Here, facies distributions were affected both by recurring eustatic sea-level changes and by rotational movements of marginal fault blocks (Fig. 10). A partially isolated, rimmed carbonate platform developed at the edge of the more rapidly subsiding, marlstone-bearing, central part of the trough. Marly limestone and siliciclastic sands formed at the eastward landward margin of the graben. During a subsequent regression, parts of the carbonate platform and some marlstonc were deeply eroded (type 1 sequence boundary). Deposition of siliciclastic sands occurred in lowstand wedges. A following major

t.P. M A R T I N I

Eq AI.

transgression established a capping carbonate platform. (2) The second system tract developed at the intersection between the main n o r t h - s o u t h trending graben and a northeastward trending secondary graben (Mores-Chilivani) (Figs. 3, 4). Its component parts are not as well defined as the first system primarily because of the more sheltered continental to inner shelf settings and poorer exposure. The lower part of this second system is dominated by fluvial and shelf sand with few local coralline biostromal bodies. The upper part contains the youngest sediments (possibly Tortonian-Early Messinian in age), has continental sands alternating with fossiliferous marine sand, and is capped by algal limestone. This upper part of the sequence represents transgressive and highstand system tracts over a type 2 sequence boundary. We cannot date the rocks accurately enough to fully prove it, but most likely some of the recorded fluctuations in sea level were eustatic in nature and can be correlated with those recognized throughout Europe for this time span (Fig. 2; Cherchi and Montadert, 1982; Haq et al., 1987, 1988). Using available age determinations, tying local vertical facies variations with the worldwide sea-level changes (Haq et al., 1987), and taking into consideration climatic and regional changes in the Mediterranean area (for instance, we have no evidence in our rocks of the Messinian "salinity crisis", hence they must be older than that), we can surmise the following sequence of events: (A) A sea-level rise in latest Burdigalian-Early Langhian (stage I1, Figs. 4, 10). (B) A major sea-level drop possibly in Late Langhian a n d / o r Early Serravallian (stage II1, Figs. 4, 10). (C) A major transgression with development of carbonate platforms such as in the FlorinasSassari area in post Early Serravallian (stage IV). (D) Highstand conditions with fluctuating sea levels and development of mixed siliciclasticcarbonate deposits (stage V) capped by algal limestones (stage VI). These last parasequences are locally preserved under Quaternary lavas such as at M.S. Elia (stage VII), but they are poorly exposed and dated, and whether or not they

S I L I C I C L A S T I C - C A R B O N A T E S E Q U E N C E S OF M I O C E N E G R A B E N S OF N O R T H E R N SARDINIA, W E S T E R N M E D I T E R R A N E A N SEA

reflect the worldwide sea-level fluctuations of Tortonian to Early Messinian age is hypothetical, only circumstantial regional evidence being available.

Acknowledgements Financial support for this research was provided by the National Science and Engineering

75

Research Council (Canada) (Grant A 7371) and the Centro Nationale delle Ricerche (Italy). The criticism of M. Brookfield, B.W. Sellwood, M. Tongiorgi and an unknown reviewer helped considerably in focusing the final manuscript. S. Sadura helped in making it all a bit more readable.

76

I.P.

MARTINI E'I

AI-

APPENDIX 1 Sedimentary facies Description

Interpretation

Algal limestone (Cat, Ca, Cam) Bioclastic algal limestone with concentrations of rhodoliths and macroforaminifera; large syndepositional slumps in unit Cat. Cam does not contain much rhodolith and no macroloraminifers.

Carbonate platfl)rm top (Ca) and slope (Cat), possibly localized by active faults. Cam forms a detached lower carbonate platform.

Coralline limestone (Cr) Bioclastic, algal limestone with abundant corals.

Small, localized limestone build-ups on siliciclastic shelf.

Marly limestone (Cm) Slightly argillaceous, fine crystalline, fossiliferous, algal limestone.

Muddy carbonate shelf.

Sandy limestone (Cs) Assemblage of highly bioturbated, fossiliferous quartzose sandstone and sandy bioclastic marly limestone.

Well aerated, shallow (few meters) transgressive sea.

Fossiliferous marlstone (Mf) Calcareous argillaceous siltstone, with regular alternation of medium strata with different fossil content (in situ and reworked fossils) and bioturbation. Rich in microforaminifera and nannoplankton.

Basinal sedimentation in water depth greater than 100 m, according foraminiferal forms. Burdigalian-Lower Langhian in age.

Black Mudstone (Mb) Dark grey, fossiliferous mudstone lenses with disseminated coarse quartz grains.

Restricted, lagoonal setting with quartz grains introduced by winds a n d / o r storm washover events.

Organic sand (So) Two sub-facies: (1) plane bedded sand with cuts and fills, alternating with organic-rich, intensely bioturbated sand containing sideritic nodules; (2) coarse quartz-rich sand, thickly bedded, cross-bedded, bioturbated (large subvertical burrows). with scattered plant fragments.

(1). Prograding coastal-plain protected seaward by (2) brackish (absence of body fossils) bars-barriers.

Marly sand to sandy marlstone (Sm) Muddy sand, highly calcareous, regularly horizontally bedded, local large scale (meter wavelength) wave forms on surface of beds, differentially cemented, bioturbated and fossiliferous with reworked fossils mixed from various bathymetric zones,

Formed in deeper parts of shelf, possibly by return turbid flows associated with storm "set-ups".

l%ssilit~'rous sand (Sf) Highly fossiliferous, bioturbated, massive, carbonate cemented, quartz-rich, coarse grained sand with sparse granitoid granules and pebbles.

Sandy shelf bottom and highly fossiliferous bars.

Cak'areous sand (Sqc) Quartz-rich, calcareous cement, rare fossil fragments, massive to cross-bedded (up to 5 m thick foresets).

Gilbert-type cross-bedded bodies (deltas a n d / o r bars) rapidly prograding in brackish coastal areas.

Quartzo-]eldspathic sand (Sqd) Coarse, quartz (65%) and feldspar (25%) rich with some kaolinitic matrix, mostly massive, locally cross-bedded (4-5 m thick sets) sand.

Formed in a variety of environments, from fluvial channel to rapidly prograding fans into lacustrine to brackish marine settings (post-Lower Langhian age).

SILICICLASTIC-CARBONATE SEQUENCES OF MIOCENE GRABENS OF NORTHERN SARDINIA, WESTERN MEDITERRANEAN SEA

77

A P P E N D I X 1 (continued)

Lithie sand (Sl) Coarse, poorly sorted, non-fossiliferous sand composed of subrounded quartz, feldspar, with minor biotite and lithic (granitoid and volcanic) fragments. Massive to cross-bedded (foresets up to 5 m thick).

Fluvial and cross-bedded delta deposits developing into lacustrine settings (absence of fossils and bioturbation in toesets).

Sandy gravel (Gs) Sandy, fine to medium quartzitic and granitoid pebble conglomerate, plane and cross-bedded, locally burrowed.

Fluvial to transgressive shore deposits.

Lithic conglomerate (GI) Reddish, sandy, metamorphic and granitoid pebble to small boulder conglomerate, local well developed pebble imbrication, massive with few cuts and fills and poorly developed cross-beds.

Braided alluvial fans.

Volcaniclastic (1I) Tuff and cinder interlayered with freshwater limestone and black chert. Rhythmic laminated bedding with local small cuts and fills. Small mammifer remains. This facies constitutes the substratum of the sequence analyzed in this paper.

Lacustrine deposits developed in sags between stratovolcanoes. M i d d l e - U p p e r Aquitanian to Lower Burdigalian (Bruijns and Rumke, 1974).

Symbols: a = algal; b = bioclastic; C, c = limestone, calcareous; d = feldspathic; f = fossiliferous; G = gravel, conglomerate; 1 = lithic; M, m = marlstone, marly; o = organic; q = quartzose; r = reefal; S, s = sand, sandy; t = syndepositional deformations; V = volcaniclastic.

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