The Basal Shaly formation of the Iberian pyrite belt (South-Portuguese zone): Early carboniferous bituminous deposits

Palaeogeography, Palaeoclimatology, Palaeoecology, 73 (1989): 233 241

Elsevier Science Publishers B.V., Amsterdam

233

Printed in The Netherlands

THE BASAL SHALY FORMATION OF THE IBERIAN PYRITE BELT (SOUTH-PORTUGUESE ZONE)" EARLY CARBONIFEROUS BITUMINOUS DEPOSITS C. M O R E N O 1 a n d L. S E Q U E I R O S : 1Departamento de Geologfa y Mineria. Seccidn de Geologia de la Rdbida, Universidad de Sevilla 21819 Palos de la Frontera, Huelva (Spain) 2Departamento de Geologla. SAFA, Ubeda, Ja~n (Spain)

(Received July 5, 1988; revised and accepted February 22, 1989)

Abstract Moreno, C. and Sequeiros, L., 1989. The Basal Shaly Formation of the Iberian Pyrite Belt (South-Portuguese Zone): Early Carboniferous bituminous deposits. Palaeogeogr., Palaeoclimatol., Palaeoecol., 73:233 241. We present here a sedimentary model for the Basal Shaly Formation in the Iberian Pyrite Belt based on stratigraphical and paleobiological data. Rises of sea level seem to be confirmed by the presence of organisms (Posidonia) with an r-strategy for colonisation, that almost form a well located association with a limited vertical thickness. A new fall in sea level caused coastal erosion and the first turbiditic cycles in onlap deposition.

Introduction T h e H e s p e r i a n M a s s i f consists of P a l e o z o i c and P r e c a m b r i a n r o c k s o c c u p y i n g m o s t of the n o r t h e r n a n d w e s t e r n p a r t s of the I b e r i a n P e n i n s u l a . All t h e s e m a t e r i a l s w e r e d e f o r m e d a n d / o r m e t a m o r p h o s e d d u r i n g the H e r c y n i a n Orogeny. The massif has been classically divided in zones (Lotze, 1945; J u l i v e r t et al., 1974), a c c o r d i n g to s t r u c t u r a l , p a l e o g e o g r a p h i c and p e t r o l o g i c a l criteria. T h e s o u t h e r n m o s t zone of the massif, n a m e d S o u t h - P o r t u g u e s e Zone (SPZ) is s u b d i v i d e d in t h r e e s e c t o r s ( C a r v a l h o et al., 1976). T h e C e n t r a l S e c t o r or I b e r i a n P y r i t e Belt, c o n t a i n s a l a r g e n u m b e r of h u g e m a s s i v e sulphide deposits, by w h i c h a w e l l - k n o w n metall o g e n e t i c p r o v i n c e is defined. Locally, however, t h e r e is little i n f o r m a t i o n a b o u t c e r t a i n m a t e r i a l s a n d / o r s t r a t i g r a p h i c u n i t s w h i c h are of no i m m e d i a t e e c o n o m i c interest, as o c c u r s 0031-0182/89/$03.50

w i t h the B a s a l S h a l y F o r m a t i o n (BSF) (Moreno, 1987). T h e a b s e n c e of specific studies on this subject h a s led to c o n f u s i o n a n d a v a r i e t y of t e r m i n o l o g y and, in cases, to the a c c e p t a n c e of " t r a d i t i o n a l e r r o r s " s u c h as to include the B S F in the C u l m facies. T h e s t r a t i g r a p h i c p o s i t i o n of t h e BSF, its l a t e r a l c o n t i n u i t y a n d r e d u c e d t h i c k n e s s , its a b u n d a n t f a u n a ( m a n y i n d i v i d u a l s of a single species), and last, its l i t h o l o g y are some of the f a c t o r s t h a t p r o m i s e its s e d i m e n t o l o g y a n d p a l e o g e o g r a p h y to be of i m p o r t a n c e . In addition, t h e s e can be c o m p a r e d w i t h e a c h o t h e r in t e r m s of t h e i r t e c t o n o g e n i c p o s i t i o n i n g , e v e n t h o u g h t h e y m a y n o t h a v e the e x a c t s a m e age (Oliveira et al., 1979). P r e v i o u s studies of the BSF in this r e g i o n are v e r y s c a r c e a n d n o n e of t h e s e p r e s e n t s a detailed d e s c r i p t i o n a n d p a l e o g e o g r a p h i c a l i n t e r p r e t a t i o n . T h e o n l y specific p a p e r on this s u b j e c t is a b r i e f n o t e by M o r e n o a n d S e q u e i r o s

(( 1989 Elsevier Science Publishers B.V.

234

(1987), which serves as an introduction to this study. This however, has not prevented many authors in their papers on the Sedimentary Volcanic Complex or in regional studies, from mentioning the BSF unit briefly. The unit has been given various names: Lower Member (Doestch, 1953), Shaly Lower Formation (Kleyn, 1960), Phyllitic Member (Van den Boogaard, 1967), Basal Stratigraphical Shale Facies (Routhier et al., 1980) Basal Black Schist (Albouy et al., 1981), Basal Series (Moreno and Vera, 1985), Basal Shaly Formation (Moreno, 1987). In this paper the study of the materials which constitute the BSF of the Southwestern Iberian Pyrite Belt and a sedimentation model are presented based on data obtained from the western part of the Spanish Pyrite Belt (area outlined in Fig.l). / :.i:(~.:~.~i.!i~::~}:.'..:" i.'(i~ •..:. :.,.'.'i:". ,,:(" ~ ~' ~'::.::;".'?:.". , : :.; :'.:'..2..

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Geological framework The South Portuguese Zone of the Iberian Massif has usually been divided into three sectors, Northern, Central and Southern, according to their stratigraphic, tectonic and paleogeographic features. The Central Sector belongs to a rich metallogenetic province named "Iberian Pyrite Belt": its location and distribution is shown in Fig.1. The most accepted and representative sequence of the Iberian Pyrite Belt is that proposed by Schermerhorn (1971). From bottom to top the following units can be seen: Phyllite-Quartzite Formation (PQ Formation), the Sedimentary Volcanic Complex (SVC) and the Culm Group: -The Phyllite-Quartzite Formation is the basement of the regional stratigraphic series. Lithologically it is made up alternatively of shales and quarzites with local Devonian conglomeratic and carbonatic lensed levels. ~

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Fig.1. Geological map of the S o u t h P o r t u g u e s e Zone s h o w i n g the location of the study area. Legend: (OMZ) Ossa M o r e n a Zone. (A) Aracena: (B) Beja; (F) Faro; (H) Huelva; (S) Sevilla; (modificated from Carvalho et al., 1976).

235

The Basal Shaly Formation (BSF)

-The Sedimentary Volcanic Complex (SVC) is represented by acid and basic volcanites belonging to a partly subaerial-partly submarine volcanism (L~colle, 1976; Routhier et al., 1980; Leca et al., 1983), and by volcanic epiclastic materials. The regional, temporal and areal disposition of its materials is distributed in different volcanic cycles: 1st acid volcanism, 1st basic volcanism, 2nd acid volcanism, 2nd basic volcanism and 3rd acid volcanism. Related to the volcanic activity, important sedimentary volcanic sulphides appear. -The Culm Group, made up of alternating shales and flyschoid graywackes, whose age ranges from Late Tournaisian to Westphalian. Between the SVC and the Culm facies turbidites (s.s.) occurs a stratigraphic sequence with varying thickness (0-200m) formed by shaly and ash layer deposits which constitutes the Basal Shaly Formation (BSF) (Fig.2).

The BSF is composed of the deposits intercalated between the SVC and the Culm facies materials. They belong to a sedimentaryvolcanic mixed sequence of hematitic and siliceous composition and with different percentages of detritic materials. They constitute the epiclastites of the 3rd acid volcanism, (top of the SVC), plus the shaly sequence located at the bottom of the turbiditic formation. The synthetic stratigraphical column of the BSF is given in Fig.3.

Description The deposits of the 3rd acid volcanism, locally named "Tharsis-Third Volcanism", have been described by several authors (Van den Boogaard, 1967; Lecolle, 1972; Strauss et al., 1977). They consist ofvolcanics and epiclas-

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Fig.2. Geological map of study area (modificated from IGME, 1982). Legend: (1) allochthonous units of North provenance; (2) PQ Formation; (3) Volcano-Sedimentary Complex; (4) Basal Shaly Formation; (5) Culm Group; (6) Postpaleozoic Sediment Cover. Towns: (A) E1 Almendro; (AL) Alosno; (G) Gibralebn; (GR) E1 Granado; (PG) Puebla de Guzmhn; (SB) San Bartolom~ de la Torre; (SG) Sanlucar de Guadiana; (T) Tharsis; (V) Villanueva de los Castillejos; (VC) Villanueva de las Cruces.

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) Shales and sandstones. 2) F i r s t b a s i c v u l c a n i s m e . 3) F i r s t a c i d v u l c a n i s m e . 4) I n t r a v o l c a n i c shales and tuffites. 5) S e c o n d b a s i c v u l c a n i s m e . 6) S e c o n d a c i d v[11canisme. 7) J a s p e r l e n s e s . 8) T u f f i t e s . 9) T h i r d b a s i c v u l c a n i s m e . 10) P u r p l e s h a l e s a n d t u f f i t e s . II) T h i r d b a s i c vulcanisme. 12) Basal Shaly Formation. 13) S h a l e s and g r a y w a c k e s .

Fig.3. Schematic stratigraphic log of the Basal Shaly Formation (approximate scale) and locatmn ot it m a general (not at scale) log of the Iberian Pyrite Belt.

tic deposits differing in grain-size and composition and, although the epiclastics prevail, true pyroclastics, lavas, volcanic breccias and even eruption centers have been described (Strauss and Madel, 1974; L6colle, 1977). The epiclastics are made up of sedimentary rocks classified as silts, sandstones and volcanic conglomerates according to the Pettijohn et al. (1972) nomenclature and hybrid arenites according to the classification of Zuffa (1980). Their sedimentary origin is determinated by their petrographic and geometric features. Although a complete vertical sequence of structures cannot be seen, due to observation difficulties, reworking of the volcanic deposits in a littoral environment can be deduced from many observations in isolated outcrops (symmetrical oscillating ripples, horizontal lamination, reactivation

surfaces, crossed planar low-angle bedding, flaser and linsen bedding, etc.), Fig.4a and 4b. Higher in the column the volcanic characteristics of the rock fragments grade into black shales with organic matter and sulphides (pyrite). According to Bitterli (1963) there is no single sedimentary environment to explain the bituminous shale deposits, but such sediments are typical of the critical paleogeographic events (orogenic events and eustatic oscillations) and cause transgressions and regressions followed by stillstands of sea level and anaerobic conditions. Bitterli's conditions (1963) are applicable to the shales with which we are concerned for several reasons: (1) A very special stratigraphical position: postvolcanic and preturbidite (preflyschoid).

237 shallow e n v i r o n m e n t close to the c o n t i n e n t with r e s t r i c t e d and quiet water. Over the c a r b o n a t e layer, or sometimes w i t h i n it, black b i t u m i n o u s shales with Posidonia becherii Newell can be found. L o c a l l y t h e y almost form a c e n t i m e t e r t h i c k lumachelle, which usually takes shape of several superposed sheets, resembling a condensed series (Fig.5a, 5b). The sizes of the Posidonia shells differ from 1 to 2 cm and from 10 to 12 in dorsov e n t r a l diameter. T h e i r p r e s e r v a t i o n is good or m o d e r a t e , and the two valves are f r e q u e n t l y present, even a r t i c u l a t e d . B o t h the moulds as well as the c o r p o r a l limonitized fossils h a v e been observed. Usually, the smaller forms are the b e t t e r preserved. T h e i r l o c a t i o n with r e g a r d to the bed is h o r i z o n t a l and one can

Fig.4. Sedimentary structures in the ashes of "Tharsis Third Volcanism". (a) Horizontal and slightly dipping lamination, crosslamination and reactivation surface. (The coin is 2.5 cm in diameter). (b) Planar tabular cross-lamination (lower part) showing opposing paleocurrent trends with wave ripple cross lamination (upper part). (2) T h e i r age in r e l a t i o n to the H e r c y n i a n Orogeny. (3) T h e i r g r e a t l a t e r a l c o n t i n u i t y , not only in the S o u t h P o r t u g u e s e Zone, b u t also in all E u r o p e a n H e r c y n i a n Belts (Bitterli, 1963; Conil, 1964; Krebs, 1969; Oliveira et al., 1979, etc.). Locally, on top of the a u t o c h t h o n o u s shales, t h e r e is a d i s c o n t i n u o u s c a r b o n a t e layer, some c e n t i m e t e r s t h i c k with different lithologic facies: a d i a g e n e t i c nodules level, a microc o n g l o m e r a t e with boulders and c a r b o n a t e cement, c a l c a r e n i t e s with badly p r e s e r v e d echinoids, brachiopods, gastropods and algal fragments. Such an association suggests a

Fig.5. Posidonia accumulation level in the Basal Shaly Formation. (The coin is 2.5 cm in diameter). (a) General aspect. (b) Detail.

238 clearly see in them a strong compaction and later deformation. Accumulation levels of Posidonia are not restricted to the BSF. In some instances they occur also in autochthonous shales marking the upper limit of the depositional sequences and/or facies cycles of the Culm turbidites. During the last decades Posidonia becherii Newell = Posidonomya Brown has been a really problematic pectinid. The paleoecological aspects have been studied in detail in many surveys, particularly in Womersley and Edmonds (1958), MacArthur (1960), Jefferies and Minton (1965), Seilacher and Westphal (1971), Clarkson (1979), Kauffman (1981) and Seilacher (1982, a, b, 1985). It is commonly accepted that Posidonia is a benthic pectinid, capable of swimming, an opportunistic bivalve with an rstrategy for colonisation. The lack of erosional tracks in the BSF, the paleoecological characteristics of Posidonia and the lithology of the sediments where it can be found, suggest that the fossiliferous levels were produced by passive accumulation. We agree with the remarks of Kidwell and Jablonski (1983): "during a fast rise in the sea level, the terrigenous contributions reaching the basin decrease considerably and a passive organism accumulation in deep waters takes place". In this shaly level one can also observe bioturbated surfaces (horizontal tracks), although never mixed with Posidonia. This relation suggests the existence of chemical oscillations in the environment that have an effect on the benthic organisms (Byers, 1982). On top of the shale levels with Posidonia, the first turbiditic cycles begin with sharp and erosive contacts, and with proximal or distal characteristics depending on the location. The presence of sharp contacts, even the erosive ones, between the BSF and the Culm turbidites (s.s.), is the consequence of a change in the conditions of deposition. The turbiditic deposit suggests a relative fall of sea level and an erosive process on the continent and the platform (Mutti, 1985). This suggests that the contact between the BSF and

the turbidites is a discordance type I of Vail et al. (1984). The geometry (cartographical form) of the BSF could be related to the SVC volcanic relief. Variation in lateral thickness is very common, as well as the wedgings and local erosions of the whole shaly series (i.e., Santa Catalina mine, Cabezas del Pasto mine, etc.), with the Culm turbidites in the case directly on top of the SVC. Discussion: a sedimentary model

The available data suggest the following sedimentary interpretation: During the deposition of the BSF a relative rise of sea level occurred and caused a progressive change of the deposits in the sequence. The base of the BSF, composed of the socalled "Tharsis-Third Volcanism", implies the existence of a littoral environment where the volcanic materials were reworked. The main features of the basin, depth, energetic factors, shape, etc., changed gradually with the arrival of new materials. At first, within a more energetic environment, the deposition of the ~'Tharsis-Third Volcanism" took place; later, the energy level decreased and the deposition of lutites occurred. Finally, Posidonia colonization and passive accumulation of its shells took place. The relative rise in sea level associated with the BSF deposit was followed by a fall in sealevel and, consequently, the onlap deposition of the Culm turbidites (Moreno, 1988). This process defines the BSF as a complete depositional sequence, according to Vail et al. (1984). The main points supporting the sedimentation model proposed, and outlined in Fig.6, are as follows: (1) There is evidence of subaerial volcanism in the deposits of the Sedimentary Volcanic Complex (SVC). The eruptions formed the underlying relief. (2) The Basal Shaly Series (cartographic form) surrounds these topographic highs. (3) The latest "volcanic" deposits are those forming the Tharsis-Third Volcanism. They are

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ashes w h i c h are fine-grained farther from the volcanic vent (cartographic wedges). (4) The sedimentary structures seen in the ashes suggest a clearly sedimentary character and reworking in the littoral environment. (5) The contact between the ashes and black

shales is gradual, vertically as well as laterally. (6) Because of the reasons explained above, and according to field data, the black shale must be considered as a u t o c h t h o n o u s sediments of the basin.

240

(7) The colonisation of Posidonia (r-strategy) took place at the same time as the first paleogeographic events of the Hercynian orogeny, related to a sea level rise. (8) The sharp and erosive contact between the turbiditic materials and the black shales implies a sudden sedimentation which caused the erosion of a part of the platform sediments. This fact explains the varying thicknesses of the autochthonous sediments in different areas. (9) The Posidonia-accumulation levels in the interturbiditic shales are located at the top of the depositional sequences. They indicate periods of tectonic stability of the basin after the repeated pulses that caused the deposition of a thick turbiditic series (Culm facies).

Acknowledgements Thanks are due to F. G. Garrido and C. Gray for the English text, to Dr. Joan Rosell for her critical reading of the manuscript and to Dr. A. Castro for the photographs.

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