Mesozoic sequence of Fuerteventura (Canary ... - GSA Publications

Mesozoic sequence of Fuerteventura (Canary Islands): Witness of Early Jurassic sea-floor spreading in the central Atlantic

Christian Steiner

Institut de Géologie et Paléontologie, Université de Lausanne, CH-1015 Lausanne, Switzerland

Alice Hobson

Institut de Minéralogie et Pétrographie, Université de Lausanne, CH-1015 Lausanne, Switzerland

Philippe Favre

CH-1985 La Forclaz, Switzerland

Gérard M. Stampfli* Institut de Géologie et Paléontologie, Université de Lausanne, CH-1015 Lausanne, Switzerland Jean Hernandez

Institut de Minéralogie et Pétrographie, Université de Lausanne, CH-1015 Lausanne, Switzerland

ABSTRACT

INTRODUCTION

The Fuerteventura Jurassic sedimentary succession consists of oceanic and clastic deposits, the latter derived from the southwestern Moroccan continental margin. Normal mid-oceanic-ridge basalt (N-MORB) flows and breccias are found at the base of the sequence and witness sea-floor spreading events in the central Atlantic. These basalts were extruded in a postrift environment (post–late Pliensbachian). We propose a Toarcian age for the Atlantic oceanic floor in this region, on the basis of the presence higher up in the sequence of the Bositra buchi filament microfacies (Aalenian–Bajocian) and of clastic deposits reflecting tectono-eustatic events (e.g., late Toarcian to mid-Callovian erosion of the rift shoulder). The S-1 sea-floor oceanic magnetic anomaly west of Fuerteventura is therefore at least Toarcian in age. The remaining sequence records Atlantic-Tethyan basinal facies (e.g., Callovian–Oxfordian red clays, Aptian–Albian black shales) alternating with clastic deposits (e.g., Kimmeridgian–Berriasian periplatform calciturbidites and a Lower Cretaceous deep-sea fan system). The Fuerteventura N-MORB outcrops represent the only Early Jurassic oceanic basement described so far in the central Atlantic. They are covered by a 1600 m, nearly continuous sedimentary sequence which extends to Upper Cretaceous facies.

The presence of Jurassic sedimentary rocks in the Fuerteventura Basal Complex, Canary Islands (Figs. 1 and 2) is confirmed in this study. We report previously undescribed outcrops of mid-oceanic-ridge basalts (MORB) interbedded with the sediments at the base of the series (Fig. 2). New data presented in this study include (1) a complete stratigraphic column with clearly defined units that allowed us, on the basis of a sequence stratigraphic analysis and on the presence of fauna, to confirm the interpretations of von Fritsch (1868; see Bourcart and Jérémine, 1938) and Rothe (1968), who suggested an Early to Middle Jurassic age for the lowermost sediments; and (2) the first proof of an early Middle Jurassic to late Early Jurassic oceanic basement for the island of Fuerteventura. The Mesozoic stratigraphy of the island is compared with previous paleogeographic information from the northwestern African continental margin and the Tethyan basins. This allows us to make precise age attributions and give new constraints on the opening of the central Atlantic Ocean. In this paper we use the Gradstein et al. (1995) time scale.

*E-mail: [email protected].

HISTORICAL BACKGROUND Prevolcanic sediments on Fuerteventura island were already known to German authors in the middle of the nineteenth century; e.g., von Fritsch (1868) and Gagel (1910; see Robertson and Bernoulli, 1982). Although the sediments were thought at that time to be shallow-water continental deposits of Paleozoic age (belonging to a

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fragment of “European-African” basement), von Fritsch described limestones of “Lower Jurassic type.” Rothe (1968) considered the main part—if not the whole—of the Fuerteventura sedimentary sequence as Mesozoic in age. He attributed a late Aptian to Albian age to the topmost part of the series on the basis of planktonic foraminifers. He also described a “Posidonia flysch” to which he attributed a Middle to Early Jurassic age on the basis of facies correlation with the Moroccan coastal basins. Robertson and Stillman (1979a, 1979b) established the true deep-water nature of the clastic sediments and interpreted their depositional environment as continental margin. As Fúster et al. (1968) had already suggested, Robertson and Stillman showed that the clastic material was derived from the ancient crystalline massifs of the African continent and that the carbonate material came from a “probable collapsed carbonate margin located offshore” (Robertson and Stillman, 1979a, p. 47). Simultaneously, Deep Sea Drilling Project (DSDP) Site 370/416 (Lancelot et al., 1977, 1980) revealed an upper Tithonian to lower Valanginian clastic and calcareous turbiditic series (unit VII), which was interpreted in the same way as the Fuerteventura series by Price (1980). Robertson and Stillman (1979a, 1979b) only mentioned an Early Cretaceous to Albian age and did not suggest a possible Jurassic age for the underlying formations. Robertson and Bernoulli (1982) completed these previous studies and were able to attribute a Hauterivian–Valanginian age to the base of the deep-sea fan sequence on the basis of the presence of an ammonite.

EARLY JURASSIC SEA-FLOOR SPREADING, CANARY ISLANDS

Figure 1. General tectonic setting of the Canary Islands, the central Atlantic, and the Northwest African continental margin (after Hinz et al., 1982; Roeser, 1982; Heyman, 1989; Verhoef et al., 1991; Holik and Rabinowitz, 1992). (1) Precambrian craton. (2) Variscan basement (folded Paleozoic, Pz). (3) Mesozoic–Cenozoic coastal basins: D—Doukala, E—Essaouira, A—Argana, ST—Souss Trough, AT—Aaiun-Tarfaya. (4) Alpine belt (upper Eocene–upper Oligocene): SA—South Atlas, HA—High Atlas. (5) Jurassic half graben, with first-order normal faults and transfer faults. (6) Offshore diapiric province and its inland continuation. (7a) Magnetic anomalies (M-series—Jurassic sea-floor spreading lineations; S-series—positive magnetic lineations, from Verhoef et al., 1991). (7b) Oceanic fracture zones (as inferred from seismic and/or magnetic data). (8a) Upper Cretaceous–lower Tertiary volcanics (diffracted upper surface and chaotic seismic facies). (8b) Eastern Canary volcanism (Oligocene) with dike swarm on Fuerteventura and offshore. (9) Deep Sea Drilling Project sites. (10) –1000 m bathymetric contours. (11) Guyots. (12) Location of studied area.

Interpretation of the Tertiary intrusions from the Basal Complex of Fuerteventura as an ophiolitictype mafic to ultramafic stratiform complex (Hausen, 1958; Gastesi, 1973; Bennel-Baker et al., 1974; Stillman et al., 1975) and the use of the ambiguous term “spilite” (Hausen, 1958, in Fúster et al., 1968) led to the hypothesis of an underlying oceanic lithosphere. Both arguments have since been abandoned (Stillman, 1987; Coello et al., 1992). The oceanic nature of the crust beneath Fuerteventura is nevertheless now supported by refraction seismic data (Banda et al., 1981), magnetic anomalies (Verhoef et al., 1991; Roest et al., 1992), and by the absence of contamination by sialic continental crust of the Fuerteventura magmas (Hoernle and Tilton, 1991; Hobson, 1995, personal commun.).

GEOLOGIC SETTING Fuerteventura is the largest of the seven islands of the Canary archipelago (Fig. 1). Together with the island of Lanzarote and the submarine Conception Bank, it forms an elongate northeast-southwest–oriented volcanic structure, called the Eastern Canary Ridge. Fuerteventura hosts the oldest subaerial volcanic rocks of the archipelago; they have been dated as 21 Ma (Schmincke, 1982). Two main lithologic units are defined in Fuerteventura. 1. The Basal Complex consists of (a) a Mesozoic sedimentary succession overlain by Tertiary volcanic sedimentary rocks containing an early to middle Oligocene fauna (Fúster et al., 1968;

Robertson and Bernoulli, 1982); (b) a series of late Oligocene to early Miocene alkaline ultramafic to intermediate intrusions (Féraud et al., 1985; Coello et al., 1992; Cantagrel et al., 1993), emplaced into (a); (c) a very dense early Miocene (24–17 Ma, Féraud et al. 1985; Cantagrel et al., 1993) sheeted dike complex, associated with (b) and intrusive into (a). 2. A succession of flat-lying volcanic flows, which overlie and are separated from the Basal Complex by an unconformity comprises (a) early Miocene flows (Basaltic Series I, Fúster et al., 1968; Féraud et al., 1985) and middle Miocene pyroclastic deposits (trachytic and syenitic breccias, Féraud et al., 1985) and (b) Pliocene–Quaternary flows (Basaltic Series II to IV, Fúster et al., 1968; Coello et al., 1992).

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Main Clastic Unit

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Pelagic Limestone Unit

Pelagic Bivalve Limestone Unit

Tertiary volcaniclastics

Mixed Clastic Unit

Tertiary dikes, Neogene basaltic flows and Quaternary deposits

Trachytic sills, Basaltic Series I

Figure 2. Map of Mesozoic outcrops, Betancuria massif, and Fuerteventura Basal Complex. Topographic grid after the Cartografía Militar de España, Mapa General, serie 5V, scale 1:25 000, Hojas 92–78 (Antigua), 91–79 (Pájara).

The term “Basal Complex” was first introduced by Bennel-Baker et al. (1974) and Stillman et al. (1975) and has since then been widely used. The Basal Complex crops out in different locations, but the main outcrop, and only outcrop containing the sedimentary rocks, is the Betancuria massif (Rothe, 1868; Fúster et al., 1968), which lies along the central west coast of the island (Fig. 2). The present geologic setting of Fuerteventura (Fig. 1) mainly results from the opening and evolution of the central Atlantic. A first rifting phase from Anisian to Hettangian time reactivated Variscan faults of the future AtlanticTethyan domain (Jansa and Wiedmann, 1982; Favre and Stampfli, 1992), creating confined basins with evaporitic and dolomitic deposits (e.g., Holser et al., 1988) and tholeiitic basaltic 1306

flows (210 ± 2 to 196 ± 1 Ma, High and Middle Atlas, Fiechtner et al., 1992; 201 ± 2 Ma, Newark Supergroup, Sutter, 1988). Sinemurian–Pliensbachian to earliest Toarcian deposits (transgressive carbonate platform, nodular limestones with black shales) point to a sudden deepening of the depositional environment and are affected by an extensive synsedimentary stress regime (Ambroggi, 1963; Jansa, 1986; Favre et al., 1991). Both features suggest the tilting and foundering of crustal blocks along the African margin, corresponding to a final stage of rifting (Lancelot and Winterer, 1980; Favre and Stampfli, 1992). The salt deposits and tilted blocks of the northwestern African margin are located in the diapiric province (Fig. 1). Sea-floor spreading in the central Atlantic was

thought to have started during Bajocian time between 170 and 175 Ma (Jansa and Wiedmann, 1982; Klitgord and Schouten, 1986). It was shortly preceded, or accompanied during Toarcian time, by the thermal uplift of the rift shoulders (Favre and Stampfli, 1992) dated off Mazagan by Huon et al. (1993) as 184 ± 4 Ma. The onset of the shoulder uplift during late Toarcian time is marked in the northwestern African basins by the following. 1. There was sudden shedding and input of clastic deposit into the Argana and Essaouira rim basins and the external Rif Basin (Choubert and Faure-Muret, 1962; Favre et al., 1991; Favre and Stampfli, 1992). 2. Erosion marked the end of the Liassic carbonate platform in Essaouira and in the Western

Geological Society of America Bulletin, October 1998

EARLY JURASSIC SEA-FLOOR SPREADING, CANARY ISLANDS

and Central High Atlas; there is evidence of local erosion (Choubert and Faure-Muret, 1962; Faugères, 1978; Favre and Stampfli, 1992; our own observation). 3. There was generalized regression in all the Atlasic domain, linked to tectonic movements in the Western Meseta (Choubert and Faure-Muret, 1962; Heyman, 1989). Generally, the erosion of the shoulder lasted until Bathonian to middle Callovian time. The oldest central Atlantic oceanic crust had been found in the Blake-Bahama Basin (Sheridan et al., 1983), in the underlying middle to upper Bathonian sediments (164 to 167 Ma, Baumgartner et al., 1995). The oldest recognized sea-floor spreading lineation is M25 (154 Ma, Oxfordian-Kimmeridgian boundary, Channell et al., 1995). In the eastern Atlantic, the Jurassic magnetic quiet zone lies between M25 and somewhere within the diapiric province, and includes the Canarian archipelago. Its only recognizable magnetic patterns are two prominent linear anomalies (S1 and S2; Roeser, 1982; Roest et al., 1992). Two similar magnetic anomalies are found inside the Jurassic magnetic quiet zone on the North American side: the Blake Spur magnetic anomaly and the East Coast magnetic anomaly, which are located at the ocean-continent boundary according to Klitgord and Schouten (1986). On the African side, S1 was long thought to be the analogue to the East Coast magnetic anomaly (Hinz et al., 1982; Roeser, 1982) but is now considered to be a sea-floor spreading lineation and the counterpart of the Blake Spur magnetic anomaly (Roest et al., 1992). This would imply that the continent-ocean boundary is situated east of S1, just before the S2 anomaly (which is not the exact counterpart of the East Coast magnetic anomaly; e.g., Favre and Stampfli, 1992). During Middle Jurassic time a connection was established between the pelagic domain of the Tethys and the Gulf of Mexico (Bernoulli, 1981; Gradstein et al., 1990). During Callovian time the rift shoulders and rim basins were submerged. A carbonate platform gradually formed from Mazagan to the Tarfaya basin (Mazagan: Hinz et al., 1984; Argana: Ambroggi, 1963; Tarfaya: Ranke et al., 1982, Heyman, 1989), and reached a maximum seaward extension in late Kimmeridgian time. Early Cretaceous time is characterized by the onset of rifting in the North Atlantic (e.g., Welsink et al., 1989; Sibuet and Srivastava, 1994) and by a major drop of sea level with associated clastic sedimentation contemporaneous with the emergence of the continental platforms. SEDIMENTARY SEQUENCE Sedimentary rocks crop out along the west coast of Fuerteventura, near and to the north of

Puerto de la Peña (Fig. 2). They form a continuous overturned series, with no repetition of lithologies. The strata are all vertical to subvertical and strike northwest to west-southwest. Polarity indicators in turbidite sequences (e.g., flute casts and fluid escape structures), biostratigraphy, and facies correlation indicate younging of this series to the north (Robertson and Bernoulli, 1982, and this study). Dike intensity ranges from 30% to 90% or even 100% of the total outcrop. The sedimentary rocks were affected by thermal metamorphism of low greenschist grade (chlorite-epidote-quartz-albite; Robertson and Bernoulli, 1982, and this study) to intermediate greenschist grade (garnetepidote-oligoclase-biotite ± apatite). The metamorphic grade in the normal (N) MORB of the Basal Unit reaches lower amphibolite facies (oligoclase–epidote–blue-green hornblende–calcic garnet) because of their proximity to the Tierra Mala pluton. Metamorphism explains the difficulty of finding microfossils in the first 700 m of the sedimentary sequence. Five sedimentological units are herein formally described on the basis of detailed geologic mapping and geochemical analyses (Fig. 3). Type sections for each of the units are defined (Fig. 4). These units correspond roughly to the division proposed by Rothe (1968), Robertson and Stillman (1979a) and Robertson and Bernoulli (1982). Basal Unit The Basal Unit is the lowermost unit of the Fuerteventura sedimentary sequence (Figs. 2 and 4). Two main subunits are recognized: a lower subunit A, which comprises sediments interbedded with basalts, and an upper subunit B, consisting of about 150 m of turbidites and pelagic sediment. Subunit A, “Basalt Dominated.” Outcrops of oceanic tholeiitic basalts are found at the Playa de Los Muertos and near the mouth of the Barranco del Aulagar, and as smaller sections inland at the Majada de la Perra and along the Barranco de la Potranca (Fig. 2). At the Playa de Los Muertos, basalt thickness ranges from a minimum of 63 m (cumulated vertical height of the outcrops) to a possible maximum of 320 m (considering the gap in exposures). The other sections are less continuous and nowhere thicker than 10 m. This study shows that the basalts are present as (1) massive extrusive flows, (2) pillow lava (Fig. 5-1), and (3) breccias; they are interbedded with fine clastic terrigenous sediments (siltstone and claystone). 1. The massive basalts have elongate vesicles and plagioclase microphenocrysts with habits typical of rapidly cooled minerals (see following). They crop out in the Playa de los Muertos as 30-m-thick homogeneous and compact masses,

too severely intruded by the Tertiary dikes to show any recognizable volcanic structures. Some 1 m ovoid pillow-like shapes are locally seen. They are interpreted as massive submarine flows. 2. The pillow lava forms isolated outcrops and cannot be stratigraphically correlated with the massive lava. The pillows are about 50 cm in diameter and have chilled margins. The interpillow spaces are filled with green sandy (hyaloclastic?) calcite-rich material. 3. The “breccias” consist of subangular basalt clasts, several centimeters in size, contained in a stratified lithic wacke composed of (igneous?) feldspar elements with a clayey matrix. The entire bed is interpreted as a pebbly sandstone of reworked basaltic material. The texture of all these basalts is microlitic to glomeroporphyritic with an aphanitic vesicular groundmass. Mineral sizes range from 1 mm for microphenocrysts, to smaller than 0.1 mm for the groundmass. Modal composition is dominated by plagioclase (~30%) and rare pigeonite. Sericitization has altered most plagioclase, preventing accurate determination of its composition. It is Carlsbad twinned, and original mineral habits include dendritic, skeletal (“belt-buckle”), and needle ending in sparrow-tails, which are typical of high degrees of undercooling. The groundmass has been entirely replaced by secondary minerals. The primary mineralogy and texture of these basalts are similar in every way to Holocene (Schilling et al., 1983; Kunz, 1993), Cretaceous (Natland, 1977), and Jurassic (Sheridan et al., 1983) Atlantic tholeiitic T to N MORB, or to Triassic–Lower Jurassic Moroccan continental tholeiites (Dupuy and Dostal, 1984). In the Blake-Bahama basin (cores 534-127 and 534128, Sheridan et al., 1983) sediments are found interbedded with the pillow lavas and basaltic flows in a way similar to that at Fuerteventura. The large ion lithophile elements (mainly K, Rb, Sr, and Ba) in the basalts from the Basal Unit show erratic behaviors in geochemical diagrams and must have been mobile during the metamorphic events that affected these rocks (see spider diagram, Fig. 3B). The use of discrimination diagrams based on alkaline elements (AFM; Kuno, 1968; De la Roche et al., 1980) is therefore excluded. Elements such as Ti, Nb, and Y (Floyd and Winchester, 1975; Pearce, 1982), or Cr (Pearce, 1980) and V (Shervais, 1982), show a clear tholeiitic affinity. However, such elements do not allow the discrimination between oceanic MORB or continental tholeiites. Fe and Ti contents are similar to those found in T-MORB to N-MORB from the FAMOUS Site (36°N) along the Mid-Atlantic Ridge (Kunz, 1993). The chondrite-normalized rare earth element (REE) patterns (Fig. 3, A1 and A2) clearly enable us to differentiate the continental from the oceanic tholeiites. The continental tholeiites

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A1)

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Ce

Nd Sm Eu

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3.9 3.7 2.6 1.9 2.8

9.9 10.7 8.9 7.2 8.6

10.4 10.7 9.8 8.8 9.1

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Ba Rb K NbLa Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

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Eu

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1.57 1.95 1.46 1.49 1.23

1.1 1.1 1.1 1.1 1.0

0.7 0.7 0.7 0.7 0.6

4.2 4.3 5.0 5.9 4.3

0.65 0.69 0.74 0.79 0.63

CH 54 CH 55 CH 106 AH 23 AH 22

Tholeiitic basalts from the Basal Unit, Fuerteventura, Middle to Early Jurassic

FU 128 FU 64 FU 90 FU 185 HM 2(flow 1) HM 2 (flow 9)

Hawaiite, Basaltic Series I, Fuerteventura, early Miocene * Alkali olivine basalt, Basaltic Series II, Fuerteventura, Plio-Pleistocene * Basanitoïd, Basaltic Series II, Fuerteventura, Plio-Pleistocene * Basanitoïd, Basaltic Series IV, Fuerteventura, Quaternary * Continental tholeiite, Morroco: High Atlas, Middle Atlas, High Moulouya, Triassic to Liassic Oceanic tholeiite, pillow formation, Maio, Cape Verde Islands, Ea rly to L a te Cre ta c eous*

M 72/25 M 72/27 N1 TR 154 10d-3 -

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N-MORB from Mid-Atlantic Ridge, 28.88 N-MORB

Ce/Yb

(La/Sm)N

2.36 2.49 1.78 1.47 2.00

0.52 0.45 0.36 0.27 0.38

56.80 20.95 50.53 47.50

2.92 2.05 3.20 3.38

19.00 14.44

2.01 1.91

1.84 2.19 2.42 17.93

0.49 0.47 0.53 2.47

#

* Data from De Paepe et al., (1987) Data from Fiechtner et al., (1992) § Data from Schilling et al., (1983) # Data from Sun, (1980)

Figure 3. (A) Chondrite-normalized rare earth elements (REE) concentrations (normalization values from Boynton, 1984). A1: Basaltic tholeiites from the Fuerteventura Basal Unit, oceanic tholeiites from Maio (Cape Verde Islands; De Paepe et al., 1974), mid-oceanic-ridge basalt (MORB) (Schilling et al., 1983), and continental tholeiites (Fiechtner et al., 1992). A2: Tertiary and Quaternary subaerial volcanics of Fuerteventura (De Paepe et al., 1971). (B) Chondrite-normalized multielement abundances (normalization values from Boynton, 1984) in basaltic tholeiites from the Fuerteventura Basal Unit, N-MORB (Saunders and Tarney, 1984; Sun, 1980), and in continental tholeiites (Fiechtner et al., 1992). Normalized data are recalculated to make (Yb)N = 10; stars: normal (N)-MORB. (C) REE concentrations in tholeiites from the Fuerteventura Basal Unit (units are in ppm), REE ratios are compared to MORB, oceanic tholeiites of Maio (Cape Verde Islands), continental tholeiites, and subaerial volcanics of Fuerteventura. show light REE-enriched patterns, like enriched potassic (P) MORB, and negative Nb anomalies (Fig. 3, A and B; Dupuy and Dostal, 1984; Fiechtner et al., 1992). The basalts of the Fuerteventura Basal Unit plotted in Figure 3B show a marked large ion lithophile enrichment but no Nb anomaly. Their chondrite-normalized REE patterns are similar to the one of the Cretaceous (Cape Verde Islands, De Paepe et al., 1974) and Holocene Atlantic N-MORB and are not similar to the Fuerteventura Tertiary subaerial volcanics and the Triassic to Lower Jurassic Moroccan continental tholeiites. REE ratios (Fig. 3C), i.e., Ce/Yb = 1.47 to 2.42 for the Fuerteventura Jurassic N-MORB; 14 to 19 for the Fuerteventura Tertiary basalts, also show this distinction. (La/Sm)N ratios from the Fuerteventura Jurassic basalts (0.27 to 0.52) are 1308

in agreement with the values from the Mid-Atlantic Ridge basalts from the same latitude (Schilling et al., 1983). Subunit B, “Sediment-Dominated.” Overlying the basalts are 130 to 150 m of alternating finely bedded dark claystones and yellow siltstones. These fine clastic deposits have a composition and structure typical of silty argillaceous turbidites from the lower lobes of a deep-sea fan (Stow, 1986). The siltstones have fine planar laminations with rare foresets at their bases, and become gradually more argillaceous up-sequence (C-D-E and D-E Bouma divisions). The lower and upper parts of the unit consist of thinning- and fining-upward cycles with centimeterscale bedding. For the most part, however, the unit consists of a stack of cycles with decimeterscale bedding (5–20 cm). Each cycle begins with

a few beds of arkosic or quartzitic arenite that sometimes amalgamate or show complete Bouma sequences, and ends with alternating siltstones and claystones. The latter locally become dominant (up to 80%). A 1-m-thick black shale interval is at the end of one of these sequences. The regularity of the siliciclastic sediments is punctuated by the sporadic occurrence of limestones. Several sparitic light-colored and entirely recrystallized limestone beds are at the base of the unit but are absent in the middle. Toward the top, this subunit is composed of cycles of two to three fine-grained calcareous and noncalcareous turbidites. These calciturbidites are light-colored and recrystallized but still show current structures at the base of the beds. Above, the calciturbidites become dominant and alternate for about 30 m with fine clay interbeds. These form beds

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EARLY JURASSIC SEA-FLOOR SPREADING, CANARY ISLANDS

1–3 cm thick with planar undulating laminations (D-E and C-D-E Bouma divisions) and are sometimes amalgamated. They are of mixed composition, sandy limestones in which the only calcareous components that are not recrystallized are microsparitic peloids (30%). The matrix is microsparitic to sparitic and secondary metamorphic minerals (garnet-quartz-oligoclase) are usually dominant (up to 70%). The sediments alone give no precise indication of bathymetry, but several criteria include (1) dominating D-E Bouma divisions, the presence of rare foresets (C division) in the siltstones and the generally fine grained sandstones; (2) rather thin turbidite beds (a few centimeters to 10 cm) arranged as thinning- and fining-upward cycles; (3) the absence of channel structures; and (4) the absence of carbonate in the thin clayey interbeds, except at the top of the unit, suggesting that deposition took place below the calcite compensation depth (CCD), in a distal or lateral part of a deep-sea fan, or as contourites. The typical depth of the top of Atlantic spreading ridges is about 2500 ± 300 m and is suggested to be the bathymetric range in which sedimentation took place.

trix and silt-sized clasts. The turbidites form small (50 cm) thickening-upward cycles showing the CD-E divisions of the Bouma sequence. A southeast-northwest current direction is inferred from the foresets in the C division. Fossil traces of the deep marine epibiontic Helminthoida [?Helminthoida crassa (SCHAFHÄUTL)] are found on the lower surfaces of some of the turbidites interbedded with fine-grained marls. Upsection, this facies rapidly gives way to marlstones and claystones. The green color of the rocks from this unit, even in the zones least affected by the dikes, as well as the epibiontic bioturbations of the marls could indicate deposition in an oxygenated environment. This facies association typically reflects open circulation in a marine environment (Gardner et al., 1977). The association of a deep-water facies (bathyal ichnofacies and weak but constant terrigenous influx) as well as the position above the CCD (dominant marls and micrites) suggest an environment between the lower part of the slope and the abyssal plain. Bositra is a biostratigraphic marker indicating a Toarcian to Oxfordian age (Jefferies and Minton, 1965; Sturani, 1967; Bernoulli and Kälin, 1984) and documents the late Early Jurassic to earliest Late Jurassic age of this unit.

Pelagic Bivalve Limestone Unit

Mixed Clastic Unit

This unit crops out along the left bank, near the mouth, of the Barranco de Ajuy and along the coast north of Puerto de la Peña (Fig. 2). It consists of 120 to 150 m of limestone and claystone to marlstone; the base is 50 to 60 m of a pelagic limestone facies characterized by beds a few centimeters to 10 cm thick of shelly or nodular limestones alternating with green marls that become dominant upsection (Fig. 5-2 and 5-5). These marls contain numerous fossil impressions of Bositra buchi (RÖMER) (Fig. 5-3). Rothe (1968) mentioned these as Posidonia bronni, but these fossils are clearly Bositra buchi. The limestones have a neomorphic microsparitic matrix and contain a small fraction (0%–20%) of silt-sized clasts that form discontinuous layers. There are some water escape structures at the base of the limestone beds. The fossil impressions usually display separated valves lying horizontally with the concave side facing down. They are 0.2 to 1 cm long and only 0.05 mm thick. They show neomorphic syntaxial rims that grew perpendicularly (less often tangential) to the shell, thus reflecting the structure of the prismatic and respectively lamellar layers of the Bositra shells (e.g., Kälin and Bernoulli, 1984, Plate 1) (Fig. 5-4). The shells are closely packed (to 80% of the thin section) and are found in both the calcareous and argillaceous beds. Toward the top of this unit, the shelly limestone gradually gives way to composite distal turbidites (3 to 8 cm thick) with a recrystallized micritic ma-

The mixed clastic unit crops out along the coast, between the south of the Caleta Negra and the mouth of the Barranco de la Peña (Fig. 2), and is about 470 m thick. The unit marks the return to more clastic sedimentation and is mainly composed of alternating sandy turbidites and claystones, and in places green marlstones and black shales, deposited in a deep-sea environment. There are two intercalations of calciturbidites in this unit. One is in its middle part, the other at the top, ending the unit. Calciturbidites are separated by an important hemipelagic to pelagic interval, consisting of alternating fine siltstones and black, green, and red noncalcareous claystones, with a layer of black shale. The turbidites are arkosic graywackes of medium to fine sand size. They locally preserve calcitic (palisade) or siliceous (chert) cements in fractures. At the base of this unit, they form small thinning and fining-upward (more rarely coarsening- and thickening-upward) cycles about 10 m thick. Each bed is 3–5 cm to 50 cm thick and shows the C-D-E Bouma intervals. Higher in the sequence only the D-E intervals are found. Some of these cycles end with a thick (up to 80 cm) black shale interval containing small slump structures. Higher in the sections, the interbeds change to greenish marlstone, which are small recurrences of the calcareous facies of the underlying unit. They alternate, between about 70 m and 100 m (Fig. 4), with 5–10-cm-thick calcilutites, which, although recrystallized, still show convo-

lute bedding and small thickening-upward cycles. These calcilutites and interbedded marlstones reflect the proximity and variations of the CCD. Between 60 and 80 m (Fig. 3) the marlstones contain Chondrites-type and U-shaped vertical burrows. Near 160 m, they become thinner, darker marlstones with the last occurrence of Bositra impressions. This association of thinning- and fining-upward cycles of medium- to fine-grained turbidites with pelagic to hemipelagic deposits documenting slope instabilities is characteristic of the lower suprafan lobes and of the middle fan-fringe areas of a deep-sea fan (Howell and Normark, 1982), even if no distributary channel is documented. The close association of carbonate and noncarbonate pelagic sediment suggests proximity to the CCD. The less common thickening- and coarsening-upward cycles are interpreted as constructional lobes. Above a few sandstone layers (at about 220 m, Fig. 4), the facies clearly become more distal. Sandstones give way to about 100 m of a monotonous succession of claystones and very thin (1 cm) siltstones. Claystones are green or black with some thicker (2 m) intervals of black shales (e.g., at 310 m, Fig. 4) and some small synsedimentary listric faults. The top and bottom parts of this succession of claystones and siltstones are characterized by striking intercalations of red claystones. This noncalcareous, hemipelagic to pelagic cyclic facies reflects the drowning of the sources of the clastic material on the continental margin. It was deposited below the CCD and away from any constructional lobe (no sandstones or channels) and can be interpreted as an outer fan in the abyssal plain (if there was a deepsea fan there at that time) or as a transgressive or highstand systems tract. Samples of these red claystones were processed for radiolarians, but without success. A palynological study was also undertaken in the black shales but gave no results because of contact metamorphism. A return to a more proximal facies is observed with the deposition of coarse-grained calcarenites and sandstones (370 m, Fig. 4). Calcarenites are largely dominant and alternate with thin beds of fine sandstones and dark claystones. The calcarenites are 3 to 10 cm thick, graded, and show the AC-E Bouma divisions. Abundant flute casts and bioturbations are found at the bottom of the beds. The calcarenites contain a variable amount of siltsized terrigenous clasts (5%–30%, to 90% in the argillaceous fraction). The microfacies of the coarse-grained fraction of the calciturbidites is (packed) biomicrite and biosparite, pelmicrite (with foraminifers), pelsparite, and oosparite. Carbonate grains are derived from different areas on the continental slope and from the outer part of a carbonate platform. The platform environment is characterized by numerous nonskeletal grains that document intense algal and bacterial activity in a

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1309

LZA4-LZB1

= facies = fauna

suggested age sequence stratigraphy LZB2 supercycle (see text) BERR

LZA3 - LZA4 LZA3

OXFORDIAN

m. Cal to m. Oxf Toa to Oxf

CALLOVIAN BATHONIAN

UAB4?

TOARCIAN?

KIM. - TITH.

Tith to u. Val Tith to Val

key age (see text)

summary

clays silts sands

samples and photo (Fig. 5) lithology

thickness [m]

LZA2 LZA1 LZA1

BAJOCIAN AALENIAN

m.Aal to Baj

BATHONIAN

STEINER ET AL.

Figure 4. Lithologic logs and type sections of the Mesozoic sedimentary sequence, Basal Complex, Fuerteventura. Location: (A) (582,81; 3140,97); (B) (585,55; 3141,02); (C) (583,32; 3140,95); (D) (584,40; 3140,00); (E) (583,15; 3141,70); (F) (582,76; 3141,66); (G) (582,74; 3141,96); (H) (582,74; 3141,82); (I) (583,80; 3143,23); (J) (583,45; 3144,02); coordinates from Cartografia Militar de España, Mapa General serie 5V, scale 1:25 000; Hojas n° 92-78 (Antigua) and n° 91-79; 92-79 (Pájara).

high-energy environment (algal balls, coated bioclastic grains, ooids, grapestones, and lumps) and by the benthic foraminifers Ophtalmidium sp. (Fig. 5-9) and Nautiloculina sp. or Mesoendothyra sp. (Fig. 5-7), which are characteristic of bioclastic shoals (M. Septfontaine, 1993, personal com1310

mun.). Other bioclasts include numerous echinoid fragments, crinoid ossicles, Dasycladaceae [Salpingoporella pygmaea (GÜMBEL); Fig. 5-6], foraminifers (Miliolidae, Textulariina, and Lituolidae), few solitary corals, brachiopod and bryozoan fragments, and thin bivalve shells. This

slope facies is characterized by scarce, undetermined calpionellids, small benthic foraminifers, and red argillaceous micrite intraclasts and white micritic lithoclasts a few centimeters in diameter containing some terrigenous silt (10%) occurring as fine discontinuous, graded, or cross-bedded

Geological Society of America Bulletin, October 1998

LZC2 LZC1

LZC3

L. ALBIAN - E. CENOM. l. Bar to Alb

LZB4 LZB3

E. to M. ALBIAN

CONIAC to CAMP.

suggested age sequence stratigraphy supercycle (see text) LZB2

VALANGINIAN

Val to Haut

HAUTERIVIAN

continue

BARREMIAN

APTIAN

key age (see text)

summary

sedimentary structure

clays silts sands

lithology

thickness [m]

samples photo (fig.5)

EARLY JURASSIC SEA-FLOOR SPREADING, CANARY ISLANDS

Figure 4. (Continued).

laminae. The intraclasts contain some basinal fauna of foraminifers and calpionellids. A deep-water environment is inferred for the deposition of these calciturbidites from the presence of epibiontic traces (Nereites facies; Seilacher, 1964) and of agglutinated foraminifers, in particular Glomospira sp. (Fig. 5-8). The obvious predominance of the latter taxon in some of the turbidites may be due to its prevalence in deep environments (lower bathyal to abyssal; Sliter, 1980). The calciturbidites record

a general increase of resedimentation upsection, as follows. 1. The lowermost calciturbidites are fine grained (calcilutites, 70–100 m, Fig. 4), less abundant, and interbedded with red followed by green claystones. 2. The second calciturbidite interval (370 m, Fig. 4) records a general coarsening-upward trend; the beds are commonly amalgamated and A-C-E intervals and flute casts are abundant. This interval ends with two to three calcirudite

beds, one of which is 1 m thick (thicker than normal), and with limestones containing abundant micritic cement and sandstones, both showing slump structures and interpreted as resulting from mudflows. Close proximity to the slope is inferred from these last deposits. Clasts contained in the slumped limestones are derived from the slope (red claystone intraclasts and calpionellidbearing plastically deformed micritic clasts). The 1-m-thick calcirudite contains imbricated micritic lithoclasts a few centimeters in size, typical

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1311

2

1

3

4

6

5

7

8 11

10 9

1312

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EARLY JURASSIC SEA-FLOOR SPREADING, CANARY ISLANDS

of redeposited conglomerates (Walker, 1984) and generally found in the upper-fan channels. The calciturbidites are therefore interpreted as basinal deposits, composed of redeposited material derived from a carbonate platform and from the continental slope. The environments of deposition gradually became more proximal (lower fan at the base, middle fan to upper fan and slope at the top) and thus reflect the progradation of the sedimentary prism from the margin onto the oceanic crust. We suggest a Tithonian to early Valanginian age for the upper part of the mixed clastic unit, on the basis of the following available chronological data. 1. The upper limit of the unit is constrained by the age of the base of the overlying unit, which is dated by a Valanginian to Hauterivian ammonite (Renz et al., 1992). 2. The fauna and flora found in the topmost calciturbidites of this unit are Jurassic to Early Cretaceous (Nautiloculina sp. or Mesoendothyra sp. and Ophatalmidium sp.) and Oxfordian to Valanginian [Salpingoporella pygmaea (GÜMBEL)] in age. The calpionellids are too poorly preserved to be determined specifically, but are known to occur from the Tithonian to the Albian. 3. There is a facies correlation with the late Tithonian (bottom hole sediments) to early Valanginian unit VII of Deep Sea Drilling Project (DSDP) Site 370/416 (Lancelot et al., 1980; Vincent at al., 1980), which consists of similar alternating sandy turbidites and calciturbidites (Robertson and Bernoulli, 1982): the sedimen-

tary sequence, the origin and nature of the calciturbidite material, and their flora (calcareous algae) and fauna (the assemblage of bathyal to shallow-water foraminifers) document not only a similar environment of deposition but also a similar evolution and source (carbonate platform and micrite- and calpionellid-bearing continental slope sediments). Main Clastic Unit This unit crops out along the coast, between the southern end of the mouth of the Barranco de la Peña and the south of the Caleta de la Peña Vieja, along the Barranco de la Peña and also in the lower parts of the Barranco de Los Sojames and of the Majada del Muerto (Fig. 2). The main clastic unit consists of 500 m of sandy turbidites interpreted as deposited mainly on the middle fan. Its base is dated by the presence of a Valanginian–Hauterivian ammonite (Renz et al., 1992) and its upper part is bounded by the overlying upper Albian to Cenomanian strata of the pelagic limestone unit. A deep-sea fan of similar inferred ages is visible in seismic profiles (K1.1 or MO-2.1: Valanginian lowstand wedge and K.1.2 or MO-2.2: Hauterivian to Aptian lowstand and transgressive systems tracts; Todd and Mitchum, 1977; Hinz et al., 1982). Robertson and Bernoulli (1982) suggested that the entire unit resulted from the redeposition of the prograding deltaic and prodelta sediments of the Tan Tan Formation of the Tarfaya basin centered round Cape Juby (Fig. 1).

Figure 5. (1) Pillow lava (sample CH-54), Basal Unit, Barranco de la Potranca. Hammer for scale. (2) Thickening-upward cycle of calciturbidites interbedded with green marls, pelagic bivalve unit, Puerto de la Peña. (3) Bositra buchi (RÖMER) impressions, semirelief in the green marl interbeds (scale: 1 cm = interval between two red lines), pelagic bivalve unit, Puerto de la Peña. (4) Thin section of skeletal limestone and marl interbed containing valves of Bositra. Valve length is 3 mm; in the marly interval, valves are 20 nm thick; in the calcareous part, neomorphic syntaxial overgrowth shows tangential growth reflecting the structure of the lamellar layer. Pelagic Bivalve Unit, Puerto de la Peña. (5) Epibiontic traces (grazing trails) of Helminthoida? crassa (SCHAFHÄUTL), Nereites ichnofacies (Seilacher, 1964); positive semirelief under the limestone beds, pelagic bivalve unit, Puerto de la Peña. (6) Cross section of Salpingoporella pygmaea (GÜMBEL), diameter = 0.75 mm; thin section of sample CH-34 (calcirudite); mixed clastic unit, mouth of the Barranco de la Peña. (7) Mesoendothyra sp. or Nautiloculina sp., size = 0.37 mm, thin section of sample CH-34 (calcirudite); mixed clastic unit, mouth of the Barranco de la Peña. (8) Glomospira sp., size = 0.5 mm, thin section of sample CH-93 (amalgamated calcarenite); mixed clastic unit, mouth of the Barranco de la Peña. (9) Ophthalmidium sp., size = 0.47 mm, thin section of sample CH-93 (amalgamated calcarenite); mixed clastic unit, mouth of Barranco de la Peña. (10) Amalgamated turbidites with fining- and thinning-upward cycles and A3- and A4-type erosion clasts (rafted and amalgamation clasts); scale: measuring tape = 1 m; polarity: top to the left; high energy and confined channel of upper fan; main clastic unit, El Golfete. (11) Thinning- and fining-upward cycle of arenitic turbidites ending with a black shale; thickest individual beds are 3.6 m; polarity: top to the left (the last big clear bed to the left in the black shale is a trachytic sill); channeled portion of the midfan, main clastic unit, Jújado. For location of photographs on the lithologic log, see Figure 4.

This unit is subdivided into two cycles, each about 250 to 300 m thick. The first cycle documents a clear progradation of the middle fan onto the lower fan; it consists of mudstones and of thin sandstones that form small (about 10 m) poorly developed fining- and thinning-upward cycles (15–25 cm to 1–3 cm). These cycles correspond to the C-D-E facies of Howell and Normark (1982); they are characteristic of the middle fan fringe to outer fan. The beds become thicker upsection (0.5 to 1 m) and start forming two or three thicker (about 30 m) coarsening- and thickeningupward cycles characteristic of the progradation of sand lobes. The channelized geometry of the middle fan is documented by a 40-m-thick fining- and thinning-upward cycle of amalgamated and very thick (to 4 m) turbidites of B-type facies (Howell and Normark, 1982) (Fig. 5-11). The turbidites contain rafted shale clasts and upsection give way to overbank deposits consisting of predominant claystones with slumped and discontinuous sandstone beds with wavy laminations. These turbidites are overlain by 100 m of thick, monotonous, alternating thin-bedded dark claystones and fine-grained sandstones to siltstones. These can be interpreted either as relatively stable interchannel deposits or as the outer-fan fringe deposits. The second cycle records a sudden return to a more proximal setting of the upper and middle fan inferred from the overlying 200 m. The presence of 1-m-thick and amalgamated turbidites containing rafted clasts and numerous amalgamation clasts (A3- and A4-type clasts of Johansson and Stow, 1995) denotes an upper-fan feeder channel setting (10 in Fig. 1). These turbidites alternate with thin-bedded turbidites containing slump structures, small accommodation faults, discontinuous beds, and small (10 cm) channels characteristic of channel levees. The top of this facies is marked by two to three sandy debris-flow deposits or shale-clast conglomerates (Johansson and Stow, 1995) that rework material from the underlying strata (mainly clayey but locally sandy intraclasts). The last 100 m of this unit consist of layers of black shale giving way in the last 40 m to light gray claystone and siltstone, interbedded with rare beds (1–4 cm) of fine-grained sandy turbidites. These mark the end of the deep-sea fan system. Considering the age of the base of the next unit, the black shales could correspond to the older (late Berriasian to Albian event; Jenkyns, 1980) of the Cretaceous oceanic anoxic events. Pelagic Limestone Unit This unit crops out in the Caleta de la Peña Vieja and in the middle part of the Barranco de la Peña and along the upper parts of the Majada de la Perra and of the Barranco de Los Sojames,

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where the contact with the underlying main clastic unit can be seen (Fig. 2). The pelagic limestone unit consists of 150 m of slope deposits, mainly chalk. They correspond to units F and G of Robertson and Stillman (1979a). Unit F has been dated as late Albian to early Cenomanian (Rothe, 1968; Robertson and Stillman, 1979a, 1979b; Robertson and Bernoulli, 1982; Renz et al., 1992) and unit G as Coniacian to Campanian (Robertson and Bernoulli, 1982). DISCUSSION The ages in Figure 4 (see “key age”) are based on the biostratigraphic data already described and on facies correlation with nearby known basinal deposits and continental margin facies (see the following). The similarities between the different Tethyan facies and the central Atlantic ones have already been emphasized (Bernoulli, 1972; Bernoulli and Jenkyns, 1974; Jansa et al., 1977; Favre and Stampfli, 1992). The Cretaceous sequences are biostratigraphically well dated, and the Jurassic age is defined by the presence of Bositra buchi (Toarcian–Oxfordian). Arguments based on regional tectono-eustatic events and on sequence stratigraphy allow us to propose more detailed ages for the Jurassic, with the available biostratigraphic and lithostratigraphic data. References to the global eustatic supercycles of Haq et al. (1987) are indicative and mentioned only where we could relate them to biostratigraphically constrained onshore events. Any tectonic influence was considered negligible because we are dealing with an oceanic crust with an a priori normal thermal subsidence (the first confirmed tectonic activity only started during Oligocene time). This detailed chronostratigraphy is based on the following five points. 1. The Presence of N-MORB This implies that the Fuerteventura sequence was deposited in a postrift environment, which allow us to propose a post-Pliensbachian to Toarcian age for the base of the series. The prerift and synrift sequences known along the continental margin (Rif: Favre et al., 1991; Favre and Stampfli, 1992. Mazagan: DSDP Sites 544 and 547, Hinz et al., 1984) are Anisian to late Pliensbachian and consist of transgressive facies that evolve from continental to marine deposits (red beds followed by salt, marine gypsum, and carbonate platforms along the proximal margin or by deep pelagic sediments along distal margins; Bernoulli and Kälin, 1984) and calcareous breccias derived from fault scarps (Jansa et al., 1984; Favre and Stampfli, 1992). The final rifting phase is late Pliensbachian in age (sudden deepening of the rift basins and first mixing of Tethyan and subboreal fauna, margari1314

tus or stockesi Zone, 192 Ma; Jansa, 1986; Favre, 1992). The next sequence (late synrift to postrift) consists of a widespread shedding of clastic deposits into the oceanic and rim basins. These deposits derived from the upraising rift shoulder (Fig. 1) and began in latest early to middle Toarcian time. They consist of limestone breccias containing shallow-water benthic foraminifers and clasts of red sandstone (Mazagan, falciferum and bifrons Zones, 187–186 Ma; Jansa et al., 1984; Riegraf et al., 1984) and continental deposits and channeled delta derived from the southwest (falciferum and bifrons Zones, 188–186 Ma; southwest Rif: Faugères, 1978; Favre, 1992; western High Atlas: Choubert and Faure-Muret, 1962; Heyman 1989). The return of an exclusively Mesogean cephalopod fauna also indicates a generalized uplift and/or regression (southwest Rif; Favre, 1992). The thermal uplift has been dated by radiometric data as 184 ± 4 Ma (Mazagan, Huon et al., 1993). This age being a cooling age (40K/40Ar method on neoformed muscovite), the actual onset of sea-floor spreading, which is contemporaneous or slightly (to 10 m.y.) younger than the thermal uplift, could be slightly older than 184 Ma, i.e., middle Toarcian age. The presence of oceanic basalts at the base of the Basal Unit of Fuerteventura allows us to propose an age that is older than the youngest dated synrift sequence, i.e., post–late Pliensbachian, and contemporaneous or younger than the onset of sea-floor spreading, i.e., middle Toarcian or younger. The presence of black shales (40 m above the oceanic basalts, Fig. 4) could eventually represent the Toarcian oceanic anoxic event (falciferum Zone, 187–188 Ma, Jenkyns, 1988). Because the overlying unit is not younger than Oxfordian, and more probably middle Aalenian to middle Bathonian in age (see the following), we can ascribe a late Toarcian to early Aalenian age to the top of the Basal Unit. This is consistent with the presence, toward the top of the unit, of calciturbidites recording the erosion of a carbonate platform. The only known carbonate platform older than Callovian is middle Early Jurassic in age, it crops out in the Essaouira-Agadir area, and probably extends to Tarfaya (Fig. 1). It was subject to a major regression during Toarcian time (bifrons Zone, Choubert and Faure-Muret, 1962; top of marly calcareous deposits with Z. anglica, Ambroggi, 1963) with local erosion in Essaouira (Faugères, 1978), and was capped by Toarcian to Bathonian and middle Callovian deltaic sediments (Ambroggi, 1963; Favre and Stampfli, 1992). From the dominance of the noncarbonate siltyargillaceous fraction, from the organization of the beds and their current-related structures and from their proposed age, we can suggest a rather high sedimentation rate (15 to 30 m/m.y.) for the Basal

Unit, pointing to deposition in the distal fan or as contourites, below the CCD. 2. The Bositra buchi Limestone Facies (Pelagic Limestone Unit) This unit indicates that this horizon is not younger than Oxfordian age. Bositra buchi is known from Toarcian to Oxfordian strata. However, this taxon is found in large quantities only in a few typical facies (e.g., lower Toarcian bituminous shales, Jeffries and Minton, 1965; Bernoulli and Kälin, 1984; Aalenian to upper Bajocian (garantiana Zone) bioclastic facies, Sturani, 1971; Baumgartner et al., 1995). The filament (B. buchi) limestone facies of Fuerteventura (5 to 50 m, Fig. 4) crops out as fine-grained nodular limestone beds with green marly interbeds, it shows abundant water-escape structures, and the density of shells in thin sections reaches up to 80%. We propose a more precise age for this facies, i.e., middle Aalenian to middle Bajocian, by the comparison with the northwestern Atlantic deep-water limestone facies known laterally. In the Ketama unit, external Rif, Morocco (the distal part of the northwestern African margin), limestone beds with an identical filament microfacies occur as a very distinct interval dated by ammonites as middle Aalenian to middle Bajocian age (Favre, 1992). Off Mazagan (core 547B-8-3, Steiger and Jansa, 1984), limestones with a similar microfacies give similar ages on the basis of nannofossils (late Toarcian–Aalenian to late Bajocian; Wiegand, 1984; Bown, 1996). Although the calcareous character of this facies is somewhat at odds with the paleogeographic situation at that time (emergence of the continental plateaus), this facies is a correlation horizon along the Rif and Mazagan margins in a similar oceanic domain, relatively close to Fuerteventura. It is probably because of its typical character that this facies was recognized very early by von Fritsch (1868) and later by Rothe (1968). Light colored to greenish marlstones with rarer occurrences of Bositra buchi follow these filament limestones (pelagic bivalve unit, 80–110 m, and mixed clastic unit, 160 m, Fig. 4). A similar change of facies occurs in the External Rif and in the Eastern High Atlas, where filament limestones and cancellophycus limestones are followed by Posidonia marlstones of Bajocian (sauzei to garantiana Zones) to mid-Callovian ages (Choubert and Faure-Muret, 1962; Favre, 1992). In the Blake-Bahama basin, similar and contemporaneous marlstones with Posidonia (middle Bathonian age, aspisoides to subcontractus Zones, Baumgartner et al., 1996) were found interdigited with oceanic basalts (cores 534-127 and 534-128, Sheridan et al., 1983). These deposits represent the oldest drilled At-

Geological Society of America Bulletin, October 1998

EARLY JURASSIC SEA-FLOOR SPREADING, CANARY ISLANDS

lantic sediments (Greenish-Grey Limestone Unit, Sites 100 and 370, Jansa et al., 1977). 3. Terrigenous Fan of the Base of the Mixed Clastic Unit These deposits, which are about 200 m thick, document major terrigenous influx during Jurassic time. They are best understood in the context of the only major event of terrigenous sedimentation in the Jurassic postrift series: the long period of emergence of the rift shoulders characterized by latest Early to Middle Jurassic continental deposits in the rim basins (about 300 m of red beds, Argana basin; Favre and Stampfli, 1992). In the adjacent area of Agadir-Argana, this event ended in middle Callovian time (coronatum Zone, 161 Ma; Ambroggi, 1963; Favre and Stampfli, 1992). Considering the possible middle Bajocian age of the underlying unit, the most probable age for these fan deposits in Fuerteventura is Bathonian– Callovian (base of LZA3, Haq et al., 1987). Farther north, on the Mazagan margin, this detrital event is reflected by a few calcareous breccias (cores 9B and 8B, DSDP Site 547, Hinz et al., 1984). However, unlike the Mazagan starved margin sequence (the Middle High Atlas troughs were trapping the sediments at that time), the Fuerteventura sequence records thicker “normal margintype” clastic sedimentation. The importance and continuity of this clastic input in the ocean basin are compatible with the position of Fuerteventura, offshore from the Anti-Atlas structural high. This more important and continuous terrigenous influx could be the reason for the small quantities of purely pelagic deposits in the entire sequence. 4. Presence and Transition of Green to Red Claystone (Mixed Clastic Unit) Between middle Callovian and middle Oxfordian time, a striking and systematic change from green to red color is observed in numerous Atlantic-Tethyan facies (central Atlantic, middle Oxfordian, Jansa et al., 1977; Subbetic rim basin, upper Callovian, O’Dogherty et al., 1995; Lombardian rim basin, middle Callovian, Baumgartner et al., 1995; Dinaric Tethys, middle Oxfordian, Gorican, 1993). Whatever the cause of this phenomenon, the appearance in the mixed clastic unit of the red clays and of the overlying shales (260–360 m, Fig. 4) is a possible marker of the upper Callovian–lower Oxfordian (LZA3) and/or upper Oxfordian (LZA4) condensed intervals. It is at that time that the two major marine flooding surfaces between Middle Jurassic and Berriasian strata are found on the Moroccan continental plateau: at Cape Ghir, continental red sandstones and siltstones are overlain by middle to upper Callovian marly limestones (Reineckeia richei followed by Rei-

neckeia anceps), corresponding to the first transgressive systems tract. The second marine flooding surface occurs with the beginning of the gray and red upper Oxfordian marls (G. transversarium Zone, 157–155 Ma, Ambroggi, 1963; Renz et al., 1975). The appearance of the red claystones (condensed sections) during the Oxfordian is reflected in the DSDP holes by the “Reddish Brown Argillaceous Limestone Unit” (Jansa et al., 1977).

tervals of the base of the Berriasian of Agadir. The late Berriasian section would then correspond to the base of the main clastic unit. Similar episodic sequences of periplatform calciturbidites are known from the same time in the Central Atlantic, offshore Morocco (DSDP Sites 370/416 and 415, Tithonian to early Valanginian, Vincent et al., 1980; DSDP Site 547, cores 6B and 7B, Kimmeridgian to late Tithonian–early Berriasian, Hinz et al., 1984).

5. Presence of the Periplatform Calciturbidite Sequences with a Tithonian to Valanginian Fauna (Top of the Mixed Clastic Unit)

CONCLUSIONS

These calciturbidites are pre-Hauterivian– Valanginian age (age of the overlying unit) and contain a Tithonian (first occurrence datum of calpionellid) to Valanginian fauna (last occurrence datum of S. pygmea). The sequence (350–450 m, Fig. 4) is composed of prograding clastic deep-water deposits derived from a platform and documents shedding of carbonate material during a late highstand (Schlager et al., 1994), followed by major erosion of the same platform and the continental slope (lowstand basin floor fan). From the evolution of the Tarfaya basin, which lies just east of Fuerteventura (Fig. 1) and is, according to paleocurrent directions, the source of the carbonate material, we can infer a more precise age for the calciturbidites. In this domain, a carbonate platform developed (Puerto Casando Formation, Ranke et al., 1982) that reached its maturity during late Oxfordian time with fully developed calcareous reefal facies. Since late Kimmeridgian time it episodically developed shelf-margin wedges (inferred from seismic data, Hinz et al., 1982) that are characterized onshore and farther north by mixed clastic-evaporitic sedimentation (sandy marls with gypsum; Ambroggi, 1963). This corresponds to the maximum seaward extension of the reef that lasted until early Berriasian time. We suggest correlating this episode (maximum sea level of the Middle to Late Jurassic marine transgression— LZA4—and subsequent sea-level oscillations— base of LZB1, Haq et al., 1987) with the first thin turbidites of the Fuerteventura sequence (360–420 m, Fig. 4). During late Berriasian and Valanginian time the platform emerged and was covered by terrigenous and continental deposits (red sandstones and siltstones with reptile footprints, Argana basin; Ambroggi, 1963; our own study). This drastic drop of sea level can be correlated with the calcirudites and the micritic limestones and claystones from the mudflows at the top of the mixed clastic unit (440 m, Fig. 4). An alternate interpretation would be to place the laminated fine-grained limestones with red claystone interbeds (460 m, Fig. 4) in the Tithonian–early Berriasian condensed section and thus correlate them with the Calpionella elliptica in-

A terrigenous and pelagic sedimentary sequence extending almost continuously from latest Early to Middle Jurassic (Toarcian–Aalenian) to Late Cretaceous crops out in Fuerteventura. It was uplifted and deformed during the Cenozoic magmatic buildup of the island. The Mesozoic sequence is strongly influenced by the proximity of the African continental margin and by sealevel fluctuations. This enables us to determine precisely the age of the Jurassic sedimentary series by using sequence stratigraphy and facies correlation and to date its base as at least late Toarcian–early Aalenian time (179 to 184 Ma, top of Basal Unit). This sedimentary sequence overlies N-MORBs. Sea-floor spreading was accompanied by the thermal uplift of the rift shoulders. This uplift began in latest early to late Toarcian time on the basis of field evidence (Favre et al., 1991; Favre and Stampfli, 1992) and radiometric ages (184 ± 4 Ma, Huon et al., 1993). The strip of oceanic crust that lies between Fuerteventura and the S1 magnetic anomaly (about 50 km) could represent 5 to 10 m.y. of sea-floor spreading (from 0.6 cm/yr according to Luyendyk and Bunce, 1973, to 1.9 cm/yr according to Klitgord and Schouten, 1986). Considering a slow spreading rate, the onset of sea-floor spreading could have been during middle Toarcian time (184 to 187 Ma). Using that value as a possible age for the Blake Spur magnetic anomaly (the analogue to the S1 anomaly on the American side of the Atlantic) we obtain a similar slow rate of 0.6 to 0.9 cm/yr for the earliest sea-floor spreading in the eastern central Atlantic. Any spreading rate greater than 1 cm/yr yields different ages for S1 and for the Blake Spur magnetic anomalies. This confirms a likely early to middle Toarcian age for the onset of sea-floor spreading in that part of the central Atlantic. ACKNOWLEDGMENTS Field work of Steiner, Favre, and Stampfli was supported by Swiss Fond National de la Recherche Scientifique (FNRS) grant 20.28943.90, and field work of Hobson and Hernandez was supported by Swiss FNRS grant 21.31098.91 and the I. Friedländer foundation.

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