Biostratigraphic events at the Barremian/ Aptian boundary in the Betic Cordillera, southern Spain

Cretaceous Research (1997) 18 , 309 – 329

Biostratigraphic events at the Barremian / Aptian boundary in the Betic Cordillera , southern Spain 1

*R . Aguado , † M . Company , † J . Sandoval and † J .M . Tavera

* Departamento de Geologı ́ a , Universidad de Jae ́ n , Escuela Universitaria Polite ́ cnica de Linares , 2 3 7 0 0 Linares , Spain † Departamento de Estratigrafı ́ a y Palaeontologı ́ a , Universidad de Granada , 1 8 0 0 2 Granada , Spain

Revised manuscript accepted 1 8 July 1 9 9 6

This paper presents an analysis of the vertical distribution of ammonites and calcareous nannofossils throughout the uppermost Barremian – lowermost Aptian interval of pelagic sections in the External Zones of the Betic Cordillera (southern Spain) . We were able to identify all the standard ammonite zones in this interval and have established for the first time a direct calibration of calcareous nannofossil events against the ammonite zonation . The results confirm that the Barremian / Aptian boundary , defined by the first appearance of Deshayesites , does not coincide with any other biostratigraphic event . Nonetheless , the sequence of events recorded in the studied sections provides alternative criteria on which to characterize this boundary with reasonable accuracy . A new nannofossil species ( Micrantholithus stellatus sp . nov . ) is also described .

÷ 1997 Academic Press Limited

K E Y W O R D S : Barremian / Aptian boundary ; biostratigraphic events ; ammonites ; calcareous nannofossils ; Betic Cordillera ; southern Spain .

1 . Introduction According to the conclusions of the Digne Workshop (Hoedemaeker & Bulot , 1990) , the Barremian / Aptian boundary in the Mediterranean domain coincides with the base of the Tuarkyricus Zone , defined by the first appearance of the ammonite genus Deshayesites . However , this criterion cannot be easily applied to pelagic sediments , where uppermost Barremian to lowermost Aptian ammonite assemblages mainly consist of long-ranging taxa (including desmoceratids , silesitids , macroscaphitids) of little biochronological value . On the other hand , deshayesitids are extremely scarce in these levels . This well-known fact has led to the frequent use of alternative events to characterize the base of the Mediterr- anean Aptian . For example , Pseudohaploceras matheroni , different species of Procheloniceras , and the genera Kutatissites and Pseudocrioceras have traditionally been considered taxa characteristic of the lowermost Aptian . However , it is now known that some of these taxa appear in the uppermost Barremian .

Not only ammonites , but also calcareous nannofossils have played an important role in the characterisation of the Barremian / Aptian boundary . Thierstein (1973) defined this boundary by the first occurrence (FO) of Chiastozygus litterarius and Hayesites irregularis and by the last occurrence (LO) of Nannoconus colomii (De Lapparent) Kamptner . It was later shown that none of these datum planes coincides with the conventional base of the Aptian , as N . colomii is found in the Upper Aptian , while H . irregularis and C . litterarius are already present in the uppermost Barremian . Nevertheless , a direct calibration of these and other calcareous nannofossil events with the standard ammonite zonation of the Barremian / Aptian boundary interval is still in progress . 1 Contribution to IGCP Project 362 : Tethyan and Boreal Cretaceous .

0195 – 6671 / 97 / 030309 1 21 $25 . 00 / 0 / cr970069 ÷ 1997 Academic Press Limited

R . Aguado et al . 310

Figure 1 . Location of the sections . 1 , section X . F (Campillo de Arenas , Jae ́ n Province) ; 2 , section X . HA (Hue ́ scar , Granada Province) (see Figure 2) ; 3 , sections X . Kv (see Figure 3) and X . Kv 2 (Caravaca , Murcia Province) ; 4 , sections X . Cp , X . Cp 1 , X . Cp 2 (see Figure 4) and X . Cp 4 (Fortuna , Murcia Province) ; 5 , section X . Fc (Sierra de Fontcalent , Alicante Province) ; 6 , section X . B 2 (Busot , Alicante Province) .

We present here the stratigraphic distribution of ammonites and nannofossils in several sections in SE Spain , covering the uppermost Barremian (Giraudi and Sarasini Zones) – lowermost Aptian (Tuarkyricus and Weissi Zones) interval . These results improve our knowledge of the true sequence of bioevents occurring throughout this stratigraphic interval and , therefore , complete the data recently presented by other authors (Cecca et al . , 1994 ; Channell & Erba , 1992 ; Channell et al . , 1995 ; Coccioni et al . , 1992 ; Delanoy , 1992 , 1995 ; Hoedemaeker & Leereveld , 1995) .

2 . Studied sections

The studied material comes from several pelagic sections located in the External Zones of the Betic Cordillera (Figure 1) . Three of these (X . HA , X . Kv and X . Cp 2 ) were taken as reference sections and analysed in more detail .

Section X . HA (Figure 2) is located in the vicinity of Cortijo del Hielo , 5 . 5 km north of Hue ́ scar (Granada province) , near the road to Santiago de la Espada . The lithology consists of rhythmically alternating marly limestone beds 5 to 70 cm thick , and marly interbeds up to several metres thick . Braga et al . (1982) published the first data on ammonite distribution in this section . More detailed sampling was carried out for the present study .

Section X . Kv (Figure 3) presents similar lithological characteristics . It is located in the Barranco de Cavila , 6 . 3 km SSW of Caravaca (Murcia province) . This section represents a magnificent record of the upper Lower Barremian to

The Barremian / Aptian boundary 311

Figure 2 . Distribution of the most significant ammonite and calcareous nannofossil species in section X . HA (Geogr . coord . : 37 8 51 9 14 0 N , 02 8 35 9 13 0 W) .

the lowermost Aptian and , in particular , the Lower Barremian / Upper Barremian boundary (Company et al ., 1995) and was , therefore , proposed as a candidate for the stratotype of this boundary at the 2nd International Symposium on Cretaceous Stage Boundaries , held in Brussels during 1995 .

Section X . Cp 2 (Figure 4) is located on the southeastern slope of the Sierra del Corque , outside Capre ́ s , 5 . 8 km north of Fortuna (Murcia province) . Here the Giraudi Zone and the lower part of the Sarasini Zone are represented by a condensed interval also including the Sartousiana and Feraudianus Zones (Company et al . , 1992a , b) . The uppermost Barremian – lowermost Aptian interval is represented by marlstone – limestone rhythmites similar to those of the sections described above , although here the marly interbeds are much thinner .


Figure 3 . Distribution of the most significant ammonite and calcareous nannofossil species in section X . Kv (Geogr . coord . : 38 8 03 9 08 0 N , 01 8 53 9 00 0 W) .

These three sections appear to represent a complete , continuous record of the uppermost Barremian-lowermost Aptian interval . Samples for ammonites and calcareous nannofossils were collected from the same beds and , therefore , direct and detailed correlations between the two groups were established .

3 . Ammonite biostratigraphy Approximately 3500 ammonites were collected from all of the studied sections . Most specimens were found in the limestone beds , although some limonitized specimens were also abundant in the marly interbeds . In general , the specimens are sufficiently well preserved to allow identification for biostratigraphical purposes .


Figure 4 . Distribution of the most significant ammonite and calcareous nannofossil species in section X . Cp 2 (Geogr . coord . : 38 8 14 9 01 0 N , 01 8 07 9 57 0 W) .

The assemblages display a marked Mediterranean character and are dominated by desmoceratids , silesitids , phylloceratids , lytoceratids and other forms with little biostratigraphic significance . Nonetheless , the presence of heteroceratids and deshayesitids in most of the sections permits recognition of all of the zones proposed for this stratigraphic interval in the Mediterranean area (Hoedemaeker & Bulot , 1990 ; Hoedemaeker & Company , 1993) .

Zonation The Giraudi Zone is well characterized in our sections by the presence of Imerites giraudi (Kilian) (Figure 5b) , together with several species of Heteroceras .

The base of the Sarasini Zone coincides with the first appearance of the index species Martelites sarasini (Rouchadze) (Figure 5d , e) , co-occurring with the last specimens of the Heteroceras baylei Reynes (Figure 5c) group . In the studied sections , as in other Mediterranean areas (Delanoy , 1994a , b ; Kakabadze & Kotetishvili , 1995) , the heteroceratids disappear in the middle part of this zone


Figure 5 . (a) Pseudohaploceras gr . matheroni (d’Orbigny) (X . HA . 6 . 2) , Sarasini Zone , section X . HA ; (b) Imerites giraudi (Kilian) (X . Cp . F . 123) , Giraudi Zone , section X . Cp ; (c) Heteroceras baylei Reynes (X . Kv . 40 . 42) , Sarasini Zone , section X . Kv ; (d) Martelites sarasini (Rouchadze) (X . Kv 2 . 90 . 1) , Sarasini Zone , section X . Kv 2 ; (e) Martelites sarasini (Rouchadze) (X . Kv 2 . 90 . 2) , Sarasini Zone , section X . Kv 2 . All figures 3 1 .

Thus , the upper part of the Sarasini Zone corresponds to a poorly characterized interval with a predominance of taxa inherited from older levels . However , a few species , such as Kutatissites edwardsi (Reynes) (Figure 6d) and ‘‘ Anahamulina ’’


glemmbachensis Immel (Figure 6c) were first observed in this interval , which probably corresponds to the Turkmeniceras turkmenicum – ‘‘ Matheronites ’’ ridzewskyi Zone of the uppermost Barremian in the northern Caucasus and Turkmenistan (Kakabadze , 1983) .

Figure 6 . (a) Kutatissites cf . bifurcatus Kakabadze (X . Cp 2 . 15 . 107) , Tuarkyricus Zone , section X . Cp 2 ; (b) Kutatissites cf . bifurcatus Kakabadze (X . Cp 2 . 15 . 75) , Tuarkyricus Zone , section X . Cp 2 ; (c) ‘‘ Anahamulina ’’ glemmbachensis Immel (X . Cp 2 . 15 . 114) , Tuarkyricus Zone , section X . Cp 2 ; (d) Kutatissites edwardsi (Reynes) (X . Cp 2 . 8 . 21) , Sarasini Zone , section X . Cp 2 . All figures 3 1 .


Identification of the Tuarkyricus and Weissi Zones is problematic , mainly because of the scarcity and poor preservation of the deshayesitids . In fact , the beds presumably corresponding to the Tuarkyricus Zone provided only one specimen of this family , identified as D . oglanlensis Bogdanova (Figure 7e) , which is a species characteristic of this stratigraphic level in the Transcaspian area (Bogdanova , 1979 , 1983) . This specimen was found in bed 15 of section X . Cp 2

(Fortuna) in association with Procheloniceras gr . albrechtiaustriae - pachystephanum (Uhlig) (Figure 7a) and Kutatissites cf . bifurcatus Kakabadze (Figure 6a , b) . These taxa were used to characterize this zone in other sections (X . HA , X . Kv , X . Kv 2 , X . Cp , X . Cp 1 , X . Fc and X . B 2 ) .

Deshayesitids are more common in higher levels . In some of the studied sections (X . HA , X . Cp 1 and X . Fc) we detected a bed containing abundant specimens of Deshayesites cf . luppovi Bogdanova (Figure 7f , g) , very similar to those described by Delanoy (1995) as Deshayesites sp . ( 5 Prodeshayesites cf . tenuicostatus in Delanoy , 1991) . According to this author’s interpretation , we tentatively consider the top of this bed as the base of the Weissi Zone , although Bogdanova (1979 , 1983) reported this species in both the upper part of the Tuarkyricus Zone and the lower part of the Weissi Zone of the Transcaspian area . In our sections , typical forms of the Weissi Zone such as D . callidiscus Casey (Figure 7c) , D . forbesi Casey (Figure 7d) , and D . gr . spathi - normanni Casey , appear slightly higher than the bed containing Deshayesites cf . luppovi .

Location of other ammonite events As already mentioned , the scarcity of deshayesitids in the lowermost Aptian in the pelagic domain of the Mediterranean area has induced palaeontologists to use other taxa to characterize the Barremian / Aptian boundary . In most cases , however , the use of these taxa is not completely justified and , in fact , some of them appear in levels quite far removed from the conventional boundary defined by the first appearance of Deshayesites .

In his revision of the Angles stratotype section , Busnardo (1965) arbitrarily located the Barremian / Aptian boundary at the base of level 197 , coinciding with the local appearance of Pseudohaploceras matheroni (d’Orbigny) (Figure 5a) . This event was subsequently adopted by other authors (Thomel , 1964 ; Moullade , 1966 ; Patrulius & Avram , 1976 ; Avram , 1983) . However , it was recently shown (Company et al ., 1992a ; Delanoy , 1992 , 1995 ; Hoedemaeker & Leereveld , 1995) that P . matheroni appears in the uppermost Barremian . In our sections , this species , or morphologically very similar forms , is already present in the Giraudi Zone .

Another taxon frequently used to characterize the Barremian / Aptian boundary is the genus Procheloniceras . P . albrechtiaustriae , for example , was considered as a species indicative of the lowermost Aptian by Kilian (1910 – 1913) and other authors (Drushchits & Mikhailova , 1966 ; Vas ä ic ä ek , 1972 ; Kotetishvili , 1986) . P . pachystephanum was used by Nikolov (1987) for the same purpose . In our sections , the acme of this group does indeed occur in the Tuarkyricus Zone , but some specimens were collected in the uppermost Barremian . We should also point out that other species traditionally assigned to this genus , such as ‘‘ P . ’’ amadei (Uhlig) (Figure 7b) or ‘‘ P . ’’ trachyomphalum (Uhlig) , were considered to be Aptian in age (Avram , 1976 ; Vas ä ic ä ek , 1981) , but were found in the Sartousiana ,


Figure 7 . (a) Procheloniceras pachystephanum (Uhlig) (X . Cp 2 . 15 . 170) , Tuarkyricus Zone , section X . Cp 2 ; (b) ‘‘ Procheloniceras ’’ amadei (Uhlig) (X . Kv . 40 . 3) , Sarasini Zone , section X . Kv ; (c) Deshayesites callidiscus Casey (X . Kv . 55 . 1) , Weissi Zone , section X . Kv ; (d) Deshayesites forbesi Casey (X . Kv 2 . 103 . 44) , Weissi Zone , section X . Kv 2 ; (e) Deshayesites oglanlensis Bogdanova (X . Cp 2 . 15 . 217) , Tuarkyricus Zone , section X . Cp 2 ; (f) Deshayesites cf . luppovi Bogdanova (X . HA . 17 . 17) , Tuarkyricus Zone , section X . HA ; (g) Deshayesites cf . luppovi Bogdanova (X . HA . 17 . 2) , Tuarkyricus Zone , section X . HA . All figures 3 1 .


Feraudianus and Giraudi Zones of the studied sections , in agreement with the observations by Delanoy (1992) and Cecca & Landra (1994) in other Mediterr- anean areas .

Something similar occurs with the genus Kutatissites . In our sections , the genus Kutatissites shows maximum abundance in the Tuarkyricus Zone , where it is represented by Kutatissites cf . bifurcatus . However , the first representatives of the genus , attributable to the species K . edwardsi , are already present in the upper part of the Sarasini Zone , as is also documented in SE France (Thieuloy , 1976 ; Delanoy & Bulot , 1990) .

Barremites strettostoma (Uhlig) and Silesites seranonis (d’Orbigny) were traditionally considered to be indicative of the Upper Barremian , but their range extends into the lowermost Aptian , as pointed out by Patrulius & Avram (1976) . This is also the case with ‘‘ Anahamulina ’’ glemmbachensis , reported by Immel (1987) and Vas ä ic ä ek et al . (1994) in the Upper Barremian , but occurring in the lowermost Aptian in the studied sections .

4 . Nannofossil biostratigraphy

A total of 75 samples from the three reference sections , together with 20 samples from additional sections were studied for calcareous nannofossils . Smear slides were prepared from raw material . The study was carried out using a light microscope with 1250 3 magnification and examining a minimum of 200 fields of view for each sample . Nannofossils are abundant in all samples , constituting about 40 – 80% of the whole rock . Preservation is clearly dependent on the lithology , varying from moderate with some overgrowth in marly limestones , to good with only slight overgrowth and / or etching in the marlstones . The biostratigraphic analysis was not affected by the effects of preservation .

Assemblage diversity is generally high , with identification of a total of 64 species . All the identified taxa are listed in Appendix 2 . The assemblages are dominated by cosmopolitan taxa , although some Tethyan taxa ( Assipetra terebrodentaria , Conusphaera rothii , Micrantholithus spp ., Nannoconus of Med- iterranean affinity) are abundant . A few Boreal taxa , such as Crucibiscutum salebrosum and C . hayi (Mutterlose , 1992a , b) were observed .

The M . hoschulzii and H . irregularis zones were recognized in sections X . Kv and X . HA , while in section X . Cp 2 the sampled interval corresponds to the H . irregularis Zone .

Calcareous nannofossil events As noted earlier , Thierstein (1971 , 1973) defined the base of his Chiastozygus litterarius Zone by the last occurrence of Nannoconus colomii and / or the first occurrences of C . litterarius and Rucinolithus irregularis ( 5 Hayesites irregularis in this study) . According to this low latitude zonation , the base of this zone coincides with the Barremian / Aptian boundary as established by Busnardo (1965) in the Angles section . However , it has since been shown that none of these three events is valid for characterization of the base of the Aptian (Noe ̈ l , 1980 ; Covington & Wise , 1987 ; Applegate & Bergen , 1988 ; Aguado et al ., 1988 , 1992 , 1993 ; Aguado , 1993 ; Erba , 1988 , 1989 , 1994 ; Hoedemaeker & Leereveld , 1995) .


In the Angles section , Erba (newsletter on the Aptian Working Group , Nov . 1993) pointed out the presence of H . irregularis several metres below the base of the Aptian proposed by Busnardo , while more recently Hoedemaeker & Leereveld (1995) located the first occurrence of this species within the upper part of the interval corresponding to the Giraudi-Sarasini Zones in the Rı ́ o Argos section . The data obtained from the distribution of calcareous nannofossils in sections X . HA and X . Kv , where H . irregularis occurs consistently , allow us to confirm these observations and to specify that the first occurrence of this species correlates with the middle part of the Sarasini Zone , approximately coinciding with the last record of heteroceratids .

C . litterarius is rare in the studied sections . Its highly discontinuous record is also typical of other Tethyan areas (Channell & Erba , 1992) . This taxon occurs below H . irregularis , and its first occurrence cannot be higher than the top of the Giraudi Zone (section X . Kv , Figure 3) . Moreover , the taxonomic interpretation of C . litterarius is complex . Very similar forms were reported from older intervals (Noe ̈ l , 1980 ; Covington & Wise , 1987 ; Applegate & Bergen , 1988 ; Aguado , 1993) . Low abundance and taxonomic uncertainties suggest that the first occurrence of this species should not be used , at least in the Tethyan realm .

As regards N . colomii ( 5 N . steinmannii in this study) , many studies have reported this species from Aptian sediments (Erba & Quadrio , 1987 ; Applegate & Bergen , 1988 ; Aguado et al ., 1988 , 1992 , 1993 ; Aguado , 1993 ; Erba , 1988 , 1989 , 1994) , so its last occurrence cannot be used as indicative of the Barremian / Aptian boundary . In our sections , N . steinmannii is found throughout the upper Barremian – lower Aptian . However , in agreement with the observations of Erba (1994) , we detected a progressive decrease in the relative abundance of N . steinmannii with respect to the group of wide-canal nannoconids (mainly N . bucheri , N . circularis and N . wassallii ) . The beginning of this trend approximately coincides with the first occurrence of H . irregularis and continues throughout the upper part of the Sarasini , Tuarkyricus and Weissi Zones . The nannoconid crisis and the marked increase in abundance of Assipetra terebrodentaria and A . infracretacea (Erba , 1994) were not detected in our sections and probably occur above the stratigraphic interval we analysed .

The first occurrence of Flabellites oblongus has been currently observed above the FO of H . irregularis (Bralower , 1987 ; Erba , 1988 ; Mutterlose , 1991 , 1992a) . However , in the studied sections , the first occurrence of F . oblongus is found in the lower part of the Sarasini Zone , clearly preceding the FO of H . irregularis . The earlier specimens of F . oblongus occurring below the appearance of H . irregularis are usually smaller (Figure 8 . 8 – 10) than those found in the Aptian to Albian interval . Specimens of normal size (Figure 8 . 11 – 18) were , however , observed along with primitive populations .

The last occurrence of Crucibiscutum salebrosum (Figure 8 . 19 – 22) is recorded slightly above the first occurrence of F . oblongus . The observed specimens of C . salebrosum may be similar to those illustrated by Aguado (1993 , pl . 2 , fig . 18) from coeval sediments and differ from the holotype (Black , 1971) on account of the base of a spine on the axial cross . This feature can only be observed under electron microscopy , and was not seen in this study . Crucibiscutum hayi (Figure 8 . 23 – 24) , which is related to C . salebrosum , was recorded discontinuously throughout the studied interval .

Typical specimens of Nannoconus truittii , which are present in higher stratigraphic levels , were not recorded in the studied sections . However , in the



lowermost Aptian the first occurrence of a group of nannoconids with morphologies intermediate between N . bucheri and true N . truittii was detected . These transitional forms , here reported as N . sp . cf . N . truittii (see taxonomic note) , were observed throughout the Tuarkyricus and Weissi Zones , in the interval with wide-canal nannoconids dominating the assemblages .

Some recent papers have reported the first occurrence of Braarudosphaera africana towards the middle-upper part of the Lower Aptian in both the Tethyan realm (Larson et al . , 1993 ; Erba , newsletter on the Aptian Working Group , November 1993) and the Boreal realm (Keupp & Mutterlose , 1994) . However , some specimens that can clearly be assigned to this species (we used B . africana in the sense of Lambert , 1986) appear in the Tuarkyricus Zone of section X . HA . Moreover , forms very similar to B . africana have been found to occur in the Giraudi Zone and are continuously recorded throughout the uppermost Bar- remian and lowermost Aptian . The main distinguishing features of these forms with respect to true B . africana are a smaller size and segments with slightly rounded positive angles (Figure 8 . 27 – 28) . They are referred to as Braarudosphaera sp . cf . B . africana in Figures 2 – 4 and 8 .

Forms with morphological features different from known taxa were observed throughout the studied interval . Specimens belonging to the genus Micrantholithus , but slightly smaller than Micrantholithus obtusus and M . hoschulzii , are characterized by highly negative intersutural angles that make them look markedly star-like (Figure 9 . 1 – 6) . These forms are described here for the first time as M . stellatus sp . nov . (see taxonomic note) . The other group of forms belongs to the genus Rhagodiscus and has morphological characteristics intermediate between those of R . asper and R . angustus (Stradner) Reinhardt . These forms , which are reported as R . sp . cf . R . angustus , are smaller and more elliptical than R . asper but less elliptical than R . angustus .

5 . Characterization of the Barremian / Aptian boundary

The results of this study confirm the problems involved in characterizing the Barremian / Aptian boundary in pelagic sediments of the Mediterranean domain . The first occurrence of the genus Deshayesites , which is the event traditionally used for the definition of the base of the Aptian , cannot be easily applied because the extreme scarcity of this taxon in this interval . Other ammonite events

Figure 8 . (1-7) Hayesites irregularis (Thierstein in Roth & Thierstein , 1972) Covington & Wise , 1987 (1 , sample X . HA . 6A ; 2-4 , sample X . Cp 2 . 6 ; 5 , sample X . Cp 2 . 12 ; 6 , sample X . Cp 2 . 20 ; 7 , sample X . Kv . 50) . (8-18) Flabellites oblongus (Bukry , 1969) Crux , 1982 (8 , sample X . Cp 2 . 12 , 45 8 to crossed nicols ; 9 , X . Cp 2 . 12 , 45 8 to crossed nicols ; 10 , same specimen as in 9 , 0 8 to crossed nicols ; 11 , sample X . HA . 4 , 45 8 to crossed nicols ; 12 , same specimen as in 11 , 0 8 to crossed nicols ; 13 , sample X . HA . 6A , 45 8 to crossed nicols ; 14 , same specimen as in 13 , 0 8 to crossed nicols ; 15 , sample X . HA . 16 . 1 , 45 8 to crossed nicols ; 16 , same specimen as in 15 , 0 8 to crossed nicols ; 17 , sample X . Kv . 42 , 45 8 to crossed nicols ; 18 , same specimen as in 17 , 0 8 to crossed nicols . (19-22) Crucibiscutum salebrosum (Black , 1971) Jakubowski , 1986 (19 , sample X . HA . 2 , 45 8 to crossed nicols ; 20 , same specimen as in 19 , 0 8 to crossed nicols ; 21 , sample X . HA . 12 , 45 8 to crossed nicols ; 22 , sample X . HA . 4 , 45 8 to crossed nicols) . (23 , 24) Crucibiscutum hayi Black , 1973 , sample X . HA . 4 , 45 8 to crossed nicols . (25 , 26) ‘‘ Vekshinella ’’ mitcheneri Applegate & Bergen , 1988 (25 , sample X . HA . 4 ; 26 , sample X . HA . 6A) . (27 , 28) Braarudosphaera sp . cf . B . africana Stradner , 1961 (27 , sample X . Cp 2 . 6 ; 28 , sample X . HA . 6A) . All figures c . 3 4000 .

Figure 9 . (1 – 6) Micrantholithus stellatus sp . nov . (1 , 2 , sample X . HA . 4 ; 3 , sample X . HA . 6A ; 4 , 5 , sample X . Kv . 42 ; 6 , sample X . Cp 2 . 6) . (7) Diazomatolithus lehmanii Noe ̈ l , 1965 (sample X . HA . 6A) . (8) Microstaurus chiastius (Worsley , 1971) Gru ̈ n in Gru ̈ n & Allemann , 1975 (sample X . Cp 2 . 12) . (9) Rhagodiscus asper (Stradner , 1963) Reinhardt , 1967 (sample X . Cp 2 . 12) . (10) Glaukolithus diplogrammus (Deflandre in Deflandre & Fert , 1954) Reinhardt , 1964 (sample X . Cp 2 . 12) . (11) Watznaueria biporta Bukry , 1969 (sampleX . HA . 4) . (12) Stradneria surirella (Deflandre in Deflandre & Fert , 1954) Aguado , 1993 (sample X . HA . 4) . (13) Palaeomicula maltica (Worsley , 1971) Varol & Jabukowski , 1989 (sample X . HA . 6A) . (14) Assipetra infracretacea (Thierstein , 1973) Roth , 1973 (sample X . HA . 4) . (15 , 16) Chiastozygus sp . cf . C . litterarius (Gorka , 1957) Manivit , 1971 (15 , sample X . HA . 12 ; 16 , sample X . HA . 4) . (17 – 20) Conusphaera rothii (Thierstein , 1971) Jakubowski , 1986 (17 , sample X . Cp 2 . 6 , low focus ; 18 , same specimen as in 17 , high focus ; 19 , 20 , sample X . Cp 2 . 12) . (21 , 22) Micrantholithus obtusus Stradner , 1963 (sample X . Cp 2 . 6) . (23) Mitosia infinita Worsley , 1971 (sample X . HA . 4) . All figures c . 3 4000 .


proposed as alternative criteria do not coincide with the first occurrence of Deshayesites . When Desahyesites is absent , the co-occurrence of Procheloniceras s .s . and Kutatissites can be used tentatively to characterize the lowermost Aptian in the Mediterranean area . Indeed , although the first specimens of these two genera appear at the top of the Sarasini Zone , they reach maximum development in the Tuarkyricus Zone not only in southern Spain , but also in SE France (Delanoy , 1995) and in equivalent levels in Bulgaria (Stoykova , 1992) and the Caucasus (Kakabadze , pers . comm ., 1995) .

On the other hand , none of the nannofossil events detected in the uppermost Barremian – lowermost Aptian coincides with the conventional boundary . The first occurrences of H . irregularis and F . oblongus were detected in the Upper Barremian , whereas the first occurrences of B . africana and N . sp . cf . N . truittii , in the lowermost Aptian , are not events that can be clearly identified . Finally , low abundance and taxonomic uncertainties suggest that the first occurrence of C . litterarius should not be used , at least in the Tethyan realm .

In the Boreal realm , the Barremian / Aptian boundary coincides with the base of the Fissicostatus Zone , defined by the first occurrence of Prodeshayesites . This genus has been occasionally reported in the lowermost Mediterranean Aptian (Dimitrova , 1967 ; Vas ä ic ä ek , 1972 ; Immel , 1987 ; Delanoy , 1991) although most , if not all of the specimens could be attributed to different species of true Deshayesites . Further studies are necessary to ascertain the equivalence between the Mediterranean Tuarkyricus Zone and the Boreal Fissicostatus Zone .

At the present state of knowledge , calcareous nannofossils do not provide precise correlation either . Indeed , as Mutterlose (1992a , b) and Keupp & Mutterlose (1994) pointed out , the appearance of several cosmopolitan species in the uppermost Barremian – Lower Aptian interval coincides with important palaeogeographic changes . According to these authors , the first occurrence of C . litterarius in the Boreal realm correlates with the base of the Aptian , whereas F . oblongus first occurs somewhat higher , in the Fissicostatus Zone , and H . irregularis higher still . In the Mediterranean domain on the other hand , C . litterarius is already present in the Giraudi Zone , whereas F . oblongus and H . irregularis appear in the Sarasini Zone , still in the uppermost Barremian . Such discrepancies cannot be easily explained simply by south – north migration . It seems likely that they are partly the result of indirect calibration of nannofossil events to the ammonite zonation in the Boreal realm , as well as indirect correlation of Boreal versus Tethyan ammonite zonations . In any case , given the present state of knowledge , none of the nannoplankton events detected provides an adequate criterion for interegional correlation .

Recently , the Aptian Working Group (2nd International Symposium on Cretaceous Stage Boundaries , Brussels , 1995) proposed to define the base of the Aptian at the base of the magnetic polarity chron M0 . This magnetic reversal has been identified in several sections in northern and central Italy and in the Atlantic , Pacific and Indian oceans (e . g ., see Erba , 1994) and is systematically located slightly above the first occurrence of H . irregularis , and therefore may coincide with the conventional Barremian / Aptian boundary . However , direct calibration of magnetic chron M0 with ammonite zonation is still unknown . Channell et al . (1995) reported the presence of Prodeshayesites sp . in the upper part of CM0 in the Gorgo to Cerbara section (central Italy) , but taxonomic identification of this specimen (figured by Cecca et al . , 1995 , pl . 2 , fig . 25) , at least in our opinion , is doubtful .


Figure 10 . Correlation of ammonite and calcareous nannofossil events within the Barremian – Aptian boundary interval in the Mediterranean region .

6 . Conclusions

The characterization of the Barremian / Aptian boundary is highly problematic in the pelagic sections of the Mediterranean domain because of the absence or extreme scarcity of deshayesitids in this interval . On the basis of the data obtained in this study , we can confirm that other ammonite or nannofossil events do not provide an adequate alternative to this problem . However , a more precise sequence of biostratigraphic events in the uppermost Barremian and lowermost Aptian can provide alternative criteria to determine the approximate position of the boundary .

The sequence of events observed in the studied sections is as follows (Figure 10) :

12 : Base of the Weissi Zone 11 : FO of Nannoconus sp . cf . N . truittii 10 : FO of Braarudosphaera africana 9 : FO of Deshayesites and Kutatissites cf . bifurcatus (base of the Tuarkyricus

Zone 5 base of Aptian) 8 : FO of Procheloniceras s .s . 7 : FO of Kutatissites and ‘‘ Anahamulina ’’ glemmbachensis 6 : FO of Hayesites irregularis 5 : LO of heteroceratids 4 : LO of Crucibiscutum salebrosum 3 : FO of Flabellites oblongus 2 : FO of Martelites sarasini (base of the Sarasini Zone) 1 : FO of Chiastozygus litterarius


This sequence may be valid for all of the Mediterranean region . By contrast , correlation with the events recognized in the Boreal domain is still problematic .

Acknowledgements

We wish to express our gratitude to Dr . T . N . Bogdanova and G . Delanoy for their suggestions and comments on some aspects of ammonite fauna , and to E . Erba and Ph . J . Hoedemaeker for critically reviewing the manuscript . This study was co-financed by Project PB94-0786 (DGICYT , Ministry of Education and Science) and Research Group 4064 (Junta de Andalucı ́ a) .

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Appendix 1

Taxonomic note (R . Aguado)

Genus Micrantholithus Deflandre , 1950 . Type species : Micrantholithus flos Deflandre , 1950 .

Micrantholithus stellatus sp . nov . Figure 9 . 1 – 6

1993 Micrantholithus cf . obtusus Stradner , 1963 ; Aguado , pl . 3 , fig . 3 . Diagnosis . Relatively small (diameter less than 7 m m) pentalith with markedly star-like shape . The segments have very negative intersutural angles producing a strongly concave margin . Derivation of name . Latin stellatus , starry , star-like shape . Holotype . Micrantholithus cf . obtusus in Aguado (1993 , pl . 3 , fig . 3) . Paratypes . Figure 9 . 1 – 6 . Type level and locality . Uppermost Barremian Micrantholithus hoschulzii Zone ; sample PA-6 , Betic Cordillera (Subbetic Zone) , Spain . Description . Small pentalith with V-shaped segments and highly negative intersutural angles . The adjacent sides of the segments form arms with nearly parallel margins that only taper at the tips . Very marked , approximately straight median sutures that meet at the centre of the form are present along the arms . Remarks . This species is distinguished from Micrantholithus hoschulzii in being smaller and having a star-like shape . It differs from M . obtusus in having segments with a much stronger concave margin and in being smaller . M . stellatus is distinguished from Micrantholithus ? parvistellatus Varol , 1991 , described from the Indonesian Berriasian , by its larger size , much more pronounced sutures , and arms with nearly parallel margins . On the other hand , M ? parvistellatus appears to have stronger affinities with the genus Polycostella Thierstein , 1971 . Micrantholithus brevis Jakubowski , 1986 bears some resemblance to M . stellatus , which , however , has stronger negative angles and lacks the small positive angle in the segments of the former .


Occurrence . This species has been recorded from uppermost Barremian – Lower Aptian sediments from the Subbetic , in the Betic Cordillera , southern Spain .

Genus Nannoconus Kamptner , 1931 . Type species : Lagena colomi De Lapparent , 1931 .

Nannoconus sp . cf . N . truittii Bro ̈ nnimann , 1955

Remarks . We include in this taxon a group of nannoconids with morphologies intermediate between N . bucheri and true N . truittii occurring throughout the lowermost Aptian (Tuarkyricus and Weissi Zones) . These forms have a suboval to nearly elongate outline and a wide and uniform axial canal . The adjacent walls are thick and their outer surfaces slightly converge towards the apical aperture . Nannoconus sp . cf . N . truittii is distinguished from N . bucheri by its shape which is less oval , the wide and uniform axial canal , and their thick adjacent walls . It difers from true N . truittii in having a suboval to nearly elongate outline .

Appendix 2

List of calcareous nannofossil taxa recognized , with author attributions and dates .

Assipetra infracretacea (Thierstein , 1973) Roth , 1973 Assipetra terebrodentaria (Applegate et al . in Covington & Wise , 1987) Rutledge & Bergen , 1994 Axopodorhabdus dietzmannii (Reinhardt , 1965) Wind & Wise in Wise & Wind , 1977 Biscutum ellipticum (Gorka , 1957) Gru ̈ n in Gru ̈ n & Allemann , 1975 Braarudosphaera africana Stradner , 1961 Braarudosphaera sp . cf . B . africana Stradner , 1961 Conusphaera rothii (Thierstein , 1971) Jakubowski , 1986 Cretarhabdus conicus Bramlette & Martini , 1964 Cretarhabdus inaequalis Crux , 1987 Cretarhabdus loriei Gartner , 1968 Crucibiscutum salebrosum (Black , 1971) Jakubowski , 1986 Crucibiscutum hayi (Black , 1971) Jakubowski , 1986 Cyclagelosphaera margerelii Noe ̈ l , 1965 Chiastozygus litterarius (Gorka , 1957) Manivit , 1971 Diazomatolithus lehmanii Noe ̈ l , 1965 Discorhabdus ignotus (Gorka , 1957) Perch-Nielsen , 1968 Ellipsagelosphaera britannica (Stradner , 1963) Perch-Nielsen , 1968 Ellipsagelosphaera fossacincta Black , 1971 Ellipsagelosphaera ovata (Bukry , 1969) Black , 1973 Flabellites oblongus (Bukry , 1969) Crux , 1982 Glaukolithus diplogrammus (Deflandre in Deflandre & Fert , 1954) Reinhardt , 1964 Grantarhabdus meddii Black , 1971 Haqius circumradiatus (Stover , 1966) Roth , 1978 Hayesites irregularis (Thierstein in Roth & Thierstein , 1972) Covington & Wise , 1987 Lithraphidites carniolensis Deflandre , 1963 Manivitella pemmatoidea (Deflandre ex Manivit , 1961) Thierstein , 1971 Micrantholithus hoschulzii (Reinhardt , 1966) Thierstein , 1971 Micrantholithus obtusus Stradner , 1963 Micrantholithus stellatus sp . nov . Microstaurus chiastius (Worsley , 1971) Gru ̈ n in Gru ̈ n & Allemann , 1975 Mitosia infinita Worsley , 1971 Nannoconus bucheri Bro ̈ nnimann , 1955 Nannoconus circularis De ́ res & Ache ́ riteguy , 1980 Nannoconus globulus Bro ̈ nnimann , 1955 Nannoconus kamptneri Bro ̈ nnimann , 1955 ssp . kamptneri Nannoconus steinmannii Kamptner , 1931 ssp . steinmannii Nannoconus sp . cf . N . truittii Bro ̈ nnimann , 1955 Nannoconus wassallii Bro ̈ nnimann , 1955 Palaeomicula maltica (Worsley , 1971) Varol & Jabukowski , 1989 Percivalia fenestrata (Worsley , 1971) Wise , 1983 Pickelhaube furtiva (Roth , 1983) Applegate , Covington & Wise in Covington & Wise , 1987 Podorhabdus gorkae Reinhardt , 1969 Reinhardtites scutula Bergen , 1994 Retecapsa angustiforata Black , 1971 Retecapsa octofenestrata (Bralower in Bralower et al ., 1989) Aguado , 1993 Rhagodiscus asper (Stradner , 1963) Reinhardt , 1967 Rhagodiscus pseudoangustus Crux , 1987 Rhagodiscus sp . cf . R . angustus (Stradner , 1963) Reinhardt , 1971


Rotelapillus laffittei (Noe ̈ l , 1957) Noe ̈ l , 1973 Staurolithites crux (Deflandre in Deflandre & Fert , 1954) Caratini , 1963 Staurolithites matalosus (Stover , 1966) Cepek & Hay , 1969 Staurolithites mutterlosei Crux , 1989 Stradneria surirella (Deflandre in Deflandre & Fert , 1954) Aguado , 1992 Stradnerlithus sexiramatus (Pienaar , 1968) Perch-Nielsen , 1984 Tegumentum stradneri Thierstein in Roth & Thierstein , 1972 Tranolithus gabalus Stover , 1966 ‘‘ Vekshinella ’’ mitcheneri Applegate & Bergen , 1988 Watznaueria barnesae (Black in Black & Barnes , 1959) Perch-Nielsen , 1968 Watznaueria biporta Bukry , 1969 Zeugrhabdotus embergeri (Noe ̈ l , 1959) Perch-Nielsen , 1984 Zeugrhabdotus erectus (Deflandre in Deflandre & Fert , 1954) Reinhardt , 1965 Zeugrhabdotus noeliae Rood , Hay & Barnard , 1971