PALYNOLOGICAL ZONATION OF MID-PALAEOZOIC. NW SPAIN 



SILURIAN GLOBAL CHITINOZOAN 

 BIOZONATIQN 



The global scheme of Verniers et al. (1995) was calibrated with 

 reference to chitinozoan index species in global stratotype sections 

 and other localities, or by reference to graptolite, conodont or 

 trilobite biozonal schemes where this information was not available. 

 To be included in the scheme, index species had to have been 

 recorded from at least two major Silurian palaeocontinents. 



Conochitina pachycephala was used as the index species for the 

 Middle-Upper Wenlock pachycephala Biozone. This species was 

 recovered in our study from one sample close to the base of section 

 at La Peral (LP3). As C. pachycephala ranges well into the Gorstian 

 and because the succeeding Upper Homerian lycoperdoides Biozone 

 was not encountered, it is not possible to increase the resolution of 

 this part of the San Pedro Formation with chitinozoan data. 



Angochitina elongata was used as the index species for the Middle 

 Ludlow biozone in the global scheme but was only recorded sporadi- 

 cally from La Peral and Geras. At Geras (Ger 9) it is possible that the 

 species is reworked as it is found associated with spores attributed to 

 the lower EC Biozone, which is regarded as Upper Pfidoli in age. At 

 La Peral, A. elongata occurs in sample LPS and is associated with 

 spores attributed to the Ludlow RS Biozone. A. echinata is noted as 

 an accompanying species in the elongata biozone of the global 

 scheme and was recovered from La Peral, La Vid and questionably 

 from Geras. Although a common component of many Middle and 

 Upper Ludlow chitinozoan assemblages it is known to range into the 

 Lower Pfidoli (Verniers et al. 1995). 



No chitinozoans were recovered from the Cantabrian Mountains 

 that could be directly attributed to the Middle Ludfordian/?/;//;/?; Bio- 

 zone or the Upper Ludfordian barrandei Biozone of the global 

 scheme. S. sphaerocephala, recovered from Geras, La Peral and Argo- 

 vejo, is shown to range through the barrandei Biozone by Verniers et 

 al. (1995), but is also noted as ranging into the Middle Pfidoli. 



The Lower Pfidoli ko.sovensis biozone is well represented in most 

 areas globally by the presence of U. iirna, which was recovered from 

 all four sections studied. The index species Fimgochitina kosovensis 

 is recorded from La Peral, but in only one sample (LP 19) at a level 

 well above the base of the following elegans Biozone. 



The elegans Biozone is well represented in our sections by the 

 index species, Margachitina elegans. in all sections, often associated 

 with P. carmenchui and U. urna. 



The index taxa for the VpperPtidoU superba Biozone (Anthochitina 

 superba Eisenack, 1971) has not been recorded in this study, with 

 only the accompanying species U. urna encountered. The top of the 

 superba Biozone and the base if the first biozone in the Devonian is 

 defined by the first occurrence of E. bohemica, regarded as being 

 coincident with the base of the Devonian (Paris, Winchester-Seeto & 

 Grahn, 2000) E. bohemica has been identified from Geras in samples 

 Ger Hand 16. 



BIOSTRATIGRAPHICAL CONCLUSIONS 



The vetustus (V) and brevicosta- verrucatus (BV) Spore Biozones 

 indicate an age no earlier than Homerian. Chitinozoan evidence is 

 sparse but the range of Conochitina rudda in sample LP3 includes 

 the Upper Homerian. There is thus a possible disconformity between 

 the BV spore assemblage ( Upper Homerian) in the La Peral section 

 and the reticulata - sanpetrensis (RS) assemblage (Ludfordian to 

 Lower Pfidoli). The base of the RS Biozone is not seen in the sections 

 studied with the possible exception of La Peral. 



135 

 Ludfordian/Pridoli boundary 



Because of differences in chitinozoan assemblages between the 

 Cantabrian Mountains and the type area in Bohemia, and possibly 

 sampling deficiencies due to the coarseness of some of the 

 sediments, it is currently not possible to define this boundary with 

 precision. However, the occurrence of several forms, such as 

 Ramochitina villosa at Geras, indicate a Upper Ludfordian age, 

 consistent with .spore evidence that places the boundary in the RS 

 Biozone. Spore assemblages from the Welsh Borderland show 

 many species in common with those from Spain, but the lower 

 Cantabrian samples (Formigoso and the lower part of the San 

 Pedro Formation) often yield poor results and the key forms from 

 the Anglo-Welsh area are usually missing or have a sporadic dis- 

 tribution. Consequently, the exact level of the lower RS boundary 

 is uncertain. The approximately equivalent LP/TS (Ludfordian/ 

 Pfidoli) boundary in the type area has not been located but the 

 spore assemblages below the RS Biozone have some comparable 

 features to the lower LP Biozone (mid- Gorstian to Ludfordian) 

 and spores similar to the nominal species of the Chelinospora 

 obscura Sub-Biozone (mid-Gorstian) and the overlying 

 cambrensis and inframurinatus Sub-Biozones occur in the RS 

 Biozone. Few other independently dated sections have been stud- 

 ied palynologically across the Ludfordian/Pffdoli boundary 

 elsewhere, so the location of the lower boundary of the Pfidoli 

 Stage must remain provisional. 



Silurian/Devonian boundary 



In the Silurian/Devonian boundary stratotype (Klonk section, Bohe- 

 mia), Eisenackitina bohemica and Angochitina chlupaci appear 

 close to the base of the Devonian. They are also found in the Geras 

 and Argovejo sections respectively, where the base of the Devonian 

 (based on spore data - Richardson. Rodriguez & Sutherland, 2000) 

 is also a little lower than the appearance of these two chitinozoan 

 species, and close to the base oi X\\e Aneurospora spp. Sub-Biozone. 

 This latter assemblage is found in three of the four Cantabrian 

 sections studied, and occurs also in the Anglo- Welsh area above the 

 thelodont Turinia pagei at a level 30 m below the base of the MN 

 Biozone. The MN Biozone occurs near the base of the Gedinnian in 

 Belgium and above the basal Devonian graptolites in Podolia 

 (Ukraine). Consequently, although further work needs to be done on 

 sections (e.g. Podolia) containing spores, chitinozoa, and key 

 macrofossils, the Aneurospora spp. Sub-Biozone has potential for 

 inter-regional correlation, and its base, on cun"ent evidence, includ- 

 ing chitinozoan work in this paper, is near, if not coincident with, the 

 Silurian/Devonian Boundary. 



Radiometric dates and Pridoli basal Devonian 

 timescales 



Recently construed radiometric dates (Tucker er a/., 1998) give ages 

 for the Upper Ludfordian (Whitcliffe) and basal Devonian 

 (Lochkovian) as 420 ± 4 Ma and 417.6 ± 1 Ma respectively. This 

 leaves a minimum of 2.4 Ma for the duration of the Pfidoli. Our 

 researches on sequences from the Cantabrian Mountains indicate 

 one spore zone in this interval with parts of two others. Previously, 

 Richardson e? a/. ( 1 984) estimated duration of c. 3 Ma for each spore 

 zone. Using roughly 1 2/3 zones this would give a figure of approxi- 

 mately 5 Ma for the duration of the Pfidoli Stage. This seems more 

 realistic for the biological changes involved. The maximum interval 

 (7.4 Ma) indicated by the radiometric dates would allow for at least 

 2.5 biozones of average duration. 



