118 



J.B. RICHARDSON, R.M. RODRIGUEZ AND S.J.E. SUTHERLAND 



chitinozoans) and miospores were extensively studied by Cramer 

 1964a. b, 1966b, c, 1967, and 1970; by Cramer & Diez 1968, 

 1975 and 1978a, b; and by Rodriguez 1978a, b, c, 1979, and 1983. 

 Several biozones based on chitinozoans (Cramer & Diez, 1978b) 

 and on acritarchs and miospores (Rodriguez, 1983) have been 

 described. These biozones were established on composite sections 

 and thus they do not constitute formal biostratigraphicai units 

 according to the rules of the International Code of Stratigraphic 

 Nomenclature. Instead they constitute respectively, chitinozoans, 

 acritarchs and spore associations, each one arranged in chrono- 

 logical order, ranging from the Upper Wenlock? to Lower 

 Lochkovian. Based on the miospore associations found in the 

 Geras Section, and on correlation with other palynological assem- 

 blages found in several different localities, Rodriguez (1983, fig. 

 13) placed the Silurian/Devonian transition about 5 m below the 

 stratigraphic level adopted in the present work. This author also 

 showed that the boundaries of this formation are highly 

 diachronous (eg. the upper San Pedro Formation is younger in 

 sections to the south and west of the 'Folds and Nappes Unit' than 

 in those closer to the Precambrian Asturian Arch). Argovejo, Geras 

 and La Vid are in the south part of this unit and all these sections 

 have Lochkovian strata at the top. 



Chitinozoans of Pffdoli and Lochkovian age from the Carazo 

 Formation, of the 'Pisuerga-Carrion Unit", east of the 'Cantabrian 

 Zone', were described by Schweineberg (1987). The Carazo Forma- 

 tion differs in lithology from the San Pedro Formation but, although 

 the precise lateral continuity between these two formations has not 

 been established, the San Pedro Formation also includes sediments 

 of Pndoli and Lochovian age. 



SPORES 



Biozonation (Figs 2-6) 



The San Pedro Formation has been sampled in four sections: at 

 Argovejo, Geras de Gordon and La Vid de Gordon (referred to in the 

 text as Geras and La Vid respectively) and La Peral. Productive 

 samples from the La Peral section occur at a lower horizon than in the 

 other sections (Fig. 5). Strata from the underlying Formigoso Forma- 

 tion were sampled in the Argovejo and La Vid sections only. The 

 samples from this formation are practically barren of spores but may 

 contain chitinozoans. The absence of smaller palynomorphs. such as 

 spores, may indicate turbulent to winnowing environments, and 

 possible reworking of at least some chitinozoans. Alternatively, there 

 may be a disconformity either within, or at the top of, the Formigoso 

 Formation (Cramer & Diez. 1978b), and possibly within the lower 

 San Pedro Formation. The La Vid Formation, overlying the San 

 Pedro strata, has been sampled only at the La Vid and Argovejo 

 sections, but all samples taken from shale intercalations in the sandy 

 dolomites were barren. 



Four new spore zones, and one subzone, are proposed. These 

 zones, with the exception of the hemiesferica Biozone, contain two 

 nominal taxa, at least one of which starts at the base of the zone. The 

 zones are described in ascending order and reference sections are 

 given for the base of each zone, or its lowest known occurrence. In 

 addition, in two sections, possible equivalents of the Synorispohtes 

 Uhyciis - Chelinospora poecilomorpha (LP) Assemblage Spore 

 Biozone (Richardson & McGregor, 1986) occur, and in the La Peral 

 Section the Artemopyra brevicosta - Hispanaediscus verrucatus 



H Sandstone 



I Shale v.Phosphadc nodules 



H Sandstones Shale Q Dolomites 



■ ~ , , ^ , H Limestones 



Sandstone / Oolites ^ 



■ Ferrous Oolites ZlQuarlziles 





-^ 



a. a a 

 o S a S 



■s 2 5. 



?, -^ 



a a a i: 







.« ,0 .C 





Cl. o 



Ae -Angochitina elongate Mid Gorstian - Early Pncioli 



At - Angochitina tsegelnjucki Lochkovian 



Eb - Eisenackitina bohemica Lochkovian 



Rv - Ramochilina villosa Late Ludfordian 



Me • Margachitina elegans Mid Pridoli 



Ss - Sphaerochitina sphaerocephala Mid Gorstian - Early Pridoli 



Pc - Psuedoclathrochitina carmenchui Mid Pridoli 



Uu - Urnochitma tima Pridoli - Earliest Lochkovian 



S- 5 S g 





-g a ^: 





OOOt^^OO^t^U^O^O^ 



= ih- 



" = j ji'ii = 



11 ih"=iii = 



.1 1 



1 



IMIHHIIIHIHH 



s_ .Fault 



Ger 18 

 Ger17 

 Ger 16 

 GerlSB 

 Ger15A 

 Ger 14 . 

 Ger 13 

 Ger 12 _ 

 Gerll 

 Ger 10 

 Ger9_ 

 Ger8 

 Ger7 

 Ger6_ . 

 Ger5 

 Ger4 

 Ger3 

 Ger2B 

 ■Ger2A 

 Ger 1 



MN 

 (na) 



(a) 



EC 



H 



-?— - 



RS 



(LP) 

 ?- - 



Eb/At 



Uu/Pc/Me 



. Ss/Ae 

 Rv 



Lochkovian 



Pridoli 



Ludfordian 



Fig. 2 Selected spore distribution at Geras. Chitinozoan zonation after Verniers et al. (1995): bo: bohemica, su: superba, ele: elegans, ko: kosovensis, ba: 

 barrandei, ph: philipi. 



