PALYNOLOGICAL ZONATION OF MID-PALAEOZOIC, NW SPAIN 



Pnull'. and ThruM 



^^^ Oiictrop ol ihc San Pnlro R)raiiiln 

 ^^^^^1 Mcso/nic / Tcnair^- 



Fig. 1 Simplified geology of the Cantabrian Region, NW Spain (based on Rodriguez, 1983). 



San Pedro Formation 



This formation was described in two main areas: a. the 'Ferruginous 

 Sandstones of Furada' (Barrois, 1882) on the northern flank of the 

 Cantabrian Mountains, and b, the San Pedro Formation (Comte, 

 1937) in the south. These two names refer to the Upper Silurian 

 Formations of the 'Folds and Nappes Unit' of the Cantabrian Zone. 

 Three of our sections were originally designated as the San Pedro 

 Formation (Geras de Gordon, La Vid de Gordon and Argovejo). The 

 fourth section was included in the Furada Formation (La Peral). As 

 both formations constitute a single lithostratigraphic unit they are all 

 referred to in the text as San Pedro Formation. 



The San Pedro Formation is predominantly sandy and overlies the 

 Formigoso Formation; the latter is Silurian in age. The Formigoso 

 Formation is composed of black shales, with alternating shales and 

 quartzites in the upper part. The San Pedro Formation appears to be 

 in stratigraphical continuity with the underlying Formigoso Forma- 

 tion in the sections near the Precambrian of the 'Narcea 

 Anticlinorium', but the contact is sharp and erosive in the sections 

 situated farther east. 



Informally three members can be distinguished in this formation. 

 The Lower Member consists mainly of red ferruginous, coarse- 

 grained sandstones, sometimes with oolites, having cross-trough 

 stratification. At some localities there are conglomeratic beds, 

 with phosphatic, sideritic and silty clasts and intraclasts; some of 

 these clasts have a volcanic origin (Van den Bosch 1969; Suarez 

 deCenti. 1988). 

 The Middle Member is composed of medium to fine grained 



sandstones with current and wave ripples, and grey to green 



intensively bioturbated mudstones and siltstones. 

 The Upper Member has fine-grained quartzitic sandstones, with 



cross stratification, interbedded with dark to black shales. 



The San Pedro Formation was deposited in a shallow epeiric sea in 

 intertidal to inner platform conditions (Suarez de Centi, 1988) in a 

 transgressive-regressive cycle (Rodriguez, 1983). In this latter inter- 

 pretation the middle member represents the deepest conditions. 



Age and biozones of the San Pedro Formation 



The age of this formation has been established on macrofauna, 

 microplankton and spores. A macrofauna of brachiopods, mol- 

 luscs, connularia, trilobites and graptolites has been found, but 

 they are not abundant and are restricted to certain beds (Comte, 

 1934, 1959; Llopis Llado. 1958. 1960. 1964; Poll. 1962. 1963, 

 1970). The lower half of the formation is of Ludlow age. based on 

 brachiopods and connularia (Comte, 1934). In some sections, situ- 

 ated to the west of the "Folds and Nappes Unit" (see Fig. 1 ), near 

 one of our localities (La Peral), Poll (1963, 1970) found graptolites 

 20 to 50 m. below the top of the formation: Saetograptus fristichi 

 fristichi, S. fr. linearis, S. chimaera sahveyi belonging to the 

 nilssoni, scanicus, and probably leintwardinensis Biozones and 

 indicating a Gorstian to Lower Ludfordian age. The highest part 

 was dated as Lochkovian on its macrofauna (Comte. 1934, 1937, 

 19j9;Po11, 1970). 



In contrast to the macrofauna, microplankton and miospores are 

 extremely abundant and well preserved. Microplankton (acritarchs. 



