LEIDSE GEOLOGISCHE MEDEDELINGEN, Deel 49, Aflevering 3, pp. 487-497, 1-12-1975

The Palaeozoic strata near Moeche in Galicia, NW Spain

BY

C.G. van der Meer Mohr

Abstract

Coral fragments, crinoidal debris and foraminifers found in limestone beds near Moeche (Galicia) are evidence of a Devonian-Silurian

Coruña Hoja age for the strata in the greenschist facies belt around Cabo Ortegal, hitherto considered on the Spanish 1:200,000 map-La

1 to currents into ancient floor is (IGME), be Precambrian in age. Their deposition by turbidity troughs on an ocean being suggested.

INTRODUCTION metamorphosed sediments considered to be of Ordovician

the to Silurian age on Mapa Geologico de Espana

The northwestern mentioned extremity of Spain, near El Ferrol, Cabo above.

Ortegal and Ortigueira is an area of igneous and

metamorphic rocks which forms part of the Galician-Castilian zone (Lotze, 1945) in western Spain. STRATIGRAPHY

Towards the East the exposed strata become progressively

in in the in the under younger age and the Cantabrian Mountains The stratigraphic succession area

Palaeozoic succession is finally buried under a thick cover consideration is very difficult to be established in the field

of Mesozoic sediments. due to the lack of palaeontological data and marker beds,

complicated folding of the strata and the metamorphism of Since 1957 petrological fieldwork has been carried out the sediments. The most pertinent palaeontological and around Cabo Ortegal by students and staffof the petrologic stratigraphical data bearing on an area close to our region departmentofthe Geological Institutein Leiden.This work His contains have beenpublished by Matte(1968). work an resulted e. g. in theses by Vogel (1967), Maaskant (1970), excellent synopsis of the stratigraphy of the Palaeozoic and Engels (1972) in a geological map by Vogel, Engels and succession in northwestern Spain. From his data, Ho Len Fat. Also the Mapa Geologico de Espana, correlations can be established between Silurian and older sheet 1, the Instituto Minero 1:200,000, by Geologico y de strata in the Cantabrian Mountains and their equivalents Espana (IGME) is most important with regard to the the flank of the near El Picon, on northern Barquero present paper. anticline (Fig. 2). This is important since the strata in the

greenschist facies belt around Moeche are almost on strike

GENERAL GEOLOGICAL SETTING with the Palaeozoic succession near El Picon.

The area around Cabo Ortegal can be divided into a The stratigraphic succession described by Matte from the

NE-SW trending complex of eclogites, granulites, northern flank of the Barquero anticline is presented in amphibolites and paragneisses with abundant catazonal Figure 3. The most important aspects of this succession are mafic inclusions considered to be of Precambrian age. in stratigraphical order:

These intruded are by ultramafic rocks and metagabbros. the Olio de volcano-detrital Faults separate this complex from intensively folded 1. Sapo formation, considered

Matte be metasediments and greenschists to the Southeast, the by to of Precambrian age (although elsewhere it

South and the Southwest. Within this belt of greenschist might be Cambrian in age); facies rocks are isolated outcrops of serpentinite. Near

Moeche (Pereiro) and Insua, halfa kilometer SE of Pena

Grande, fossiliferous limestone beds can be found associated * within serpentinite (Fig. 1, Photo 1). Eastward Geological Institute, Department of Stratigraphy and

Garenmarkt The of the greenschist facies belt lies an area of very slightly Palaeontology, lb, Leiden, Netherlands 488

Complex. Ortegal

Cabo

the

of East

the

to strata Palaeozoic

the

of map sketch Geological

1.

Fig. 489

Fig. 2. Stratigraphic cross-section of the Lower Palaeozoic strata between Cabo de Peñas and the anticline of Barquero close to the Cabo

Ortegal Complex. Data after Radig (1962) and Matte 1968 (modified).

2. a thick sequence of Ordovician strata ofpredominantly light green to grey coloured, finely laminatedschists with believed abundant and lenses. in the shales with a pronouncedinterval of quartzites to quartzveins They occur

be equivalent to the Culebra quartzites (Arenig.); greenschist facies belt as well as on top of the serpentinite

of metavolcanites show 3. a sequence of shales, intercalated with limestone lenses, body near Moeche. Several the a

greenish quartzites, rhyolitic tuffs and, near the top, relict variolitic texture (Photos 2 and 3). The cavities have

limestones containing crinoids and siliceous shales with been generally filled with quartz and feldspar.

Monograptus halli Barr. and Rastrites linnaei Barr.

THE STRATA NEAR MOECHE From Matte's cross-section through the Barquero

the of and anticline, Spanish geological map El Coruna, Serpentinite bodies occur around Moeche, Pena Grande mapping done by Hoogervorst (internal report, 1973) it can and near Ladrido. They consist of greyish-green rock be deduced that Silurian the rhyolitic tuffs are probably contains composed ofa mass of serpentine which no longer analogous with a distinct quartzite zone that can be

any remnants ofolivine or pyroxene. Only near Ladrido did followed It many kilometers inland (Fig. 1). changes its Ho Len Fat (internal report, 1968) find a in which the original SSW strike just about SE of Moeche. After that this quarry serpentinite still contained some remnants of pyroxenite. quartzite zone continues in a northwestern direction

In quarries near Moeche (Photo 1) and Pena Grande around the Cabo Ortegal complex. It finally strikes limestone beds occur interlayered with serpentinites. northwards to Punta Corbeira at the Atlantic Coast (see Repetition of this sequence is probably due to recumbent IGME-map). folding.

These limestones of which several samples were

The stratalying tothe West and to the North ofthis quarzite collected in the Moeche quarry and Pefia Grande are

to the facies zone up greenschist belt couldbe Silurianinage lightgreen, slightly fossiliferous, well stratified carbonate

according to Matte's data. They are marked as phyllites on rocks.

the geological et al. map by Vogel (1967). In thin section they turned out to consist of a mass of

microsparite with about 5-10% calcite 'porphyroblasts'

The greenschists are from the phyllites probably separated which in many cases show the outlinesofcrinoid columnals by faults. Faults also border this facies belt towards the of these crinoid (Photo 4). Many fragments show up as northeast forming its contact with the amphibolites and the ghost-structures inside the porphyroblasts. We have to do

gneisses. here with crinoid fragments which often became The chlorite-schists and greenschists are mainly surrounded by a rim of syntaxial calcite cement and in that metavolcanites. are According to Hoogervorst (1973) they fashion withstood the severe conditions, including 490

and laminar Tabulatathese beds cannot even be older than

Middle Ordovician. However, since we know from the

work by Matte (1968) and Nissen (1959) that the Silurian

strata near El Picon contain crinoidal limestones, an

identical age for the Moeche and Pena Grande limestone

beds seems to be the most likely possibility.

There of also is, course, a possibility that the limestones

might even be younger in age; perhaps they might date from

Devonian times. However the nearest Devonian strata

known to-day are at a considerabledistance from Moeche.

Yet it be ruled that Devonian cannot out strata might once have been deposited far more to the West than they are

and in anotherfacies presently found, than they are present in the Cantabrian Mountains. Corals are abundantly

present in the Devonianstrata in the CantabrianMountains

but do not occur in the underlying pre-Devonian, marine

deposits.

The emplacementofthe fossiliferous limestonesis another

The problem. average percentage of crinoid fragments in

these limestones is in no way comparable to the amounts

found in shallow water carbonates known from so many other Neither places. did we get the impression that the

corals the foraminifers or were originally present in great

numbers. It must be however admitted that shearing and

recrystallization could have caused the destruction of the

skeletal structure of many bioclasts, particularly those of

corals or Bryozoa. Turbidity currents, sweeping material

from a nearby platform, could have caused the

of bioclastic material into transportation deeper parts of a sea. There such detritus could have become deposited in calcareous muds overlying lavaflowsof a basic, and mantle

slices of an ultrabasic composition.

CONCLUSIONS

The preceding observations and deductions lead to the

conclusion that the greenschist facies belt is of

Silurian-Devonianage and that theirstratamight have been

deposited in deep troughs.

A Caledonianfoldingfor the Gafician-Castilian zone has Fig. 3. Stratigraphic succession of Lower Palaeozoic strata near been et His idea isbased on the Barquero anticline, after Matte (1968). suggestedby Wagner al. (1974).

a pre-Upper Wenlockian uncornformity reported by

Martinez-Garcia(1972) from NW-Zamora.However, inthe

concrete vicinity of Cabo Ortegal we do not find any metamorphism, to which they were submitted afterwards. unconformable evidence of a Caledonian orogeny. The Apart from crinoid fragments some foraminifers contact, in northern Spain, between Ordovician ■ and (Glomospira?)i (Photo 5) were observed in the thin sections Silurian strata, can be explained by uplifts of a foreland and as well as fragments of tabulatecorals (Photos 6 and 7). The block faulting. latter were found in a limestone breccia collected in the

Pena Grande quarry.

ACKNOWLEDGEMENTS

These findings offossils in the limestones near Moeche and in Pena Grande rule out the previously suggested The author feels much indebted to Prof. Dr. E. den Tex and Precambrian of age the strata in the greenschist facies belt Drs. P. W. C. van Calsteren for their stimulating dis-

(see IGME-map). cussions and criticism and to Drs. H. Koning for his Since neither echinoderms nor corals have ever been cooperation in the field. Last but not least he wishes to found in Precambrian rocks have with thank we to do the Spanish people for their friendliness and Phanerozoic strata. Considering the presence ofbranching hospitality during his stay in their country. 491

REFERENCES

The'catazonal rocks of Engels, J. P., 1972. poly-metamorphic Comparación con series vecinas importancia orogénica. Acta structural and study. Ph. Geol. 7/4, 104-108. Cabo Ortegal fNW Spain), a petrofabric Hisp., pp.

D. thesis in: Leidse Geol. Med., 48, pp. 83-133. Matte, Ph., 1968. La structure de la virgation hercynienne de

1973. mikros- Galice Hoogervorst G. H. T. C, Makroskopische en (Espagne). Geologie Alpine, 44, pp. 157-280. rond kopische beschrijving van de gesteenten uit het gebied Nissen, H. U., 1959. Deformation und Kristallisation im de zuidelijke punt van het Cabo Ortegal Complex, (internal Nordwest Spanischen Küstengebirge bei Vivero. Diss. Math. report). Naturw. Fak. Univ. Munster, 300 pp.

Ho Len Fat, 1968. Petrografïsche en mineralogische Radig, F., 1962. Ordovizium/Silurium und die Frage beschrijving van het gebied ten oosten van het Cabo Ortegal Pravariszischer Faltungen in Nordspanien. Geol. Rundschau, 52,

Complex, (internal report). pp. 346-357.

D. Lotze, F., 1945. Zur Gliederungder Varisciden der Iberischen Vogel, E., 1967. Petrology of an eclogite- and pyrigarnite-

Meseta. Geotektonische Forsch., 6, pp. 78-92. bearing polymetamorphic rock complex at Cabo Ortegal, NW

Maaskant, P., 1970. Chemical petrology of polymetamorphic Spain. Ph. D. thesis in: Leidse Geol. Med., 40, pp. 121-213. ultramafic rocks from Galicia, NW Spain. Ph. D. thesis in: Leidse Wagner, R. H. & Martinez Garcia, E., 1974. The relation

between and foreland in Geol. Med., 45, pp. 237-325. geosynclinal folding phases movements

Martinez 1972. El Silurico de San Vitero (Zamora). northwestern Studia 131-158. Garcia, E., Spain. Geologica, VII, pp.

PHOTOGRAPHS —￿ 492

Photo 1. Light coloured fossiliferous limestone beds interlayered with darker

coloured serpentinites in a disused quarry near Moeche.

texture. Photo 2. Metavolcanite from the serpentinitebody near Moeche showing relict variolitic x5, crossed nicols.Thin section 7—c4—107.

Printed with permission of G. H. T. C. Hoogervorst. 493 494

and feldspar crystals (albite) filling variolites. x50, ordinary Photo 3. Same thin section as on Photo 2 showing quartz light. Printed with permission of G. H. T. C. Hoogervorst.

Photo 4. three crinoid columnals. Peña Grande x50. Photo is of thin section. Axial cut through quarry. a negative 495 496

6. Photo 5. Glomospira-like foraminifer from a fossiliferous Photo Fragment of a tabulate coral fragment in a limestone of limestone bed near Moeche. x350. breccia (?) in the Pena Grande quarry. x50. Photo is a negative

thin section.

Photo 7. of in Grande x50. Fragment a tabulate coral or Bryozoa (?) in a limestone breccia (?) the Peña quarry. 497