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Geological Setting and Genesis of Coticules (Spessartine Metapelites) in the Lower Ordovician of the Stavelot Massif, Belgium

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Geological Setting and Genesis of Coticules (Spessartine Metapelites) in the Lower Ordovician of the Stavelot Massif, Belgium Journal of the Geological Society, London, Vol. 143, 1986, pp. 253-258, 8 figs, 2 tables Printed in Northern Ireland Geological setting and genesis of coticules (spessartine metapelites) in the Lower Ordovician of the Stavelot Massif, Belgium J. LAMENS’, F. GEUKENS’ & W. VIAENE2 Instituut voor Aardwetenschappen, Katholieke Universiteit Leuven, Redingenstraat 16, B-3000 Leuven, Belgium ’Afdeling Fysico-Chemische Geologie, Katholieke Universiteit Leuven, Celestijnenlaan 200C, B-3030 Heverlee, Belgium Abstract: The LowerOrdovician in the CaledonianStavelot Massif (Belgium) containsa coticule member, probablyArenig in age, composed of redhematite-rich shales alternating with thin yellowish coticulebeds. These coticules are fine-grained highly manganiferousmetamorphic rocks mainly composed of spessartine garnet, mica and quartz. Similar rocks of about the same age occur in several localities along the Appalachian and Caledonian fold belts. The Belgian coticules are thought to haveoriginated from marly sediments which had beendeposited rapidly, probably by density currents, and in which CaCO, had been diagenetically replaced by MnCO,. During metamorphism, spessartine garnets- were formed from this MnCO, and mica. Manganese and iron are thought to have originated from volcanic-exhalative processes. The Stavelot Massif is one of the five Caledonian Massifs in could be used as a key lithology for correlation along the Belgium. It consists of MiddleCambrian to Lower Appalachian and Caledonian fold belts. Similar rocks have Ordovician strata (Fig. 1, Table 1). The southeastern partof also been described from older (Precambrian) and younger the Stavelot Massifis at a low metamorphicgrade. The strata(Roy & Purkait 1968; Vearncombe 1983; Minnigh LowerOrdovician of the Stavelot Massifis called 1979). Most of these spessartine-rich rocks, however, have “Salmian”.It has been subdivided into Salmian 1 (Sml), morequartz and less mica than the Belgian coticules. Salmian 2 (Sm2) and Salmian 3 (Sm3) by Geukens (1965); Several origins havebeen proposed for these quartz-rich these units are lithostratigraphic. The Sml is Tremadoc in coticules: Mn-rich sand layers (Clifford 1960); cherts ageasindicated by the presence of the graptolite originating from chemical precipitation of silica and Dictyonema flabelliforme. The Sml consists mainly of black, manganesesalts (Schiller & Taylor 1965; Doyle 1984); grey andgreen shales, siltstones and sandstones; it is detrital sediments rich in manganese oxides (Roy & Purkait thought to representa gradual transition from a basin plain, 1968); chemically precipitatedMnCO, (Woodland 1939); characterized by turbidite deposits, through a depositional and Mn-rich volcanic tuffs (Kramm 1976). slope to a shelf (Lamens 1985). The Sm2 and Sm3 contain This paper discusses the origin of coticules in the Belgian no macrofossils; judging from acritarchs they are attributed type area based on their sedimentological and geochemical tothe Arenig and Llanvirn, possibly also the Llandeillo characteristics and on a study of stratigraphically equivalent (Vanguestaine 1973). The Sm2, which is present in the strata in the non-metamorphic area of the Stavelot Massif. Stavelot Massif only, consists mainly of reddish pelitic rocks rich in iron and manganese. Geukens (1965) subdivided the Sm2 of the metamorphic area into Sm2a, Sm2b and Sm2c. The coticule-bearing Sm2b strata in the The Sm2a is composed of redsiltstones and shales. The metamorphic area of the Stavelot Massif Sm2b, also known as the coticule member, consists of red The Sm2b in the metamorphic area consists of red shales shales alternating with coticule beds of 1-15 cm; these are alternating with coticule beds of 1-15 cm (Fig. 2). The red fine-grained highly manganiferous metamorphic rocks which red shales are composed of fine-grained mica, quartz and are mainly composed of spessartine (Mn-garnet), mica and hematite; they also contain small amounts of spessartine quartz. The Sm2c consists of violet and green chloritoid-rich garnet and some andalusite and kaolinite blasts. They show shales. The Sm3 is composed of grey and black shales and a regular horizontal millimetre-scale lamination. Coticules siltstones. are on average composed of about 40 wt% garnet, 30 wt% Coticules werepetrographically described forthe first mica and 30 wt%quartz. They are almost devoid of time in Belgium by Renard (1878). They used to be mined hematite. The rock is very fine grained;the garnets are and sold as whetstones; they owe this quality to their high mostly less than 20 microns diameter. There are also more content of very small garnets. Similar spessartine-rich rocks quartz-richvarieties of this coticule: these consist of a havesince been found inseveral other countries. These millimetre-scale alternation of greenish quartz-rich laminae rocks occur mainly in the lowerOrdovician of the and yellow garnet-rich laminae. Rarer varieties are rich in Appalachian and Caledonian fold belts;they have been andalusite or chloritoid. found in New England, Massachusetts,Newfoundland, The contact between the coticules and the shales is usually Nova Scotia, Ireland, Wales and Norway (Kennan Lk very sharp. In contrast to the shales, the coticules and their Kennedy 1983). Kennan & Kennedy suggest that coticules quartz-rich varieties show many sedimentary structures. The 253 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/253/4893080/gsjgs.143.2.0253.pdf by guest on 24 September 2021 254 J. LAMENS, F. GEUKENS & W. VIAENE Fig. 1. (a) Caledonian Massifs in Belgium. (b) Geological map of the Stavelot Massif (after Geukens 1984). most frequent structures are recumbent folds and low-angle slump structures. The slump direction is towards the south. faults(Fig. 3), which areinterpreted here as small-scale Primary small-scale cross-lamination is also a typical structure. Figure 4 shows garnet-rich oblique laminae in a quartz-richlayer between horizontal quartz-rich and garnet-rich laminae. Figure 5 shows a sequence starting with 1 a cross-laminated quartz-rich lens atthe base, passing Table 1. Lithostratigraphic units inthe Stavelot upwards into horizontal lamination and recumbent folds. It Massif in correlation with the Cambro-Ordovician stratigraphic scale. After Geukens (in Robaszynski is noteworthy that the lower part of a coticule bed is often & Dupuis 1983) quartz-rich whereas the upper part is garnet-rich. This may reflect primary compositional differences caused by graded Ashgill Caradoc bedding. The bases of the beds sometimes show small load Llandeillo casts or flame structures or may be slightly erosional. Some Llanvirn SalmianLlanvirn 3 beds contain small red mudstone fragments (Fig. 6). Arenig Salmian 2 Salmian Arenig The coticules are typically very rich in manganese and Tremadoc SalmianTremadoc 1 poor in iron when compared to the associated shales which are much poorer in MnO and richer in Fe,03 (Table 2). The UPPER Revinnian contact between shales and coticules is also geochemically CAMBRIAN (f1900m) very sharp (Fig. 7). The coticule member is thickest (40-50m) near Vielsalm MIDDLE Devilliarl (Fig. 1). It thins towards the north and towards the west. It CAMBRIAN (+600 m) is only a few metres thick near MaImCdy in the north and about 10metres near Lierneux in the west; it seems to be Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/253/4893080/gsjgs.143.2.0253.pdf by guest on 24 September 2021 COTICULESMASSIFSTAVELOT OF THE 255 Fig. 6. Coticule bed showing slightly erosional base and small red mudstone fragments. totally absent further west. NearLierneux some thin (ca 1m) volcanic rock layers interstratified with Sm2b red shales and coticules havebeen recently discovered (Lamens & Geukens 1985). Theyare greenish porphyritic rocks of intermediate composition which belong to the calc-alkalic series. Fig. 2. Red shales and yellow coticule bed. Note the sharp contact between the two lithologies. The Sm2b strata in the non-metamorphic area of the Stavelot Massif The Chevron syncline in the non-metamorphic area (Fig. 1) contains strata which, with respect to the well-defined Sml-Sm2 boundary,appear to be situated atthe same stratigraphic level as the coticule member (Sm2b) in the metamorphic area. This stratigraphic interval consists in its lower part of an alternation of red shales, siltstones and coarse-grained beds of 1-20 cm thick. These beds are either structureless or show horizontal lamination andgraded bedding. They are composed of fragments 1-5 mm in size in Fig. 3. Coticule bed showing recumbent slump fold and low-angle a hematite-rich matrix (Fig. 8). Most fragments are wholly fault. or partlycomposed of carbonates. The most conspicuous I Fig. 4. Thin section of coticule showing oblique garnet-rich laminae (dark) in quartz-rich layer between horizontal quartz-rich and garnet-rich laminae. 1 3 5 7 9 % 2'6 'lO'% Fig. 5. Coticule bed showing cross-laminated quartz-rich lens at the Fig. 7. Geochemical profile in the Sm2b showing the sharp contrast base, horizontal lamination in the central part andrecumbent folds in Fe and Mn contents between coticule beds (white) and shales at the top. (black). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/253/4893080/gsjgs.143.2.0253.pdf by guest on 24 September 2021 256 GEUKENSJ. LAMENS,F. & W. VIAENE ponents also occur: they are carbonate-richmudstone fragments,probably intraclasts, and well-rounded quartz, quartzite and volcanic rockfragments. The carbonate mineral present in such high amounts
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