The precambrian turbidite-tempestite transition as displayed by the amphibolite-facies Puolankajärvi Formation, Finland

Sedimenta(v Geology, 58 (1988) 195 216

195

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

The Precambrian turbidite-tempestite transition as displayed by the amphibolite-facies Puolankajarvi Formation, Finland K. LAAJOKI

and E. KORKIAKOSKI

*

Department of GeoloKv, University of Oulu, Linnanmaa, 90570 Oulu (Finland) Received May 18, 1987: revised version accepted J a n u a ~ 11, 1988

Abstract Laajoki, K. and Korkiakoski, E., 1988. The Precambrian turbidite tempestite transition as displayed by the amphibolite-facies Puolankaj~irvi Formation, Finland. In: M.J. Jackson (Editor), Aspects of Prolerozoic Sedimentary Geology. Sediment. Geol., 58: 195-216. The Puolankaj~irvi Formation (PjF) forms the lowermost unit of the progradational Central Puolanka Group. It is at least 2200 Ma old metamorphosed to amphibolite facies, displays polyphase deformation, and is commonly near vertical or overturned. The PjF is about 1500-2000 m thick and 30 km in strike length but grades into gneisses both below and at its northern extension. Thus, the original sediments could have been much thicker and perhaps hundreds of kilometres long in strike length. The lower part of the PjF consists of graded-bedded mica schists and associated massive or graded arkosites, at least 500-1000 m thick, which represent metamorphosed turbidites deposited by low-concentration and high-concentration turbidite currents, respectively. The turbiditic unit is overlain and partly interfingers with semipelitic mica schists containing combined-flow-origin hummocky cross-stratification and related structures indicating that these rocks, about 100-200 m thick, were originally fine sands and silty muds deposited by storm waves and other shelf processes. Large-scale cross-bedded quartzite interbeds at the top of the PjF indicate a progradational change into the overlying Akanvaara Formation of shallower-water origin. The PjF shows a change of relatively thin-bedded turbidites to tempestites which is interpreted as representing either the distal and middle parts of a relatively steep shelf or the upper slope and the distal-middle part of a narrow shelf.

Introduction

little deformed or metamorphosed epi/pericontinental

Sedimentological

studies of Precambrian

sedi-

Shield and

platformal and

cover rocks of the Canadian

the Kaapvaal

craton. This paper

mentary rocks have increased enormously over the

ports an attempt

p a s t few y e a r s a n d m a n y d e t a i l e d a n d c o m p r e h e n -

logical study of a more metamorphosed

sive p a p e r s

formed sequence.

concerning

the lithology

and

sedi-

mentology of ancient sedimentary rocks from the Precambrian Among

shield

areas

are

now

available.

those rocks studied in most detail are the

The Puolankaj~irvi Formation a

psammitic-semipelitic-pelitic it

now

and metapsammites

0037-0738/88/$03.50

© 1988 Elsevier Science Publishers B.V.

and de-

( h e r e a f t e r P j F ) is unit

metamor-

phosed to upper amphibolite grade and complexly deformed;

* Present address: Geological Survey of Finland, P.O. Box 77, 96101 Rovaniemi, Finland.

re-

at a similar detailed sedimento-

tion.

Despite

the

consists

of

metapelites

folded to a near vertical posiubiquitous

porphyroblasts

of

s t a u r o l i t e , g a r n e t , a n d a l u s i t e , b i o t i t e , etc., in t h e

196 metapelites, the pervasive schistosities and the complicated refolding many outcrops still show enough primary sedimentary features for a conventional facies analysis to be applied. The main problem encountered is the uneven distribution and relatively small size of the outcrops which hinder the measurements of continuous stratigraphic sections and lateral tracing of stratigraphic units. Despite these drawbacks and shortcomings, we can confidently outline the sedimentology of the PjF. The most interesting result is that the lowermost turbiditic metasediments of the PjF, which are of deep-water origin, are abruptly prograded by rocks showing evidence of storm activity. This work has significantly increased our regional understanding of the Puolanka area; so much so that we feel systematic sedimentological studies should routinely accompany Precambrian stratigraphic and tectonic work.

closely associated with each other are grouped into a facies association. In this usage a facies/subfacies and facies association reflect, a particular process and environment or sub-environment, respectively (cf. Walker, 1979; Reading, 1986). Bed thickness definitions follow Ingram (1954) and turbidite terminology follows Bouma (1962). Lithostratigraphic and lithodemic terms are used as defined by the North American Commission on Stratigraphic Nomenclature (1983) except that the term bed is used both for an individual stratum representing a single sedimentation event and a lithostratigraphic unit smaller than a member, as well as for the smallest lithodemic metasedimentary unit which can be delineated from its surroundings.

Terminology

The PjF forms the lowermost formation of the r e g r e s s i v e / p r o g r a d a t i o n a l Central Puolanka Group (CPG) described recently by Laajoki (1986a). This group outcrops on the western margin of the early Proterozoic Kainuu Schist Belt where it underlies the quartzites of the Jatuti tectofacies (Fig. 1). The basement to the group is not exposed, but the PjF grades metamorphically into the gneisses mapped as a lithodemic unit called the Kettukangas Paragneiss (KP) (Figs. 2 and 3). The upper contact of the PjF is gradational into the Akanvaara Formation (AvF) which is a cross-bedded metapsammite unit (see Laaj0ki. 1986a). The KP, which occupies the area west of the PjF, represents the more metamorphosed lower part of the PjF. This lithodemic KP unit consists of thin to medium-thick banded feldspar gneisses with less frequent mica schist beds. Depending on the grade of metamorphism and intensity of deformation, the KP rocks may show typical gneissic structures without any distinctive sedimentary structures (Fig. 4). The original stratigraphic thickness of the KP cannot be measured accurately, but the sparse outcrop information integrated with structural interpretations suggest that it may have been at least 1000 m.

All the rocks studied are metamorphic and therefore standard metamorphic rock names are applied when describing their present lithologies. However, in the sedimentological part of this study the terms sand, mud and silt are used to refer to what are thought to have been the original sediments. Due to the pervasive recrystattization and neomineralization, the psammitic rocks are granoblastic and the pelitic rocks lebidogranoblastic and so, it is not possible to determine the original grain sizes. However, original grain size has been estimated from the sizes of quartz and plagioclase. The grain-size scale of Wentworth (1922) is used for all except clay because all clay minerals have been neomineralized to micas, whose sizes are much greater than the original ones. Mud is used to refer to those mica-rich parts of a rock thought to have originally contained both clay and silt in about equal amounts. The term facies is used descriptively to refer to a sedimentary rock body which can be distinguished from its surroundings by grain size, primary structures and other distinctive features (cf. Selley, 1982, p. 264; Pickering et al., 1986, p. 79; Reading, 1986, p. 4). Those facies which are

Stratigraphy and lithological distribution

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Fig. 3. R e c o n s t r u c t i o n of the s t r a t i g r a p h i c section of the PjF a n d its l i t h o d e m i c derivatives from the geological m a p of Figs. 1 and 2. The effect of the p o s t - s e d i m e n t a r y H u o s i u s l a m p i F a u l t on the s p a c i n g of the S u k s i h a r j u a n d H u o s i u s l a m p i is a m i n i m u m estimation.

The PjF is about 1000 m thick in its northern parts and is estimated to be more than 1500 m thick in the south. It consists of the following metamorphic lithologies: (1) massive or graded arkosites, (2) porphyroblastic mica schists, which often show grading, (3) semipelitic, feldspar-rich mica schists, which are often cross-bedded or rippled, and (4) cross-bedded arkosite and quartzite interbeds in mica schists (unit 4 is restricted to the upper part of the formation). These rocks make up primary sedimentary Facies 1, 2, 3 and 4 as shown in the legend of Fig. 2.

Where not tectonic, the upper contact with the A v F appears to be g r a d a t i o n a l - - t h e amount of cross-bedded quartzite increases gradually until this lithology becomes dominant in the AvF. There are four major areas where the PjF is well-enough exposed to permit detailed lithological and stratigraphic analysis. These are, from south to north, Kapustasuo, Suksiharju, Honkaniemi and H o n k a v a a r a (Fig. 2). The information from these four areas has been condensed into two summary stratigraphic columns showing the main variation between the northern and southern parts

Fig. 4. A K P p a r a g n e i s s s h o w i n g mica-rich a n d feldspar-rich lithologies, 2 k m west of H u o s i u s l a m p i . P h o t o K. Laajoki. C o m p a s s for scale.

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Fig. 5. Simplified stratigraphic column of the southern (Koivikko-Kapustasuo)and northern (Ketttukangas-Honkaniemi)parts of the PjF and interrelations of the sedimentary facies. Notice that the southern column is a composite one with large stratigraphic gaps. (see Fig. 3).

of the area (Fig. 5). In the south, the PjF is divided into three members. The lower member consists of interbedded lithologies 1 and 2, the middle member of alternating sharply defined units of lithologies 2 and 3 and the upper m e m b e r mainly of mica schists with cross-bedded quartzite interbeds (Fig. 5). In the north, at Honkamemi, the middle member appears to be absent and the PjF is here divided into the lower and the upper members. At H o n k a v a a r a (further north), however, solitary outcrops of lithology 2 occur within lithology 3. The overlying A v F is at least 800 m tl~clc and consists almost entirely of cross-bedded arkosites and quartzites. It is a maturing-upwards sequence interpreted as representing an inner-shelf-coastal environment (Laajoki, 1996a). The A v F is overlain by the P~trekangas Formation (PkF) of wavy bedded or graded bedded phyllites and mica schists with cross-bedded quartzite interbeds. This se-

quence which is at least 500 m thick seems to be tidal (Laajoki, 1986a). The PjF-AvF-PkF sequence, totalling at least 2500-4000 m. forms a prograding sequence, which accumulated at some time between about 2540 Ma and 2200 Ma ago as indicated by zircon determinations from interbedded and cross-cutting basic rocks (Laajoki, 1986a). The total strike length of the PjF and the KP is about 60 km (Fig. 3). The overlying AvF and the PkF continues to Paltamo in the south (Fig. l). PjF-type rocks occur near Paltamo and also south of Lake Oulujarvi suggesting a m i m m u m strike length of 120 k m for the original sedimentary system. As discussed by Laajoki (1986a) these rocks m a y correlate with the Lapponian rocks in Lapland, about 300 k m north of Puolanka. If so. the PjF m a y have formed part of a huge sedimentary system interpreted by Laajoki (1986b) as either a narrow sea or a large inland basin. In the following sections the sedimentary facies

201 and facies associations of the PjF are described and a model is presented for the basin in which they were deposited.

Primary sedimentary facies of the PjF On the basis of their primary structures and estimated primary grain sizes the PjF metasediments are divided into four major facies. These are numbered 1 to 4 in the order in which they appear in the stratigraphic column. Facies 2 and 3 contain many subfacies and Facies 1 grades into Facies 2.

Facies 1: massiue sands Description This facies consists of light brown sandstone beds which are mostly tens of centimetres thick but sometimes up to 1.5 m thick. They usually occur either as thick amalgamated sand units or as single beds within Facies 2 rocks (Figs. 6 and 7). At their lower contacts, flame or load cast structures occur, whereas the upper contact is mostly planar. The beds are predominantly massive, but m a n y of the thicker beds show faint planar laminations in their upper parts, overlain in places by wavy lamination (Fig. 8). It is not always possible to determine whether this lamination is primary or due to the schistosity parallel with the bedding. The grain size within a single bed ranges mostly from coarse sand at the base to medium

sand at the top. Some of the thickest beds at Nuottim~iki contain a thin muddy upper seam which is now sericite schist and at Honkavaara mudstone-capped sandstones occur representing a transition to Facies 2b (Fig. 8). Sole structures have not been found, but most bed contacts are sheared. The main minerals are quartz and plagioclase with minor biotite and muscovite. As a whole the facies is composed of rather clean quartz-plagioclase sand.

Interpretation Facies 1 is probably equivalent to the "arenaceous facies" of Mutti and Ricci Lucchi (1978). the "massive sandstone facies" of Walker (1978) and "facies B I . I " of Picketing et al. (1986); although dewatering structures are not present in these Finnish examples. Picketing et. al. (1986) interpret their facies BI.1 as representing rapid deposition from a high-concentration turbidity current by the freezing of a dense cohesionless suspension. Because our Facies 1 occurs within, and grades into, beds of undisputable turbiditic origin this interpretation is also favoured for the PjF. The massive and graded sandstone beds are T~ turbidites and those with laminated upper divisions, a r e T:,b or T,h , turbidites. The rare m u d d y upper drapes may represent sediments deposited from suspension from the waning tail of the current or represent background fallout.

Fig. 6. Single Facies 1 beds (white) amidst staurolite-mica schist and garnet-mica schist. Honkaniemi. Photo K. Laajoki.

202

Fig. 7. Three thick, amalgamated Facies 1 beds at Koivikko, Nuottimaki. Top to the left upper corner. Photo K. Laajoki.

Facies 2: graded muddy sands This facies consists of very-thin-bedded to thin-bedded mudstone-capped graded sandstone or muddy sandstone beds. On the basis of the metamorphic mineral content, which reflects the differences in the original clay content, two subfacies (2a and 2b) are delimited. Facies 2b in-

cludes mudstone-capped sandstone interbeds gradational into Facies 1 sandstones.

Facies 2a." clay-richer (porphyroblastic) muddy sands This facies is characterized by graded bedded mica schists with beds usually only 0.5-3 cm thick and the bedding thickness is fairly constant which

Fig. 8. Stacked Ta-Ta, turbidite beds of Facies 1 at Honkavaara. T o p to the left. The bed left of the label (16 cm long) shows wavy lamination interpreted as a C division. Photo E: Korkiakoski.

203

Fig. 9. A typical outcrop of Facies 2a, showing constant bedding thickness. The beds are overturned and their tops are towards the lower left corner. The pen is 13 cm. Photo K. Laajoki. gives the rock a varve-like a p p e a r a n c e (Fig. 9). T h e lower halves of the g r a d e d s t r a t a consist of feldspar- a n d quartz-rich fine s a n d s t o n e or siltstone, whereas the u p p e r p a r t s were originally rich in clay as i n d i c a t e d b y their p r e s e n t richness in micas, s t a u r o l i t e / a n d a l u s i t e a n d g a r n e t (Fig. 10). In thicker b e d s (i.e. m o r e than 3 c m thick) the lower sand p a r t is d o m i n a n t . R a r e w a v y l a m i n a tion (climbing ripples?) a n d small-scale cross-

l a m i n a t i o n are present. F a c i e s 2a c o m m o n l y forms u n i f o r m units u p to tens of meters thick or, in the south, it m a y occur as thin units o n l y a few tens of c e n t i m e t r e s thick in b e t w e e n F a c i e s 1 a n d 3 rocks. In the north, rocks of this subfacies are m o s t l y s t a u r o l i t e - b e a r i n g a n d the s e p a r a t i o n b e t w e e n the s a n d i e r lower p a r t s a n d the m i c a c e o u s u p p e r p a r t s is clearer (Fig. 11). This m a y be p a r t l y d u e to the different m e t a m o r p h i c histories of these two areas.

Fig. 10. Facies 2a Suksiharju rock showing development of abundant andalus~te (puffed up) in the upper parts of the graded strata: top to the left. The hoe is 73 cm. Photo K. Laajoki.

204

1

Fig. 11. Facies 2a rock at Honkaniemi showing fluctutation in bed thickness. Top to the right (east). Note: the sandy beds in the section covered by the scale (16 cm) showing distribution grading and very thin upper muddy parts: and the very thin ( < 0.5 cm) graded beds between the right-hand end of the scale and the sandy bed on the right of the photo. Dark spots are staurolite. Photo E Korkiakoski.

Subfacies 2a also includes mud-rich varieties which alternate with the sandier beds containing rare plane- or cross-laminations (Fig. 12).

Facies 2b: clay-poorer (non-porphyroblastic) graded muddy sands A typical Facies 2b rock is a non-porphyrobtastic mica schist showing graded bedding with a

feldspar and quartz-rich lower part and a biotite and muscovite-rich upper part (Fig. 13). Because these rocks do not contain porphyroblasts their primary chemical composition can not have been as rich in aluminium as that of Facies 2a. The bed thickness is very thin or thin but the beds are in general thicker than in Facies 2a and their lower sandy subdivisions are slightly coarser grained.

Fig. 12. A sandy Facies 2a bed about 10 cm thick showing parallel and wavy laminations. Photo K. Laajoki.

205

Fig. 13. A Facies 2b sandstone bed about 5 cm thick showing distribution grading and a very thin mud cap. Photo K. Laajoki.

Interpretation Although grain-size determinations are only approximate it is most likely that our Facies 2 is equivalent to facies C2.3, D2.1, D2.3 or E2.1 of Pickering et al. (1986). All of these are interpreted by them as having been deposited by low-concentration turbidity currents. The PjF rocks are mostly Tae turbidites with rare Tabe and Tabce turbidities, which are characteristic of those facies in the scheme of Pickering et al. (1986).

Facies 3: cross-stratified fine sands and accociated facies Facies 3 consists of metasemipelites relatively poor both in micas and porphyroblasts. These are interpreted as metamorphosed equivalents of cross-stratified and rippled fine sandstones. Based mainly on the character of the primary structures, Facies 3 has been subdivided into five subfacies. Facies 3a is dominant in the north and the rest are encountered in the south.

able sand laminae and separated by mud drapes with sharp upper contacts. Three styles of cross-stratification occur. First, is cross-stratification with subhorizontal or slightly undulating first-order truncation surfaces and low-angle even lamination dipping mostly to the north, but also showing opposite dip directions (Fig. 14). The original sediment was a micaceous fine sand. The first-order surfaces are veneered by thin mud drapes. The spacing of the undulation is rather short: from about 0.5 m up to 1.5 m. Secondly, Fig. 15 displays a low-angle cross-stratification which differs from the undulating type in that the first-order surfaces are mainly subhorizontal, hummocky-swale structures are either poorly-developed or lacking and the number of mud drapes is greater. Thirdly, in those cases where the m u d / s a n d ratio is close to 1 : 1 the sand produces bedforms with internal very low-angle bi-directional cross-laminations overlain by form-conformable even sandstone laminae or sandstone occurs as beds displaying flat or undulating laminations.

Facies 3a: hummocky cross-stratified sands This subfacies, preserved only at Honkaniemi, includes fine-grained micaceous sandstone with sets from a few centimetres up to 10-15 cm thick of low-angle or smooth, convex-up cross-stratification. The sets are often overlain by formconform-

Facies 3b: complexly cross-laminated sands Tabular or near-tabular sets (around 10 cm thick) of complexly cross-laminated fine-grained micaceous sandstone capped by thin mudstone

206

Fig. 14. A. A Facies 3a rock showing smooth low-angle cross-stratification and low-angle truncation. Photo K. LaajokJ. B. Line drawing. The thicker lines indicate smooth first-order truncation surfaces and thinner ones the third order laminar surfaces. Notice the low-angle second-order truncation surfaces and the low dip angles of the even lamination.

layers (now andalusite-rich mica schist) (Fig. 16) characterize this subfacies. It is the d o m i n a n t facies in the Suksiharju area. Internally, the sets are cross-laminated in a complex way. Typical of this internal structure are very thin wavy or s m o o t h laminations and m a n y irregular erosional and second-order truncation surfaces separating the different laminae bundles (Fig. 17). Mudstones s e p a r a t i n g the sandstone sets form either continuous layers or shorter trough fillings,

whose m u d content m a y increase gradually towards the sharp u p p e r surface (Fig. 17B).

Facies 3c: Tabular planar cross-bedded sands This subfacies occurs only at K a p u s t a s u o where it is characterized b y planar cross-bedded sandstone sets f r o m a few centimetres to twenty centimetres thick. The sets are solitary and are separated f r o m each other by Facies 3e (see below) (Fig. 18).

207

Fig. 15. A. Facies 3a rock showing bidirectional low-angle cross-stratification capped by mud drapes. Photo K. Laajoki. B. Line drawing. Mud drapes are hatched. Notice the subhorizontal first-order truncation surfaces (thicker lines) and the low-angle third-order laminar surfaces. The diagonal thin lines outlines S4 foliation and andalusite veins are shown by letter A. Notice the antiformal structure just above the label "top".

Facies 3d: Ripple cross-laminated sands O n e outcrop at Suksiharju c o n t a i n s a u n i t of rippled s a n d s t o n e a b o u t 2 m thick. The ripple c r o s s - l a m i n a t i o n s are u n i d i r e c t i o n a l b u t their upper b o u n d a r i e s are relatively symmetrical a l t h o u g h asymmetrical profiles also occur (Fig. 19). The ripple sets are d r a p e d by thin m u d d y flasers or seams a n d their lower b o u n d a r i e s are straight or slightly u n d u l a t o r y .

Facies 3e: Horizontal laminated silty muds Facies 3c sets are separated b y mica schist beds a few centimetres or tens of centimetres thick which are mostly so schistose that their p r i m a r y structures c a n n o longer be seen (Fig. 18). I n rare cases, however, they display horizontal l a m i n a t i o n with l a m i n a thickness of a b o u t 0.5 cm or less a n d faint g r a d i n g in grain size.

208

!

. . . .

~ ....................

]

Fig. 16. Five tabular Facies 3b sets (numbered from 1 to 5) separated by mud layers (now rich in andalusite). The beds are upside down. Photo K. Laajoki.

Fig. 17. A, A close-up of a facies 3b rock showing internal cross-lamination in three sets separated by mud drapes. Photo K: Laajoki. B. Line drawing showing first-order surfaces (" master bedding") (thicker lines) and wavy third-order laminar surfaces. Notice the andalusite aggregates (A) in the lower drape, the graded appearance of the second drape and the metamorphic segregation (MS) in the upper fight comer.

209

Fig. 18. Stacking of Facies 3c and 3e sets at Kapustakangas. Photo K. Laajoki. Top to the left.

In terpretation Facies 3a: Based mainly on comparisons with published examples of hummocky cross-stratified sandstones, Facies 3 a is interpreted as a hummocky cross-stratified facies. Our undulating cross-stratified beds (Fig. 14) are closely comparable to those illustrated by Dott and Bourgeois (1982), especially their H, HF, and H F M sequences: and also to those illustrated by de R a a f et al. (1977, fig. 8), especially their type 11. Our low-angle cross-stratification (Fig. 15) more closely resembles that described by Nottvedt and Kreisa

(1987) which, although resembling hummocky cross-stratification, was formed by combined-flow, not just oscillatory flow. The bedforms in our sequences which are richest in mudstones appear identical to the FXM model described by Dott and Bourgeois (1982, fig. 24). The origin of hummocky cross-stratification is variably ascribed to: the action of oscillatory storm waves (e.g. de Raaf et al., 1977; Dott and Bourgeois, 1982); combined-flow storm currents (Swift et al., 1983; Allen, 1985: Nottvedt and Kreisa, 1987); or turbidity currents (Walker et al.,

Fig. 19. Ripple cross-laminated and m u d draped Facies 3d. The length of the label is 16 cm. Photo E. Korkiakoski.

210 1983). Because symmetrical hummocky-swale structures are less common than the low-angle cross-stratification with preferred northern dip, we favour a largely combined-flow model for the origin of our Facies 3a. Although Walker et al. (1983) ascribe h u m m o c k y cross-stratification to turbidity currents, we have not identified their B and P divisions in our rocks; so we see little evidence of turbidity current action in Facies 3a. The mud drapes and layers represent either deposition from wainmg storm currents or from suspension during fair weather. Facies 3b: We have not found any description in the literature of facies with which this facies could readily be compared. The m a n y second-order truncation surfaces and the wavy nature of the laminations indicate, however, that flow conditions were variable and probably of the combined type. In Nottvedt's and Kreisa's (1987) bed-form phase diagram (their fig. 4) this facies may represent energy conditions a little lower than those which form hummocky cross-stratification. Facies 3c: This facies represents a bedform formed by unidirectional traction currents. Facies 3d: On the basis of the symmetrical ripple profiles this facies is interpreted as representing mostly wave generated bedforms, but because other diagnostic features of a wave origin

L

(de Raaf et al., 1977) seem to be lacking combined flow origin is not totally excluded ~cf. Harms. 1969. 1979). Facies 3e: Because the internal structure of the laminae of this facies are not preserved its origin cannot be deduced. It m a y represent a high-energy plane-bedded facies. General As a whole, in comparison with Facies 2. Facies 3 represents clearly higher-energy deposits, formed mostly in fine-grained sand under combined flow conditions. The restriction of Facies 3a to the north indicates that energy conditions were higher there than in the south where Facies 3b-3e dominate. Facies 4: cross-bedded medium sand Description This facies is only exposed at Huosiuslampi and east of Honkaniemi in the uppermost part of PjF (Fig. 2) where overturned and refolded crossbedded quartzite beds occur within strongly schistose mica schists lacking primary structures. The sands are trough cross-bedded with sets up to a few tens of centimetres thick and the cosets up to about 0.5-1 m thick (Fig. 20).

.

Fig. 20. A close-up of an outcrop of Facies 4 showing trough cross-bedding. The beds are inverted. Photo K. Laajoki.

211 Interpretation Facies 4 represents sands deposited by traction currents. A more detailed interpretation is hindered by the poor outcrops. Rocks associated with Facies 4 The uppermost rocks in the PjF which contain the interbeds of Facies 4 are so strongly deformed that their original facies cannot be determined. They are mica schists or feldspar-rich metasemipelites. A few of the latter show relict bedding structures of probable turbidite origin and some ripple cross-lamination.

Facies associations of the PjF The facies described above can be grouped into four broad facies associations called A, B, C and D in stratigraphic order from the oldest to the youngest (Fig. 3). Facies Association A: alternation of Facies 1 + Facies 2 Description The lower part of the PjF is characterized by the repetition of Facies 1 or Facies 2 rocks. This is exemplified best at Nuottim~iki in the south where these two facies alternate through 500 m of section (Figs. 2 and 5). Here Facies 1 forms either solitary beds or units up to 50 m thick within Facies 2a rocks. Facies 2b has not been identified here but it is common at Honkaniemi, 20 km to the north, where Facies 1 alternates with Facies 2a and Facies 2b. The underlying KP also seems to be composed of this facies association. Interpretation There can be little doubt that this facies association represents part of a turbiditic system, but the main question is whether the sediments accumulated in a basinal, deep-water fan, slope or shelf environment. The apparent "distal" and "basinal" nature of Facies 2 suggests a basin plane deposit, but turbidites in those are typically base-truncated Tcde turbidites (e.g. Mutti, 1977), and these are very rare at Puolanka. However,

there are no coarse-grained turbidites, indicating that the facies is not proximal. As discussed in the next section, Facies 2a is closely associated with Facies 3 which shows evidence of abundant storm-wave activity, thus suggesting that Facies 2a probably represents a relatively shallow-water sediment deposited somewhere below storm wave base. Facies 1 and Facies 2 rocks resemble some slope deposits such as those described by Mutti and Ricci Lucchi (1978) and Lundegard et al. (1985), but the sequences at Puolanka lack slumps which are diagnostic of slope deposits. Taking into account these facts we propose that Facies Association A probably represents either upper slope or outer shelf deposits, which according to Mutti and Ricci Lucchi (1978) are difficult to distinguish one from another. Facies Association B: alternation of Facies 2a and Facies 3 Description The best established stratigraphic relations in the PjF are that Facies 2a and Facies 3 are interstratified at Suksiharju and Kapustasuo (Figs. 2 and 5), and that Facies 1 and Facies 3 do not occur in association. The thickness of alternating Facies 2a and 3 units varies from a few tens of metres to about 200 m at Kapustasuo. The transition from Facies 2a to Facies 3, where observed, is sharp. Intrepretation The alternation of "basinal" Facies 2a and storm-generated Facies 3 indicates deposition close to the zone of storm-wave base and, secondly, that the height of the wave base was suddenly and drastically changed, raised and lowered repeatedly at least five times. Facies Association C: Facies 3a 3e Description This association comprises only Facies 3 deposited in the area between storm-wave base and the shallower environment in which the AvF was deposited. It contains m a n y different sub-associa-

212 tions. At Kapustakangas there are regular repetitions of Facies 3b, 3c and 3e; in the Suksiharju area Facies 3b and 3d are dominant, while at Honkaniemi Facies 3a is prominent. Obviously these differences between the north and south relate to amount of storm influence during sedimentation and probably reflect slightly different water depths in these parts of the basin. Interpretation On the basis of the common occurrence of storm-generated deposits and the shallow-water nature of the overlying AvF it is concluded that this association was deposited on an inner shelf shorewards of the line marking the base of the storm waves. Facies Association D: Facies 4 and associated rocks

A convincing interpretation for this facies cannot be presented due to the lack of environmentally diagnostic features. However, an origin as subtidal sandwaves, close to the palaeoshoreline, would not be unreasonable in this stratigraphic situation. Basin models The PjF represents a fragmentary and arbitrary section of the lower part of the progradational sedimentary fill (the Central Puolanka Group) of the lowermost allochthonous western basin of the early Proterozoic Kainuu Schist Belt. Its depositional basement is not known, neither are its northern and southern extensions. All these, together with the fact that outcrops are sporadic (Fig. 3) make any basin reconstruction extremely difficult. Consequently, we have to be content with giving only a two-dimensional basin model and discussing briefly the tectonosedimentological context of the PjF in terms of the early Proterozoic sedimentary evolution of the Fennoscandian Shield in northern Finland. Taking into account the stratigraphy, the facies relations and their interpretations given in this and an earlier paper (Laajoki, 1986a), the PjF was deposited in a prograding system whose deeper parts were occupied by sediments deposited by turbidity currents, the middle part by storm-gener-

ated and related fine sands and the upper part by shallow-water sands. These kinds of progradational turbidite-shallow-water sediment transitions are common in the ancient sedimentary rock records and many are well documented. They have mostly been interpreted as representing either submarine a n d / o r delta deposits (e.g. Link and Welton, 1982: Pickering, 1982), outer shelf-pro-delta sediments prograded by delta complexes (e.g. Allen, 1960) or transitions from a turbidite basin to a shallow shelf (Van de Kamp et al., 1974; Graham, 1982; Lamens, 1985). Because the PjF turbidites are monotonously rather thin-bedded and fine-grained and the transition from distal basinal turbidites to coarse proximal channel deposits cannot be established a canyon-fed submarine fan seems not to have existed at Puolanka. Although the AvF has not yet been studied in detail it is known to be composed of uninformalty cross-bedded sands without any upwards coarsening cycles typical of deltaic sequences, so that a delta-fed system seems unlikely. The tempestites of Facies 3 prove that the Puolanka system included at least a narrow shelf, so the two most probable models to explain the turbidite. shallow-water transition displayed by the PjF are a slope-shelf or an outer-inner shelf transition. The choice between these two possibilities depends largely on the interpretation of Facies Association A; does it represent (a) upper-slope or (b) outer-shelf turbidites? Since there is insufficient proof Of either~ Fig. 21 gives two models based on these alternatives. In the first model (Fig. 21A) Facies Association A represents distal outer-shelf turbidites and minor background sediments. The turbidites may be related to the storm activity which produced the tempestitic Facies 3 (cf. Hamblin and Walker. 1979). We have not, however, identified stormgenerated sand beds that could be expected to occur in the outer-shelf laminated muds like those, for instance, from recent shelves l e.g. Reinecke and Singh, 1980; Aigner and Reinecke. 1982; Allen, 1982) or those from ancient ones Ifor references, see Marsaglia and Klein. 1983: Johnson and Baldwin, 1986; cf. Soegaard and Eriksson.

213

Formation

~ Akanvaara

~L

Formation

=

Shelf

Formation Tidal flat

outer

Sea level

Parekangas

inner

=L j j

Facies ass, D

S t o r m wave base Facies ass. C

G

Facies ass. B Facies association A

Shelf

Upper slope Sea level

1

2

Tidal flat

inner

outer

Facies ass. D

Shelf brake = storm wave base Facies ass. C Legend : Facies ass.B

Facies 4 -' 3 " Facies association A

2 1

Fig. 21. Two arbitrary sections of the PjF showingthe lateral facies association distributions. A. Rather steep shelf model. B. Narrow shelf model. 1985). This model presumes to explain the thick amalgamated Facies 1 units at Koivikko and Honkaniemi in particular as the results of repeated and prolonged major storm periods with relatively insignificant background sedimentation. This model agrees with that of Graham (1982) who wonders whether many of the turbidites could not be storm generated and formed as sheet-like deposits down slopes devoid of typical fan morphology. The second choice is a narrow-shelf model (Fig. 21B) in which case the storm-wave base and shelf break/slope neck must have been at about the same depth. The Facies 3 sediments were deposited on the outer parts of the narrow shelf and probably some of the sediments spilled over the shelf break onto the upper slope. In this model Facies Association A could represent a lev6e com-

plex of a deep-water fan whose feeder channel and lobe systems were outside the PjF outcrop area or are not exposed and upon which Facies 3 repeatedly prograded. In this connection it is pertinent to remember that the majority of the ancient turbidite system, of which the PjF forms only a small fraction, is now represented by the West Puolanka Paragneiss (Fig. 1) and its northern and western extensions. These compose one of the major crustal shortening zones of the Fennoscandian Shield. This of course leaves the door open for many speculations. Like Shanmugan et al. (1985) we stress that the turbidite facies association scheme based on modern fans may not always be appropriate for interpreting ancient submarine fan environments. Secondly, recent studies show that a turbidite system

214

may be more complex and varied than the traditional one point source fan model; see for instance the basin types 1-3 of van de Kamp et al. (1974), the submarine ramp facies model of Heller and Dickinson (1985), the line and multiple point source models of Chan and Dott (1983) and Lundegard et al. (1985) and the discussions by Miall (1984, p. 305) and Reading (1987). Clearly, a new approach how to classify turbidite systems is needed; the facies analysis technique (e.g. Pickering et al., 1986) is useful for descriptions, but for environmental analyses to know lateral variabilities within the system is essential. The rapid sea-level changes indicated by Facies Association B call for closer discussion. The sealevel changes are generally attributed either to eustatic or local causes or both. Of the global sea-level changes glacioeustatic and tectonoeustatic ones are considered the most important for the pre-Quaternary (Harms, 1984). Shanmugam et al. (1985) attribute the control of the growth of Phanerozoic submarine fans to eustatic sea-level changes caused by glaciation: with growth occurring mainly during glacials (low sea level). Early Proterozoic glaciations are known from the Fennoscandian Shield (Marmo and Ojakangas, 1984) and from North America (e.g. Young, 1973). Consequently, glacioeustasy may be one possible control for the repetition of Facies 2 and 3, although the regular and sharp nature of this repetition indicates that this is not very likely. For the same reason tectonoeustasy also seems unlikely and so it is felt that these changes were probably caused by more local tectonic factors, e.g. basement faulting. Finally, in terms of regional stratigraphic evolution Laajoki (1986b, 1988) has recently divided the Karelian formations in Finland into four cycles or tectofacies: the Sumi-Sariola, the Kainuu-Lapponi, the Jatuli and the Kaleva which correspond to the continental rift, the narrow sea (the Red sea type), the open sea and the foredeep stage. In this scheme the PjF represents the rocks of the second cycle (Kainuu-Lapponi), which were transgressed by the Jatuli. R~siinen (1986) states that the Lapponi quartzites, which show evidence of tidal activity and are correlative with the AvF, form part of a transgressive sequence. The Pre-

cambrian stratigraphy of Lapland is, however, poorly understood and the rocks underlying the Lapponi quartzites are not sufficiently well exposed that the transgressive or regressive nature of the complete cycles could readily be established. Furthermore, the relationship between the Lapponi and Jatuli and the palaeoenvironment of the abundant volcanic rocks and associated iron-formations of the upper Lapponi have not yet been completely clarified. Because of the vertical bedding positions the overall progradational/regressive nature of the CPG at Puolanka is well founded, although the topmost PkF may also indicate some deepening of the basin at the final stage of sedimentation of the CPG. The present observations, therefore, support the idea that the CPG was deposited during the opening stage of the Karelia sea on a rifted continental margin and later transgressed by the open-sea stage Jatuli sediments. Conclusions

The main results of our study can be condensed into the following conclusions. (l) A conventional sedimentological approach can be applied with reasonable success to rather highly metamorphosed and complexly deformed Precambrian strata and should routinely accompany stratigraphic and tectonic work on metamorphic, supracrustal terrains. (2) As has been discussed in many recent papers (Graham, 1982; Link and Welton, 1982; Chan and Dott, ]983; Heller and Dickinson. 1985, Lamens, 1985; Lundegard et al., 1985; Shanmugam et al., 1985; etc.), and confirmed here by the PjF, many ancient turbidite sequences are not readily modelled by the classical canyon-fed deep-sea fan system (e.g. Mutti and Ricci Lucci, 1978). (3) The PjF is one of the oldest (at least 2200 Ma old) metasedimentary formations from which storm-generated deposits have so far been described. Because tempestites have a key role in reconstructing ancient continental margins special attention should be paid to mapping them in Precambrian shield areas. (4) The palaeoenvironmental setting of the PjF was either the outer to middle parts of a relatively

215 broad,

steep

shelf, o r t h e u p p e r

slope and

the

o u t e r p a r t s o f a n a r r o w shelf.

Acknowledgements T h i s s t u d y is b a s e d nanced

by

contribution

o n a r e s e a r c h p r o j e c t fi-

the Academy to IGCP

of Finland

and

is a

160. K . L . is g r a t e f u l t o K.

E r i k s s o n , R. O j a k a n g a s a n d A. S i e d l e c k a f o r f r u i t ful discussions during the IGCP

160 e x c u r s i o n t o

these outcrops. The English of the manuscript has been

checked by Mrs.

Sheila Hicks, Ph.D.

The

r e f e r e e s K. E r i k s s o n , M . J . J a c k s o n a n d K. T u c k well m a d e m a n y v a l u a b l e s u g g e s t i o n s w h i c h g r e a t l y h e l p e d to c l a r i f y a n d i m p r o v e s e v e r a l p o i n t s i n t h e paper.

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216 ancient upper continental slope. J. Sediment. Petrol., 52: 171-186. Pickering, K.T., Stow, D.A.V., Watson, M.P. and Hiscott, R.N., 1986. Deep-water facies, processes and models; a review and classification scheme for modern and ancient sediments. Earth-Sci. Rev., 23: 75-174. Ras~inen, J., 1986. On the Lapponia quartzites in the Sodankyl~-Savukoski area, northern Finland. 17e Nordiska Geologm6tet, Helsingfors Universitet, May 12-15, 1986, Abstracts, pp. 166. Reading, H.G., 1986. Facies. In: H.G. Reading (Editor), Sedimentary Environments in Facies. Blackwell Scientific Publications, Oxford, pp. 4-19. Reading, H.G., 1987. Fashions and models in sedimentology: a personal perspective. Sedimentology, 34: 3-9. Reinecke, H.-H. and Singh, I.B,, 1980. Depositional sedimentary environments with reference to terrigenous clastics. Springer-Verlag, Berlin, 549 pp. Selley, R.C., 1982. An Introduction to Sedimentotogy. Academic Press, London, 417 pp. Shanmugam, G., Damuth, J.E. and Moiola, R.J., 1985. Is the turbidite facies association scheme valid for interpreting ancient submarine fan environments? Geology, 13: 234-237. Soegnard, K. and Eriksson, K.A., 1985. Evidence of tide, storm, and wave interaction on a Precambrian siliclastic

shelf: the 1,700 m.y. Ortega Group, New Mexico. J. Sediment. Petrol., 55: 672-684. Swift, D.J.P., Figueiredo, A.G., Jr., Freeland, G:L. and Oertel, G.F., 1983. Hummocky cross-stratification and megarippies: a geological double standard? J. Sediment. Petrol.. 53: 1295-1317. van de Kamp, P.C., Harper, J.D., Conniff, J.J. and Morris, D.A., 1974. Facies relationships in the Eocene-Oligocene in the Santa Ynez Mountains, California. J. Geol. Sot., London, 130: 545-565. Walker, R.G., 1978. Deep-water sandstone facies and ancient submarine fans: models for exploration for stratigraphic traps. Am. Assoc. Pet. Geol. Bull., 62: 932-966. Walker, R.G., 1979. Facies and facies models, 1. General models. In: R.G. Walker (Editor), Facies Models. Geosc. Can., Reprint Ser., 1: 1-7. Walker, R.G., Duke, W.L. and Leckie, D.A., 1983. Hummocky stratification: significance of its variable bedding sequences: discussion and reply. Geol. Soc. Am. Bull., 94: 1245-1251. Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments. J. Geol., 30: 377-392. Young, G.M., 1973. Tillites and ahiminous quartzites as possible time markers for middle Precambrian (Aphebian) rocks of North America. Geol. Assoc. Can. Spec. Pap., 12: 97-127.