The sequence in the lower Allochthon of Jämtland

 

 

55

THE SEQ

UENCE IN THE

L

OWER

A

L

L

O

CHTH ON

O

F

JÄM

T

L

AN D

Lars

Karis

Tremadoc

In places where black shales are strongly folded or tectoni­

cally disturbed, it is difficult to distinguish between

Upper

Cambrian

However, in several areas the presence of the

and Tremadoc shales.
Tremadoc has been proved. The recorded maximum thickness of the
Tremadoc black shales is a bout 5 m but mostly the thickness is less
1.5 m; whether these figures represent the primary thickness or
Dictyonema Shale only the

not cannot be safely esta blished.

In the

than

Zone of

Dictyonema flabelliforme has been proved, with Q. fla belliforme,

Bienvillia wimani and

Boeckaspis mo bergi.

In the

Moholmen section,

central

storsjön area, the presence of the

Upper

Tremado c

Ceratopyge

Shale

(

Zone of

Platypeltoides incipiens) is indicated by

Peltocare cf.

norvegicum and

Parabolinella n.sp. (with a pronounced ocular ridge)

(

Karis, unpu blished).

Recently a thin

(0. 3 m) black bituminous limestone was found between

the

Upper Cam brian dark shale and the

Latorp

Limestone in a road­

seetian about l km west of

Oviken new church, south-west

storsjön

area.

The fossil content has still to be fully studied, but appears

similar to the

Upper Tremadoc at

Moholmen

(

Karis, unpublished).

Latarp Limestone

In the

Lower

Allochthon of

Jämtland, the

Lower

Arenig

Latarp

Limestone is generally thin.

It varies from about l .5 m

in the south-west

storsjön area

(Österåsen and Fjällsågen, 10- 15 km

west of

Oviken) to about 0.5 m in the

Andersön section. This unit

seems persistent across large areas and can be identified in seetians

as far west o f

the

Caledonian thrust front as

Föllinge and at localities

on the western shore of

20 km due west of
Allochthon the zones of Megistaspis

storsjön

Östersund.

In the

(

Ekeraspis) armata

eastern

Lower

and

Megistaspis

(

Varvaspis) planilimbata are present, while in the

western seetians

(

Andersön) only the former has been recognised

(

Tjernvik

1956).

The

base of this limestone is glauconitic in many localities.

5

6

Töyen

Shale

This shale is apparently developed throughout the

Lower

Allochthon and in places attains an estimated thickness of about

15m.

The shale facies evidently began earlier in the

Lower

Allochthon than

in many areas of the Autochthon.

The zones of

Tetragraptus phyllograp­

toides and

Didymograptus balticus have been recorded from

Andersön

(

Tjernvik

195

6), where higher zones are proba bly missing.

In adjacent

localities, graptolites of the

Zone of

found (e.g. the

Bynäset section, west

Didymograptus �undo have been
Östersund).

Frösön, 7 km west of

The

Töyen

Shale o ccurs through a comparable interval in

the south-west

storsjön area

(Österåsen and

Svartbodarna road, west of

Oviken).

The dating of this unit is generally difficult since the shale is

strongly folded and cleaved, and fossils can only be obtained from a

very fe w levels.

Specimens of the trilobite

Gog have been found at

several localities, (e.g. at

Sanne, Hackås and on

Andersön

(

Fortey

1975) and

Frösön, from the zones of

Tetragraptus phyllograptoide� and

fhyllograptus densus).

Isö

Limestone

The grey, generally thick-bedded, calcilutitic

Isö

Limestone is widely distributed in the

Lower

Allochthon, from the

lowermost nappes in the east, to the vicinity of

As and

Aspås in

the western part of the central

storsjön area, and Föllinge north

of Östersund

(similar, undated rocks appear in the antiforms west of

Are).

The type area is the island of

Isön, in

storsjön immediately

south- west of

Andersön.

There the whole sequence is strongly

folded,

in parts inverted, but

without any major breaks.

The upper and lower

boundaries, with the

Andersö and

Töyen

Shales respectively, are

exposed.

The estimated thickness is about

40 m.

Only a few levels within the

Isö

Limestone have yielded diagnostic

faunas .

The basal part of the limestone in the type area falls

within the Zone of

Didymograptus hirundo, and in a nodular calcilutite

about 5 m above the base, trilobites from the

Zone of

Asaphus

(

Asaphus)

expansus have been recovered.

In the uppermost

2m of the

Isö

Lime­

stone, Paraceraurus exsul

(

Beyrich) and

Pseudomegal��pi� patagiata

(

Törnquist) are common, indicating beds equivalent to the

Folkeslunda

Limestone.

The

Isö

Limestone has the same age range throughout most

of the central

storsjön area.

57

To the north of

Östersund in the

Lower

Allochthon of the eastern

Häggenås area, the

Holen, Segerstad, Folkeslunda and lower

Dalby

equivalents can be recognized within the

Isö

Formation in the

Örån

seetian upstream of the road to Storhögen.

An almost identical

development is seen in the

Autochthon of

Högfors seetian

(Stop 3:4)

about 3 km south-east of

Häggenås.

In the south-west

storsjön area the thickness of the

Isö

Limestene

decreases rapidly.

Across a distance of approximately

2 km within a

tectonic unit, the thickness changes from about

1 2m to

1. 2- 0.8 m;

in seetians through the same interval about 8 00 m further north-west,

the unit is missing.

This thinning-out can be seen from the village

Kläppe

illong the road to

Svartbodarna, about

12 km WNW of

Oviken.

The

thin limestene wedge is rich in

rnaerafossils

(trilobites, brachiopods),

which indicate the

Zone of Asaphus

(

Asaphus) expansus

(

Karis, unpublished).

To the west the limestene is replaced

by the

Föllinge

Turbidites.

Andersö

Shale

This formation, formerly termed

Ogygiocaris

Shale, is

widely distributed in the

Lo wer

Allochthon.

In the type area on the

northern shore of

Andersön the formation rests on the pro bable

Folkes­

lunda equivalent of the Isö

Limestone.

According to Hadding

(1913)

the lower part comprises a dark shale with limestene lenses

(

Zone of

Glyptograptus �eretiusculus, about

6 m), the middle is dark, bedded

limestene

(l .5- 2 m), and the upper part is dark shale with lenses of

limestene and sandy interealatians becomin g particularly common towards

the top

(more than

10 m; Zone of

Nemagraptus gracilis and

Zone of

Dicranograptus clingani in the upper part).

The identification of the

Zone of

Dicranograptus clingani has been quest�oned and seems improbable.

West, north-west and north of the central

storsjön area the shale is

generally thinner.

The

Zone of Glyptograptus teretiusculus is the most

widely developed.

Throughout the outcrop area, the formation commonly

shows graded bedding indi cating turbidite sedimentation.

The thinly

laminated units of the central

storsjön area represent more distal

turbidites.

Limestene lenses or layers commonly separate graded units

on

Andersön and

Norderön.

On

Andersön and especially on northern

Norderön, eyeles of graded siltstone 1- 5 cm thick occur throughout the

sequence.

On Norderön there are also thicker eyeles 1 0- 15 cm thick in

58

the lower part, and a few beds of greywacke 0.5- 0.8 m thick in the upper

part of the formation.

On

Norderön channel-fillings, l m wide and filled

with

cross-bedded coarse silt, have been observed.

The turbidite sedi­

mentation in the Andersö

Shale appears to be

related to the

Oviken and

other antiforms

(see also

Föllinge formation).

In the beds of the

Zone of

Glyptograptus teretiusculus some limestone

lenses are rich in shelly ma crofossils, parti cularly trilobites such as

Ogygiocaris sarsi regina

Henningsmoen, Q. �rsi lata

Hadding,

Pseudo­

megalaspis patagiata

rörnquist), Nileus sp., Botryaides brann� (

Sars),

Triarthrus humilis

Hadding, Robergia microphthalm� (

Linnarsson}, and

Telephina

(

Telephi_na) bi cus..PJ2 (Angelin).

In the lower part of the

Zone of Nemagraptus gra cilis, the dark bedded limestone contains a mixed

shelly and graptolite fauna, chara cterized by

Telephina

(

Telephops)

biseriata

(Asklund) and was previously referred to as

Biseriata

Limestone.

From the shaly equivalents of the

Zone of Nemagraptus gracilis,

Ogygio caris sarsi lata, Botryaides efflozescens

(Hadding), Robergi�

mi crophthalma, Telephina

(

Telephop�) granulata

(Angelin) and other

trilobites have been recorded.

In the area west and north-west of

Hammerdal, 30-4 0 km north of

Östersund,

a sequen ce of dark shales resting on

Isö

Limestone has re cently been

shown to

contain graptolites of the

Zone of Didymograptus murchisoni,

and thus

comparable to the

Upper

Didymograptus

Shale.

However, on

lithological

criteria this shale is diffi cult to distinguish from the

Andersö

Shale and it is mapped within this formation.

The fairly soft

shale is isoclinally folded throughout the area.

In seetians along a

road south-east of

Björvallen, 1 2-1 5 km west of

Hammerdal, the thi ck-

ness of the unit is estimated to be at least

1 2 m.

There are indi cations

of a

comparable shale in the lower part of the

Lower

Allochthon in the

Östersund area.

The lateral facies relationships of the shales with

limestone and turbidite developments are not yet studied

(

L. Karis,

u n pu b l i s hed).

The upper boundary of the Andersö

Shale is at present difficult to

define.

In same areas the shale is overlain by

Dalby equivalents

(see below) while in

other areas, for example at

Lugnvik in

Östersund,

the

Andersö

Shale grades inta the

Örå

Shale without an intervening

59

limestene

{Stop 3:6).

Dalby equivalents

On the islands of

Frösön and

Andersön, and pro­

bably also on
lain by a limestene unit of argillaceous bedded and nodular calci­

Verkon farther to the west, the

Shale is over­

Andersö

The lower, bedded part is very poor in fossils whereas the
lutites.
upper, nodular limestene has yielded fossils indicating a earrelation

with the upper

Dalby

Limestone.

The limestene is overlain by

Örå

Shale.

In the western

Lit-Häggenås area, about

15 km north of

Östersund, the

Andersö Shale is overlain

by bedded, dark grey limestene with distinct

argillaceous intercalations.

The limestone, at least

5. 4 m thick,

contains

Asaphus

(Neoasaphus) cf. ludibundus and

Echinosphaerites

aurantium.

The apparently limited distribution of upper

Dalby equiva­

lents may chiefly be a reflection on the generally poor exposure through

this interval.

At

Lugnvik {Stop 3: 6), the complex of bentonitic

beds, which in the

central confacies belt marks the

boundary between the

Dalby and Skagen

Limestones, occurs

within a

continuous sequence of dark shales.

A few

tephra layers from the same camplex have recently

been recorded in the

Örån seetian

{Stop 3: 3).

Örå

Shale

The

Örå

Shale

consists of dark grey to

black, partly cal­

careous, silty, graptolitic shale with interealatians of dark grey

limestones.

In the type section, about

100 m downstream from the bridge

where the

storhögen road

erosses the river of

Örån

(Häggenås area;

Stop 3: 3), the unit is at least 9 m thick.

The

lower boundary is not

exposed, and the centact with the overlying

Slandrom

Limestene is

obscured

by falding and thrusting.

In the central

Lockne area, shales

of similar lithology and fauna overlie

D�by Beds, and there are also

largely comparable relationships in the

Lower

Allochthon.

The

Örå

Shale occurs widely in the central and northern

storsjön areas

without significant lithological variation.

Thorslund

(

1940) used the

term Upper

Chasmops

Beds for this unit.

Its equivalents in the Siljan

district and in

Östergötland are the Skagen and

Moldå topoformations.

A silty shale-mudstone in approximately the same stratigraphical position

60

as the

Örå

Shale e ecurs beyond the area where the Slandrom Limestene

is developed

(e.g. at

Lugnvik, Stop 3: 6 and localities 1 5 and 25 km

north-west of

Lugnvik). However, the stratigraphical range of this

mudstone facies is uncertain, and it may incorporate equivalents to

the Fjäcka, Slandrom and upper

Dalby Formations. For practical

purposes these shales are included in the

Kogsta

Formation.

The graptolite fauna in the

Örå Shale indicates the

Zone of

Dicrano­

graptus clingani {

Thorslund

1 940 recorded the index fossil itself)

and possibly also the upper part of the

Diplogr�ptus multidens.
Trilobites are not uncommon in the inter bedded limestones, particularly

Zone of

Triarthrus linnarssoni Thorslund, Ampyxell� cf. aculeata

(

Angelin), and

others.

Slandrom Limestene

The lithology is similar to that in the autoch­

thonous sequence.

In the

Lower

Allochthon the Slandrom Limestene

eecurs mainly in two regions, in the

Skute and

Lit-Häggenås areas.

In intermediate areas the corresponding portion of the sequence is

developed as a dark silty shale with thin inter ealatians of dark

limestone.

At

örån the thickness of the formation is 7.3 m {Thorslund

1940).

Föllinge greywackes and turbidites

The formation is camposed of a

rnanotanous sequence of greywackes and silty mudstones, mostly showing

graded bedding

{Stop 4:2).

In some areas the thickness is in the order

of several hundreds of metres.

The formation has its maximum develop­

ment and coarsest sediments

(locally including conglomerates) in the

south-west, around the

Olden and other antiforms

Oviken antiform, and in the north towards the
Östersund and

(Fölling-Holmsö area north of

Tåsjö area, north

in the
areas is a belt extending roughly from Östersund to

Jämtland and south

Lapland).

Between these

Offerdal along

Lake Näldsjön in which equivalent

beds are considera bly finer grained.

The oldest dated Föllinge beds occur east of the
Iffelnäs, approximately 4 km south-east of Hallen church, basal Föllinge

Oviken antiform.

At

greywackes resting on the

Töyen

Shale have yielded graptolites of the

Zone of P_�_�_l_9_g_�_ap�� �ngust� fo�� �� ongatu�.

On the road from

Kl äppe

to

Svartbodarna, approximately

1 2 km WNW of

Oviken, the

Föllinge beds

61

rest on

Töyen Shale of

Didymograptus hirundo

Zone age.

Over large areas

the

Föllinge turbidites are divided by a tongue of

Andersö Shale

west of

(mainly the

upper part.

Zone of Glyptograptus teretiusulus) into a lo wer and an
The tongue of shale can be seen at Hallen church, 28 km
10 km west of Föllinge, where it divides
Föllinge turbidites into parts of approximately equal thickness.

Östersund, and about

the
In the transition from turbidites to shaly siltstones of the
Formation in the
shale development has not been observed elsewhere.
10 m thick, and graptolites indicate the
than

Mattmar area a fossiliferous shale is developed.

The shale is less
Diplograptus

Zone of

Kogsta

This

multidens.

In the

Alsen area

15 km north of

Mattmar and 3 km north­

west

öf

Kogsta the transition from the

Föllinge grey wackes

Kogsta

to the
Formation has yielded graptolites indicating the

Siltstone covers about 3m of beds, and the basal

Zone of

Dicranograptus

Kogsta

clingani.

Kogsta Siltstone

In the

west the

Kogsta Siltstone rests on the

Föllinge

turbidites, and is developed as a
siltstones, about 30-40 m thick.
about 1 2 km south-west of

rnanotanous sequence of dark, shaly

The type seetian is in the

Alsen area,

Offerdal, where the basal part of the forma­

tion has yielded graptolites indicating the

Zone of

Dicranograptus

clingani and the top is overlain by the

Ede

Quartzite.

Recent callec­

tians here suggest that the upper

Kogsta beds contain a Hirnantian

brachiapad fauna

(M.G. Bassett, pers.

comm.), and similar faunas are

now known from several localities across the
1 2 km south-west of

Änge, Tretaspis seticornis and

Lo wer

Allochthon.

About

Diplograptus pristis

have been found in the lo wer part of the

Kogsta

Formation.

In western areas where the Slandrom

Limestone is developed, silty

Kogsta beds rest directly on the limestone. However, in eastern areas

(e.g. in the rail way seetian at

stengärde, Skute area), a dark shale is

developed above the Slandrom

Limestone and

within the basal

Kogsta

Formation.

This shale is equivalent to the

Fjäcka

Shale and the higher

siltstones are roughly equivalent to the

Jonstorp Formation in the

Siljan district, except that they probably also include beds of early

Hirnantian age.

The lo wer

Kogsta

Formation in siltstone facies is mostly poor in fossils;

in addition to

Tretaspis seticornis and

Diplograptus pristis, it includes

6 2

Tretaspis latilimbus

(

Linnarsson) and

Dionide euglypt� (

Angelin).

The

upper part of the formation is known to contain

Dalmanitina

(�ucronaspis)

mucronata

(

Brongniart) together with

Hirnantian brachiopod faunas.

Within the

Upper

Ordovician part of the

Kogsta

Siltstone a conspicuous

limestone unit is developed locally.

It has its maximum development

in and immediately north of

Östersund.

In the north-eastern outskirts

of

Östersund

Dicellograptus camplanatus has been recorded from under­

lying beds which suggests that the limestone may be roughly equivalent

to the

Öglunda

Limestone of the

Jonstorp Formation.

The

Kyrkås

and

Ede

Quartzites

Coarse quartzite siltstones of two

different developments conclude the

Ordovician sequence.

The

Kyrkås

Quartzite occurs in the east; its westernmost occurrences are at

Landsom, approximately

15 km north-west of

Östersund, and on the islands

of

Andersön and Frösön.

The main development of the

Ede

Quartzite is

in the west; a similar thin quartzite unit is

known from south-east of

the westernmost occurrences of the

Kyrkås

Quartzite, for example at

stengårde in the

Skute area.

West of its major outcrop area, Ede

quartzite is present below the

Middle

Allochthon

Are amphibolites.

Törnebohm’s

(1896) interpretation of nappe displacement in

Jämtland

involved his recognition of this quartzite unit here.

The maximum known thickness of the

Kyrkås

Quartzite is

45 m.

Of the

35 m at

Rannåsen

{Stop

3:5), at least the lower

15 m is

Ordov ician.

The formation is composed mainly of
itic siltstones with interealatians of fine, shaly siltstone.

grey, thin bedded, coarse, quartz­

The

bedding is disturbed by occasional shallow channels filled with mud­

flake

conglomerates.

Sedimentary structures include oscillation and

current ripples, megaripples in some thicker bedded portions of the

sequence, cross-bedding and mud cracks.
large-scale slumping has been observed about 1 2 m above the base of

Storhögen, west of

At

Kyrkås,

the formation.

The

Ede

than

6 m

Quartzite
thick. Generally its lo wer part, 1.5 to 2m thick (rarely

(Stop 4:1) is a much thinner unit, usually not more

up to

3.8 m, e. g. Mattmar), is a bluish-grey, massive

(primarily thi ck

bedded) quartzite.

The

upper part

consists of

thin bedded siltstones,