2. INTRODUCTION
The Billefjorden Fault Zone is a major tectonic lineament striking north-south through central Spitsbergen. The fault zone is covered by water or sediments for much of its length. A region where it is well exposed is between Austfjorden and Billefjorden. This project was based south of Austfjorden, near a glacially dammed lake, Alandsvatnet.

Many Geologists have suggested Late Devonian (370 Ma) strike-slip movement across the Billefjorden Fault Zone^{1 - 4} (similar to the present day San Andreas Fault - California). Their evidence includes metamorphic facies correlation and the absence of Devonian rocks to the east of the fault zone. Movements as large as 1000km have been suggested. More recently, Geologists have concluded that there is no evidence for such motion, and thrust faulting has been suggested^{5 - 7}.

The rocks come in roughly north-south striking bands . Devonian Old Red Sandstone lies in the west of the area. It becomes overturned next to its eastern, faulted-bounded contact with the metamorphic Hecla Hoek. The Hecla Hoek forms a central, fault-bounded sliver. In the east of the area are Carboniferous (310 Ma) shallow marine and river deposits.

There are several strands of the Billefjorden Fault in the area. All trend approximately north-south.

The terrain ranges from 100m to 962m. The large glacial valley of Alandsdalen was covered with Recent alluvium. The mountain sides had varying degrees of exposure, and otherwise were steep scree. Ridges provided valuable lines of consistent exposure. Ice covered a significant area only in the south, otherwise it was restricted to gullies, with the exception of the south-east side of Odellfjellet where there was a mini-glacier.

3. ROCK TYPES

3.1 Hecla Hoek
The Hecla Hoek is a 1100 Ma metamorphic unit that belongs to the Harkerbreen Formation². It forms a sliver from the north-east of Odellfjellet southwards, passed the east face of Sentinelfjellet. It reaches a maximum thickness of 800m along 78^o56'45''N.

3.2 Devonian Old Red Sandstone
The Devonian has been subdivided as follows (after Reed et al 1987):

Old Red Sandstone	Grey Hoek Group	Gjelsvikfjellet Formation
	Skamdalen Member
	Wood Bay Group	Stjordalen Formation Keltiefjellet Formation Kapp Kjeldsen Formation

Wood Bay Group
The Wood Bay Group consists of a single, large-scale fining-up sequence, from sandstones to mudstones. Reed et al (1987) suggested that the sandstones are from a braided river setting. This evolved to a flood-plain and finally estuarine setting. There are root horizons and other terrestrial indicators such as a calcrete bed (representing a fossil soil).

Grey Hoek Group
The Skamdalen Member of the Gjelsvikfjellet Formation outcrops for only 200m on a ridge in the north-west corner of the mapping area. My recognition is based entirely on the description of Reed et al (1987), that is: predominant (90%) grey or green mudstones and shales.

Unit X
I recognised the unit as a grey quartzite interbedded with a grey or red shale. This unit is often highly deformed and the thickness ranges from zero to 1000m.

3.3 The Carboniferous
The Carboniferous has been subdivided as follows (after Reed et al 1987):

Carboniferous of Spitsbergen	Gibsdalen Group	Ebbadalen Formation	Member c Member b Member a
	Billefjorden Group	Svenbreen Formation Horbybreen Formation

The Carboniferous is less deformed than the Old Red Sandstone. I was able to make a stratigraphic log of the sequence.

3.4 Recent Sediments
Drift covers most of the large valley floors, and mountain sides. Alluvium, lacustrine deposits and moraine are all found in the valley floors. The mountain-side cover is talus, alluvium, and moraine - talus being by far the major constituent.

4. STRUCTURE
The geological structure in the mapping area consists of major north-south trending faults, and more localised structures.

4.1 The Karnakfjellet Fault
The Karnakfjellet fault is present from the north to the south of the mapping area. It divides the Kapp Kjeldsen Formation to the west from the Unit X to the east. The fault strikes 002^o and dips 52^o east. The fault's last movement was extensional.

4.2 The Balliolbreen Fault
The Balliolbreen fault is present from the north to the south of the mapping area. It has a cliff of Hecla Hoek to its east except when it is beneath valley floor drift, ice, or the Horbybreen formation. It is the principal strand of The Billefjorden Fault Zone. It shows net thrust movement, and divides the Old Red Sandstone from the Hecla Hoek. The fault trends 355^o and dips 71^o to the east.

4.3 The Odellfjellet Fault
The Odellfjellet Fault separates the Hecla Hoek from the Carboniferous. The fault never outcrops. The evidence only points to the fault trending approximately north-south, and dipping steeply. The dip direction remains unknown.

4.4 Internal Structures of the Devonian
The Devonian shows many local deformation structures. The majority of these show east-west shortening.

4.5 Internal Structures of the Carboniferous
The Carboniferous is less deformed compared to the Devonian. It lies in uniformly dipping blocks. The two major blocks in the mapping area were separated by the Alandsvatnet Fault. Bedding in both blocks is horizontal on 330^o. The northern block dips at 50^o to the east while the southern block dips at 28^o to the east.

5. GEOLOGICAL HISTORY
Approximate ages are taken from Harland et al (1974). This section contains terms that may not make sense from this text, and sequences of events that I have not justified here - they are all justified in the full geological mapping report.

1050-475Ma	(i) The Harkerbreen Formation was deposited, buried and metamorphosed.
475-395Ma	(ii) The Harkerbreen Formation was uplifted to the surface.
395-359Ma	(iii) A Devonian Basin formed3 and the Old Red Sandstone was deposited. The Billefjorden Fault Zone was at the eastern edge of this basin, and the sandstone thinned rapidly to the east in the area7. The Wood Bay Group was laid down as part of a large scale fining-up sequence from sandstones to siltstones.
359-345Ma	(iv) After sedimentation, the area underwent a period of east-west compression. This folded and overturned the Grey Hoek and Wood Bay Groups. The Hecla Hoek basement was brought up relative to the Old Red Sandstone, and juxtaposed next to the deformed Wood Bay and Grey Hoek Groups. Firstly the Balliolbreen Fault was active. Then the Karnakfjellet Fault broke through at a lesser angle. (v) There then followed a period of uplift and erosion.
345-245Ma	(vi) During the Carboniferous, east-west extension occurred. This formed a graben that filled with sediments. The Karnakfjellet fault juxtaposed the Horbybreen Formation and the Kapp Kjeldsen Formation. The Odellfjellet fault was also active, and a the thick pile of syn-sedimentary deposits on its east were laid down. These thin rapidly east and south4. (vii) Extension continued causing faulting within the Carboniferous itself - such as the Alandsvatnet Fault, and the faults on the east ridge of Sentinelfjellet.
245-5Ma	(viii) Faulting ceased at the end of the Carboniferous, and the Permian was laid down uniformly across the area. The Permian is not observed in the area, but it can be seen to the south, and to the west, on Grapigen and Abeltoppen. (ix) There is no geological record from the Permian to the Quaternary.
5-1.5Ma	(x) During the late Tertiary the area was glaciated, and eroded.
1.5Ma-Now	(xi) During the Quaternary, the icesheets have retreated to leave the present topography, and deposit the Recent drift.

6. CONCLUSIONS
During July 1993 I mapped 17km², around Alandsdalen. Particular care was taken to record structural features that would indicate the regional strains. From interruptation of the outcrop pattern and structural information, it is possible to constrain the sense, and time, when various faults within The Billefjorden Fault Zone were active.

The results of this analysis broadly support the results of the most recent publications on the structure of the area, that is:

• Any strike-slip movement on the Billefjorden Fault Zone must have been pre-Devonian. Such movement would account for the remarkabe linearity of The Fault Zone.
• There was Late-Devonian post-sedimentary compression, causing thrust faulting, and deformation of the Old Red Sandstone.
• The last episode of deformation was during the Carboniferous, when an extensional half-graben formed, and was filled with sediments.