Notes on the structure of the Hornsund area - a history of prolonged reactivation.

8/1998. Compiled for Saga trip with Lance.

Harmon D. Maher Jr., University of Nebraska-Omaha, harmon@cwis.unomaha.edu

Introduction

The Hornsund area displays a long and complex history of reactivation (Dallmann, 1992) that includes: Caledonian nappe formation, Devonian and Lower Carboniferous basin development, local Serpukhovian (?) fold and thrust development (Adriabukta phase of Birkenmajer), subsequent Middle Carboniferous local basin formation with local inversion of part of the earlier basin and consequent formation of the Sorkapp-Hornsund high, development of Tertiary fold-thrust structures, and subsequent dissection by high-angle normal faults. This document points out major structures in the area that demonstrate portions of this history. More detailed descriptions can be found in Dallmann (1992) and various publications by Birkenmajer (e.g. 1964, 1977, 1984).

Figure 1: Scanned portion of Dallmann et al. (1993) map showing postion of localities mentioned. a= site of exposure of Sergeijevfjellet Fm. strata beneath Triassic strata, b=Lidfjellet and site of repetition of Mesozoic strata by thrusts, c= Savitsjtoppen and site of Liddalen fault, d=Samarinbreen syncline and lineament, e=Hyrnefjellet and site of Adrtiabukta phase deformation, f=Treskelodden, g = StarostinFjellet and site of overturned fold in Cretaceous strata, h=Rastupet and Caledonian fold nappe.

Caledonian tectonism

In the south fjord wall of Rastupet (Fig. 1, h for location) a photogenic fold nappe structure is well displayed. This structures occurs in Lower to Middle Ordovician carbonates within the basement sequence. The general basement structural grain (as defined by prevailing strikes and fold traces) is NNE-SSW, and likely influenced subsequent tectonism. Further south in this mountain complex flatlying Triassic rocks rest unconformably on these highly deformed rocks. It is interesting to note that similar age and lithology basement rocks are exposed on Bjørnøya, underlying the Stappen High.

Adriabukta deformation phase

Of particular interest is the Adriabukta phase of deformation, which will be one of the focuses of the University of Tromsø and Nebraska mapping expedition this summer. The 'type' locality (Birkenmajer, 1964) of this phase is western Hyrnefjellet (Fig. 1), where fold and thrust structures involving basement and Adriabukta Fm. (Visean) rocks, are interpreted to be unconformably truncated by Hyrnefjellet Fm. clastics (Birkenmajer, 1964). The underlying Adriabukta Fm. strata are severely deformed and locally exhibit shear zone structures. The area has been complicated by both Tertiary fold-thrust and normal fault structures.

Dallmann (1992) describes to the S the large regional Samarinbreen syncline and documents folds in Devonian strata clearly truncated by overlying Triassic strata. In 1996 we also mapped a tight syncline of Hornsundneset quartzites truncated by overlying Triassic rocks in the footwall of the Liddalen fault. These structures could also belong to this deformation phase, suggesting it is more widespread than previously known. However, evidence for similar Adriabukta phase structures elsewhere in Svalbard is unknown, even though appropriate localities exist. The larger tectonic significance of this event is uncertain, although coeval events are known from Greenland.

Development of Middle Carboniferous basins and the Sørkapp-Hornsund High

Much of this story is in the sedimentology and stratigraphy, which is addressed by Worsely (1989) and Dallmann (1992) and by others on this field trip. Exposed structures directly related to Middle Carboniferous basin formation are all but unknown to the author. However, a structural lineament can be inferred to exist to the S in the Samarinbreen area (Fig. 1), by the fact that to the E a notable Carboniferous section exists, and to the W Triassic rocks rest on basement rocks. An E-side-down, synsedimentary, normal fault could account for this disparity and some of the distinctive basin facies (coarse fanglomerates). This lineament defines the eastern margin of the high. It is also interesting to note that dissimilar basement rocks occur on either side of this glacier (Winsnes et al. 1992), suggesting possible control by an older Caledonian fault.

The western margin of the Sørkapp-Hornsund high can be easily seen along the northern shores and slopes of the Hornsundneset region as one sails into Hornsund. In the upper portion of Sergeijevfjellet, to the west, Triassic rocks rest unconformably on the Billefjorden Group, Sergeijevfjellet Fm. strata (Fig.1,2). This unit is truncated to the east where the Triassic rocks rest on the underlying Horsundneset Fm., and even further E this unit is truncated and the Triassic rocks rest on the basement. This pattern of a W dipping tilt-block has been attributed to Carboniferous half graben development with an unexposed master border fault to the W (Fig. 2). This geometry would be somewhat similar to the pattern exposed on Bjørnøya and the Stappen High (Worsley et al. in press), except there the truncating unit consists of Tempelfjorden Group strata and coeval Gipsdalen Group basin fill is preserved. In this case, both highs are primarily a result of footwall uplift in half-graben development.

Figure 2: In this schematic cross section the configuration of the Inner Hornsund trough borders is highly speculative. Also, a condensed or truncated Permian section can be found resting on the Gipsdalen Group clastics in the east.

An alternate, but quite speculative possibility is that the Hornsund-Sørkapp high originated in the 'compressional' Adriabukta event, and subsequent normal fault reactivation along an existing steep fold limb created the Inner Hornsund trough in Middle Carboniferous times.

Tertiary fold-thrust development

The more thick-skinned (basement-involved) zone of the fold-thrust belt is found well exposed in inner Hornsund. The photogenic Hyrnefjellet anticline (Fig. 3) is a primary and well known example. There is shallow structural plunge to the S that characterizes the termination of this portion of the fold and thrust belt to the S. Across the fjord at Starostinfjellet is a large E verging overturned syncline cored by a thrust (Fig. 1, 4). This structural style, of large overturned synclines dissected by thrusts is somewhat characteristic of the zone of Tertiary, thick-skinned tectonism (e.g. at Berzeliustinden, and at inner St. Jonsfjorden, Welbon and Maher, 1992). Considering that tectonic transport to the E has occurred it seems quite reasonable to root some of these Tertiary thrusts in the vicinity of the Samarinbreen lineament, consistent with control by older structures as is the case for other parts of the thick-skinned Tertiary tectonism (Maher & Welbon, 1992). There is also an interesting structural disparity across the fjord from Treskelodden to Meranpynten that is not yet fully understood.

On the north side, to the west of the Hyrnefjellet anticline, is a very complex overturned syncline that Dallmann (1992) has interpreted to be dissected by out-of-sequence thrusts. Out-of sequence thrusting has also been documented N of Isfjorden (Bergh et al. 1997) in the Broggerhalvøya area, where it can be related to a late phase of fold-thrust belt wedge development that tends to have orogen-oblique transport.

Mesozoic strata on Lidfjellet (fig.1) in southern Hornsundneset (visible from the entrance to Hornsund) are repeated by several thrusts. This represent the end of the offset continuation of Tertiary fold-thrust tectonism to the south, which is known as the Lidfjellet-Øyrlandsodden fold zone. Work in the southern portion suggests an orogen-parallel slip component may be accomodated along this zone (Bergh, pers. comm.), in keeping with a model of transpressive decoupling with orogen-parallel slip concentrated more in the hinterland area, and orogen-perpendicular in the foreland area. Given that the two fold-thrust zones flank the Hornsund-Sørkapp High it seems likely that fold-thrust development bifurcated around this mechanical impediment.

Fig. 3: Looking N from the S plunging fold closure of the Hyrnefjellet anticline to this fold's expression in the Hyrnefjellet mountain side. Ch=Hyrnefjllet Fm., Ct=Treskelodden Fm., Trv=Vardebukta Fm., J=Janusfjellet Group.

Fig. 4: View to S of thrusts (white dashed lines) on Starostinfjellet. Kh=Helvetiafjellet Fm. and Kc=Carolinefjellet Fm.. Note the steeply dipping strata are overturned.

Tertiary normal faulting

NNE to N trending normal faults are common in the Hornsund area. One of the best exposed examples is found in the slopes of Savitsjtoppen (Fig. 1) and is easily seen from outer Hornsund. Here the dark Triassic shales overlying the light grey Billefjorden Group quartzites, are dropped down some 100 m on the W side of the fault. Similar normal faults are also seen further NE along strike (e.g., at Renarodden in Bellsund, Dallmann et al. 1990), but usually are more common in the hinterland, and not as widely distributed as in the Hornsund area. The easiest interpretation is that these are related to the later transtensive motion of the Barents Shelf and Greenland as they finally parted ways.

References:

- Bergh, S. G., Braathen, A. & Andresen, A., 1997, Interaction of Basement-Involved and Thin-skinned Tectonism in the Tertiary Fold-Thrust Belt of Central Spitsbergen, Svalbard; American Assoc. of Petroleum Geologists Bulletin, v. 81, p. 637-661.

- Birkenmajer, K., 1964: Devonian, Carboniferous and Permian Formations of Hornsund, Vestspitsbergen. Studia Geologica Polonica, 11, 47-122.

- Birkenmajer, K., 1977, Triassic sedimentary formations of the Hornsund area, Spitsbergen; Studia Geologica Polonica, vol. 51, 1-74.

- Birkenmajer, K., 1981: The geology of Svalbard, the western part of the Barents Sea and the continental margin of Scandinavia. in Nairn, A. E. M., Churchkin, M., & Stehli, F. G. (eds.), The Ocean Basins and Margins, v. 11, the Arctic Ocean. Plenum Press, New York, 265-329.

- Dallmann, W. K., Hjelle, A., Ohta, Y., Salvigsen, O., Bjornerud, M. G., Hauser, E. C., Maher, H. D., Jr. & Craddock, C., 1990: Geological Map of Svalbard 1:100,000

- Van Keulenfjorden Sheet, Spitsbergen. Norsk Polarinstitutt Temakart nr. 15, 58 p, 1 plate.

- Dallmann, W. K., 1992, Multiphase tectonic evolution of the Sørkapp-Hornsund mobile zone (Devonian, Carboniferous, Tertiary), Svalbard; Norsk Geologisk Tiddskrift, vol. 72, pp. 49-66.

- Dallmann, W. K., Andresen, A., Bergh, S., Maher, H. D., Jr, & Ohta, Y., 1993, Tertiary fold-and-thrust belt of Spitsbergen Svalbard: Norsk Polarinstitutt Meddelelser # 128, 46 p. and 2 plates.

- Maher, H. D., Jr., and Welbon, A., 1992, Influence of Carboniferous structures on Tertiary tectonism at St. Jonsfjorden and Bellsund, Western Svalbard; Norsk Geologisk Tiddskrift, 72 p. 67-75,

-Welbon, A. & Maher, H. D., Jr., 1992, Tertiary tectonism, the Vegard thrust complex and basin inversion of the St. Jonsfjorden region, Svalbard; Journal of Structural Geology, 14, p. 41-55.

- Winsnes, T.S., Birkenmajer, K., and Dallmann, W. K., 1992, Geologic Map of Svalbard 1:100,000 Sørkapp, Sheet C13G; Norsk Polarinstitutt Temakart # 17.


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