OR/14/040 Geological Structure

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Stone, P. 2014. Recent contributions on Falkland Islands bedrock geology, with an inventory of representative lithostratigraphical specimens held by the British Geological Survey. British Geological Survey Internal Report, OR/14/040.

An unusual addition to the structural features of the Falkland Islands was the east-west sinistral shear zone noted by the author in February 1999, coincident with the steep shoreline of Ordnance Point to the north-east of Stanley (Figure 6). The strata affected are quartzites of the Port Stanley Formation. Steeply plunging to vertical folds, tight to isoclinal and ranging in wavelength and amplitude from 1 m up to about 20 m, occur in an east-west zone at least 30 m wide. The smaller folds commonly form syncline-anticline pairs indicating a consistent sinistral shear sense. The larger fold hinges (Figure 7) are generally isolated within an anastomosing fault network. The D1 fold hinges elsewhere in the area are sub-horizontal, but appear to steepen as the shear zone is approached. No details of the Ordnance Point shear zone have been published, but a summary report was lodged with the Department of Mineral Resources, Stanley, dated 15 February 1999. That report is reproduced here (see Appendix 3 – Notes on the zone of steeply plunging folds at Ordnance Point).

Figure 6 Location map for the Ordnance Point sinistral shear zone. Note that Yorke Point is currently inaccessible (within an Argentine minefield from the 1982 invasion) and so could not be checked for a likely continuation of the shear zone structures.

Generally in recent years, the examination of the onshore structural geology of the Falkland Islands has been complementary to the more extensive analysis of the Late Jurassic and younger offshore basins. Much offshore seismic data is now available but most of the interpretations arising from their analysis are commercially confidential and unlikely to be generally available for some time.

Figure 7 Two examples of steeply plunging fold hinges within the Ordnance Point sinistral shear zone.

Combining the available offshore data with onshore observations, Hyam and others (2000)[1] established a structural history for the Falkland Sound Fault. They stressed the geological contrasts between East and West Falkland and from these established factors, together with new data on thermal maturity and kerogen facies, calculated a structural relief of 6–8 km across the Hornby Mountains anticline at the eastern margin of West Falkland (Figure 1). This has developed by intermittent downthrow to the south-east across a major sub-anticline fault from Devonian to Jurassic times. Velocity and gravity data from a seismic line to the south–west of the Falkland Islands was interpreted to show a similar basement structure to that underlying West Falkland. On this evidence the Falkland Sound Fault — or more properly the Hornby Mountains Fault — continues offshore as a series of en echelon fault segments for at least 60 km to the south–west of the archipelago. The conclusions presented were partly based on work carried out as part of a PhD project by Hyam (1997)[2] at Southampton University.

Structural contrasts between East and West Falkland were also investigated by Thomson and others (2002)[3] who used apatite fission track analysis and vitrinite reflectance data to establish a thermal history for the Falklands sedimentary succession. Three discrete episodes of heating and cooling were established. Initial cooling in the Late Permian was restricted to West Falkland and was the result of differential uplift and erosion of West Falkland relative to East Falkland. Subsequent Early Jurassic cooling of both East and West Falkland was associated with the plume-related thermal uplift that preceded the break-up of Gondwana and the opening of the Atlantic Ocean. There was then renewed heating during the Late Jurassic to Early Cretaceous interval that Thomson and others relate, most probably, to renewed burial, with the implication that the offshore basins originally extended onto the present-day onshore area. Late Cretaceous to Early Tertiary cooling followed as uplift and erosion accompanied regional structural developments at the margins of major offshore features on the Falklands Plateau.

There are two potentially significant adjuncts to the Late Jurassic to Early Cretaceous phase of heating indentified by Thomson and others (2002)[3]. Firstly, the discovery of Early Creatceous dykes since, at the time they wrote only Early Jurassic minor intrusions were known from the Falkland Islands. Subsequently, an Early Cretaceous dyke swarm was identified by Stone and others (2008)[4] and correlated with magmatism in the offshore area by Richards and others (2013)[5].

This extensional magmatism may well have contributed to an elevated Early Cretaceous heat flow. More information on the Cretaceous dykes is give below, in the next section of this report.

The second point to be made in the context of the thermal history proposed by Thomson and others (2002)[3] relates to their proposal that the offshore basins originally encroached onto what is now the onshore area. As currently interpreted, there are no post–Permian strata present on the Falkland Islands, whilst the oldest strata known from the offshore basins are Jurassic. Here, speculatively, attention is drawn to the outcrop on the north coast of East Falkland of the Limpet Creek Member of the Port Stephens Formation. This unique unit was defined by Aldiss and Edwards (1998[6], 1999[7]) who nevertheless noted its anomalous position and lithology — a relatively soft, brown micaceous sandstone contrasting with and apparently underlying the much harder quartzo-feldspathic sandstone of the main Port Stephens Formation. The latter is no younger than Early Devonian in age and may well be Silurian. The stratigraphical relationships are not well-defined and the outcrop of the Limpet Creek Member lies within a plexus of faults tranding north-west to south-east that would appear to link with the offshore faults defining the southernmost extremity of the North Falklands Basin (Figure 8). Perhaps it is just possible that the Limpet Creek Member is of Permian or Mesozoic age and comprises the youngest onshore strata, forming a faulted outlier linked to the succession in the offshore North Falkland Basin, rather than the oldest onshore strata, as a position at the base of the Port Stephens Formation would imply. Further investigation might prove fruitful.

Figure 8 The onshore-offshore geological relationships at the north coast of East Falkland showing the spatial association of the Limpet Creek Member, supposedly forming the base of the Port Stephens Formation (cf. Figures 2a and 2b), with the fault-controlled southern extremity of the North Falklands Basin. Figures provided by Dr Phil Richards.

In addition to the published studies of Falklands structural geology discussed above, the theme was also addressed in an unpublished PhD thesis by Hodgkinson (2002)[8], completed at the University of Birmingham. Hodkinson recognised four phases of deformation (D1–4) with particularly significant results reported in association with the first and third. The main, south-verging fold and thrust belt that dominates the structure of northern East Falkland includes developments of foliated cataclasite within the thrust zones. Secondary, syn-tectonic mica within a cataclasite zone at Cow Bay (Figure 1) gave an Ar-Ar radiometric date of 278±8 Ma confirming initial (D1) deformation of the Falklands succession to have been active in the Early Permian. Hodgkinson’s second deformation phase (D2) was associated with formation of the Falkland Sound Fault. Thereafter, the intrusion of Early Jurassic dykes was syn-tectonic with a transtensional phase (D3) so that shear was imposed on the dykes during cooling and influenced the formation of minerals that were to be fundamental to palaeomagnetic modelling. This, Hodgkinson thought, cast doubt on the reliability of results previously used to establish the rotation of a Falklands microplate (Taylor and Shaw 1989)[9], results that he was unable to replicate. Hodgkinson related the east-west sinistral shear zone at Ordnance Point and dextral shear along Falkland Sound Fault as conjugate D3 effects. The final phase of deformation (D4) was thought to involve normal reactivation of the D1 thrust structures as part of the regional, Cretaceous extension responsible for the development of the North Falklands Basin.

References

  1. HYAM, D M, MARSHALL, J A E, BULL, J M, and SANDERSON, D J. 2000. The structural boundary between East and West Falkland: new evidence for movement history and lateral extent. Marine and Petroleum Geology, Vol. 17, 13–26.
  2. HYAM, D M. 1997. The Falkland Island and their position within Gondwana. Unpublished PhD thesis, University of Southampton.
  3. 3.0 3.1 3.2 THOMSON, K, HEGARTY, K A, MARSHALLSEA, S J, and GREEN, P F. 2002. Thermal and tectonic evolution of the Falkland Islands: implications for hydrocarbon exploration in the adjacent offshore region. Marine and Petroleum Geology, Vol. 19, 95–116.
  4. STONE, P, RICHARDS, P C, KIMBELL, G S, ESSER, R P, and REEVES, D. 2008. Cretaceous dykes discovered in the Falkland Islands: implications for regional tectonics. Journal of the Geological Society, London, Vol. 165, 1–4.
  5. RICHARDS, P C, STONE, P, KIMBELL, G S, MCINTOSH, W C, and PHILLIPS, E R. 2013. Mesozoic magmatism in the Falkland Islands (South Atlantic) and their offshore sedimentary basins. Journal of Petroleum Geology, Vol. 36, 61–74.
  6. Aldiss, D T, and Edwards, E J. 1998. Geology of the Falkland Islands. Solid Geology 1:250 000. Two sheets, East and West. British Geological Survey for Falkland Islands Government.
  7. ALDISS, D T, and EDWARDS, E J. 1999. The Geology of the Falkland Islands. British Geological Survey Technical Report, WC/99/10. 135pp.
  8. HODGKINSON, R. 2002. Structural studies in the Falkland Islands, South Atlantic. Unpublished PhD thesis, University of Birmingham.
  9. TAYLOR, G K, and SHAW, J. 1989. The Falkland Islands: New palaeomagnetic data and their origin as a displaced terrane from southern Africa. In: Hillhouse, J W. (ed.) Deep structure and past kinematics of accreted terranes. Geophysical Monographs, No. 50, 59–72.