OR/14/040 Appendix 3 Notes on the zone of steeply plunging folds at Ordnance Point
| 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. |
Field report deposited with the Department of Mineral Resources, Stanley, Falkland Islands, in February 1999. The four stereograms that accompanied the original report are reproduced here as Figure 10.
a. The regional fold attitude as illustrated by the Mount Challenger area, 20 km west of Stanley.
b. Fold attitude in the southern part of the Ordnance Point Promontory.
c. Fold attitude close to the Ordnance Point sinistral shear zone.
d. Fold attitude within the Ordnance Point sinistral shear zone.
Steeply to vertically plunging, tight to isoclinal folds occur in an east-west zone trending west from Ordnance Point, the headland to the north-west of Yorke Bay and Gypsy Cove. The folds are seen at the foot of the cliffs, which form the southern margin of the zone, and in the rocks around sea level. The northern margin of the zone is not seen but it is at least 30 m wide, across strike. The folds range in wavelength and amplitude from less than 1 m up to about 20 m. One isolated area of anomalous bedding strike suggests the presence of a larger fold with wavelength up to about 50 m. The larger folds tend to be cut up by a broadly east-west, anastomosing and vertical fault system so that only isolated hinges are seen. The smaller folds commonly form complete anticline-syncline pairs with a consistent sinistral vergence, i.e. they are of "S" pattern looking down plunge. Fold style is mostly tight to isoclinal but there are some more open flexures of strike, still with sinistral vergence.
The Fl fold geometry south and inland from the Ordnance Point sinistral zone is the same as that seen elsewhere along strike in the folded Port Stanley Formation rocks. Stereogram 1 shows comparative data from the Mount Challenger area with data from the area south of the Ordnance Point zone summarised in stereogram 2. There is no significant difference in the overall F1 fold pattern, which has been illustrated elsewhere by, for example, Curtis and Hyam. However, close to the Ordnance Point zone there is a tendency for the Fl hinge plunge to increase to moderate and westerly. This is illustrated in stereogram 3. Some of the spread in bedding poles plotted on steteogram 2 might be a precursor to this development. There is then an abrupt northwards change in hinge plunge into the Ordnance Point zone; in one case an Fl hinge plunging west at about 30 degrees is only 2 m across strike from a series of vertically plunging folds. Structural geometry in the zone of steeply plunging folds is illustrated in Stereogram 4. The plunge of some minor hinges is quite variable.
There are two possible mechanisms for the production of the Ordnance Point sinistral zone:
Progressive but irregular steepening of F1 hinges, northwards across strike, until they become vertical. There has apparently been some such steepening towards the Ordnance Point zone. It would most likely have occurred in association with lateral east–west shear but the sense of that shear would not be recorded in the preserved fold geometry.
Folding subsequent to Fl in association with sinistral shear, perhaps with some steepening of adjacent F1 hinges.
The evidence is permissive of either explanation but I favour the second on two rather weak counts. Firstly, the intensity of folding (and local variation in hinge plunge) is much greater in the Ordnance Point zone than is the case in F1 fold zones elsewhere; this reasoning is a bit subjective. Secondly, there is one possible example of a minor Fl hinge with a moderate plunge refolded about a steeply plunging hinge; the relationships are not clear and could do with careful re-examination. If the east–west sinistral shear interpretation is correct, it probably occurred as a D3 phenomenon conjugate with the dextral, NE–SW lateral shear recognised particularly in the Falkland Sound area. The regional east–west strike of the Port Stanley Formation quartzite beds would have provided a strong pre-existing control on movement so the Ordnance Point zone could be contemporary with NW–SE sinistral D3 faults which have been sporadically recorded elsewhere. However, I do not think that steeply or vertically plunging folds have been previously observed so they may represent an additional structural element in the regional pattern.
Phil Stone
15 February 1999