The Ahlmannryggen group, the viddalen formation and the associated igneous rocks in the viddalen area, western droning Maud land, Antarctica

Abstract

Nunataks within an area of approximately 3 400 km2 between latitudes 71°50' and 72°35' south and longitudes 1°30' and 4°00' west were mapped geologically by the 12th South African National Antarctic Expedition during the summer of 1971-72. The area is underlain mainly by subhorizontal arenaceous and argillaceous sediments, extensively intruded by thick mafic sills and dykes of Precambrian age. An isolated succession comprising agglomerate, tuff and lava flows with intercalated sediments in its upper part is exposed in the east and south. A new lithostratigraphic subdivision is proposed, based on detailed investigation of field relationships combined with subsequent petrological studies and radiometric age determinations. The oldest rocks in the area belong to the Pyramiden Formation which consists mostly of feldspathic graywacke and siltstone. Correlates of this formation were discovered at Kjolrabbane, Babordsranten and at Nashornet and Viddalskollen. At the latter locality the sequence is subdivided into a Lower Member, 140 m thick, deposited under low-energy, deep-water conditions, and an Upper Member, 250 m thick, which was deposited in shallower water under higher energy conditions .. The overlying Grunehogna Formation has the widest distribution of all sedimentary formations in the area and consists of alternating persistent beds of light-coloured arenite and dark-coloured argillite. This succession is conformably overlain by the H6gfonna Formation which is subdivided into a lower, Borgmassivet Member, consisting of feldspathic quartzite, shale and jasper-bearing conglomerate, and an upper, Jekselen Member which consists of calcareous, jasperbearing arenaceous sediments. All the formations mentioned above are assigned to the Ahlmannryggen Group. It is suggested that the Raudberget Formation, which is not exposed in the area investigated, is also included in this Group. The volcanic-sedimentary succession at Tindeklypa and Istind is believed to overlie the Hogfonna Formation conformably and is collectively called the Viddalen Formation. It is subdivided into two conformable members: a lower, Tindeklypa Member, some 500 m thick, which consists of agglomerate with subordinate tuff and andesitic lava flows, and an upper, Istind Member, 340 m thick, consisting of alternating feldspathic quartzite, agglomerate, tuff and lava flows. (ii) A continuous upward gradation from plutonic through hypabyssal to volcanic rock (near-surface intrusions) was detected in the 1 700 My mafic sill (Krylen Intrusions) at Jekselen. The lava flows in the Viddalen Formation are believed to be the volcanic equivalent of the Krylen Intrusions. Two samples of Krylen Intrusions from Jekselen and Istind were submitted for 40 Ar/39 Ar wholerock age determinations. Ages of 1 339 ±. 55 My and 603 ±.. 12 My respectively were computed, both being minimum ages resulting from complete overprinting by metamorphism. The majority of the large mafic intrusions in the area are believed to have an age of around 1 000 My and are called Ytstenut Intrusions. Minor felsic intrusions are also included under this heading as they were found to represent only local remobilised differentiation products formed by the reaction between the mafic intrusions and the sediments. Two samples of Ytstenut Intrusions from Grunehogna and Ytstenut were submitted for 40 Ar/39 Ar whole-rock age determinations. Minimum ages of 83 2 .±. 2 and 924 .± 4 My respectively were obtained; the latter age was regarded by the analysts as being close to the real date of intrusion. The youngest rocks in the area are represented by minor dolerite dykes of Jurassic age and are called Post-Ytstenut Intrusions.