Discussion on relationships between the Cumberland Bay and Sandebugten Formations, South Georgia, and some tectonic implications

Abstract

P. W. G. Tanner writes: In their review of the Cumberland Bay (CBF) and Sandebugten (SBF) formations on South Georgia, Turnbull & Craw (1988) draw many conclusions regarding the sedimentology, petrography, and metamorphism of the two sequences with which I am in agreement. However, they neglect to consider all possible models for explaining the structural and stratigraphical relationships between the two units and discuss only two possibilities: either the turbidite sequences are broadly contemporaneous and derived from separate source areas on the continental (SBF) and island arc (CBF) sides of the basin; or they represent different structural levels within a single sequence derived solely from an evolving island arc and its sialic foundations. Historically, the debate has been more wide-ranging and opinion has fluctuated over the past 70 years between the idea of a single, conformable sequence and that of two sequences of different age separated by an unconformity or thrust (for reviews, see Stone 1980 and Tanner 1989 ). The basic problem is that the CBF and SBF consist of grey, turbiditic sandstones and slates which appear similar in the field and can only be separated by careful structural and lithological mapping, and thin-section examination. A down-plunge profile drawn nearly parallel to, and a few kilometres northwest of Turnbull & Craw's section BB' (fig. 8) and based upon the computed means of planar and linear data ( Tanner 1989 ) displays the main features of the two formations (Fig. 1). Significantly, there is no evidence of a downward structural or stratigraphic transition between the