Abstract

Drs C. R. Dalgarno & R. P. Coats write: We disagree with the hypothesis presented by Burns et al. (1977) for the origin of the breccias of the Blinman Dome and other Flinders Ranges structures. Uplift and erosion of breccia cores occurred during deposition of the rim rocks, as indicated by marginal upturning with local unconformities and onlapping conglomerates. This is associated with stratigraphic thinning on the flanks of structures during late Precambrian (Sturtian) and Lower Cambrian sedimentation. Burns et al. suggested that the conglomerates within the rim sediments ‘may have been emplaced by mass movement from topographic highs composed of Willouran rocks’, although they do not envisage origin of the (Blinman) domes until the late Cambrian orogeny when they describe a ‘brecciation front which simply migrated upwards through the Callanna Beds and overlying Sturtian tillite’. This concept is considered by them to explain blocks of basement granite within the breccias as derived in situ from Sturtian tillites containing erratics. Our alternative suggestion is that the granites have been plucked off the basement by diapirs. The Geological Survey considers that the diapir model applies to the whole province in which cross sectional views of some structures are afforded by steep dips. Burns et al. also suggested that the breccias are flat plates or tabular rafts, and that the features are similar to those found in sedimentary breccias caused by gravitational sliding. These characters do not apply to the breccias described by Coats (1964) and Dalgarno & Johnson (1968), which are related

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