Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia

Abstract

An apparently conformable sequence of metaclastics forming the upper part of the type Kanmantoo Group (Cambrian) has been mapped along 50km of coastline between Tunkalilla Beach and Middleton Beach. Sporadically occurring black carbonaceous and sulphide-rich phyllites are confined to and characterise the Brown Hill Sub-group. Associated immature "flysch-like" metasediments were deposited very rapidly in an actively subsiding basin by currents flowing mainly from the NW. Numerous pebble beds, similar to those in the underlying Inman Hill Sub-group, reflect the continuity of the Kangarooian Movements. The cross-bedded Middleton Sandstone at the top of the overlying Wattaberri Sub-group was deposited by currents directed mainly towards the east. Kanmantoo Group rocks younger than the Middleton Sandstone are unknown in the Mt. Lofty Ranges. Because the formation is intruded by the Encounter Bay Granites, a thick cover must have been present at the time of intrusion and metamorphism. Worm casts were the only fossils located in the study area and are suggestive of a marine environment of deposition. The sequence mapped forms the eastern limb of a regional anticline overturned towards the NW. Two phases of folding are recognised. F1 folds plunge shallowly towards the SSW or NE. Eastwards from the core of the highly appressed and asymmetrical regional anti­ cline, F1 folds in metasandstones become progressively more open, symmetrical and upright. Mesoscale F2 folds with E to SE plunges are confined to the eastern part of the type section. A weak crenulation of S1 is observable near Coolawang Creek, and becomes progressively stronger towards the east. The Encounter Bay Granites were intruded and had crystallised prior to the main phase of F1 deformation because thin granite sheets, concordant with bedding, developed the S1 and S2 schistosities during the folding episodes. Stability fields for the observed metamorphic mineral assemblages show that the metamorphism of the Kanmantoo Group was effected at moderate temperatures (below 540°C) and low pressures (below 3 kb). This is indicated also by the well preserved sedimentary structures found throughout this sequence of andalusite-staurolite grade rocks. Cordierite, restricted to the proximity of the granite, records the highest grade of metamorphism within the type section. Petrographic evidence suggests that cordierite crystallised during the pre­ to early syn-F1 phase, a conclusion in harmony with the suggested pre-tectonic emplacement of the Encounter Bay Granites. Petrographic evidence indicates that most of the andalusite formed during the late syn-F1 phase of metamorphic crystallisation. In the post-F1 and pre-F2 static phase of metamorphic crystallisation, some andalusite, garnet, hornblende, scapolite and chlorite grew across the S1 schistosity. The S2 crenulation cleavage, where developed, deformed porphyrohlasts of this age. Albitisation postdates the S2 schistosity. A peculiar "striped" layering, post-S1 and pre-S2 in age, occurs within the Petrel Cove Formation west of Rosetta Head and resulted from alteration adjacent to tensional fractures. Two distinct groups of basic dykes, post-S1 and pre-S2 in age, cut Kanmantoo Group metasediments.