216 



bounded on the east by the Pre-Cambrian massif of the 

 Murray and Broken Hill regions, and on the western side by 

 a similar massif that formed the great Pre-Cambrian plateau 

 of Eyre Peninsula. The protracted subsidence of this area 

 permitted a vast accumulation of sediments of various kinds 

 which would lie thickest in the centre of the geosynclinal 

 trough, and gradually thinned towards the lateral margins, 

 where successive overlaps of sediments would take place. This 

 transgression of the Cambrian sea reached the horst-like 

 massif of Yorke Peninsula only at a late stage in the develop- 

 ment of the trough — indeed, not until shortly before or 

 during the Archaeocyathinae stage — which will explain the 

 absence of all the sediments that in the eastern areas form 

 the Lower Cambrian and lower portion of the Upper Cam- 

 brian series. 



A different explanation from the above might be given 

 by supposing that the missing beds were laid down within 

 this area and then subsequently removed by denudation 

 before the Archaeocyathinae limestones were deposited. Such 

 an explanation would involve an unconformity for the 

 Archaeocyathinae beds, in relation to the missing beds, which 

 is not borne out by comparison with other localities. Else- 

 where the Archaeocyathinae beds are uniformly both under- 

 lain and overlain by thick strata of the purple slates series 

 without any sign of interrupted sedimentation. This may 

 b>e taken as presumptive evidence that no such unconformity 

 exists on Yorke Peninsula as is implied in the above sug- 

 gestion. It is assumed that the explanation based on the 

 supposition of transgression and overlap is more in consonance 

 with the evidence. 



The Cambrian terrain on Yorke Peninsula began its his- 

 tory when the transgressing sea reached the level of the 

 ancient horst and spread itself over the prepared plateau. 

 The sea was shallow, and the advancing waves operated upon 

 the granitic and felspathic outcrops of the older rocks over a 

 wide littoral. The breaking down of these rocks led to the 

 formation of coarse felspathoid grits and beds of conglomerate 

 which are congenetic with similar arkose grits and con- 

 glomerates that form the basal beds of the Cambrian on the 

 Mount Lofty district. The chief lithological difference in the 

 two areas is that, on the Mount Lofty side the Pre-Cambrian 

 and the basal grits are rich in ilmenite, while on Yorke 

 Peninsula the latter mineral is not so extensively present 

 either in its original condition, in the Pre-Cambrian rocks, 

 or in its secondary form, as a clastic product, in the basal 

 sediments. 



