PALEOZOIC CORDILLERAN GEOSYNCLINE 



79 



been clearly noted, they generally rest in angular unconformity on de- 

 formed, metamorphosed, and intruded rocks. 



The zone of maximum subsidence extends through the center of British 

 Columbia with reported thickness ranging from 10,000 feet at the south- 

 ern border to 24,000 feet in the northern part of the province. 



The Cache Creek strata have yielded Upper Permian fossils in a num- 

 ber of places but lower beds in the sequence may be Carboniferous. 



Shuswap Terrane and Orogeny 



A large complex of metamorphosed rocks in southern British Columbia 

 is known as the Shuswap terrane. Its location is shown on the map of 

 Fig. 17.14. The metamorphism has long been attributed to Mesozoic 

 batholithic processes, but now certain positive information indicates that 

 extensive parts were metamorphosed in Pre-Cache Creek time. An au- 

 thoritative summary of the nature of the Shuswap terrane by Cairnes 

 (1939) is quoted below in which he leans toward metamorphism in 

 Mesozoic time but recognizes that early metamorphism may have oc- 

 curred. 



The rocks of this Shuswap terrane are a metamorphic complex, and their 

 transformation is attributed to processes connected with Mesozoic batholithic 

 intrusions, of which those of the Nelson batholith of the West Kootenay region 

 have played a principal part. The nature of these processes is, however, not 

 entirely clear, though certain probable conditions may be surmised from the 

 available evidence. On the one hand it is apparent that, in part and over large 

 areas, the Nelson batholith, together with other adjacent or comagmatic in- 

 trusives, has been emplaced to the accompaniment of much deformation in the 

 invaded formations. On the other hand it seems equally plain that, within the 

 broad areas occupied by much of the Shuswap terrane, the mechanics of batho- 



j lithic intrusion have been of a quite different sort. There is little evidence here 

 of those pronounced deformations with which batholithic invasion is so gen- 

 erally associated in mountainous regions; nor of that abrupt shouldering aside of 

 formations flanking the irruptive mass which elsewhere characterizes the in- 

 vaded strata bordering the Nelson batholith. On the contrary, batholithic inva- 

 sion within the Shuswap terrane has apparendy progressed under conditions of 



h comparative stability by a process or processes of gradual soaking of the super- 

 incumbent rocks with tenuous and mobile products from the underlying magma 

 reservoir. The nature of these products can perhaps best be judged from the 

 occurrence of abundant bodies of pegmatitic granite throughout the Shuswap 

 terrane; from the many associated aplitic dykes; and from the aplitic injection 

 material that is such an important constituent of the gneissic members of tire 



Shuswap complex. The fact, too, that large areas of massive granite contain 

 many bodies of pegmatitic granite of precisely the same mineral composition as 

 the granite, and show every textural gradation into these pegmatitic bodies, is 

 further indication of the character and composition of the magmatic products 

 effecting the transformations in the Shuswap terrane. These products are be- 

 lieved to have been essentially of the nature of pegmatitic and aplitic differenti- 

 ates, high in volatile constituents and extremely mobile. The principal processes 

 have seemed to involve a gradual upward seepage of this material, infiltration 

 along bedding planes, replacement or partial replacement of intervening rock 

 matter, and the growth, in situ, of perhaps much of the pegmatitic granite. In 

 places the continued supply of magmatic material resulted in the complete con- 

 version of large bodies of the original strata into massive granitoid rock which, 

 under the conditions of transformation, became partly plastic or molten and, 

 where subjected to local stresses, behaved much as a normal intrusive rock in 

 its contact relations with adjoining rock masses. 



An important fact in the history of the Shuswap rocks, and one that has been 

 stressed adequately by Daly, is the great depth at which their transformation 

 has been achieved. Unquestionably the Shuswap terrane at that time was 

 deeply buried, and unquestionably the temperatures within the zone of trans- 

 formation were extremely high and long sustained. That this zone lay, in part 

 and at times, within the zone of plastic flow is indicated in many places by 

 numerous local sigmoid folds in which the Shuswap gneisses are involved. That 

 temperatures within the metamorphic zone were high is indicated alone by the 

 abundant and widespread occurrence of pegmatitic bodies everywhere within 

 the terrane. That this condition of deep burial may, as Daly points out, afford 

 an explanation of why the Shuswap terrane as a whole has escaped the severe 

 deformations effecting more superficial formations (such as are now found bor- 

 dering the Shuswap area), must be kept in mind in any interpretation of the 

 origin and mode of formation of these rocks. That conditions implied by depth 

 of burial would be most effective on the stratigraphicallv oldest formations is 

 evident from the fact that for any sizable area of Shuswap rocks it is the oldest 

 formations, or basal strata, the alteration of which has been most complete. 

 Thus it is quite probable that within the principal area of the Shuswap terrane. 

 as about Shuswap Lake, die formations principally effected are, as suggested 

 also by the general structural trend of their foliation, of pre-Cambrian (Beltian?'* 

 age. In other areas, however, it is known that metamorphism lias extended up- 

 ward to include late Paleozoic and probably Triassic formations, but that the 

 effects of this metamorphism have been less intense as, in general, the depth of 

 burial has decreased. 



Since 1950 evidence has been accumulating that points to the conclu- 

 sion, if not the fact, that the Permian strata rest unconformably on the 

 Shuswap and are not affected by the same orogenic and intrusive activity. 

 Reesor (1957) summarizes recent opinion as follows: 



