Paleomagnetic Results from the Jurassic Topley Intrusions near Endako, British Columbia: Discussion

Abstract

Although Symons' (1973) study of the Topley Intrusions is an important contribution to unraveling the Jurassic polar wander path for North America, several difficulties become apparent on reading his paper. He states that a major object of his study was to add data to help clarify the Jurassic portion of the polar wandcr curve. However, he is apparently unaware of our contribution in that area, which was published last year (Steiner and Helsley 1972). Quite to the of his assertion that no confidence limits are given for the Kayenta and Sumrnerville poles, in that paper all relevant statistical information is given for the data of our earlier abstract (Steiner and Helsley 197 1 ) . In comparing his results to other published Jurassic data, Symons makes several inaccurate statements regarding the White Mountain Magma Series data (Opdyke and Wensink 1966), referring to it as reliable and saying that it predates the Topley pole by 40 m y . Since many workers, in comparing their data to published data, consider the White Mountain result reliable, issue is taken here with that concept. As was discussed in our paper, the White Mountain Series pole: ( I ) is not 180 m.y. old, ( 2 ) spans a time period of up to 100 may., from 199 to 96 m.y. (Foland 1969; Foland et al. 1971; Armstrong and Stump 1971 1, and (3) is derived from widely scattered site mean poles, indicative of either secondary component remaining or individual plutons having been magnetized at different times during the long time between Triassic and Cretaceous spanned by the intrusions. The White Mountain data are suspect in that the normal results in the mixed polarity study are associated mainly with monzonites and all give a present pole position while the reversed results are all from gabbroic compositions and give a streak of poles away from the Cretaceous pole. Therefore, the White Mountain pole cannot be taken to represent a reliable Jurassic pole to which other data from North America can be compared. Furthermore, the White Mountain pole does not predate the Topley Intrusions but includes them within its age span. Since the currently accepted age for the boundary of the Jurassic and Cretaceous Periods is 136 m.y. (Geol. Soc. Phanerozoic time-scale 1964), the Topley Intrusions, within their error limits (139 * 4 m.y.), fall on the Jurassic-Cretaceous boundary. Therefore, Symons' statement that his pole position, intermediate between those of the Jurassic and Cretaceous, is contrary to the static TriassicJurassic pole position interpretation of Steiner and Helsey (1971 ) makes no sense. The age of his pole position is at the end of the period to which we referred. Moreover, we would likc to reiterate our findings (Steiner and Helsley 1971; 1972) that the pole relative to North America apparently remained in its Triassic position until post 150 m.y. (Summerville time) and then moved very rapidly to its Cretaceous position. New data of ours to be published from uppermost Jurassic sedimentary rocks of the Colorado Plateau support that conclusion. Although the Topley Intrusions pole has a rather large confidence ovd, the position of the pole is in agreement with the concept of rapid movement in the time post 150 m.y. (His pole, at about 139 m.y., is almost half way between Triassic and Cre-