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and Goose Tickle group samples are shown plg(led individually on a diagram revised in
this IlWIller with all samples now plg(ting in the aclin or passive margin r.elds.
Since there is abundarlt independenl evidence 10 show that the Lower lIead
Formation and Goose Tickle group were deposited in an 'active margin' setting, the
plolling of a significant number of samples. particularly from the Mainland formation.
in the passive margin rICld, mUSI be addressed. lbtre are three possible hypotheses to
explain this observalion:
I. Most of the sands which pig( in the passive margin field ate from the Mainland
formation. and .several of those from near the inferreG lOp of the section are parlicularly
rich in silica. It is possible that towards the top of the Mainland formation a gradual
change in provenance to a source richer in silica occurs, as originally suggested by
Schillereffand Williams (1979). However, this hypot~sis is not favoured because not
all of the silica rich sands occur at the lop of the seclion and in addition. sandstones (rom
unitS stratigraphic:aJly above the Mainland forrn:tion show a similar scalier on either sitk
of the passive/active margin boundary, althougt: admiutdly the (bta are sparse (see
chapter 6, figure 6.11);
2. A facies variation ffiloy be responsible for the chemical variation. The Mainland
formation, particularly near its tOil, is characterised by several coarse and very coarse
grained sands which have clearly been reworktd and have had some of Ihe fine material
removed. Carbonate cement is common in these better sorted arenites and may he