The Permian sequence of Western Canada is examined stratigraphically, paleontologically and petrologically as an example of a condensed shelf sequence of clastic, phosphatic, carbonate and chert rocks which contains widespread, thin, persistent lithologic units and displays three principal associated paraconformities: one separating Middle Pennsylvanian and Lower Permian, one within the Upper Permian and one separating the Permian and Triassic. The latter contact corresponds to a marked lithological and faunal change in the succession.

Permian sandstones consist of quartz, chert, phosphatic and carbonaceous fragments, plus silicified sulphate grains; varieties of silica cement are dominant. All uppermost Paleozoic rocks contain negligible amounts of true clay material, either in the sandstones or in interbedded shaly-weathering intervals. If clastic clay had been present, its silica component might have contributed to the abundant silicification, but virtual absence of alumina in any form remains a problem. Triassic clastic components are similar, although feldspar and clay occur in variable (but noticeable) quantity; carbonate cement is dominant.

This study indicates the existence of three main derivative provenances: 1) many-cycled, clastic sedimentary rock from the hinterland; 2) local gypsum from an arid shoreline, and 3) phosphate produced on the shelf and characteristic of a starved, condensed sequence. The clastic products from these provenances have been modified by weathering, transport and diagenesis in differing continental, coastal and shelf environments of sorting, deposition and burial.

Condensation occurs ideally at a laterally mobile hinge-line separating the subaerial and shoreline sequences from the basinal sequence. The position of the hinge-line may change to produce demonstrable disconformities, paraconformities and possible diachronisms. The condensed sequence, with or without time-indicative fossils, may be complete or partially complete due to nondeposition, erosion and reworking or corrosion. All these factors make precise time relationships difficult to demonstrate.

Deposition of laterally persistent lithologic units is controlled by major factors such as eustatic and tidal cycles or regional epeirogenesis. There can be no such thing as an absolutely continuous sequence of sediments, and frequently, allocation of sedimentary sequences to transgressive or regressive cycles is open to more than one interpretation.

Orogenic activity of the earliest stages of the Tahetanian orogeny (approximating to the Permian-Triassic boundary) is indicated in several areas in the eugeosynclinal rocks of British Columbia. Specifically, this

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includes regional metamorphism, ultramafics, angular unconformity and immature basal Triassic conglomerates in the Greenwood area (southern Omineca geanticline), Princeton-Ashcroft area, Skeena River and Yukon Plateau, and plutonism in the Coast Ranges and Omineca geanticline.

The above involves appreciation of the evolving paleogeography of the hinge line region due to the effects of distant, shifting diastrophism from Permian-Triassic time onwards.

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