FOSSILS AND STRATA OF THE HARPER RANCH GROUP AND THEIR SIGNIFICANCE TO THE TECTONIC HISTORY OF THE QUESNEL TERRANE

Strata of the Quesnel Terrane in the Kamloops region, British Columbia represent three successive island-arc successions of Devonian to Jurassic age, the Harper Ranch, Nicola, and Rossland groups. The Late Paleozoic, siliciclastic, volcaniclastic, and limestone succession of the Harper Ranch Group, representing the basement of Quesnellia in the area, comprises two members, a lower volcanic arc succession and an upper carbonate platform succession.

The lower member, represented by Late Devonian (Famennian) shoreface sandstone and chert pebble conglomerate overlain by interstratified mudstone and volcaniclastic debris flows in turn overlain by carbonate build-ups of Late Mississippian age, records the inception of arc activity, the growth of an arc edifice, and finally volcanic quiescence and fringing reef construction. The upper member consists of a deepening-up sequence of interbedded, variously argillaceous wackestone, packstone, and grainstone of Sakmarian to Wordian age; an unconformity, representing the Pennsylvanian, separates the two members. Successive Mesozoic island-arc sequences, the Upper Triassic Nicola Group and the Lower Jurassic Rossland Group, unconformably overly the Harper Ranch Group.

The biostratigraphic framework devised for the Harper Ranch Group, which is based on conodont and fusulinid data, has been refined with recent collections leading to a better-constrained Late Paleozoic tectono-stratigraphic history for the succession. Conodont and ammonite collections from the overlying Mesozoic successions provide evidence for the duration of the Nicola Group and further the identification of an unconformity between the Triassic and Jurassic strata in the area.

Comparison of the tectono-stratigraphy of the Harper Ranch Group with those of other Cordilleran terranes, such as the Yukon-Tanana, East Klamath, and Chilliwack terranes, demonstrates broad synchroneity of major events and may infer a common tectonic regime.