Geology of the Harper Ranch Group (Carboniferous-Permian) and Nicola Group (upper Triassic) northeast of Kamloops, British Columbia
Creators: name: Smith, Randall Blain
Publisher: name: University of British Columbia
Year: 2010

description: The "Cache Creek Group" as previously mapped in the Kamloops area actually consists of two sequences of different ages, the Late Paleozoic Harper Ranch Group (new name), and the Upper Triassic Nicola Group. The lower part of the Harper Ranch Group is a 5 km-thick sequence of hemipelagic mudstone and redeposited tuff with rare lenses of shallow marine limestone which yield fossils of Late Mississippian to Middle Pennsylvanian age. This sequence is disconformably overlain by several hundred metres of Lower Permian limestone forming the upper part of the group. Only the lower 2.3 km of the section were studied in detail. In this portion, andesitic to dacitic tuffs consist of ash produced by shallow marine eruptions, then reworked and redeposited in deep water by turbidity currents and high concentration subaqueous flows. A thin Upper Mississippian bioclastic limestone with a diverse marine fauna is found near the base of the sequence. It accumulated during a period of volcanic quiescence and shallowing of the basin floor. The Harper Ranch and Chilliwack Groups were probably parts of a Late Paleozoic volcanic arc which formed above an east-dipping subduction zone. East of the arc was an "oceanic" back-arc basin bounded on the east by orogenic lands formed by the Late Devonian to Mississippian Caribooan orogeny. This orogenic terrane supplied the lithic-rich elastics of the Late Paleozoic Anarchist Group, Mt. Roberts Formation, and the Eastern assemblage of Monger (1977). Pre-Late Triassic deformation and low grade metamorphism of the Late Paleozoic eugeoclinal rocks of southern British Columbia may have been caused by Permo-Triassic closure of the back-arc basin, and collision of the arc with the old orogenic terrane to the east. A new volcanic arc formed in the Late Triassic is represented by volcanic flows and breccias of the Nicola Group west of Kamloops. East of Kamloops, the Nicola Group consists of 3 km of sediments and volcaniclastics which accumulated in deep water within a back-arc basin. Pelagic and hemi-pelagic mudstones dominate the section, but are interbedded with redeposited tuff, lithic sandstone and conglomerate, and limestone, all of which were deposited by turbidity currents and high concentration density flows. Massive and pillowed basaltic or andesitic volcanic flows occur near the base and top of the sequence, which has been subdivided into five lithologic units. Conodonts extracted from limestones yield Karnian ages. Redeposited tuffs in the Nicola Group were probably derived from the volcanic arc to the west, and also from submarine volcanoes in the basin to the east. Lithic sandstones and conglomerates contain sedimentary and volcanic detritus, including abundant chert and cherty mudstone. These may have been derived from accreted oceanic rocks of the Cache Creek Group, exposed in the emergent Pinchi geanticline west of the volcanic arc. Detrital blue amphiboles in fine-grained turbidite limestones suggest this sediment was also derived from shallow waters surrounding the Pinchi terrane. The Nicola volcanic arc therefore seems to have been built on an east-facing paleoslope. The back-arc basin was floored by older eugeoclinal rocks, and stretched from the arc eastward to the miogeocline. The Harper Ranch and Nicola Groups are separated by a northwest-trending vertical fault, probably of Late Mesozoic or Early Tertiary age. East of the fault the lower Harper Ranch Group forms an east-facing homocline with few discernible mesoscopic folds. To the west, lithic units in the Nicola Group outline a pair of faulted northwest-trending folds: an upright syncline on the east, and a westward-overturned anticline on the west. Deformation probably took place during the Latest Triassic to Earliest Jurassic Inklinian orogeny. The Triassic rocks are intruded by the Paul Peak Stock, a zoned intrusion ranging from pyroxenite to granite in composition. This pluton is similar to zoned Alaskan-type mafic-ultramafic intrusions in composition and tectonic setting, and is probably part of the 200 m.y. plutonic suite of southern British Columbia. Numerous andesitic to rhyolitic dikes of probable Eocene to Oligocene age cut all other rock units. Prehnite-pumpellyite facies metamorphism has affected all rock units in the area, including the dikes. Alteration of volcaniclastic rocks in the Harper Ranch Group is much more complete than in the younger rocks, suggesting that the Paleozoic rocks were affected by the Permo-Triassic low grade metamorphic event recorded elsewhere in southern British Columbia.