CHAPTER IV HISTORICAL GEOLOGY

The following statement of the geological history of McConnell Creek map-area is based on data recorded in foregoing sections of this report, and is provisional to the extent that many of the basic data are admittedly incomplete or inconclusive.

In Lower and Middle Permian time sediments, lavas, and pyroclastic rocks were laid down in a marine basin. This basin extended many miles southeast of the map-area and may have originated in pre-Permian time. Vuleanism probably prevailed during the Lower Permain epoch, but sedi- mentation was the principal process during Middle Permian time.

The strata provide no recognized record of conditions in Upper Permian or Lower and Middle Triassic time. Probably uplift at the close of the Permian period initiated a Triassic interval of erosion that removed some of the Permian formations. Upper Triassic time opened with the relatively slow, quiet, sub-aqueous deposition of argillaceous sediments mixed with a little voleanic dust or ash. Vulcanism prevailed thereafter, and pyroclastic material accumulated at an ever increasing rate in thicker and thicker beds of progressively coarser material to culminate in the chaotic deposition of extremely coarse agglomerates accompanied by the outpouring of great thicknesses of lavas.

In places a temporary pause in vulcanism allowed erosion to act on the youngest members of the Upper Triassic assemblage, but elsewhere they were followed by Jurassic voleanie rocks without recognized inter- vening erosion. The Jurassic was a period of accumulation over at least the southwestern half of the map-area, and probably most of the strata were laid down in water. Vuleanism prevailed during Lower Jurassic time, when great thicknesses of pyroclastic rocks were deposited, partly at least in marine water. By Middle and Upper Jurassic time the sea occupied a basin of unknown width that extended northwesterly from Scallop Mountain probably to upper Niven River. Although volcanic materials were still extruded at intervals, most of the materials then laid down were derived from the attrition of sedimentary and volcanic rocks and consolidated to form greywackes and related sedimentary types. The occurrence of coal seams, in places close to strata carrying marine fossils, suggests that prob- vbly in late Jurassie time greywackes and argillaceous sediments were deposited in shallow marine water that receded from time to time to permit the accumulation of vegetable matter in shoreline swamps.

During late Upper Jurassic or Lower Cretaceous time the Omineca batholith was emplaced, the area uplifted, folded, and probably faulted, and marine waters completely and finally withdrawn. Active erosion commenced, and by the close of Lower Cretaceous time had exposed Per- mian, Upper Triassic, and Jurassic strata and parts of the batholith. Inasmuch as no Lower Cretaceous strata were recognized, it must be assumed that the products of erosion were swept beyond the map-area, probably westerly away from an upwarped area bordering and overlying the batholith.