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.