Mesozoic and (?)Tertiary rocks of Quatsino Sound, Vancouver island, British Columbia
Authors: Jeletzky J A 
Published: November 2010

The report-area is underlain largely by the more than 25,000 - foot - thick succession of Triassic and Lower Jurassic volcanic and sedimentary rocks of the Vancouver Group. The base of the group is not exposed . The Vancouver Group is subdivided into the
armutsen Subgroup below and the Bonanza Subgroup above ·with the Quatsino Limestone included in the latter. The broad Karnian (i.e. Karmutsen and early Quatsino time) marine basin, which exitended well to the west and east of the report-area, was transformed into a relatively narrow,
north-trending depositional trough in the early Norian and then fragmented by source areas (?islands) in the late Norian. The resulting complex facies pattern of elastic and calcareous sediments (Thinly bedded member, Arenaceous member, Sutton limestone) was terminated by the latest Triassic (?early
Rhaetian) orogenic phase which flexed the Triassic rocks and faulted them judging by the concurrent outburst of volcanism. The crests of at least two north-trending anticlines (i.e . Port Alice - Coal Harbour and Southwestern tectonic lands) became source areas supplying pyroclastics, lavas and
coarse volcanic elastics to the intervening Neroutsos and West Coast residual marine troughs. The latast Triassic orogenic phase and the accompanying volcanism did not affect the northeastern side of Vancouver Island, the Queen Charlotte Strait, and the Queen Charlotte Islands where the deposition
of marine argillites and limestones (upper parts of Parson Buy and Quatsino formations) continued into the Jurassic. This resulted in the subdivision of the Insular Tectonic Belt into two zones. The southwestern tectonically and volcanically active zone included all of the report area and the
adjacent parts of northwestern Vancouver Island. The tectonically and volcanically quiescent zone comprised areas to the southeast, east, northeast, and north of the report-area. This depositional-structural pattern persisted to the end of Early Jurassic time. Some 10,000 to 20,000 feet of mainly
andesitic volcanics and sediments of the Volcanic division of the Bonanza Subgroup were deposited during the latest Triassic and Early Jurassic in the report-area and adjacent parts of the tectonically and volcanically active zone. The Volcanic division is subdivided , in ascending order as follows:
Hecate Cove Formation (new), Lower Jurassic volcanic unit, Cherty limestone unit, Grey volcanic unit, Matthews Island Formation (new), Uppermost Sinemurian volcanic unit, Uppermost Sinernurian argillite unit, ?Dark grey volcanic unit, and ?Pliensbachian-Toarcian greywacke Wlit. Only 2,000 to 3,500
feet of argillites and limestones (Harbledown and Maude formations) were simultaneously deposited in the quiescent zone which obviously was sediment-starved. The sedimentary interbeds of the Volcanic division appear to be the tongues of the Harbledown and Maude formations of the quiescent zone. The
Early Jurassic depositional-structural pattern was annihilated by the Middle Jurassic orogeny and accompanying batholithic Coast Intrusions. The
Vancouver Group of the report - area was flexed into an open, north-trending and west-dipping homocline apparently comprising the western limb of the anticlinorium of Vancouver Island. This basically simple Mid-Jurassic tectonic pattern was strongly complicated locally by the influence of largely
syntectonic Coast Intrusions. The Mid-Jurassic orogeny transformed all of the Insular Tectonic Belt into a tectonic land and caused the "migration" of volcanism (Yakoun volcanics) from the Early Jurassic active zone into the Early Jurassic quiescent zone. The Coast Intrusions of the report-area are
small to large dykes, plugs, and rarely small stocks characterized by fine to coarse porphyritic, amygdaloidal or felsitic textures. These dioritic to gabbroic plutons appear to be extremely shallow to hypabyssal apophyscs and cupolas of larger hypocrystalline intrusive bodies still hidden at depth.
These hidden intrusives are believed to be intimately connected with or to represent a direct continuation of the principal Coast Intrusion bodies of the Coast Mountains. The stratigraphical-paleontological data indicate the occurrence of only one, geologically brief (?Bathonian) major epoch of the
Mesozoic Coast Intrusions in the report-area and elsewhere on the Vancouver Island . This contradicts the now fashionable hypothesis of a more or less continuous igneous activity in the Western Cordillera. No Late Jurassic or Berriasian to mid-Vaianginian rocks wore found in the report-area. This
part of Vancouver Island apparently remained above sea level and was undergoing erosion throughout this time interval following a regional uplift of
the Insular Tectonic Bolt in the wake of the Mid-Jurassic Orogeny. The Vancouver Group and the Coast Intrusions are overlapped unconformibly by more than 10,000 feet of Cretaceous strata. These exclusively sedimentary, partly marine rocks apparently covered originally most or all of the report-area
but were removed by the post-Cretaceous erosion, except for a few outliers preserved within
downfaulted blocks or synclinal structures. An unnamed late Valanginian to late Barremian group, equivalent to the Longarm Formation of Queen Charlotte Islands and the Cretaceous part of Relay Montain Group of the Tyaunghton Trough, forms the basal part of the Cretaceous sequence. This 3,000 - foot
to 3, 500 - foot thickness of rocks is subdivided into (ascending order): Buchia crassicollis greywacke, Hauterivian siltstone unit, Upper Valanginian to ?Hauterivian greywacke-conglomerate unit, Inoceramus colonicus calcarenites, Barremian fine-grained greywacke unit, and Barremian variegated
elastic unit. These Neocomian rocks were deposited in a relatively shallow marine embayment confined between the Brooks Peninsula Landmass in the southeast and the Northeast Coast Landmass in the northeast. The pulsating late Valanginian to late Barremian transgression gradually advanced eastward
and southeastward over the worn down northwestern part of the Mid-Jurassic tectonic land. The transgression culminated in the late Barremian when all of the report-area was flooded and the flanking source areas essentially peneplaned. The Neocomian rocks are overlain, apparently conformably, by a
succession of predominantly coarse-grained, almost exclusively nonmarine elastics, as much as ?8,000 feet thick, named the Coal Harbour Group (new). These Aptian to Albian rocks. are subdivided into a Coarse arenite unit below, ?800 to ?5, 390 feet thick, and the overlying Blumberg Formation (new),
at least 3,000 feet thick. The nonmarine Coal Harbour Group is equivalent to the Jackass Mountain Group of the Tyaughton Trough. It registers the occurrence of the interregional Aptian (?late Aptian) orogeny which strongly uplifted the previously peneplaned MidJurassic tectonic land and represents
piedmont deposits laid down in an embaymentlike
subsiding basin on its northwestern side. The bulk of coarse elastics was apparently supplied by the strongly elevated Brooks Peninsula Landmass to the southeast of the report-area. Some coarse elastics were, however, derived from the less elevated tectonic land occupying the northeastern side of
Vancouver Island . Unlike the Tyaughton Trough, the report-area apparently was not flooded by the Albian sea. At least 680 feet of Cenomanian and Turonian marine shale with minor amounts of coarser elastics occur locally in the report-area. This Upper shale unit is a small erosional remnant of the
southeastward onlap of the late Haida Sea of Queen Charlotte Islands onto the subsiding northwestern part of the Aptian-Albian tectonic land of Vancouver Island. The areal extent of this early Upper Cretaceous transgression is uncertain. The Nanaimo Group is represented by a unique, small outlier
?unconformably overlapping the Barremian rocks on the eastern side of Neroutsos Inlet. Its arkosic sandstones, more than 280 feet thick (top faulted), are equivalent only to the late Campanian Cedar District Formation of the Nanaimo-Comox area . The Neroutsos outlier represents the central part of
the northern embayment of the late Nanaimo Sea. This sea advanced from the northwest or ?west, did not extend southward beyond the line defined by Suquash River, head of Neroutsos Inlet, and Kwakiutl Point. Its eastern limit was presumably situated somewhere beneath the Queen Charlotte Strait. A few
small to medium-size dyke- and plug-like plutons invade the Cretaceous rocks of the report-area. Because of their post-Nanaimo (i.e. postCampanian) age and the localization along the planes of the presu mably early Tertiary faults, these plutons are tentatively correlated with the Sooke Intrusions
of southeastern Vancouver Island and assigned an early Oligocene age. The mineralogical composition of the ?Sooke Intrusions ranges from granitic to gnbbroic. The small size of the plutons, the prevalence of fine to coarse porphyritic, amygdaioidai, and felsitic texture, and the abundance of
metasomatically altered Cretaceous sediments (thermal halos) in their vicinity indicate their being extremely shallow to hypabyssal apophyses and cupolas of larger plutons still hidden at depth. No outcrops of Tertiary sedimentary rocks were seen in the report area. The allegedly ?late Tertiary
volcanics outcropping on the northern shore of Holberg Inlet appear to be a strongly indurated and silicified facics of the Volcanic division of the Bonanza Subgroup. Several principal faults trending northwest or west-northwest and delineating four or ?five structurally discontinuous large to
medium fault blocks dominate the present day structure of the report-area. A multitude of closely spaced major and minor faults striking in several directions dissect all of the principal fault blocks. Normal and ?strike-slip faults predominate whereas high angle thrust faults are rare. With one
exception, all observed faults transect indiscriminately all Mesozoic rocks, including those of the Nanaimo Group. However, they do not seem to disrupt the presumably early Oligocene ?Sooke Intrusions. The age of these faults is more likely early Tertiary than latest Cretaceous. However, they may be
younger since all major faults on the west coast of Vancouver Island are known to transect the Tertiary
(Oligocene and early Miocene) strata. The predominent fault block tectonic style of the report-area and other parts of the Insular Tectonic Belt is caused by a somewhat peculiar basement control. The ubiquitous presence of a thick and rigid "shield" of Karmutsen volcanics must have preYented the
development of tight to overturned folds by Late Triassic and Early Jurassic time. Before the end of mIiddle Jurassic time this "volcanic basement" was further strengthened by the invasion of numerous small and large Coast Intrusions and the accompanying induration or metamorphism of the sediments
and volcanics of the Vancouver Group. The resulting ''magmatic basement" determined the subsequent tectonics style of the report-area and other parts of the Insular Tectonic Belt by yielding to the stresses of all post-Middle Jurassic tectonic movements by large and small scale faulting only. The
relatively thin post-MIiddle, Jurassic sediments were protected to a large extent from compressive stresses and could not be even moderately folded, except in the proximity of major or principal faults. This predominant block fault tectonic style seems to be out of place in an otherwise typical
geosynclinal. (=orthogeosynclinal or eugeosynclinal) region. However, other examples of a similar tectonic style are becoming known elsewhere in the Circumpacific Orogenic Belt.