Abstract |
- Seven formations of the Upper Cretaceous Nanaimo Group
sedimentary rocks are exposed on Saturna and Tumbo Islands. These
formations are, from oldest to youngest, the Extension-Protection,
Cedar District, DeCourcy, Northumberland, Geoffrey, Spray, and
Gabriola. The rocks represent four successive cycles of deltaic
sedimentation, the oldest and the youngest of which are incompletely
exposed in the thesis area. A composite section of maximum thickneis
for the formations exposed on Saturna and Tumbo Islands is
9, 776 feet; however, the formations vary significantly in thickness
along strike.
The Extension-Protection Formation, consisting of conglomerates
and lithic wackes, represents the upper part of a delta complex.
The lower part of the cycle is not exposed in the thesis area. The
conglomerates are interpreted as having been deposited in the bed
load and point bar subenvironments of highly competent streams. The
sandstones overlying the conglomerates are interpreted as marine
topset sands.
The Cedar District Formation, the lower part of the Cedar
District-DeCourcy deltaic cycle, overlies the Extension-Protection
Formation with an angular discordance of about 50. The Cedar District
Formation consists of repetitively interbedded and normally
graded sandstones, siltstones, and mudstones, which were probably
deposited as turbidites. The Cedar District strata are interpreted as
delta-slope deposits.
Conformably overlying and intertonguing with the Cedar District
Formation are the channel-mouth bar arkosic and lithic wackes of the
DeCourcy Formation. Distributary channel conglomerates overlie the
marine sandstones which are in turn overlain by marine channelmouth
bar sandstones. Pa leocurrent data indicate that the Cedar
District-DeCourcy deltaic complex prograded in an east-southeasterly
direction.
The Northumberland Formation, the lower part of the
Northumberland-Geoffrey deltaic cycle, conconformably overlies the
DeCourcy Formation. Most of the Northumberland Formation was
deposited as prodelta mudstones. However, the lower and upper parts
of the formation were deposited in delta-slope environments, indicating a transition from and to the marine sandstones of the DeCourcy and
Geoffrey Formations, respectively. The Geoffrey Formation intertongues
with the Northumberland Formation and consists almost
entirely of channel-mouth bar sandstones with local distributary channel
conglomerates. The source of the sediments for the
Northumberland-Geoffrey deltaic cycle was to the west or northwest.
Paleocurrent data indicate that the sandstones of the Geoffrey Formation
were distributed in a north to south direction around the mouths
of distributary channels by longshore currents.
Overlying and intertonguing with the Geoffrey Formation is the
predominantly mudstone Spray Formation, most of which is not
exposed in the thesis area. The Spray Formation was deposited as
turbidites and/or seasonal flood deposits in a delta-slope environment.
Overlying and intertonguing with the Spray Formation is the
Gabriola Formation, the youngest formation exposed in the thesis
area. The Gabriola Formation consists of bed load and point bar
fluvial conglomerates, marine channel-mouth bar arkosic and lithic
arenites, and topset conglomerates. Lithologies of the sandstones,
pebble counts of the conglomerates, and sedimentary structures indicate
a paleocurrent dispersal direction to the east or northeast for the
Spray-Gabriola deltaic cycle.
At least two major episodes of structural deformation have been
recorded on Saturna Island. The first episode of structural deformations
resulted in a series of north-south trending faults. The second
episode of structural deformation resulted in a series of east-west
trending faults and related folds (the Kulleet Syncline).
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