Geoscience Field Trip – Sidney and Island View Beach
Dr. T.S. Hamilton, Camosun College
Meet at Wilna Thomas Staff Parking Lot off of Foul Bay Road at 8 AM Sunday
Sept 15, 2013 and ride to Armstrong Point north of Sidney for 10:00 AM
Return to campus by 2 PM. Bring, water, lunch, sturdy footwear, raingear as
appropriate. Spouses, mature children or friends may attend if you drive them. No
horseplay as this can lead to accidents!
I) Armstrong Point (near Iroquois Park between Roberts Bay and All Bay in the
north part of Sidney. Access off Resthaven and All Bay Road between head of
Roberts Bay and Shell Station, or continue past marina and Blue Peter Restaurant
into residential area to last beach access by sign saying “Big Rock Beach”)
A) At foot of beach access note dark fragmental and partially porphyritic mafic
volcanic rocks mapped as Permo-Triassic “San Juan Volcanics”. These are most
probably correlated to the Upper Triassic Karmutsen Formation. Note the
orientation and thickness of the beds. Mueller mapped the trace of the Malahat
Fault passing through here. Do you see any evidence for this fault in these local
outcrops?
B) Cutting the mafic lavas are some thin and nearly vertically oriented felsic dykes
of Middle Jurassic (circa 168 Ma) Saanich Granodiorite. When did the tilting
and folding of the older mafic volcanics happen relative to the timing for the
emplacement of the granodiorite? Was the intrusion environment up in the
brittle upper crust or more plastic in the middle crust? What evidence is there
for this in the boundaries and inclusion relationships of the felsic dykes?
C) Proceed NE along the beach to the tombolo, a spit fixed at both ends that
connects the main part of the land to the rocky outcrops that make up
Armstrong Point. This modern sedimentary deposit is built and reworked at
high tides by reversing tidal currents and oblique wind directions from SE
winter storms. Compare the grain size and sediment composition on the 2 sides
of the Tombolo.
D) Examine the outcrops of conglomeratic sandstone that makes up the basal
Saanich Sandstone member of the Comox Formation of the Nanaimo Group.
This is the lowest member of the Nanaimo Group, an Upper Cretaceous
sedimentary fore arc basin. Note the bedforms, shapes and orientations of
bedding here. Look for bedforms, imbrications, cross beddings and fossils to tell
way up in this package. What kind of contact is it with the older volcanics? Is
this a fault or an unconformity and which kind is it? What are the cobbles made
of in the basal conglomerates. Go upsection to the NE and examine the siltstones
and shales. What kind of sedimentary bedforms do you find here and what do
they tell you about paleo slope and sediment transport directions?
E) Draw a local stratigraphic section that shows the geological sequence of events
including all the units noted above.
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II) Island View Beach: Proceed back along All Bay Road and merge heading South
on Resthaven. Take this to Beacon Avenue and turn right at the lights, then left onto
the Pat Bay Highway southbound. Pass through the construction and by the lights
near the Waddling Dog. By the lights at Michel’s Farm Barn Market turn left onto
Island View Beach Road and take it to the parking lot at the end and meet by the
boat ramp before heading south. The foreshore here is private property and our
access is along the beach below the breakwater and dike, southwards to the cliffs.
A) From the late 1950’s until October 2005, the modern beach sediments were
notably coarse cobbles to the south of the boat ramp with gravels and sands on
the beach and spit to the North. In the early1950’s this was all a sandy beach. In
attempt to stop wave cut erosion of the cliff, junked cars and later rip-rap blocks
and most recently log cribwork has been used along the cliff base to slow erosion
and slumping. This turned off the sedimentary source for the modern beach
until a big wet October storm brought several meters of the cliff face down in
2005. Recent slumps in the last 2 years have just delivered a fresh load of sand to
the southerly beach. The longshore northerly current and summer southeasterly
storms have continued to move all the fine-grained material to the north and
towards Sidney Channel. See if you can see how thick the sand is here and where
the underlying cobble pavement begins.
B) Also examine the rip rap (large blocks) and dike built to try and halt or slow
down erosion. Note how well they are working.
C) The base of the cliffs often expose failures, eroded alcoves with outcrops of the
older glaciomarine Cowichan Head Formation at the base of the cliffs. Runoff
often creates little ephemeral deltas on top of the beach and recent landslide
deposits as the cliffs regress and erode. Often blocks of this older glacial material
contain fossils of wood, shells and the odd mammoth tooth or tusk!
D) The oldest stratigraphy here exposed at the base of the cliffs south of the boat
ramp is Pleistocene Cowichan Head Formation of glaciomarine diamicton
named for the stratigraphy at this type locality. This is a dark grey dominantly
fine grained compacted dense clayey sediment with dispersed bands of
dropstones. It also has varved and deformed finer grain beds which may be
glaciolacustrine silts just above high tide line in the middle of the cliff. Most of
the lithologies are diamictons (matrix supported gravel to clay) with sparse
broken shell and wood debris. It is pre-Wisconsinan (undatably old by 14C) and
has been compacted by overlying ice of the latest glaciation. It is also called
“Capilano Sediments” by correlation to deposits of similar lithology in North
Van. Sediments like this have been cored and recovered from most of the
shallow banks and eroded submarine cliffs beneath much of Haro Strait,
Georgia Strait, Juan de Fuca Strait, and adjacent Fjords and include up to 3
prior glacial cycles. The shell and wood debris are mainly from extant species.
The magnetic remanance is normal (but the Brunhes is a long time). At present
there is no way to improve the geological call that these are at least from the
prior glacial cycle if not an even earlier one.
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E) Above this forming the local cliffs is dominantly coarse silty, sandy and gravelly
sediments of the Quadra Sand Formation, named by John Clague for the
prominent, light coloured, cliff forming, relatively clean quartzose sands of
Quadra Island and most other islands along Georgia Strait, Haro Strait, Eastern
Juan de Fuca Strait and Puget Sound. This unit is interpreted to have been
deposited as a proglacial, glaciofluvial outwash deposit in advance of and
particularly along side of, the advancing Wisconsinan ice sheet. (Clague
originally thought that high frequency flat seismic reflectors beneath the deep
marine troughs were equivalent fluvial sands, but coring has subsequently
shown these to be deep proglacial turbidites from receeding ice phases and
uncorrelated.) The advancing ice proceeded to carve, sculpt and remove much of
the former (circa 17 Ka) extent of its own forward deposits, leaving only narrow
strips and island cliff exposures of this formation, such as here in Saanich and
across on James Island. The unit is diachronous with the oldest beds in the north
and the youngest at Commencement Bay in Tacoma, Washington. This unit
contains some viable aggregate and locally has been quarried, such as in the
Butler pit south of Keating Cross Road, near Old West Saanich Road. Victoria
would have a century’s supply of aggregate, however municipal decisions prefer
to build houses, highways and leave hobby farms on top of this potentially
valuable deposit.
F) Drowned or subsided mid-Holocene Peat deposits are intermittently exposed in
the beach face north of the boat ramp, and to the south just by the end of the
man made dyke and the foot of the cliffs. This unit contains stumps of conifers
and terrestrial organic peats dated at a few thousand years (~4ka). This is a
conundrum as most of post-glacial sea level rise was complete by 6 Ka and
cedars don’t grow below sea level. Alternate explanations are that a local
landslide might have dropped a block with trees on it, or that sedimentary
subsidence due to loading or tectonic subsidence along a local fault may have
lowered this former forest to sea level. How much subsidence would it take to
take the closest trees and lower them down to this intertidal zone level? How
might you test these 3 hypotheses?
G) Near the north end of the cliff base is a contact. Is this an unconformity or a
fault and what is the evidence? Further south along the cliff there are X shaped
minor faults with short displacement. What do you think might have caused
these deformations? Can you put any constraint on their timing?
H) Try and draw a cartoon cross section here that shows the correct relative
superposition and contact relationships between the aforementioned units in A
through D.
I) Examine the eroding cliffs and local remediation measures to preserve the cliff
top waterfront property. What role does water play in this erosion process?
Where did this cliff stand 500 or 5000 years ago?
J) Go back to the cars and pull off in the first cul de sac and examine the houses
and landscaping on this eroding cliff face. What purpose did the forest serve
here? What is the risk of erosion and slumping here now? How might this slope
be best stabilized? Of what use are retaining walls? What value is there in
intercepting and diverting water off the land and cliffs?
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