Victoria waterfront geology 3: Dallas Road Waterfront Trail

By Gerri McEwen, Victoria, BC (*short bio at bottom of page) and Dave Tucker. Thanks to Glenn Jasechko for providing some of the photos.

Clover Point to Holland Point, with Finlayson Point in the middle. Google Earth screen capture. Click any image to enlarge in a new window.

This is the third installment in a series on the waterfront geology of Victoria. (1. Harling Point;  2. Inner Harbour.) Walking along the beachfront paralleling downtown Victoria’s Dallas Drive from Clover Point to Holland Point you traverse a 200 million-year-old sequence of outcrops with a very interesting and complex formational history.  The rocks belong to the Jurassic-aged West Coast Crystalline Complex (WCC) which crystallized kilometers beneath the surface but is exposed now due to uplift from collisions that have occurred and continue to occur along the western margin of the island.  It is difficult to envision the nature of the forces required to bring these rocks up from depth, but geologists must figure out how to wrap their minds around such concepts. The rocks along this shore are near the contact between two accreted terranes. Wrangellia makes up most of Vancouver Island. The WWC is the southern portion of Wrangellia. The Crescent Terrane was later thrust over these rocks- the Crescent terrane makes up the bulk of the Olympic Peninsula and is Paleocene and Eocene in age. Slow motion collision and accretion of these two terranes folded, faulted and distorted the seafloor rocks lying between them. These deformed rocks are best visible at Harling Point, to the east of Clover Point. More detail about the regional geology is in the online field guide by Steve  Johnson and others, linked at the bottom of this page. If the geology confuses you, that’s OK. Look at the cool rocks, try to puzzle it out, and at least you’ll have fine views out over the Straits of Juan de Fuca.

Dark dike of magma invaded a larger body of pale felsic magma. Both rocks were pretty hot, and the dike disaggregated. Now, isnt’ that so cool looking? This is at Holland Point. Photo by Gerri McEwen.

Getting there: You can use Victoria Mass Transit to reach Clover Point, the southern-most point on Victoria’s south shore. There is a parking lot on the point. It is a 2.4 km (1.5 mile) walk along the waterfront trail on top of the bluff towards Holland Park, veering down to the latter on the Holland Point Shoreline Trail . You can substitute a walk on the gravel beach as far as Finlayson Point if the tide allows. The shore from Finlayson west to Holland Park is rocky and not a stroll. You could easily do this field trip in different legs as your time permits.

Footwall and hanging wall of a fault. The terms are from mining lore. A miner’s lantern is hung on the hanging wall of a mineralized fault, while your feet are on the ‘footwall’.

The WCC consists of a complex mixture of leucocratic (light-colored, silica-rich) granitoids and mafic (dark-colored) diorites formed during the assembly of the southern tip of the island. 

The thrust fault has placed older rocks on top of younger rocks. Though this illustration shows ‘layer cake’ sedimentary units, a thrust could just as well shove older seafloor volcanic rocks over younger intrusions. To relate this model to southern Vancouver Island, south would be to the left.

 

Mylonite at Clover Point is sheared and broken in a fault. Enlarge the photo to see the streaky texture.

On the east side of Clover Point, you will find mangled metasedimentary similar to those at Harling Point. On the west side you will find fault-zone mylonites in marking the top of the footwall of a shallow south-dipping thrust fault responsible for placing older rocks such as those seen at Harling Point structurally above the younger WCC.  Mylonite is a microbreccia formed along faults. It typically has a streaky or banded texture. Walking westward, you progress downsection through the footwall deeper into the Earth’s crust toward the magma chamber exposed at Holland Point. If you walk along the beach rather than paved trail on top of the bluff, you’ll see that the bluff consists of Pleistocene glacial till and some outwash, with dark soil on top.

Glenn’s photo nicely shows the diagonal shearing in this Clover Point mylonite detail.

Polished and grooved rocks at Finlayson Point. Holland Point is in the distance.

Finlayson Point, midway along the traverse, exhibits fine-grained mafic diorites and coarse-grained granodiorites intruded by bimodal dikes. This region is interpreted as the base of the WCC.  The granitoids at Holland Point are light-colored (‘leucocratic’) – there are very few dark mafic minerals.  The mafic minerals such as olivine, pyroxene, and biotite crystallized early in the history of the magma chamber and sank out of solution leaving the melt enriched in silica (‘felsic’ composition, rich in feldspar minerals and silica)).  A spectacular feature of this outcrop is the dark, pillowy blebs marring the otherwise leucocratic rock.  The blebs are due to a process known as magma mingling.  The felsic melt was occasionally injected with fresh mafic magma from an adjoining chamber. Rather than mixing, the two liquids were immiscible and acted as oil and water. This allowed the mafic magma to remain more or less intact and invade the felsic body as coherent dikes.

Look around for younger east-west and north-south trending dikes crosscutting the leucocratic rock from later injection of mafic melts following complete crystallization of the felsic chamber. If you can get down on your knees right at the contact of the rock and the overlying till, you may see a very freshly exposed, highly polished and scratched reminder of the abrasive power of gritty glacial ice.

Disrupted dikes (?) within felsic rocks at Holland Point.

Holland Point exposes striking, photogenic examples of intermingled early Jurassic mafic and felsic magmas, all smoothed by recent glacial erosion. In particular, look for dark blobs and streaks of mafic magma, caught in the act of intruding the light felsic rock. Some of the dikes appear disturbed and broken, perhaps due to convective motion within the host rock. In other places, it appears that the light rock invades the dark rock. One pale dike is chock full of blocks of the invaded dark mafic rock, broken off and surrounded by the intrusion. It is apparent that  multiple pulses of magma of differing compositions were intruded into the crust beneath an oceanic volcanic chain before these rocks came to rest (for the moment) on the outermost edge of North America.

Looking west down the Straits from Holland Point.

Field Guide. Click the link at right to open the pdf.

A technical overview of southern Vancouver Island geology, including a walking tour along the same shoreline described here was written by S.T. Johnston, D. Canil, G. McEwen & M. Pope (School of Earth and Ocean Sciences, University of Victoria). They have given permission to post the field guide on Northwest Geology Field Trips.13 11 16 CTG Field Guide . The field trip description begins with Harling Point  on page 6. The guide includes directions to Swan’s Pub for post-field trip discussion and reflection.

*Gerricompleted her undergraduate geology honors thesis at University of Victoria in 2013. Her work focused on the rocks at Harling Point. She is beginning a PhD program, also at UVic. She will continue with her focus on aspects of the rocks described on this webpage. Gerri has provided a pdf of her senior thesis, which you can download here: McEwen_Gerri EOS499 Honors Thesis