GEOLOGY OF THE SOUTHERN CHILCOTIN MOUNTAINS

Geologically speaking, the first things noticed by most visitors to the Southern Chilcotin Mountains (SCM) are the unusual mix of rugged and smooth terrain as well the montage of color. In close proximity, pyramid shaped mountains such as Mt. Vic rest near to smoothened earthy ridges like “Sheba Ridge” and plane-like features such as the Dil-Dil Plateau. Upper Warner Ridge, with its fine collection of geologic color, is one example of South Chilcotins coloration.

 

Colorful Denain Spur as seen from Warner Ridge

Colorful Denain Spur as seen from Warner Ridge

Nadila Lake overseen by lofty Mt. Vic (3004m) on right

Nadila Lake overseen by lofty Mt. Vic (3004m) on right

The rainbow colors of the Grant Creek - Sluice Creek divide

The rainbow colors of the Grant Creek – Sluice Creek divide

Looking closer, moraine and other glacial features, lava flow remnants, earth flows, and abundant fossil bearing formations give rise to a geologists paradise. Thus, it should not be surprising that the South Chilcotin Mountains are one of the most geologically studied areas of our province. Here’s what geologists think happened to create this beautiful part of BC:
Plate tectonics play a huge part in the geological history of the South Chilcotin. For hundreds of millions of years, land mass and oceanic crust have been scraped off onto the North American continent. Today, beneath the Pacific Ocean the oceanic crust continues to dive and scrape below this continent. These segments of scraped off rocks and sediments, called terrains, accumulated to extend the west coast of N. America west to where it lies today. Some terrains are composed of many different types of rocks. One example of the geologic complexity of this region is the small area of upper Taylor which is believed to be generally comprised of 2 different terrains plus plutonic rock (igneous intrusive) comprising at least 19 different types of rocks!

 

Upper Taylor Basin – looking SE from Taylor Creek Trail at 'Eldorado Pass' with Camel Pass and Harris Ridge on right

Upper Taylor Basin – looking SE from Taylor Creek Trail at ‘Eldorado Pass’ with Camel Pass and Harris Ridge on right

 

Debris flows are a hazard in the South Chilcotin Mountains. This event completely blocked Paradise Creek temporarily and left deposits up to 3m deep.

Debris flows are a hazard in the South Chilcotin Mountains. This event completely blocked Paradise Creek temporarily and left deposits up to 3m deep.

Weathered serpentinite along the Taylor-Pearson Trail

Weathered serpentinite along the Taylor-Pearson Trail

Over time, the numerous terrains of this region deformed, moved along faults, folded and rose up to become mountains. Later volcanic activity, glaciation and general erosion further shaped these mountains.

FOSSILS

One extremely useful tool in deciphering the complex geology of the South Chilcotins is the presence of fossils here. Many of the rock types found in the Southern Chilcotin Mountains are sedimentary and contain fossils specific to the rocks origin. So by studying the fossils, much information can be gained about the rocks they reside in.
An incredible variety of fossils have been identified in the South Chilcotin. These include: radiolarian, conodonts, plants, ammonites, bivalves, belemnites, gastropods, nautiloids, scaphopods and sponges. For most of these fossil types, several species have been identified. The overall age range of these fossils is approximately 90-250 million years so some of these creatures lived during the time of the dinosaurs!

 

Plant fossil in Eldorado basin

Plant fossil in Eldorado basin

Leaf fossils in Eldorado basin

Leaf fossils in Eldorado basin

Most of these fossilized creatures existed in a marine environment. Some fossils like the bi-valves can be found in beds (outcrops) more than 1,000 m thick, while other types like belemnites or ammonites mainly exist as fragments on the grounds surface. Others, such as sponges, can be found protruding from weathered limestone rock.

 

Well preserved Buchia bi-valves near Spruce Lake

Well preserved Buchia bi-valves near Spruce Lake

Belemnite fossil amongst bivalves in Spruce Lake area

Belemnite fossil amongst bivalves in Spruce Lake area

 

Bivalve (Cassianella?) fossil fragments in limestone in upper Paradise basin

Bivalve (Cassianella?) fossil fragments in limestone in upper Paradise basin

Belemnite fossils in Tyoax Pass area

Belemnite fossils in Tyoax Pass area

 

Ammonite fossil fragments in the Tyaughton Creek - Paradise Creek divide area

Ammonite fossil fragments in the Tyaughton Creek – Paradise Creek divide area

OTHER GEOLOGIC FEATURES: BASALT COLUMNS, ASH, CONCRETIONS, TUFA AND GEODES:

BASALT COLUMNS

Hexagonal shaped basalt columns are formed in lava flows which cool slowly. Lava flows once covered large parts of the N portion of the map area and are an extension of the vast Chilcotin Plateau to the N. Oddly enough, remnants of these lava flows can be found today at the top of Tepee Mtn, Relay Mtn, Cardtable Mtn, Castle Peak; and along the sides of “Sheba Ridge”, Fortress Ridge and the Dil-Dil Plateau.

 

Wall of basalt columns along Fortress Ridge’s S side

Wall of basalt columns along Fortress Ridge’s S side

Cap of Basalt columns atop Tepee Mtn

Cap of Basalt columns atop Tepee Mtn


VOLCANIC ASH

White volcanic ash, called tephra, is abundant in the SE corner of the map area. This ash, called the Bridge River Tephra, is believed to have erupted from Mt Meager about 2,300 years ago, which was the most recent volcanic eruption in SW BC. The ash was deposited in the SE portion of the South Chilcotins and beyond to the E by the prevailing winds. We have found some ash pieces being 3-4 cm in diameter but most of this ash is granular and loose existing in beds up to 0.5 m thick at surface or just below the soil. This ash can be easily identified in upper Taylor and Pearson basins, lower Gun Creek, lower North Cinnabar Trail/Route, and around Pearson Pond and Mowson Pond.

 

Mt. Meager as seen from Pemberton valley - the source of the Bridge River Ash

Mt. Meager as seen from Pemberton valley – the source of the Bridge River Ash

Bridge River Ash, from the last eruption of Mt. Meager, in upper Taylor basin

Bridge River Ash, from the last eruption of Mt. Meager, in upper Taylor basin

In W portion of map area it is likely that ash from other volcanoes exists here – but deeper down and in thinner beds. Mt. Mazama, a stratavolcano, erupted about 6,800 years ago from a volcano in SW Oregon and the ash was forced NE to cover a wide area including the SE portion of the Southern Chilcotin Mountains. This volcano eventually collapsed to produce Crater Lake, one of the world’s best known calderas.
To see the volcanic ash distributions within SW BC including the South Chilcotins go to www.gsc.nrcan.gc.ca

For more info and pictures of the Mt. Meager eruption check out www.volcano.si.edu

CONCRETIONS

Odd shaped and sometimes colorful concretions are common in some parts of the map area. These nodules of sedimentary rock, usually <4 cm diameter, form in a marine environment when cementing minerals (usually carbonate or silica) fill a void within the sediments. It is thought that concretions form around a nucleus of organic material (i.e. tooth, leaf, shell fragment). Some concretions from the map area are elongated (cylindrical) with a square hole down the center – what could have been the nucleus? Colors range from grey to brown to reddish to black. Many of these concretions are round or ovoid. Look for concretions near the Castle Pass, “Relay-Cunningham Ridge”, “Little Graveyard Pass”, and Tyoax Pass areas.

Concretions amassed near Castle Pass

Concretions amassed near Castle Pass

The varying shapes of concretions found in the Southern Chilcotin Mountains

The varying shapes of concretions found in the Southern Chilcotin Mountains

TUFA

Tufa deposits form when spring water with a high concentration of calcium carbonate reaches the surface. The calcium carbonate then precipitates out to form a porous grey/brown heap of rock. There are 2 tufa deposits in lower Lizard Creek valley right alongside the trail (and Lizard Creek).

Tufa deposit alongside Lizard Creek in lower Lizard Creek valley

Tufa deposit alongside Lizard Creek in lower Lizard Creek valley

GEODES

Geodes are rock cavities containing crystal formations. The host rock is often limestone. Some geodes have been found in the Castle Pass area.

 

REFERENCES

Garver, John, I., Schiarizza, P., and Gaba, R.G.
Stratigraphy and Structure of the Eldorado Mountain Area, Chilcotin Ranges, Southwestern British Columbia
BC Ministry of Energy, Mines and Petroleum Resources. Geological Fieldwork, Paper 1989-1, 1988

Haggart, James W.
Mollusks: Exotic Shells from Cretaceous Seas
Life in Stone. Edited by Ralph Ludvigsen. 1996

Monger, James W.H.
The Origin and Evolution of Canada’s Western Mountains
Life in Stone. Edited by Ralph Ludvigsen. 1996

Nasmith, H., Mathews, W.H. and Rouse, G.E.
Bridge River Ash and Some Other Recent Ash Beds in British Columbia
Canadian Journal of Earth Sciences, Volume 4. 1967

Schiarizza, P., Glover, J.K., and Garver, J.I.
and Mineral Occurrences of the Tyaughton Creek Area
BC Ministry of Energy, Mines and Petroleum Resources. Geological Fieldwork, Paper 1989-1, 1988

Thompson, Ida
National Audubon Society Field Guide to North American Fossils. 1982

Umhoefer, Paul, Schiarizza, Paul and Robinson, Matt
Relay Mountain Group, Tyaughton-Methow basin, southwest British Columbia: a major Middle Jurassic to Early Cretaceous terrane overlap assemblage. 1982

Vancouver Paleontology Society
Tyaughton 2001 Field Guide