The Canadian La Societe canadienne Geotechnical Society de Geotechnique River Raft Trip September 21, 2008 Field trip stops and points of interest along North Saskatchewan River W S N Whitemud Drive Terwillegar Park 1 M E Ft. Edmonton 2 3 N Saskatchewan R 7 4 HP L 6 Whitemud Drive 5 8 Av Jasper 82 Av 82 HLB JMB 9 Landing 5km Modified from image prepared by Alberta Geological Survey. Compiled from Alberta 2001 natural colour mosaic and Alberta 2002 IRS Mosaic provided by (Photosat.ca). Alberta STRM purchased from US Geological Survey EROS Data Centre L- Lunch stop; M - muster/assembly area; HP - William Hawrelak Park; HLB - High Level Bridge; JMB - James Macdonald Bridge; - Exit 1. Field trip stops and points of interest along North Saskatchewan River M. Rally and muster point at Terwillegar Park Parking Lot Geological setting, discussion of what we will see on trip, logistics, lunch and rest stops River Safety and Donning of clothing and and River Put In. Stop 1. Outcrops of Horseshoe Canyon bedrock units with valley incision, preglacial sand, glacial sediments - till and lake sediments Stop 2. Whitemud Road landslide and mechanisms for failure. Stop 3. Groundwater discharge from the buried New Sarepta Valley; Pass by Fort Edmonton; pass by Jewish Culture Centre (Former Hillcrest Country Club) Stop 4. Laurier Park Lunch Stop; gold panning demonstration. Stop 5. Keillor Road instability and slope failure. Stop 6. Hawrelak Park - reclaimed former gravel pits Stop 7. Slope instability along Summit Drive and landscape armouring Stop 8. Mazama Ash at High Level Bridge; sequential terrace development recorded in 4 terraces; Pollard Brick Yards. Stop 9. Put out at Rafters Landing; Grierson Hill instability, rip-rap along bank, reclaimed landfill of Connor Hill ski club and Muttart Gardens; coal mines and Edmonton Convention Centre. 2. The ancestral North Saskatchewan River has been flowing across the prairies for millions of years within a broad, shallow-sloped valley, named the Beverly Valley. Parts of that ancestral valley underlie the central part of Edmonton, the deepest parts beneath the municipal airport and the Northern Alberta Institute of Technology (N.A.I.T.) About 27,000 years ago, a major glacier from the Canadian Shield advanced over the Edmonton region and it, along with Glacial Lake Edmonton, deposited thick sediment, completely burying the Beverly Valley and masking any present-day surface expression. The part of the river valley that is presently exposed in the City of Edmonton is only about 12,000 years old, which is relatively recent in geologic time. It was formed by the re-establishment of the regional drainage following the retreat and melting of the glaciers, but this time along a different path than the Beverly Valley. Over the last 12,000 years or so, the river has carved down through soft sediments deposited by the glaciers, and into the harder Cretaceous sedimentary rocks, forming a series of relict terraces, separated by relatively steep erosional walls. Stop 1. A record of time at Terwillegar Park More than 60 million years of time are exposed in the geological record along the banks of the North Saskatchewan River at Terwillegar Park, spanning the Age of the Dinosaurs to the arrival of man in the oldest units are at the bottom of the outcrops, and the youngest are at the top (figure 1). All of the major rock units that one is likely to encounter in the Edmonton area are visible in the Terwillegar The oldest rock unit exposed at Terwillegar is called the Horseshoe Canyon Fm. which is an assem- blage of sandy and muddy sediments deposited by rivers flowing into an inland sea during the Creta- ceous Period (~100 ma). These are the light grey rocks shown in figure 2. Lush vegetation growing in quiet backwaters along these rivers were later buried by mud, eventually turning into coal. Episodic outbursts from active volcanoes to the west, deposited thin layers of ash on the river sediments. These ash layers later transformed into clays through weathering processes and are referred to as bentonites or bentonitic clays (figure 3). The ash layer marking the time of the asteroid impact that ended the dinosaurs has been eroded away in the Edmonton area. As the young Rocky Mountains grew, the inland seas drained and the exposed sediment was eroded by ancient river systems that flowed north and eastward. During the next 45 ma (Tertiary Period), as much as 3 km of sediment were eroded by these rivers. As these paleo (ancient) river channels migrated across the landscape, they carved new valleys and transported sand and gravel derived from rocks in the emerging Rocky Mountains. These deposits are named the Empress Forma- tion and consist mainly of quartzite, the only rock hard enough to survive the long journey by river onto the plains. 3. Stop 1 ( ). A record of time at Terwillegar Park Two of these ancient (paleo) river valleys are present in the Terwillegar Park area: the Stony Valley, which is exposed in the park, and the New Sarepta Valley, which is exposed downstream of the park (figures 4 and 5). The sand and gravel deposits that rest on the floor of the bedrock channel are saturated with groundwater, and are able to supply drinkable water to In the last 2 million years a number of major continental glaciers advanced from the northeast and covered most of Alberta. At Terwillegar there is a record of only the latest glaciation which occurred about 27,000 years ago. Ice in these glaciers was as much as 1.5 km thick in places to sink many metres. The sediment deposited by the glaciers is called till, which is composed of a mixture of clay, silt, sand, gravel and boulders. Because these sediments were subjected to the weight of more than 1 km of ice in the Edmonton area, they are quite dense and stiff, and form vertical columnar faces on river outcrops, as shown in figure 6a. Almost all of the large boulders we see today along the banks of the North Saskatchewan River are eroded from glacial till and most are derived from igneous and metamorphic rocks (granite, gneiss) located in the Canadian Shield northeast of Edmonton. They are referred to as erratics because their original source is quite distant from where they are found today. About 12,000 years ago the great ice sheet began to melt and the position of the glacier margin retreated northward. The eastward drainage of large volumes of glacial meltwater, combined with drainage from the mountains to the west, was impeded by the glacier mass, causing large glacial lakes to form along the ice margin. One of the largest was Glacial Lake Edmonton, the record of which is preserved in the form of layers of interbedded light colored silt and darker colored clay which were deposited on the surface of the underlying glacial till (figure 6b.) Glacial Lake Edmonton sediments cover most of the Edmonton area, accounting for heavy soils in our gardens. In places rivers flowing along the base of the glacier spilled their contents of sand and gravel within channels carved into the glacier ice. After the ice walls melted, mounds of coarser river sediments, called kames, were left along the lake margin. One such kame deposit is located directly east of Terwillegar Park in the area known as Whitemud Road. Glacial Lake Edmonton eventually drained via a major spillway as it overfilled its shallow basin, and as the restraining ice mass collapsed during melt. In the first few thousand years after the retreat of the continental glaciers the ground surface began to rebound upwards in response to the removal of the weight of the glacier ice. Eastward-flowing rivers became re-established, adjusting to new gradients and changes to the postglacial landscape. In places the North Saskatchewan River reoccupied its former preglacial valley, but elsewhere it charted its own path establishing a new drainage route. In last 12,000 years or so in the Edmonton area, the river has eroded down through the soft sediments deposited by the glaciers, and into the harder Cretaceous rocks, forming a series of terraces, separated by relatively steep erosional walls. Erosion continues today, but at a much slower rate than following the glacier retreat. Continuing adjustment of the landscape in response to the slow but steady erosion is demonstrated by the numerous landslides along the river banks. 4. Stop 1, figure 1. A cartoon of the rock record in the Edmonton Area 12 ka - Today 12.5 ka - 12 ka 27 ka - 12.5 ka ~2 ma - 28 ka Erosion 65 ma - 2 ma ~100-65 ma (modified from R. Mussieux Geological Wonders of Alberta, Provinical Museum of Alberta) Stop 1, figure 2. Exposed bedrock of the Horseshoe Canyon Formation Glacial sediments ~30,000 to 12,000 years B.P. Horseshoe Canyon Fm. >65 million years B.P. (bedrock) River floodplain sediments 12,000 years B.P. to Pesent (photo M. Fenton, 2000) 5. Stop 1, figure 3. Bentonite - A Slippery Little Devil (modified from R. Mussieux Geological Wonders of Alberta, Provinical Museum of Alberta, p. 207) Who would have thought that thin layers of weathered volcanic ash could cause so much grief in the Edmonton area? clay formed by the weathering of volcanic ash into a mineral called montmorillonite, a member of the smectite family of clays that are known for one property - failure in the Cretaceous bedrock units in the Edmonton area, particularly after heavy rainfalls when the clays become saturated. make bentonite ideal for protective linings in landfills and waste pits, and for sealing pipes in the ground such as protecting water-wells from (photo L.