The Central plains of Alberta represent the regression of the Bearpaw Sea, the Sea produced a vast fluvial shoreline deposition system which spread towards the North and Northwest of the Bearpaw Basin. Most of the Horseshoe Canyon formation is strata that accumulated during the continued succession and retreat of the Bearpaw Sea.
The Bearpaw Sea was a portion of the Western Interior Seaway, other areas that were formed by the same Seaway include Bisti/De-Na-Zin in New Mexico and Dinosaur Provincial Park 45 minutes down the road.
The upper layers of the Horseshoe Canyon Formation were formed by rivers and flood plains after the Western Interior Seaway closed, which also consists of several coal seams from the wetland type of environments. The top section of the Hoodoos is part of the Horseshoe Canyon Formation, as it's represented by a lighter colour than the Bearpaw Formation below which is characterized by its dark brown colour.
Weathering and Erosion have created the well-known badlands over thousands of years. The erosion has also exposed layers of Coal which was the reason the area was heavily mined, the most well-known Mine in the area is the Atlas No.3 Coal Mine.
The Canadian Badlands are mostly comprised of sedimentary rock strata while some Volcanic Tuff is present in the layers. The Battle Formation sits above the Horseshoe Canyon layer and contains well preserved Volcanic Tuff horizons and organic-rich sediment but is not visible from the Hoodoos Trail.
The younger 75-73 million-year-old Bearpaw Formation (Late Cretaceous) is comprised of Marine Siltstone from the shallow Bearpaw Seaway, which was a portion of the Western Interior Seaway during its last succession. The upper Bearpaw Formation is comprised of mudstone which is characterized by the brown colouring at the contact of the Horseshoe Canyon Formation.
Fine-grained Sandstone in the formation is relatively rare but does exist, as the Bearpaw formation was deposited in shallow seas.
The base layers of the Hoodoos are formed from the Bearpaw Formation, the top layer of the formation is younger towards the southeast as that was the direction the seaway was retreating.
HORSESHOE CANYON FORMATION
The upper layer of the Drumheller Hoodoos lies within the Horseshoe Canyon Formation which is apart of the Edmonton Group. The formation was being deposited 73 to 67 million years ago during the late cretaceous. The member associated with the hoodoos is called the Drumheller Member (below under members).
The sediments in the Horseshoe Canyon Formation were deposited by fluvial action like Meandering Rivers and floodplains, along with deposition from a broad coastal plain associated with the retreating Western margin of the Western Interior Seaway. The sediment dispersal in the Formation is orientated towards the East-Southeast while the shoreline was once orientated towards the North-Northwest-South-Southwest.
Most of the Coal in the Horseshoe Canyon Formation is located in the lower portion of the formation, it consists of thin continuous coal seams split by clastic sediments. During that time Peat accumulation was widespread due to low sediment deposition.
The Horseshoe Canyon Formation is split into 7 members:
. Strathmore Member
73-74 million years old and is 0-80 meters thick. Centimetre to meter-scale beds of fine to very-fine grained sandstone and carbonaceous mudstone, shale and coal. This member was deposited within a shallow-marine environment.
. Drumheller Member:
73-71 million years old and is 80-200 meters thick. The member is comprised of isolated thin coal-beds, paleochannel sandstone that has re-filled an old river channel, tubular shallow-marine sandstone that also contains trace-fossils, mudstone from marine/non-marine and brackish environments, beds of medium-grained sandstone that is 1-cm to meter-scale in thickness, greenish/grey-black clay, silt, sandy carbonaceous mudstone along with carbonaceous shale and coal. Siderite, iron-cemented sediment along with altered volcanic ash (bentonite) and altered mudstones also occur. Litharenite sandstone (25% detrital rock fragments, more fragments then feldspar) in the Drumheller/Horsethief members is 36% quartz, 17% feldspar, 45% rock fragments. The formation was deposited within a wet/warm coastal plain to a river delta setting.
. Horsethief Member
50-90 meters thick and 71.5-71 million years old. Small pebbles have formed a layer at the top, greenish-grey/black sandy clay and siltstone, medium-grained beds of sandstone centimetre to meter-scale in thickness, carbonaceous mudstone shale and lignitic (resembles lignite low-level coal) to sub-bituminous coals are also present. Local siderite and iron-cemented sediment, centimetre and below thickness of altered volcanic ash (bentonite) and altered mudstones.
. Morrin Member
35-100 meters thick 71-70 million years old. The member is comprised of centimetre-to meter-thick layers of fine-to very fine-grained sandstone, greenish-grey clay and sandy mudstones and rare carbonaceous shale in thin beds and sub-bituminous coal. Siderite, iron-cemented sediment and altered volcanic ash (Bentonite). Fossil brackish oysters have been documented in the member along with other mollusks which represent coastal plain, estuarine (River reaching the ocean), marginal marine to small past river channels (paleochannels).
. Tolman Member
25-50 meters thick and 70-68 million years old. Comprised of centimetre-to meter-thick layers of fine-to very fine-grained sandstone, greenish-grey clay and sandy mudstones, siderite, iron-cemented sedimented and altered volcanic ash (Bentonite). Reworked fossil oysters have been documented in the member along with other mollusks. The lower part of the member represents paralic (deposited on the landward side of the coast) and estuarine deposits. The upper part of the member is dominated by non-marine and coastal plain deposits.
. Carbon Member
The member is 24-40 meters thick and is 68 to 67 million years old. This member once went by the name Carbon Tongue but has since been abandoned. This member has very good exposures of the Carbon and Thompson coals (Carbon-Thompson coal zone) within the Red Deer River Valley. Paleochannels filled by sandstones, massive mudstones, organic coals and shales. Pebbles/cobbles are also common within the member.
. Whitemud Member
The member is 1.6 to 6 meters thick and is 67.5 to 67 million years old. The member is comprised of fine-grained sandstone that's rich with bentonite, kaolin clays and contains coaly fragments and is light-grey to white in colour. Other rock types in the member contain siltstone and claystone that ranges from centimetres to meters in thickness. The top few meters are altered by chemical weathering and were once deposited in alluvial-paludal environments, extensive wetlands and meandering rivers.
The hoodoos are eroding at a rate of 1cm per year, this is from wind and water erosion taking place on the soft sedimentary rocks, these formations are very delicate and are the fastest eroding geologic formations in the world.
The Sandstone Caprock that sits on top of the Hoodoos is held together by 40% calcite cement, this means that it's more resistant to weathering and protects the softer rock layers underneath. Once the caprock has eroded away the column that is left behind erodes much faster until there is nothing left of the formation.
The layers of rock that form the hoodoos were deposited 70 - 75 million years ago in a marine/shoreline setting. All the layers are sedimentary minus some ash layering that is not visible near the hoodoos.
Weathering & Erosion
The weathering and erosional processes that have shaped the Hoodoos Trail have formed the badland type of landscape. This is by differential weathering, where some layers erode out faster than others leaving behind spires and hoodoos. The Siltstone and Shale in the area erode out faster than the Sandstone caps that form the top of the Hoodoos in the Horseshoe Canyon Formation.
Chemical Weathering and erosion by water is also a big factor in the formation of the landscape, this is when water erodes the rock layers and starts to create small channels in the weak rock. Those channels start to widen and erode back. The chemicals in the water can create reactions that also speed up the process of weathering.
Differential Weathering (Sandstone spire surrounded by Siltstone)
Chemical Weathering creating channels in Siltstone
Geological Survey of Canada, Bulletin no. 578, 2004, 188 pages
Heritage Resources Management Information System
Eberth, D. A., & Braman, D. R. (2012). A revised stratigraphy and depositional history for the Horseshoe Canyon Formation (Upper Cretaceous), southern Alberta plains. Canadian Journal of Earth Sciences, 49(9), 1053–1086.
Geological Survey of Canada (Calgary), 3303 - 33 Street N.W. Calgary, Alberta T2L 2A7 Published by Natural Resources Canada
B. Hathway, C.J. Banks, D.C. Hay, G.J. Prior, S. Mei, D. Chen and J.A. Weiss Measured Outcrop Section T27-R17W4-01 of the Bearpaw and Horseshoe Canyon Formations, Dorothy, Red Deer River Valley, Southern Alberta (NTS 82P/08)
Energy Resources Conservation Board Alberta Geological Survey