Big Raven Formation

Stratigraphically, the Big Raven Formation is the youngest unit of the late Cenozoic Mount Edziza volcanic complex in northwestern British Columbia, Canada. By 1988, the Sheep Track Formation had been reassigned as a member of the Big Raven Formation; its recognition as a geological formation has since been abandoned. The Big Raven Formation is the least voluminous geological formation of the MEVC, consisting of of volcanic material. The Big Raven Formation overlies the Klastline, Kakiddi, Edziza, Ice Peak, Spectrum, Nido, Armadillo and Raspberry formations, all of which are older units of the MEVC. It is separated from the Klastline Formation by a layer of moraine that was deposited by retreating glaciers at the end of the last glacial period. The Big Raven Formation overlies the Ice Peak Formation north and south of Mount Edziza. An outlier of the Big Raven Formation is separated from the Spectrum Formation by a thick layer of unconsolidated felsenmeer and talus. Another outlier of the Big Raven Formation overlies the Nido Formation east of Mount Edziza. Big Raven breccia on the plateau surface east of Mess Lake directly overlies trachyte of the Armadillo Formation. At the mouth of Tennaya Creek valley on the eastern side of the Mount Edziza volcanic complex, the Big Raven Formation overlies trachyte of the Kakiddi Formation. ==Age and lithology==

The exact age of the Big Raven Formation is unknown, but its oldest rocks were probably deposited during the Last Glacial Maximum at least 20,000 years ago which corresponds with the beginning of the latest magmatic cycle of the MEVC. A Holocene age for its younger rocks has not been wholly determined from geologic dating, but rather from the lack of evidence suggesting that they were overridden by the Cordilleran Ice Sheet which retreated from the area about 11,000 years ago. Tephrochronological, radiocarbon and fission track dating has yielded ages of 6520 BCE ± 200 years, 750 BCE ± 100 years, 610 CE ± 150 years and 950 CE ± 6000 years for some Big Raven volcanic rocks at Mount Edziza. Alkali basalt and hawaiite are the main volcanic rocks comprising the Big Raven Formation, having erupted from at least 30 vents along the entire north–south trending axis of the MEVC. They are in the form of lava flows and pyroclastic cones which largely comprise two lava fields on the northern and western flanks of Mount Edziza and Ice Peak, respectively. Isolated pyroclastic cones and lava flows occur at the northern and southern extremities of the Big Raven Formation, as well as on the eastern flank of Mount Edziza and along Walkout Creek. Trachyte represents a small volume of the Big Raven Formation and mainly comprises air-fall pumice of the Sheep Track Member. ==Locations==

contains the largest extent of Big Raven Formation lava flows and cones The Big Raven Formation is widespread throughout the MEVC, occurring on or adjacent to the Kitsu, Arctic Lake and Big Raven plateaus. Further south near the middle of the MEVC is the Kitsu Plateau which is bounded on the north by Raspberry Pass, on the west by the Mess Creek Escarpment and on the south by the Spectrum Range. The Arctic Lake Plateau is a nearly flat upland at the southern end of the MEVC between the Spectrum Range to the east and Mess Creek valley to the west. Big Raven Plateau The Big Raven Plateau contains the Desolation Lava Field on the northern flank of Mount Edziza which consists of several lava flows that issued from at least 10 separate Big Raven vents. Another area of Big Raven lava flows called the Snowshoe Lava Field occurs on the western flank of Ice Peak; it issued from at least 12 separate vents. The Sheep Track Member overlies much of the southern end of the Big Raven Plateau, including the Snowshoe Lava Field. At least three Big Raven vents occur on the eastern flank of Mount Edziza and comprise another volcanic zone called the east slope centres. An isolated Big Raven vent called Kana Cone occurs on the extreme northern slope of the plateau while two cinder cones occur on the southern slope of the plateau in Walkout Creek valley. Arctic Lake Plateau Two isolated occurrences of the Big Raven Formation occur on the Arctic Lake Plateau, Nahta Cone near its northern edge being the southernmost vent of Big Raven age. This cone overlies a glacially-scoured limestone hill and contains a crater that is breached to the east. At least five small conelets comprise Nahta Cone, the largest of which has a topographic relief of approximately . The breach in the eastern crater rim served as a passageway for at least two highly fluid lava flows that travelled north along a drainage system. Extending north and west of Nahta Cone are two tephra deposits, suggesting it was active twice during different wind directions. It covers an area of about and contains three separate vents of Big Raven age. The two oldest vents are marked by slightly eroded pyroclastic cones whose craters and cone-shaped structures are still apparent despite being somewhat rounded. Lava flows from both cones travelled westward to the edge of the Mess Creek Escarpment where they most likely cascaded into Mess Creek valley, but no evidence of this phenomenon has been found on or below the escarpment. At the southern end of the Mess Lake Lava Field on the steep, south-facing side of Nagha Creek is the third separate vent called The Ash Pit. This deep and in diameter conical depression is the youngest vent of the Mess Lake Lava Field; it may also be the youngest vent of the entire MEVC. The Ash Pit was the source of a wide and long tephra deposit on the Kitsu Plateau that extends to the northeast. It was also the source of a lava flow that travelled down Nagha Creek valley towards the eastern side of Mess Lake, portions of which are buried under stream gravel, outwash and braided channel deposits. ==See also==