| Ice Peak Formation | |
|---|---|
| Stratigraphic range:Pleistocene Approx.1 Ma | |
 East side ofMount Edziza; the two lateral exposures of rock in the upper left corner are part of theupper assemblage of the Ice Peak Formation. | |
| Type | Geological formation[1] |
| Unit of | Mount Edziza volcanic complex[2] |
| Sub-units | Upper assemblage[3] Lower assemblage[3] |
| Underlies | Pillow Ridge Formation,Edziza Formation,Kakiddi Formation,Big Raven Formation[1][4][3] |
| Overlies | Armadillo Formation,Nido Formation,Pyramid Formation[3][5] |
| Lithology | |
| Primary | Alkali basalt,hawaiite,mugearite,benmoreite,trachyte[6] |
| Other | Trachybasalt,tristanite[3] |
| Location | |
| Coordinates | 57°41′25″N130°38′08″W / 57.69028°N 130.63556°W /57.69028; -130.63556[7] |
| Region | British Columbia[1] |
| Country | Canada[1] |
| Type section | |
| Named for | Ice Peak[5] |
| Named by | Southeret al., 1984[1] |
 Paleogeological map of the Ice Peak Formation at the end of theIce Peak eruptive period | |
TheIce Peak Formation (IPF) is astratigraphic unit ofPleistocene age in northwesternBritish Columbia, Canada. It is the seventh youngest and fifth most voluminous of the 13 recognizedgeological formations comprising theMount Edziza volcanic complex (MEVC), which consists of lateCenozoicvolcanic rocks. The IPF overlies the 6.3-million-year-oldArmadillo Formation, the 4.4-million-year-oldNido Formation and the 1.1-million-year-oldPyramid Formation, all of which are older units of the MEVC. Overlying the IPF are the younger and much less voluminous Pillow Ridge,Edziza,Kakiddi andBig Raven formations of the MEVC; all of these formations were deposited by volcanic eruptions in the last 0.9 million years. Thestratigraphic position of the Ice Peak Formation suggests that it is about 1 million years old.
The IPF is subdivided into two units informally called the upper and lower assemblages. The upper assemblage consists of randomly orientedalkali basalt,hawaiite,trachybasalt,tristanite,mugearite,benmoreite andtrachyte which are in the form oflavas andpyroclastic rocks. This assemblage conprises the upper portion ofIce Peak, as well asCamp Hill,Cache Hill,Ornostay Bluff,Koosick Bluff andThe Neck. The lower assemblage consists mainly of alkali basalt and hawaiite, but also includes small volumes of trachybasalt, mugearite and tristanite. It comprises the bulk of the Ice Peak volcanic pile and occurs on the surroundingBig Raven Plateau.
The Ice Peak Formation was first defined byJack Souther,Richard Lee Armstrong and J. Harakal in 1984.[1] It was mapped as one of 15geological formations of theMount Edziza volcanic complex, a group of lateCenozoicvolcanic rocks in northwesternBritish Columbia, Canada.[8][9] In 1988, Jack Souther mapped the IPF in more detail and the number of geological formations comprising the volcanic complex was dropped to 13; theSheep Track and Kounugu formations were reassigned asmembers of theBig Raven andNido formations, respectively, and are no longer recognized.[3][10][11]
Stratigraphically, the IPF is subdivided into two units informally called the upper and lower assemblages. The lower assemblage is the most widespread, having been largely buried under younger rocks andfelsenmeer,till, glacial andfluvial outwash, as well assolifluction deposits. Most exposures of this assemblage occur along the western and eastern edges of theBig Raven Plateau inridges and at the head of valleys.[3] The upper assemblage is exposed along theMess Creek Escarpment at the southwestern end of the Big Raven Plateau, along the north side ofSezill Creek valley at the western end of theplateau, at the head of valleys at the northwestern end of the plateau, at the southern end ofMount Edziza in the middle of the plateau, and on ridges east of Mount Edziza.[3][12]
The IPF is the seventh-youngest unit of the MEVC and has a volume of 76.7 cubic kilometres (18.4 cubic miles), making it the fifth-most voluminous geological formation of the MEVC. It is also the youngest geological formation of the MEVC that contains more than 70 km3 (17 cu mi) of volcanic material.[6] The IPF overlies the 6.3-million-year-oldArmadillo Formation, the 4.4-million-year-old Nido Formation and the 1.1-million-year-oldPyramid Formation, all of which are older units of the MEVC.[3][13] Overlying the IPF are the much less voluminous Pillow Ridge,Edziza,Kakiddi and Big Raven formations; these are younger geological units of the MEVC deposited by volcanic eruptions in the last 0.9 million years.[1][3][14]
Lithologically, the IPF is the most diverse geological formation of the MEVC.[15] It isstructurally andpetrographically complex, containing a significant volume of volcanic rocks ofintermediate composition such astristanite,trachybasalt,mugearite andbenmoreite.[16][17] The only volcanic rocks ofmafic composition arealkali basalt andhawaiite, whereas the main volcanic rock offelsic composition istrachyte.[3][18]Basalt of the IPF is similar to older basalts throughout the MEVC with the exception of itsvesicular texture; it contains open vesicles rather thanamygdules filled withcalcite orsilica.[16] IPF trachyte is similar in composition to the younger Edziza Formation trachyte, but they are separated by anerosion surface.[19] Most of the volcanic rocks comprising the IPF were erupted fromIce Peak, theprominent south peak of Mount Edziza.[5][7] It is the western rim of a smallcaldera which formed on the summit of astratovolcano whose northern flank is buried under the younger, 2,786 m (9,140 ft) high stratovolcano of the Edziza Formation.[20][21]
Randomly oriented alkali basalt, hawaiite, trachybasalt, tristanite, mugearite, benmoreite and trachytelavas andpyroclastic rocks comprise the upper assemblage. In addition to occurring on the gently sloping surface of the Big Raven Plateau, these rocks also formCamp Hill,Cache Hill,Ornostay Bluff,Koosick Bluff,The Neck and the upper portion of Ice Peak.[3][22] Camp Hill is a smallvolcanic cone rising about 180 m (590 ft) above the southwestern portion of the Big Raven Plateau near the Mess Creek Escarpment. About 10 km (6.2 mi) to the southeast, the volcanic cone of Cache Hill rises about 120 m (390 ft) on a ridge between the Big Raven Plateau in the north and theKitsu Plateau in the southwest.[23][24] The Ornostay and Koosick bluffs are on the lower western flank of Ice Peak adjacent to thehead of Sezill Creek.[25] On the northern side ofSorcery Ridge east of the Big Raven Plateau, the roughly 300 m (980 ft) in diametervolcanic plug of The Neck rises 215 metres (705 feet) above an east–west valley immediately south ofIdiji Ridge.[3][26]
Highly irregular lava flows up to 90 m (300 ft) thick are exposed in the summit region of Ice Peak. They are mixed with equal amounts ofepiclastic fill and pyroclasticbreccia which, together with the lava flows,dip gently to the west, northwest and southwest from the destroyed summitcrater. Most of the breccia consists of randonly orientedbombs andblocks, butash andcinders are locally preserved under the lava flows asbeds or thick deposits. Basalt, mugearite and trachybasalt are the main volcanic rocks comprising the lava flows, whereas trachyte is present in smaller volumes. The trachybasalt is slightlyporphyritic which contrasts with the highly porphyritic basalts and mugearites.[22] Two lobes of trachyte originating from under the centralice cap comprise the Ornostay and Koosick bluffs, both of which are similar in composition andgeomorphology.[25] The steep sides and unusually large thicknesses of these two bluffs is attributed to them having been extruded through glacial ice.[27]
Extensive erosion of the stratovolcano on its eastern side has exposedsills,dikes and irregularintrusions, all of which are compositionally identical to the upper assemblage lavas.[28] The smallest dikes are only a few centimetres wide, whereas tabular intrusions reach thicknesses of 120–150 m (390–490 ft) and lengths of more than 1.5 km (0.93 mi).[29] At the head ofTennaya Creek is a series ofspurs containing white recessive bands ofsilt and ashy clay.[30] These well-stratified bands, intermixed withcobbles and pebbles of Ice Peak lava, probably formed whenejecta from the volcano deposited in a periodiccrater lake at the summit. Exposed in the summit area of Ice Peak are several smallnormal faults, the largest of which occurs just south ofTennaya Glacier. A nearby normal fault steeply dips to the east and has displaced thick, flat-lying lava flows andinterbeddedlacustrine deposits by about 60 m (200 ft).[31]
Most of the lithology of Camp Hill is hidden due tocolluvium covering much the volcanic edifice.[3][23] However, exposures of its internal structure occur on the southwestern flank and in moderately incised, radial meltwaterchannels elsewhere.[23] A lower unit oftuff breccia containing partiallypalagonitizedsideromelane is locally interbedded with pillow breccia andpillow lava, as well aspahoehoe with quenched lava lobes.[3][23] They are thought to have been deposited when Camp Hill first erupted under remnants of glacier ice on the Big Raven Plateau.[24][32] The overlying upper unit is an assemblage of bombs,scoria,tephra and irregular lava flows deposited by volcanic activity after the surrounding ice had melted away.[33] Both units are composed of alkali basalt or hawaiite characterized byplagioclase,pyroxene andolivinephenocrysts.[3]
Cache Hill consists of pillow lava, tuff breccia and several lava flows with a basaltic composition that overlie a gravel deposit up to 135 m (443 ft) thick.[34] A coarsely porphyritic basalt flow comprises the base of the volcano and is sporadically exposed along its southwestern flank. It contains tabularfeldspar phenocrysts up to 1.5 cm (0.59 in) long that are clear to pale amber in colour, which contrasts with the medium greymatrix. Overlying the basal flow are about seven vesicular basalt flows, all of which areaphyric, fine-grained and dark grey. All of these relatively thin lava flows contain well-developedcolumnar joints and are interbedded with scoria deposited bylava fountaining.[23] Like Camp Hill, the basalt at Cache Hill is either alkali basalt or hawaiite with phenocrysts of plagioclase, pyroxene and olivine.[3]
Trachyte is the main rock comprising The Neck, which consists of two parts. The outer part is a 2.5–3.5 m (8.2–11.5 ft) thick cylinder of fine-grained,foliated trachyte that is in the form ofconcentric shells. In contrast, the inner part is made up of coarse-grained trachyte that is in the form of gently curved tabular or vertical, closely stacked planar bodies. Looselyagglutinated basaltic tephra of the Beta Peak eruptive centre of the Nido Formation surrounds The Neck.[26]
The lower assemblage consists mainly of alkali basalt and hawaiite, but also includes small volumes of intermediate lava such as trachybasalt, mugearite and tristanite.[3][35] It comprises the bulk of the Ice Peak volcanic pile and is largely in the form of an asymmetrical basalticshield volcano. The basalt is in the form of relatively thin lava flows that are normally less than 3 m (9.8 ft) thick. These thin, columnar-jointed lava flows extend at least 16 km (9.9 mi) from the central vent in all directions with the exception of the southern flows, which travelled south for only about 5 km (3.1 mi).[35] The intermediate lavas comprise several thick flows at the eastern end of a ridge extending east fromNanook Dome where they overliePliocene basalt of the Nido Formation.[3][35]
The northeastern flank of the Ice Peak volcanic pile contains up to 78 m (256 ft) of sideromelane tuff breccia and pillow lava.[36] These deposits are exposed on two ridges and are believed to have formed when basaltic lava of the lower assemblage ponded against stagnant ice incirques.[37] The lowermost basalt flow of the lower assemblage along the northern side of Sezill Creek valley contains pillows at its base.[38] It also directly overlieshyaloclastites and is brecciated and deformed, suggesting it may have been extruded onto a glacier or anice sheet.[32][39]
Potassium–argon dating of IPF hawaiite from an unnamed ridge at57°41.4′N130°32.1′W / 57.6900°N 130.5350°W /57.6900; -130.5350 has yielded an anomalously old age of 3.7 ± 1.0 million years, which contains a large margin of error and has therefore been disregarded.[3][40][41] The second-oldest potassium–argon date, 2.8 ± 0.2 million years, comes from IPF hawaiite northwest of Cache Hill.[41] A nearly identical potassium–argon date of 2.8 ± 0.1 million years has been obtained from IPF trachyte at the head of Sezill Creek valley between the Ornostay and Koosick bluffs.[3][41] Massive trachyte in the upper part of Ice Peak has yielded potassium–argon dates of 1.5 ± 0.4 million years and 1.5 ± 0.1 million years.[42] Similarly, a potassium–argon date of 1.6 ± 0.2 million years has been obtained from trachyte of The Neck.[26] The youngest IPF potassium–argon date of 1.2 ± 0.1 million years is from trachyte on top of Idiji Ridge southeast of the summit of Ice Peak.[3][41] These dates being older than those of the underlying 1.1-million-year-old Pyramid Formation may be partially due to excessargon in IPF rocks; therefore the dates are considered unreliable.[6][16][43] The true age of the IPF is estimated to be about 1 million years old due to its stratigraphic position under the 0.9-million-year-old Edziza Formation.[44]
