Unit Name: Rosella Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Early Cambrian (542 - 513 ma)
Age Justification: Fossils collected from strata of the Rosella Formation northwest of Major Hart River include trilobites and archaeocyathids indicating the Nevadella and Bonnia-Olenellus zones of the Lower Cambrian (Gabrielse, 1998).
Province/Territory: British Columbia

Originator: Fritz, 1980.

Type Locality:
The type section for the lowest part of the formation is in the upper part of the lower segment in section 1. The next part of the type section i sin the upper segment of section 1, which is located 2 km to the east-southeast and along strike with the strata in the lower segment. The distances measured in a northwesterly direction from the lower (1a) and upper (1b) segments of the type section 1 to a point on Highway 37 near the northwestern edge of Good Hope Lake is 2.7 km and 4.2 km respectively. The base of the formation in 1a is at latitude 59°16'51'', longitude 129°13'04'' and the top of the segment is at latitude 59°16'48'', longitude 129°13'08''. The base of segment 1b is at latitude 59°16'46'', longitude 129°10'52'' and the top of the segment is at latitude 59°16'37'', longitude 129°10'55''. The upper part of the Rosella Formation is located in reference section 5 in the upper middle and upper segments of that section. The upper middle segment is located 4 km northwest of the Major Hart River. The upper segment is located 5 km northwest of the Major Hart River on a southwest facing dipslope. The midpoint of the upper middle segment is at latitude 58°55'30'', longitude 128°26'15'' and the midpoint of the upper segment is at latitude 58°55'45'', longitude 128°26'45'' (Fritz, 1980).

Distribution:
Cassiar Mountains, north-central British Columbia (Fritz, 1980). The composite section measured northwest of Major Hart River is about 850 m thick but this may be somewhat excessive because of structural complications. Southeast of the river the strata may be as much as 500 m thick. The formation south of Turnagain River is probably not more than 200 m thick and in places seems to be much thinner. There, the formation is commonly bounded by faults and thicknesses may be minimal. North of Wolverine Lake estimates of thickness range from 15 to about 45 m (Gabrielse, 1998).

Lithology:
Resistant, light grey, thick bedded to massive limestone dominates typical exposures of this formation. Subordinate intervals containing thin bedded limestone or siltstone and shale are generally covered (Fritz, 1980). Resistant carbonates of the Rosella Formation form one of the most distinctive stratigraphic markers in the miogeoclinal succession. Its light grey colour contrasts strongly with the dark-weathering strata of the Boya Formation below and its resistance contrasts markedly with the incompetent rocks of the Kechika Formation above. A complete section is exposed northwest of Major Hart River from where the rocks can be followed southeastward to along and near Turnagain River. A thin sequence of the formation outcrops southwest of the Cassiar batholith north of Wolverine Lake (Gabrielse, 1998).

Relationship:
Along with the Boya Formation, the Rosella Formation is part of the Atan Group. The Rosella Formation may be conformable with the underlying Boya Formation although the contact is relatively sharp. Fritz (1980) suggested the possibility of a minor disconformity separating the formations in the type area. A disconformity between the Rosella and Kechika formations is indicated by the lack of Middle cambrian fauna anywhere in the region. Correlative strata occur in the Pelly Mountains, Yukon Territory where the Atan Group is unconformably overlain by Upper Cambrian rocks in Read (1980). Rocks correlative with the Rosella Formation in the Cariboo Mountains are named the Mural Formation by Campbell et al. (1973) (Gabrielse, 1998).

History:
The Rosella Formation is named for Rosella Creek, which is 23 km northwest of the closest point on the type section (Fritz, 1980).

References:
Campbell, R.B., Mountjoy, E.W., and Young, F.G., 1973. Geology of the McBride map-area, British Columbia (93 H) (Report, 5 figures and Map 1356A); Geological Survey of Canada, Paper 72-35, 104 p. + "A" Series Map 1356A, Geology, McBride, British Columbia, Scale: 1:250 000, NTS 93H.
Fritz, W.H., 1980. Two new formations in the Lower Cambrian Atan Group, Cassiar Mountains, north-central British Columbia; in, Current research, Part B, Geological Survey of Canada, Paper 80-1B, p. 217-225.
Gabrielse, H., 1998. Geology of Cry Lake and Dease Lake map areas, north-central British Columbia; Geological Survey of Canada, Bulletin 504, 147 p.
Read, R.C., 1980. Lower Cambrian archaeocyathid buildups, Pelly Mountains, Yukon; Geological Survey of Canada, Paper 78-18, 54 p.

Source: LEXICON_BC
Contributor: Michael Pashulka
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 21 Apr 2011