Unit Name: Hailstone Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: early Lochkovian - early Eifelian (416 - 395.6 ma)
Age Justification: Biostratigraphy. Most fossil collections from the Hailstone Formation range in age from late Early Devonian (Pragian) to early Middle Devonian (earliest Couvinian, earliest Eifelian). However conglomerate clasts indicate older ages. They are interpreted as having been eroded from older platform strata to the east. One breccia conglomerate bed, 100 m above the Duo Lake Formation, yielded late Silurian fossils. By comparison, chip samples from 30 to 90 m at the type section indicate a late Early Devonian or younger age. Conglomerate clasts immediately above the Duo Lake Formation at the reference locality for the base of the Hailstone yielded fossils of an early Early Devonian age (early Lochkovian; early Gedinnian). At another locality two collections indicate different ages. One is from macrofauna from outcrop and gives an early Early Devonian age (late Lochkovian, late Gedinnian). The other is from felsenmeer derived from the same outcrop and gives a late Early Devonian, Pragian age (Cecile, 2000).
Province/Territory: Northwest Territories; Yukon Territory

Originator: Cecile, 2000.

Type Locality:
The type locality of the Hailstone Formation is immediately north of the study area in the Bonnet Plum (106-B/01) map area. The section was measured across an outcrop on the east side of a tributary of the Middlecoff River at UTM co-ordinates 426600 E, 709850 N, Zone 9. At the type section the base of the section is not exposed. A reference locality for the base of the Hailstone Formation is given at UTM co-ordinates 432100 E, 7089450 N, Zone 9 (Cecile, 2000).

Distribution:
Occurs within the Selwyn Basin. It is known to outcrop only in the northeast part of the northeastern Niddery Lake map area and to extend north into the adjacent 106B/01 map area (Cecile, 2000).

Lithology:
At its type section the Hailstone Formation consists of approximately 70 m of 80 to 90 per cent calcareous black shale with medium thick beds of grey-white weathering bioclastic limestone at the bas; followed by a middle unit of 50 m of grey to white weathering. , thin bedded limestone; and finally by 70 m of black shale with medium thick beds of grey-white weathering bioclastic limestone. Interestingly, the upper 25 m has rusty shales and reacts to field tests for zinc. One specimen assayed 0.27% ZnO. The Hailstone Formation is a recessive unit and is typically represented by metre-scale outcrops of resistant limestone. Because it is recessive it is possible that all limestone-dominated outcrops are at the position of the middle limestone unit, or the uppermost shale with bioclastic limestone found at the type section. In most of these outcrops the limestone is crystalline, fine to medium grained, shaly, and commonly contains some bioclastic crinoid material including ossicles with twin canals. Coarse conglomerate-breccia beds of older, shallower water facies were observed at the base and within the Hailstone Formation. The Hailstone Formation is generally thin bedded. It contains bioclastic limestone beds, and some thick breccia-conglomerate beds (Cecile, 2000).

Relationship:
The lower contact of the Hailstone Formation is generally not exposed. However at the contact reference locality (UTM 432100 E, 7089450 N, Zone 9; NTS 105-O/16) it is disconformable. At this locality the base of the Hailstone Formation is a 1 m thick conglomerate/breccia bed. The large clasts are shallow-water limestone with early Early Devonian, early Lochkovian fossils. The conglomerate/breccia bed rests directly on the Duo Lake Formation chert with Early Silurian, late Llandovery graptolites. The Hailstone Formation is a basin-and-slope facies correlative to the Grizzly Bear Formation to the northeast. In the southwest it is correlative with chert and siliceous shale of the Misfortune Formation. It is also time equivalent to parts of the Sapper Formation, a silty limestone and shale unit mapped in the south part of the northeastern Niddery Lake map area and southern Niddery Lake map area (Sd1 unit of Cecile and Abbot, 1992) and in the Nahanni map area (Gordey and Anderson, 1993) (Cecile, 2000).

History:
Hailstone Formation is named after Hailstone Creek in the northeast of the northeastern Niddery Lake map area (NTS 105-O/16) (Cecile, 2000).

Remark:
The Hailstone Formation is a deeper water debris fan derived from the Girzzly Bear platform carbonates. Exposures near the Grizzly Bear outcrops tend to have more conglomerate and bioclastic limestone than those farther southwest (Cecile, 2000).

References:
Cecile, M.P., 2000. Geology of the northeastern Niddery Lake map area, east-central Yukon and adjacent Northwest Territories; Geological Survey of Canada, Bulletin 553, 120 p.
Cecile, M.P., and Abbott, J.G., 1992. Geology of the Niddery Lake map area (NTS 105-O) at 1:250 000; Geological Survey of Canada, Open File 2465, 1 geological map (Scale 1:250 000) + 1 geological legend.
Gordey, S.P. and Anderson, R.G., 1993. Evolution of the northern Cordilleran miogeocline, Nahanni map area (105I), Yukon Territory and District of Mackenzie; Geological Survey of Canada, Memoir 428, 214 p.

Source: GEOLOGICAL SURVEY OF CANADA, CALGARY
Contributor: Michael Pashulka
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 02 Dec 2010