Unit Name: La Martre Falls Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Cambrian - Middle Ordovician (542 - 460.9 ma)
Age Justification: Sparse bryozoans, graptolites, echinoderm fragments, brachiopods, cephalopods and trilobites of Middle Ordovician age recorded from the upper 10 m (33 ft) of the formation in the type area. A few inarticulate brachiopods and trilobites of Middle Cambrian age collected by Cominco geologists west of Hottah Lake from beds estimated to be 61-91 m (200-300 ft) from the top of the formation (Fritz, 1969). Fucoidal markings are recorded from the Mazenod Member.
Province/Territory: Northwest Territories
Originator: A.W. Norris, 1965.
Type Locality:
Along a 2.4 km (1.5 mi) stretch of the Rivière la Martre canyon immediately below La Martre Falls about 26 km (16 mi) northwest of the north end of Marian Lake northwest of the north endof Great Slave Lake, 63'08'N, 116 deg 50'W.
Distribution:
Discontinuously exposed in a narrow sinuous belt at and near the eastern margin of Paleozoic rocks extending from Redrock Point on the west side of the North Arm of Great Slave Lake to beyond latitude 64 deg N. Thickness diminishes to zero on the flanks of Precam-brian knobs and ridges. Elsewhere, recorded thicknesses range from 27 m (90 ft) on Old Fort Island to between 91 and 122 m (300 and 400 ft) in the northern part of the belt. Thickness of the Mazenod Member is estimated to be 21+ m (70+ ft).
Lithology:
In the south it consists of green and dusky red shale, greenish-grey quartzose sandstone, and dark brownish-grey silty and sandy dolomite. In the north it consists of yellowish-grey sandstone, red shale with salt crystal molds, red sandy dolomite, and yellowish-brown, shaly, silty and sandy dolomite. The name Mazenod Member was proposed by A.W. Norris (1965) for a thin unit consisting of dolomite and oolitic dolomite occurring within the lower part of the formation in the northern part of the Great Slave Lake region.
Relationship:
Sharply overlies sandstone of the Old Fort Island Fm. Locally, as in the area west of Faber Lake, it nonconformably overlies Precambrian basement rocks. It is transitionally but sharply overlain by dolomite of the Chedabucto Lake Fm. The lower part of the La Martre Falls Fm is in mappable continuity with the Mount Cap and Saline River formations of Cambrian age which have been traced along the margin of the Canadian Shield as far north as the Arctic Coast (Balkwill, 1971). In the subsurface west of Lac La Martre in the southwestern part of the Great Slave Plain the term La Martre Falls Fm is applied to erosional remnants of the Mount Cap Fm, Saline River Fm and the Franklin Mountain Fm equivalents (Meijer-Drees, 1975).
Other Citations:
Balkwill, 1971; P. Fritz, 1969; Meijer-Drees, 1975; A.W. Norris, 1965.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 2, Yukon Territory and District of Mackenzie; L.V. Hills, E.V. Sangster and L.B. Suneby (editor)
Contributor: D.W. Morrow; A.W. Norris
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 29 Apr 2003