Lower Mississippian Ostracodes from the Banff Formation, Alberta

Ostracodes are common in the Lower Mississippian Banff Formation and equivalent strata in the Rocky Mountains and Peace River region of Alberta.

The type Banff Formation-divisible into three members-contains three, possibly five, ostracode zones; a major faunal change takes place near the top of the middle member. Northward from Banff a calcarenite unit, called Pekisko, is intercalated between the middle and upper members. Ostracode faunas from above the Pekisko unit in the Mountains and in the Peace River region are correlative with those of the type upper member.

The basal Banff sandstone at Jasper contains a conodont fauna of earliest Mississippian age. The ostracode faunas of the overlying Banff lower member are thus considered to be of Kinderhook (Chouteau) age. The type Banff middle member faunas are also considered to be Kinderhookian, and the Kinderhook-Osage boundary is placed at the level of the faunal change near the top of the middle member. The type upper Banff, Shunda-equivalent, strata are considered to be of early Osage (Fern Glen) age.

Type Banff Formation strata are predominantly fine- grained limestones with variable amounts of silt-size quartz. Chert nodules nd siliceous segregations are common. The carbonate percentage increases upward through the formation, and averages 78 per cent. Five main rock types are recognizable:

1. laminated, unfossiliferous micrite,

2. micrite with fine-grained organic detritus,

3. micrite with up to 10 per cent larger bioclastic fragments,

4. micrite with 10 to 50 per cent larger bioclastic fragments, and

5. biocalcarenite with fine-grained matrix.

Six rock units were established in the type Banff Formation on the basis of these lithologic facies. Ostracodes are absent from lithofacies 1 and 5, and are most common in lithofacies 3. The five lithofacies reflect fluctuations in current sorting; the mean size of ostracode valves varies with lithofacies, indicating current modification of initial ostracode assemblages. Comparison of size-frequency distribution data for theoretical death assemblages and observed assemblages indicates that current-modified death assemblages may be recognizable.

Statistical analysis applied to the ostracode data differentiates three generic groups of ostracodes:

Group I comprises Amphissites, Begrichiopsis, Coryellina, Craspedographylus, Cribroconcha, Editia, Kirkbya, Knoxina, Mammoides, Paraparchites, Rectobairdia, Silenites, Tetrasacculus and Waylandella;

Group II comprises Cornigella, Glyptopleura, Kirkbyellina and Monoceratina;Group III comprises Bairdia and Geffenina.

Group I genera tend to be not associated with Group II genera, and are prevalent in the type Banff upper member and in Plains sections.

Group II genera occur predominantly in the type Banff lower and middle members and in member A at Jasper. They are absent from Plains sections. Regional differences in ostracode morphology also exist. These are consistent, although not complete, and are believed related to environmental differences, the more highly ornamented forms of any one taxon always being associated with the darker, deeper water carbonates. It is concluded that Groups I and III forms represent shallow-water shelf environments, possibly on a lime silt-sand substratum, and that Group II forms represent a deeper water, shelf-margin environment, possibly on a lime silt substratum.

The ostracode faunas are classified into 130 forms, of which 99 are new, and 18 are conspecific or comparable with previously described species.