The Cretaceous Section at Lanigan in South-Central Saskatchewan
Watkins, Russell Allen
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The Cretaceous section in the second Alwinsal potash mine shaft at Lanigan is a thin, discontinuous sequence of entirely terrigenous clastic rocks, 327 m thick, with an age-span of Middle Albian to Early Campanian. The Cretaceous rocks rest disconformably on Jurassic rocks and are overlain disconformably by Pleistocene sediments. The basal Cantuar Formation of the Mannville Group is composed of fluviatile deposits; the overlying Pense Formation of the Mannville Group introduces the shallow-water marine deposits that persist to the top of the section. Succeeding formations, identified as the Joli Fou, Viking, 'Unnamed Shale', Upper White-Speckled Shale, and Lea Park Formations, were deposited in littoral, infralittoral, and circalittoral environments (or a combination of these) and represent the deposits of locally and regionally transgressive and regressive marine waters. The Cretaceous section is intermittently fossiliferous; there are thick sequences barren of megafossils. In terms of the established molluscan zones usually recognized in the Western Interior of Canada and of the United States, the Joli Fou Formation almost certainly falls within the Inoceramus comaneheanus Zone, the Upper White-Speckled Shale carries the CZioscaphites vermiformis and Inoceramus cordiformis Zones, and the Lea Park Formation carries at least the Baculites perplexus and Hoploscaphites gilli Zones. The lower Joli Fou Formation contains a varied molluscan fauna hitherto unrecorded in Canada; it has most in common with the fauna of the Late Albian Kiowa Formation of Kansas. The presence of this fauna, which has both distinctive boreal and tethyan elements, provides important new evidence on the date of establishment of the Western Interior seaway in North America. The Cretaceous section at Lanigan contains at least one important unconformity (expressed as a paraconformity): the Early to Middle Santonian Upper White-Speckled Shale sits directly on the Late Albian (and possibly also Late Cenomanian) 'Unnamed Shale', and rocks of Turonian and Coniacian age are totally absent. Other paraconformities also may be present.