Revision of Colorado Group on Sweetgrass Arch, Montana

Abstract

The writers propose that the rocks formerly assigned to the Colorado shale on the Sweetgrass arch of northwestern Montana be divided into a lower unit, the Blackleaf formation with four named members, and an upper unit, the Marias River shale with four named members. The Blackleaf formation, of Early Cretaceous age, consists of marine and non-marine rocks that include much sandstone and sandy shale. It is divided into the Flood, Taft Hill, Vaughn, and Floweree members. The oldest member, the Flood, consists of brown-weathering ledge-forming sandstone beds and dark gray fissile shale probably marine in origin. It is 138 feet thick at its type section. The Taft Hill member, a marine sequence that is 240 feet thick as its type locality, consists of greenish gray glauconitic sandstone and gray soft bentonitic siltstone and shale. The Vaughn bentonitic member is characterized by light-colored bentonitic and tuffaceous siltstone that contains small crystals of orange-red heulandite. This non-marine unit is 97 feet thick at its type locality. The Bootleg er member, 330 feet thick at its type locality, is a marine sequence of interbedded sandstone, siltstone, and shale, but includes some layers of bentonite and chert-pebble conglomerate. The Marias River shale, of Late Cretaceous age, consists of 900-1,200 feet of dark gray marine shale. It is divided into the Floweree, Cone, Ferdig, and Kevin members. The Floweree (basal member) is mostly non-calcareous shale and siltstone; it is 63 feet thick at its type locality. The Cone calcareous member, 54 feet thick, consists of calcareous shale, thin beds and concretions of limestone, and some layers of bentonite. Sandy non-calcareous shale characterizes the Ferdig shale member which is 225 feet thick at its type locality. The Kevin shale member, 620 feet thick, contains numerous gray, yellow, and brown limestone concretions and dusky red dolostone concretions as well as many thin layers of bentonite. The middle of the member is marked by the MacGowan concretionary bed, a thi unit of dolostone and limestone that contains pebbles of chert and phosphatic siltstone.