The strata discussed in this paper belong to the shale and lenticular sandstone sequence which has been called the Heath formation of Mississippian age in the Williston Basin. This unit is of economic importance as a source of oil in southwestern North Dakota.

Information has accumulated to indicate that these sediments are Pennsylvanian in age and are correlatable, at least in part, with the Tyler sandstone in the Sumatra area of central Montana. The name Tyler formation is, therefore, proposed for these sediments in North Dakota.

This formation is found over much of western North Dakota and may possibly be equivalent to sediments which occur over much of South Dakota. The Tyler of the Williston Basin in North and South Dakota rests unconformably on truncated Big Snowy (Otter and Kibbey) and on truncated Paleozoic strata. It is overlain by carbonates of the Minnelusa and (or) Amsden formations, and locally by sands and shales of the Jura-Triassic.

A dark gray to brown fossiliferous limestone which overlies the Tyler sediments in parts of North Dakota is believed to be a stratigraphic equivalent to the Alaska Bench member of the Amsden in Montana.

The Tyler formation varies from zero to 300 feet thick in North Dakota. A detrital deposit composed of pre-unconformity rock types has been recognized in cores and samples from the base of the Tyler. Over much of North Dakota the formation is black, carbonaceous, fossiliferous shale; however, over many areas the upper fifty per cent of the formation is dark red shale. Locally red, maroon, yellow and green shales occur in the upper twenty to forty feet of a predominantly black shale interval. Sands are developed more extensively in the black shale unit, but occur in some areas in the red shale. Toward its outer limits to the north and east in North Dakota, the Tyler becomes more sandy and red colors are dominant. In areas where the Tyler has red shales in the upper portion and dark gra to black shales below, the sequence appears to be lithologically similar to the Tyler formation in the Sumatra area of central Montana.

Apparent correlatives of the Tyler formation in South Dakota exhibit varicolored shales, and tan carbonates in the upper half, with dark green and black shales and dark colored limestones and sand in the lower half. A sand at the base, called Fairbanks by earlier workers, lies across the truncated edges of Madison and older Paleozoics.

The sands in the Tyler appear to be bar-type sand lenses which are channeled into the basal detrital beds, and occur interbedded with black to red and varicolored shales in higher parts of the unit. A near-shore lagoonal, fresh water to marginal marine environment seems apparent. Fossils indicate a Pennsylvanian age for these sediments in North and South Dakota and suggest correlation with "true Amsden" at least in part.

Tyler sand fields, in southwestern North Dakota have produced, since the first discovery in 1954, a cumulative total of 364,615 bbls. of oil as of July, 1959.

Cores from this formation on the Nesson Anticline have had good oil shows in well developed porous sands, and suggest that more oil is yet to be found in these strata.

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