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T. S. Dyman

Publications and source records attributed to T. S. Dyman.

At least 37 records · Page 2Linked to original sources
USGS1995

Measured sections and correlations of mid-Cretaceous rocks near the Lombard thrust of the Helena Salient in southwestern Montana

Two surface sections that generally characterize mid-Cretaceous strata in the central to southern part of the Helena salient of the Montana thrust belt (Figs. 1 and 2) are presented here. We have not published detailed descriptions of the lithic units of these sections, and the stratigraphic correlations are tentative. The sections are presented in this preliminary form because they may aid interpretations of seismic and borehole data of petroleum exploration companies active in the salient.

Montana↗
USGS1994

Cretaceous rocks from southwestern Montana to southwestern Minnesota, northern Rocky Mountains, and Great Plains

In Montana, Wyoming, North and South Dakota, and Minnesota, Cretaceous strata are preserved in the asymmetric Western Interior foreland basin. More than 5,200 m (17,000 ft) of Cretaceous strata are present in southwestern Montana, less than 300 m (1,000 ft) in eastern South Dakota. The asymmetry resulted from varying rates of subsidence due to tectonic and sediment loading. The strata consist primarily of sandstone, siltstone, mudstone, and shale. Conglomerate is locally abundant along the western margin, whereas carbonate is present in most areas of the eastern shelf. Sediment was deposited in both marine and nonmarine environments as the shoreline fluctuated during major tectonic and eustatic cycles. A discussion of Cretaceous strata from southwestern to east-central Montana, the Black Hills, eastern South Dakota, and southwestern Minnesota shows regional stratigraphy and facies relations, sequence, boundaries, and biostratigraphic and radiometric correlations. The thick Cretaceous strata in southwestern Montana typify nonmarine facies of the rapidly subsiding westernmost part of the basin. These strata include more than 3,000 m (10,000 ft) of synorogenic conglomerate of the Upper Cretaceous part of the Beaverhead Group. West of the Madison Range, sequence boundaries bracket the Kootenai (Aptian and Albian), the Blackleaf (Albian and Cenomanian), and the Frontier Formations (Cenomanian and Turonian); sequence boundaries are difficult to recognize because the rocks are dominantly non-marine. Cretaceous strata in east-central Montana (about 1,371 m; 4,500 ft thick) lie at the approximate depositional axis of the basin and are mostly marine terrigenous rocks. Chert-pebble zones in these rocks reflect stratigraphic breaks that may correlate with sequence boundaries to the east and west. Cretaceous rocks of the Black Hills region consist of a predominantly marine clastic sequence averaging approximately 1,524 m (5,000 ft) thick. The Cretaceous System in eastern South Dakota (457 to 610 m; 1,500 to 2,000 ft thick) consists of a marine shelf sequence dominated by shale and limestone. Major sequence boundaries in South Dakota are at the base of the Lower Cretaceous Lakota Formation, Fall River Sandstone, and Muddy Sandstone, and bracket the Upper Cretaceous Niobrara Formation.

Book chapter↗
USGS1989

Lower Cretaceous bentonitic strata in southwestern Montana assigned to Vaughn Member of Mowry Shale (East) and of Blackleaf Formation (West)

The Vaughn Member, newly assigned to the Mowry Shale in this report, comprises strata that crop out in the Greenhorn, Gravelly, Madison, and Gallatin ranges, and the Centennial and Beartooth mountains of southwestern Montana. Herein the member is correlated with the Vaughn Member of the Blackleaf Formation, which crops out to the west in the Lima Peaks area, Snowcrest Range, and Pioneer Mountains. Strata assigned to the Vaughn Member of the Blackleaf Formation in southwestern Montana exhibit the same contrasting relationships that exist in northwestern Montana. The Vaughn Member of the Mowry is late Albian in age, determined by bracketing with shallow water marine bivalves in the Muddy Sandstone below and palynomorphs in Mowry strata above. Palynomorphs from the Vaughn Member itself are typically mid-Cretaceous, but do not permit a more exact determination of age. -from Authors

Mountain Geologist↗