Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

R.D. LeFever


The Newcastle Formation (Lower Cretaceous) consists of sandstone with interbedded shale, siltstone, and mudstone facies. This work shows that the regional stratigraphy of the unit is detailed enough for the interpretation of its depositional history and environments in eastern Montana. This work also illustrates the possible lowstand position of the Western Interior Seaway along with probable source areas providing sediments into the study area.

A total of 3446 geophysical wireline logs were used to determine formation tops of the Greenhorn, Mowry, Newcastle, Skull Creek, and Inyan Kara Formations. The log character of the Newcastle Formation was also described for all gamma-ray logs. All well logs and formation tops were exported from a LogSleuth TM program and imported into Petra TM for Windows where unit thicknesses were calculated and cross sections were created. These cross sections were arbitrarily labeled 1-38, A-L respectively, and were used in selecting missing formation tops not previously picked in LogS leuth TM. All isopach maps were created using Surfer™ for Windows to ensure uniformity and consistency in the stratigraphic data. Log character distribution maps were also constructed in Surfer TM for Windows and aided in the interpretation of the depositional history and environments.

The regional stratigraphy of the Newcastle Formation in eastern Montana is stratigraphically detailed enough for the interpretation of its depositional history, and depositional environments. Prior to the Newcastle deposition, the Western Interior Seaway covered the Western Interior of North America. During this time (99 Ma), the Skull Creek Shale was deposited. The sea retreated to the north and west, reaching a maximum lowstand position around 98 Ma. This maximum lowstand position extended from present day central Daniels County, Montana, southwestward to northern Rosebud County, Montana. The Skull Creek Shale was exposed to erosion, and a major drainage system developed. During this time the Newcastle Formation was deposited. The sea transgressed across the study area where it reached a maximum highstand in western Minnesota. Beginning at this time, (96.5-96 Ma) the Mowry Shale was being deposited. The Newcastle Formation contains evidence of two distinct depositional environments: 1) Shallow marine deposits, 2) Valley-fill deposits. The shallow marine sediments were deposited while the Western Interior Sea was at its maximum lowstand position and exhibit a predominant coarsening-upward log character. The valley fill sediments were deposited as a result of the major drainage system and display a fining-upward log character. The sediments that comprise the Newcastle Formation were derived from source areas to the southeast and north. During this time, the Dakota Sandstone was exposed in southeastern South Dakota. Also, in southeastern South Dakota, the Sioux Quartzite is shown to have been a topographic high from which sediments probably contributed to the deposition of the Newcastle Formation in the southern half of the study area. Sediments that contributed to the deposition of the Newcastle Formation in the northern half of the study area probably came from source areas in northern Saskatchewan and Manitoba. During the time of Newcastle deposition, there may have been Precambrian rocks exposed throughout the Canadian Shield.

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