Date of Award

2004

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

R.D. LeFever

Abstract

The Ordovician Winnipeg Group is composed of three formations; in ascending order, the Black Island Formation, the Icebox Formation, and the Roughlock Formation. The Icebox Formation is largely composed of shale. This work shows that discontinuities and coarser facies exist within the Icebox Formation and that they are traceable throughout the subsurface of North Dakota and eastern Montana. This work also illustrates possible sea-level changes during the deposition of the Icebox Formation along with probable source areas providing detrital sediments into the study area.

Gamma-ray wireline logs were used to distinguish the formations of the Winnipeg Group from each other, and isopach maps were constructed for each formation to ensure consistency and uniformity in the stratigraphic data. Most gamma-ray curves within the Icebox Formation interval tend to extend off-scale. A total of 365 gamma-ray curves were extracted from a LogSleuthTM program and converted into a digital format using Surfer™ for Windows. The individual log segments were traced and adjusted to eliminate the off-scale effect. The digitized well logs were then converted into a numerical format using UN-SCAN-IT™ for Windows and the x-y coordinates were imported into Petra™ for Windows. Nine west-east and nine north-south cross-sections were constructed across the study area. Traceable discontinuities were established by correlating digitized wireline log curves using similar gamma-ray signatures within the same stratigraphic interval. These discontinuities and coarser facies were arbitrarily labeled D1 through D40 and classified as extensive or isolated discontinuities. The extensive discontinuities were separated into three groups: major, intermediate, and minor discontinuities. Attribute and isopach maps were then constructed to display thicknesses and distributions of the discontinuities.

Traceable discontinuities exist within the Icebox Formation of the Winnipeg Group throughout the subsurface of North Dakota and eastern Montana and these discontinuities probably consist of bioturbated silt and sand. The discontinuities are mappable on both large and small scales as illustrated by the isopach and attribute maps such as major, intermediate, and minor discontinuities. During the deposition of the Icebox Formation, the study area was covered by a warm, shallow, epicontinental sea. The Icebox Formation probably represents an offshore marine environment that was below normal wave base adjacent to a nearshore marine environment in the northwestern part of the study area. The extensive discontinuities could represent detrital sediments, probably from Deadwood deposits or reworked Precambrian rocks; which were deposited from distal sources such as the Canadian Shield and a southern deltaic source. Major, intermediate, and minor discontinuities indicate that the rate of sediment input varied throughout Icebox Formation time. The thick extensive discontinuities in the northwestern part of the study area could be sediment deposits from a paleoshoreline along which sea-level fluctuated throughout the deposition of the Icebox Formation. The isolated discontinuities were probably deposited on topographically high areas such as the Nesson Anticline, the Burleigh High, and the Stutsman High. The Ordovician sea was deeper relative to sea-level during the deposition of the extensive discontinuities, but still shallow enough for sediments to be deposited.

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