Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

R.D. LeFever


The Interlake Formation is a succession of Silurian carbonates of large areal extent found in the Williston Basin. The Interlake attains a maximum thickness of about 1,100 feet in western North Dakota and thins to an erosional edge in eastern North Dakota,

The main purpose of this study was to interpret the depositional and diagenetic history of the Interlake Formation. Approximately 2,400 feet of Interlake core and 300 thin sections were studied, The Interlake was then divided into twelve lithotypes representing deposition in low-energy sublittoral, high-energy sublittoral, littoral, and supralittoral epeiric sea environments. In addition, paleosols are present,

Rocks that represent low-energy sublittoral environments contain abundant skeletal allochems, some in growth position, High-energy sublittoral environments are represented by rocks that contain ooids, grapes tones, and incraclascs. Peloids, gastropods, intraclasts, and columnar stromatolites are common in rocks from littoral environments, Rocks from supralittoral environments contain either anhydrite (nodular and laminated) and flat-lying stromatolites or peloids, gastropods, fenestral porosity, together with flat-lying stromatolites. Storm deposits, containing imbricate and abraded skeletal allochems, are found associated with supralittoral rock types, Hypersaline pond subenvironments of littoral flat complexes are represented by rocks that contain intraclasts, hypersaline ooids, and transported skeletal debris. Paleosols are classified as calcretes and ferricretes and display severe alteration and brecciation of: previously deposited Interlake lithologies. The paleosol horizons contain significant amounts of secondary porosity and are good potential reservoir rocks,

Each core studied showed a different vertical succession of lithotypes, with some lithotypes specific to only one core. This extreme variability of lithotypes makes lateral correlation of individual lithotypes impossible. However, correlation of thick successions of littoral flat complex carbonates suggest that littoral flat complexes existed on islands which developed on structurally controlled topographic highs, such as the Nessen and Antelope anticlines, in the Williston Basin.

Local migration of subenvironments and regional migration of major environments on these islands produced the complex mosaic of lithotypes seen in the Interlake. Paleogeographic reconstructions of the Interlake suggest that the islands underwent several stages of inundation, progradation, and prolonged subaerial exposure.

Interlake rocks were dolomitized by hypersaline brines on littoral flat complexes, by mixing of sea water and fresh water in the shallow subsurface, and by pressure solution after deep burial. Porosity in the Interlake formed during depositional (interparticle) and eogenetic (fenestral, vugular) diagenetic stages, Fibrous and bladed calcite cements occlude primary porosity and formed in marine phreatic environments. Equant calcite cement formed in meteoric phreatic environments. Saddle dolomite cement formed after deep burial of Interlake rocks.

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