Date of Award

1984

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

A.M. Cvancara

Abstract

The sedimentary structures, trace fossils, and lithology of the Upper Cretaceous Fox Hills Formation in Bowman County, North Dakota, were studied during the summer of 1979 and the spring of 1983. Twenty-three stratigraphic sections were measured and described and lithologic samples were collected for textural and mineralogical analysis, also, the major outcrops of the formation in the Dakotas and Montana were visited.

The formation in the study area was previously defined as a 27-mthick sandstone unit, containing three members--ascending, Trail City, Timber Lake, and Colgate--that was conformable with the underlying Pierre and overlying Hell Creek Formations. The Fox Hills, as here defined, is a tabular, upward-coarsening unit, typically 37 m thick, of a newly included 10-m-thick basal silt-clay unit and an overlying unit of muddy, subarkosic to sublithic, very fine to medium sand that represents the formation as previously defined. The conformable Pierre-Fox Hills contact marks the horizon above which: clay changes to silt-clay; mixed or interbedded strata occur; and trace fossils become plentiful. The Hell Creek-Fox Hills contact remains at the base of the lowest substantial carbonaceous bed.

The Fox Hills Formation contains three membem, that correspond to three sedimentary structure facies, as follows (from the base): Trail City Member (massive-hummocky facies; 10 m thick); Timber Lake Member (hummocky bedded facies; 19-22 m): and Colgate Member (crossxiii bedded facies; 6-9 m). The Trail City and Timber Lake Members (lower Fox Hills), dominated by hummocky bedding, contain a limited suite of trace fossils; two species of the trace fossil Ophiomorpha are the most abundant. The Colgate Member (upper Fox Hills), separated from the strata below by an erosional surface, contains root molds and leaves at its upper contact with the Hell Creek. The Fox Hills Formation in Bowman County differs from that in the type area in South Dakota in that hummocky bedding is plentiful, the strata are one-third as thick, body fossils are absent, and the Bullhead strata are absent.

In a model based on the storm-origin interpretation of hummocky bedding and the occurrence of trace fossils, the Fox Hills Formation represents shallow marine regressional deposits, predominately of storm origin, that were laid down in depths of less than 37 m, on a broad shelf, marginal and seaward of the advancing Hell Creek delta system. Deposition occurred: (1) steadily, from suspension fallout, on the outer shelf (Trail City Member); (2) episodically, in the wake of storms, on the inner shelf (Trail City and Timber Lake Members); and (3) continually by currentdominated shoreline or tidal(?) channel processes (Colgate Member). In contrast to the depositional conditions that existed later to the east in the type area (i.e., deep water and subsidence or sea level rise), deposition on the southwest basin rim was characterized by rapid progradation over a shallow shelf under local tectonic quiescence.

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