Date of Award
Doctor of Philosophy (PhD)
The strata of the proposed Killdeer Formation (Stone, W. J., 1972) in the Killdeer Mountains of Dunn County, North Dakota is composed of a sequence of tuffaceous, calcareous, dolomitized and silicified flat lying sedimentary rocks of Tertiary age. It is the purpose of this report to describe and integrate the petrology and geochemistry of these sediments.
Fieldwork was conducted during the summer of 1972. Forty-three stratigraphic sections were measured and representative samples of each were collected for further analysis. Seven section were selected as representative of the area, and samples from these were studied in thin section with the petrographic microscope; the thin section slabs were stained to differentiate the carbonated minerals and were also studied with binocular microscope. Many samples were analyzed by qualitative x-ray diffraction.
The Killdeer sediments characteristically crop out in a series of benches of alternating well exposed indurated sediment and concealed poorly indurated strata reaching a maximum thickness of about 220 feet. In order of decreasing abundance the lithologies are tuffite (includes covered intervals), limestone, dolomite, chert, and rare highly altered vitric tuff. All the rock types are gradational and most are characterized by poor lateral and vertically continuity. A petrographically and morphologically distinct unit called the Wormy Marker Bed is located near the base of the sequence and forms an excellent stratigraphic marker.
Sedimentary structures include planar bedding, ripple cross-bedding, fluvial dune cross-bedding, oolites and pellets composite grain pellets, intraclasts, concretions, mudcracks, and intraformational breccias.
The Killdeer rocks contain a rich ichnofaunal assemblage of pellets, burrows, and other bioturbation structures. A few ostracods have been noted in one petrographic thin section.
Tuffite is commonly a silicified or dolomitizad clastic rock composed of phenocryst fragments and shards of rhyolitic composition. These clasts were transported by the wind to the drainage basin and admixed with other detritus before final deposition. Rhyolitic shards suggest a Pliocene age for the sediments.
Limestone, mostly aphanocrystalline but ranging through medium crystalline, is dominant in the lower part of the sequence. It is formed by precipitation in shallow water followed by rapid lithification and aggrading recrystallization.
The dolomite contains more abundant allochems, tends to have a rhombic granular texture, occurs in thinner units with coarse nodular surfaces, and is located higher stratigraphically. This rock type is often tough, dense, compact and aphanocrystalline. The dolomite is formed mostly by dolomitization of limestone which occurred early after limestone formation and has undergone less aggrading recrystallization than the limestone.
Highly silicified brittle chert and porous chalky chert occur as thin units that commonly have nodulose surfaces and are intimately associated with tuffite. The chert is relatively impure, with abundant clasts and is composed of opal and chalcedony. Much of the chert contais evidence indicating a carbonate replacement origin.
A porous, chalk-like, highly altered vitric tuff occurs in at least two stratigraphic intervals. It is primarily composed of the zeolite offretite [=erionite] and chabazite.
The mineralogy of these rocks is relatively simple. Clasts are composed of quartz, shards and other volcanic rock fragments, K-feldspar, plagioclase, hornblende, altered biotite, and zircon(?). Some of the volcanic clasts have been replaced by zeolites, quartz varieties, and carbonates. Authigenic minerals include calcite, dolocmite, opal, chalcedony, megaquartz, pyroluaite(?), rare clay, and zeolites.
Clinoptilolite is the most common zeolite occurring in most of the rock types. Offretite and chabazite occur in intimate association; offretite rarely occurs alone in some of the rocks.
The paragenetic sequence for the Killdeer sediments includes the following: (1) formation of limestone, (2) dolomitization of limestone, (3) silicification of carbonates and tuffites, and (4) minor silicification, deposition of secondary calcite, formation of zeolites.
The essential ions for the formation of the carbonate rocks were transported to the depositional basin by an intermittent fluvial system and concentrated by evaporation until precipitation occurred. Limestone formation increased the. Mg/Ca ratio, resulting in dolomitization by downward percolating magnesium-rich brines. Silica-enriched solutions, derived from the alteration of volcanic debris in a highly alkaline environment, replaced some carbonate rocks, cemented some tuffites, and selectively silicified various allochems. Silicification occurred under reducing conditions and precipitation was largely controlled by decreasing pH.
The rocks formed in a complex depositional environment encorporated lacustrine, eolian, and minor fluvial deposition in a playalike lake that was probably enriched with sodium carbonate.
Delimata, John J., "Petrology and geochemistry of the Killdeer carbonates" (1975). Theses and Dissertations. 73.