Date of Award:

5-1973

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

Raymond L. Kerns, Jr.

Committee

Raymond L. Kerns, Jr.

Committee

Robert Q. Oaks, Jr.

Committee

Donald R. Olsen

Abstract

Twenty-five piston cores 6 to 12 feet long were obtained from the deepwater sediments of Bear Lake, Utah-Idaho. Analyses of these cores revealed that the deep-water sediments of the lake are divided into two major sequences: a younger sequence rich in carbonate minerals, here called the carbonate sequence, and an older sequence rich in silicate minerals, referred to in this paper as the silt sequence. The carbonate sequence is composed of clay-sized quartz, aragonite, calcite, dolomite, montmorillonite, illite, kaolinite, chlorite, and amorphous material. The silt sequence consists of both silt- sized and claysized particles of quartz, calcite, dolomite, montmorillonite, illite, kaolinite, chlorite, and amorphous material. Aragonite is absent in the silt sequence. The carbonate sequence is rich in ostracod exoskeletons and pollen grains. Small quantities of woody material and dark, organic-rich wnes occur within the silt sequence. Chemical analyses for Mg, Ca, Fe, Mn, K, Zn, Na , and Sr were reformed on the sediments. Unusually high concentrations of Fe (8.25 percent) were found in the silt sequence, and of Sr (0 .110 percent), in the carbonate sequence. Isotopic analyses for o18 and c13 in the lake sediments indicate that formation of the authigenic carbonate minerals occurred under normal lake-bottom conditions.

From the data collected, the following conclusions or inferences are made. The carbonate sequence was deposited in water depths similar to, or greater than, those of the present. Within this sequence, aragonite is precipitating at present from solution in such quantities that it is responsible for the inversion of the average Ca/ Mg mole ratio of inflowing water of 2:1 to a Ca/ Mg mole ratio of 1:5 in the lake water. The high concentration of Mg +2 and possible high concentration of Sr+2 in the lake water have resulted in conditions favorable for development of protodolomite. Atypical X-ray diffraction patterns for calcite and dolomite, and the relative abundances of aragonite, calc ite, and dolomite reveal that protodolomite probably is, or has been, forming in Bear Lake.

The silt sequence was deposited in water shallow enough for rooted plants to establish themselves . In this shallow environment detrital sediments rich in kaolinitic clay derived from the· Bear Lake Plateau were altered to sediments rich in montmorillonitic clay and amorphous materials.

The sharp contact between the silt sequence and the overlying carbonate sequence apparently represents abrupt termination of widespread swampy depositional conditions in the Bear Lake graben, caused by flooding, which possibly resulted from the most recent major episode of downfaulting of Bear Lake Valley.

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