Date of Award:

5-1978

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

Robert Q. Oaks, Jr.

Committee

Robert Q. Oaks, Jr.

Committee

Peter T. Kolesar

Committee

Donald W. Fiesinger

Abstract

Detailed geologic mapping and subsurface study of late Pleistocene and Holocene sediments in northern Bear Lake Valley show at least four episodes of deposition of fluvial, marsh, bay, and lacustrine sediments. from oldest to youngest, these are the Ovid, Liberty, Wardboro, and Lifton episodes. These episodes are substantially different than those proposed by previous investigators. The informal term Bear Lake Formation is formally redefined here as the Bear Lake Group, and includes the newly defined Ovid Formation, Liberty Formation, Lanark Formation, and Rainbow Gravel. The overlying Wardboro Loess, also defined here, provides a probably age of 11,000 to 8,000 years B. P. for widespread post-Wisconsinan deposition of loess in northern Utah and southern Idaho.

The Ovid Episode began prior to 27,400 years B.P., during a warm, dry, climatic interval. Sediments deposited during the early part of the Ovid Episode include marsh and bay deposits of the lower part of the Ovid Formation in northern Bear Lake Valley, probably similar deposits of the lower part of the Lanark Formation west of the Bloomington Scarp (on the west side of Bear Lake Valley), and marshy deposits beneath Bear Lake in southern Bear Lake Valley. West-sloping pediments at the north end of Bear Lake Valley, between Bennington and Georgetown, Idaho, and old alluvial fans, also may have formed at this time. The northern outlet of the valley was near 5990 feet at this time.

Downfaulting along the Bear Lake fault zone on the east side of Bear Lake Valley, and probable eastward tilting affected the central valley during the Ovid Episode. At this time, deposition of deep-water carbonates, beneath Bear Lake, probably began in southern Bear Lake Valley. Later, cooler- and moister climatic conditions of a Late Pleistocene glacial interval (Pinedale?) resulted in a shall& extension of this lake into northern Bear Lake Valley during the Liberty Episode. Progradational, shallow-water sand deposits of the Liberty Formation show that Bear Lake attained its most recent maximum areal extent at this time. Simultaneous deposition of the Rainbow Gravel at the entrance of the Bear River into the valley, near Dingle, Idaho, and of the sandy, deltaic upper part of the Lanark Formation along the west side of the valley, reflected the increased addition of sediments, probably due to glaciation and higher stream discharges. The valley outlet was at an altitude near 5945 feet.

Downcutting of the valley outlet and waning moist climatic conditions led to exposure of lake beds, increased effectiveness of the wind, and deposition of the Wardboro Loess during the Wardboro Episode. This loess probably is correlative with the Niter Loess in Thatcher Basin (Gem and Gentile valleys, Idaho). It is slightly more than 8,000 C14 years old, and probably less than 11,000 year old.

Recurrent faulting along the east margin (Bear Lake fault zone) and west margin (Bloomington Scarp) of the valley at the onset of the Lifton Episode led to a brief re-expansion of Bear Lake, and then a recession southward to its present position near 5923 feet. During this time, a series of beach ridges, successively younger southward, and undifferentiated sediments of marsh, bay, and stream origin, formed in northern Bear Lake Valley. The present valley outlet is near 5873 feet.

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