Masters Theses

Date of Award

3-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Steven G. Driese

Committee Members

Kula Misra, Nicholas Woodward

Abstract

Deposition of the Lower Silurian Rockwood Formation occurred within a subtidal, shallow marine environment. The shallowest, most nearshore portion of the shelf was dominated by megarippled and rippled sand shoals deposited above fairweather wave-base, and subtidal, lower shoreface storm sands deposited below fairweather wave base. In this shallow, nearshore environment, conditions were favorable for the transportation and concentration of iron. Tidal currents and both fairweather-wave and storm-wave flows affected this environment. The effect of storms was to erode this nearshore environment, and transport sand and skeletal debris (some of which was replaced by iron) out into deeper portions of the shelf below fairweather wave-base.

Conditions below fairweather wave-base were for the most part quiet and placid with little agitation reaching this environment. Oxygen was typically in limited supply, which restricted the occupancy by soft-bodied infauna and calcified species. Occasionally storm flows entered this environment, depositing sharp-based sandstone and bioclastic limestone beds. The abundance of skeletal debris within the storm beds implies skeletal sand banks must have been present in the nearshore environment. These were intimately associated with the megarippled and rippled sand shoals. The storm beds deposited above storm wave-base exhibit features indicative of combined-flow currents such as form-discordant wave ripples, HCS, micro-HCS, and some graded bedding. Within this realm, both the action of the unidirectional flow transporting the material and storm waves impacting the sea bottom influenced the resultant sedimentary structures. Once the storm flows reached below storm wave-base, the effects of oscillatory currents were greatly reduced. In this locality, beds were typically graded and contain horizontal laminations. The most probable transporting mechanism was by storm-surge-ebb currents that evolved into geostrophic flows.

Fairweather flows, such as tidal and alongshore flows, transported sand in the nearshore environment above fairweather wave-base. Effects of storm flows were probably intense in this environment, but reworking by fairweather tidal and wave processes have erased any trace. Below fairweather wave-base, during periods of non-storm conditions, deposition was dominated by mud.

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