Jerry Alvin Lineback

Origin of regional geomagnetic variations recorded by Wisconsinan and Holocene sediments from Lake Michigan, U.S.A., and Lake Windermere, England

The declination of the Earth9s paleogeomagnetic field, determined from sediments in Lake Michigan deposited during the past 11,500 C14yr, exhibits fluctuations east and west of the mean declination with a period of about 2,090 C 14 yr. Inclination and intensity measurements do not exhibit similar fluctuations. Each extreme in declination occurs in the same stratigraphic position in cores from different parts of the lake. The variations in declination in Lake Michigan are similar to those found in sediments from Lake Windermere, England, deposited during the same time span, but the period of the Windermere cycles is 2,800 C 14 yr. Plots of the paleogeomagnetic poles for inclination-declination pairs representing each east or west extreme for the two lakes are quite different, indicating that the geomagnetic effects were not a result of shifts of the main dipole field. In a model of the standing nondipole field, the foci, represented by radial dipoles located one-fourth of the Earth9s radius from the geocenter, are each allowed to oscillate with a characteristic period. This model gives a satisfactory explanation of the principal features exhibited by the declination and inclination records at the two lakes.

Glacial and postglacial sediments in Lakes Superior and Michigan

The lacustrine sediment sequence above the glacial till under Lake Superior consists of red clays overlain by gray varved clay and nonvarved gray and brown clay. The Lake Michigan sequence also contains red clay at the base and brown and gray clays above. The red clays in Lake Michigan are found in areas overridden by glaciers of Valderan age as well as in the far southern end of the lake. Deposition of red clay took place in the northern part of the lake as the Valderan glaciers were melting and continued during the Algonquin stadial when the ice front reached a stillstand on the northern peninsula of Michigan. Red and gray clay outwash dumped into Lake Superior may also have escaped from the Superior Basin into the Lake Michigan Basin through the Au Train–Whitefish channel and other channels across the upper peninsula of Michigan. The connection of the two lakes began between about 11,000 C14 yr B.P. and ended about 10,000 C14 yr B.P., when the connection was broken by rebound or by lowering of water levels near the end of the glaciation in the Lake Superior Basin.

Paleoecological Analysis of Corals and Stromatoporoids in a Devonian Biostrome, Falls of the Ohio, Kentucky–Indiana

A single broadly exposed bedding surface in the coral zone of the Middle Devonian Jeffersonville Limestone at the Falls of the Ohio, Kentucky and Indiana, displays a virtually in situ contemporary population of colonial corals and Stromatoporoids. The positions, sizes, shapes, orientations, and subfamilial or generic identities of 14,233 unbroken colonial coelenterates were recorded by detailed quadrat mapping of the bedding surface. The colonial coelenterate fauna (exclusive of branching Favositinae) consists of 70.8 per cent tabulate corals, 25.5 per cent Stromatoporoids, and 3.7 per cent compound rugose corals. These coexisted with numerous solitary rugose corals and pelmatozoans. Stromatoporoids and some tabulate corals concentrated in north–south linear bands in response to ecological conditions, whereas colonial rugose corals were apparently randomly distributed Concentrations of branching Favositinae fragments mark the growth-sites of large colonies. Significant positive and negative linear correlations of paired taxonomic groups indicate tendencies of preferred distribution and concentration among numerically abundant groups. The preferred orientation of the long axes and distal ends of recumbent elongate and massive corals is east–west. Combined evidence suggests that a gentle east–west tidal current, with a stronger easterly component, was responsible for the orientation. The distribution of colonies in the Devonian biostrome is in some ways analogous to the distribution patterns of modern non-reef corals on the South Florida carbonate platform.

Palynology and sedimentology of Holocene deposits in southern Lake Michigan

An examination of the fossil pollen in the sediment shows that the upper part of the Waukegan Member (Holocene) of the Lake Michigan Formation can be divided into four pollen assemblage zones on the basis of subtle changes in percentages of the dominant types of pollen and in total numbers of pollen grains per gram of sediment. Three of these zones are distinguished on the basis of the relative abundance of pine and oak pollen, and the fourth is characterized by a high percentage of ragweed pollen. Zone 1, the oldest, has lower percentages of pine and higher percentages of oak than the other zones. The total pollen concentration in the sediment is highest in zone 1. Zone 2 is distinguished from zone 1 by a lower concentration of total pollen. Zone 3 has the highest percentages of pine. Zone 4 is distinguished by high percentages of ragweed, a result of forest clearance for agriculture about 1840 A.D. On the basis of the ragweed increase, calculated sedimentation rates in southern Lake Michigan for the postsettlement period are 0.49 mm/yr for the central area of the southern basin of the lake and 0.9 to 1.02 mm/yr for areas along the east side of the lake. In agreement with results of studies of the other Great Lakes, the pre settlement sedimentation rates in Lake Michigan were generally lower than the postsettlement rates.

Sedimentary Facies and Distribution of Biota in Coupon Bight, Lower Florida Keys

Coupon Bight is a 9-sq-km bay separated from an offshore Inner Reef Tract by low-lying islands and a bay-mouth bar. Coupon Bight has less wave activity and less complete tidal exchange than Inner Reef Tract waters, and is no more than 250 cm deep. Temperatures and salinities in Coupon Bight are more variable and usually higher than those in waters of the Inner Reef Tract. The harsher environmental conditions of Coupon Bight are reflected in its less diverse biota, compared to that of similar areas in the Inner Reef Tract. Environmental gradients persisting between the various parts of Coupon Bight produce more pronounced faunal variations than are found in the more homogenous Inner Reef Tract. Physical and chemical conditions, sedimentary facies, and biofacies of the bight are closely related. Several major environments can be recognized in Coupon Bight, each characterized by a particular assemblage of organisms. Open bay, nearshore, restricted bay, baymouth bank, and tidal channel environments are present. The unconsolidated sediment of Coupon Bight is mainly autochthonous skeletal debris, with minor contributions washed in from the Inner Reef Tract and from eroded limestone bedrock. The texture of bight sediments ranges from sand to mud, with most of the mud facies found in bank-like deposits associated with thick growths of the marine grass Thalassia. Most of the fine-grained sediment in Coupon Bight is the product of disintegration of coarse shell debris. Fine materials are winnowed out by waves and currents and either trapped on Thalassia covered mud banks or carried out of the bight. The variability of and relations between environments, sedimentary facies, and biofacies that can occur in a small area such as Coupon Bight are clarified by the data collected during this study. As a result, similar variability in shallow-water marine carbonate rocks can be better interpreted.