Petrography of Northeastern Lake Michigan Bottom Sediments

Abstract

ABSTRACT A direct relationship between sediment properties and their environment of deposition, source material, and post-glacial history was shown by textural and mineralogical studies of cores and dredge samples obtained from northeastern Lake Michigan. The lake bottom topography of the area, which may be characterized as a valley-and-ridge province, exerts a very strong control on the distribution of sediments. In general the coarse sediments are found on the ridges and their adjacent flanks and fine sediments are restricted to the hollows between ridges. There is a regular variation in size with depth, but no regular variation in size with relation to distance from shore. The majority of bottom sediments are well sorted, but most of the samples from the lower portion of the flanks of troughs show poor sorting, probably the result of slumpage of coarser sediments from the higher portions on the flanks. Evidence for the Lake Chippewa low-water stage is shown by the following: Core samples taken from depths of water less than 275 feet contain coarse sediment overlying, underlying, or included within a sequence of much finer grained silty clay or clayey silt. The presence of shells of shallow-water forms in some of these coarse sediment zones and a peat deposit found in association with this coarse sediment give additional evidence of a low-water condition. The similarity of the mineralogy of the detrital sand-sized material in the lake sediments to that of the glacial drift from the adjacent shore shows that the lake sediments were derived mainly from that source. The general order of abundance of clay minerals in both the lake sediments and glacial till material from the adjacent shore is illite, mixed-layer chlorite and illite materials, and chlorite. Little variation in the clay mineral composition with changes in depth of water, distance from shore, or depth of burial was observed. In general the carbonate content of the lake bottom samples increases with a decrease in the median diameter of the sediment. There is, however, a lower carbonate content in the dark-gray sediments found in the deep troughs and in Little Traverse Bay: this is attributed to the solution of calcium carbonate as a result of a decrease in the pH within the sediment. Pyrite, in association with calcareous shell material, was found in many core samples, but not in glacial till samples from the adjacent land areas. The pyrite is interpreted as secondary, resulting from local reducing conditions produced by the decay of soft parts of the shelled organisms.