The varves and climate of the Green River epoch

Abstract

The Green River formation is a series of lake beds of middle :Eocene age which occupies two large intermontane basins, one in Colorado and Utah, the other in Wyoming. The formation averages about 2,000 feet in thickness and covers an area of more than 25,000 square miles. Many of its beds of marlstone, oil shale, and fine-grained sandstone contain varves. As the origin of these varves is closely linked with the climate the writer has attempted rough quantitative estimates of several elements of the climate of the Green River epoch. These esti mates are based largely upon the relative area of the lake and its drainage basin. A climate is 'postulated which was char acterized by cool, moist winters and relatively long, warm Bummers. Presumably the temperature fluctuated rather widely from a mean annual temperature that was of the order of 65 0 F. The rainfall varied with the seasons and probably also fluc tuated rather widely from a mean annual precipitation between 30 and 43 inches.One type of varve predominates. This consists of a pair of laminae, one of which is distinctly richer in organic matter than the other. The contacts between the two parts of the varve and between successive varves are generally sharp. The varves differ considerably in thickness according to the type of rock in which they occur and range from a minimum of 0.014 millimeter in the beds of richest oil shale to about 9.8 milli meters in the beds of fine-grained sandstone. The average thickness of the varves, weighted according to the quantity of ·each type of rock in the formation, is about 0.18 millimeter.The assumption that the pairs of laminae are varves is tested, fir::;t, by analogy with the varves in the deposits of modern lakes and, second, by calculation of the thickness of annual laminae to be expected in the ancient Green River Lake based upon data of present stream loads.The bipartite character of the varves is explained by postu . lating a more or less continuous sedimentation of mineral and organic constituents, with first a peak in the production of the carbonates and then a peak in the production of the plankton, both peaks apparently occurring during the summer, and by assuming that the primary difference in composition was accen tuated by the differential settling rates of the two principal constituents. The preservation of the varves suggests that the lake water was thermally stratified and that the lake may not have been more than 75 or 100 feet deep where the varved deposits accumulated. Three cycles of greater length than the varve cycle are sug gested by fairly regular recurrent variations in the thickness of the varves and in the thickness and character of certain b.eds and by the fairly regular spacing of certain salt-mold layers. The first of these cycles averaged a little less than 12 years in length and appears to correspond to the cycle of sunspot num bers. The second cycle had an average length of about 21,600 years and suggests the average period of about 21,000 years which is the resultant of the cyclic...

Chattanooga shale and related rocks of central Tennessee and nearby areas

“…Bradley (1929) and Rubey (1930) described apparent varves in the finely laminated Eocene and Upper Cretaceous shales of Wyoming and nearby areas. Both writers called attention to the alternation of laminae that are dominantly of mineral matter with those of organic matter; they mentioned the persistence of these layers as evidence of their accumulation in quiet water, presumably below wave base.…”

Cyclic Sedimentation along the Continental Margin of Northwest Africa

“…Sediment types range from Pleistocene deep-sea sediments, to Eocene and Triassic lacustrine sediments (Bradley, 1929;Van Houten, 1964;Anderson, 1964), Miocene cherts (Bramlette, 1946;Anderson, 1964), Cretaceous limestone and chalk (Gilbert, 1895;Winkler, 1926), Pennsylvanian limestone (Anderson, 1964), and Permian evaporites (Anderson et al, 1972).…”

Sedimentary cycles and volcanic ash beds in the Lower Pliocene lacustrine succession of Ptolemais (NW Greece): discrepancy between 40Ar/39Ar and astronomical ages

“…Orbital influences are not restricted to open marine systems, but are found in all types of sedimentary environments, including lacustrine successions (Bradley, 1929;Van Houten, 1964;Fischer, 1980;Anderson, 1982). Ideally, sections with a continuous cyclic pattern and good time control are used in studying the role of orbital forcing.…”