Contributions to Geology 11.1

Two Late Pinedale advances in the southern Medicine Bow Mountains, Colorado

EUGENE P. KIVER Department of Geology, Eastern Washington State College, Cheney, Washington 99004

Pages
1-8

Keywords
Pinedale, Medicine Bow, Colorado, glacial, moraines

Abstract
Four Pinedale advances in the study area are recognizable by using downvalley extent of moraines, terminal moraine slope angles, and preliminary results of two new correlation techniques. One technique uses weight percent differences of fine material in moraine soils and the other involves measurement of sound pulses emitted by granite moraine boulders struck with a hammer. The five meter vertical separation of Pinedale 3 and 4 terraces suggests a shorter non-glacial interval than the eight to ten meters between Pinedale 2 and 3 terraces. Average moraine-front slopes for Pinedale 1 through 4 are 23 degrees, 18.3 degrees, 25.7 degrees, and 16.2 degrees respectively and differ from each other at the .05 confidence level.

The regression line, y = -.246 + 0.438x, describes the downvalley extent of Pinedale 4 (y) relative to Pinedale 3 (x) glaciers from the nine major cirque headwalls considered. The correlation coefficient of +.79 between x and y is significant at the .05 level. The regression line slope is inseparable at the .05 level from a similar equation (Graf, 1970) for the Beartooth Mountains. Thus, this relation may have wide applicability in the Rocky Mountains.

The three to six meter high Pinedale 4 moraines are easily overlooked. Their location on or near a relatively flat high level erosion surface in the study area is fortuitous and conducive to moraine preservation. Thus, additional Pinedale 4 glaciation evidence should be particularly sought where high ice caps existed. Formal proposal of a fourth Pinedale stade should await the time when strong stratigraphic, rather than mainly geomorphic, evidence becomes available.

The systematics of Cretaceous Ichthyosaurs with particular reference to the material from North America

CHRISTOPHER McGOWAN Royal Ontario Museum, Toronto, Ontario

Pages
9-30

Keywords
Cretaceous, Ichthyosaurs, fin, Platypterygius

Abstract
A Review is made of the Cretaceous Ichthyosaurs. It is shown that only four of the species recorded from Eurasia are valid, and that these are to be referred to the longipinnate genus Platypterygius.

The North American material is discussed, and the most completely known fore fin is described. It is shown that this is longipinnate and that the North American specimens should be referred to the single species americanus. Hitherto unpublished photographs of Australian material are given. Comparisons made between Australian, North American and Eurasian species show them to be so closely similar that they must be included within the same genus, Platypterygius.

Mafic dikes of the Clear Creek drainage area, southeastern Bighorn Mountains, Wyoming

THEODORE J. ARMBRUSTMACHER U. S. Geological Survey, Menlo Park, California 94025

Pages
31-40

Keywords
dikes, Clear Creek, Bighorn Mountains, Wyoming, mineralogy

Abstract
Tholeiitic magma was emplaced as dikes at two different periods in the southeastern Bighorn Mountains. The older metadolerite dikes (~2.5 b.y.) are sheared and metamorphosed, resulting in clouded minerals and amphibolitized margins. The primary mineralogy of these dikes is characterized by either orthopyroxene or pigeonite with augite and an exsolved pyroxene phase; these pyroxene assemblages are characteristic of the middle stages of tholeiitic fractionation. The younger quartz dolerite dikes (about 2 b.y.) postdate the metamorphism and contain only augite-pigeonite. Some thicker quartz dolerite dikes have coarse-grained gabbroic cores which represent more extreme differentiation, in the direction of enrichment of alkalies and silica, than found in the older dikes.