Contributions to Geology 7.1

Dendritic surge marks ("Dendrophycus") along modern stream banks

LEE R. HIGH, JR. Department of Geology, Oberlin College, Oberlin, Ohio
M. DANE PICARD Department of Geology, University of Nebraska, Lincoln, Nebraska

Pages
1-6

Keywords
dendrophycus, rill-mark, Wyoming, Colorado, dendritic, surge marks

Abstract
Dendrophycus was originally described as the imprint of a marine alga of uncertain affinities. Although finds were few and scattered, three species were ultimately described. Because of its lack of resemblance to any living or fossil algae, Dendrophycus has long been suspect and is now generally considered to be a rill-mark. We have seen several structures identical to Dendrophycus along creek banks in Wyoming and Colorado. These structures were observed to form as flood waters surged against the bank and eroded a modified dendritic pattern. The requirements for formation are: 1) steep slope, 2) semi-consolidated silt or fine sand, 3) standing water, and 4) surging currents. These conditions occur in marine and paralic settings, as well as fluvial. Because the structure is inorganic, the name "Dendrophycus" should be abandoned; in its place we suggest the term dendritic surge marks.

A computer program for representation of mineral chemical analyses in terms of end member molecules

KENNETH PERRY, JR. Department of Geology, University of Wyoming, Laramie, Wyoming

Pages
7-14

Keywords
chemical analysis, molecular, mineral, oxides

Abstract
This program is designed to compute a given mineral chemical analysis in terms of a specific collection of molecular members or molecular end members. The method employed is quite general and may be extended to a mineral chemical analysis composed of any number of oxides. The program as written, however, is limited to those analyses composed of at most eleven oxides.

The program is written in Fortran IV language for the Philco 2000 computer.

The stratigraphy and volcanic history of the Karmutsen Group, Vancouver Island, B.C.

RONALD C. SURDAM Department of Geology, University of Wyoming, Laramie

Pages
15-26

Keywords
Karmutsen, Vancouver Island, volcanic, breccia, Triassic, Buttle Lake, Canada, British Columbia, Mesozoic, Cordilleran, eugeosyncline

Abstract
The Karmutsen Group is part of the volcanic rich Mesozoic section which characterizes much of western British Columbia, Canada. A study of the Karmutsen Group adds significantly to our knowledge of the volcanic activity which characterized the early stages of the Cordilleran eugeosyncline. Furthermore the Mesozoic volcanic sequence exposed in the central part of Vancouver Island is particularly well suited for investigation, for it is part of a very thick and complete succession that is neither highly metamorphosed nor strongly folded.

Seventeen separate lithologic units were mapped in the Buttle Lake area of central Vancouver Island. In this area the Karmutsen Group is approximately 18,000 feet thick and can be subdivided into at least five mappable units. The Karmutsen Group conformably overlies a sequence of interbedded Triassic(?) argillites and volcanic flows, and it is conformably overlain by the Upper Triassic Quatsino Formation. The stratigraphy in the Buttle Lake area suggests that the minimum depth which the top of the Karmutsen Group has been buried was 5,000 feet and the maximum depth was probably never greater than 15,000 to 20,000 feet.

During the time when the Karmutsen Group was formed the Cordilleran eugeosyncline in western British Columbia was characterized by massive volcanic activity. This volcanic activity was cyclic in nature; generally each cycle can be divided into four distinct parts: 1) volcanic quiescence, during which time biogenetic limestone was deposited, 2) accumulation of close-packed pillow lavas, pillow breccias, and aquagene tuffs, 3) extrusion of thin-layered volcanic flows, and 4) extrusion of thick-layered massive volcanic flows. These four parts can be interpreted as being the result of a steady increase in extrusion rates during each volcanic cycle.

An Ichthyosaur skull from the Cretaceous of Wyoming

ALFRED SHERWOOD ROMER Museum of Comparative Zoology, Harvard University, Cambridge, Mass.

Pages
27-42

Keywords
ichthyosaur, Mowry Shale, Cretaceous, temporal, euryapsid, Wyoming

Abstract
An ichthyosaur skull from the Mowry Shale of Wyoming -- the only good ichthyosaur skull so far known from the Cretaceous -- is described and figured. Study of the well-preserved temporal region leads to the conclusion that the dermal element usually described in ichthyosaur skulls above the quadratojugal is non-existent; that, hence, the large element bounding the temporal opening is the true squamosal, and the opening is thus of a typical euryapsid pattern. Illustrations of a Liassic ichthyosaur prepared by D. M. S. Watson are appended.

The effect of fire on geomorphic processes in the San Gabriel Mountains, California

DONALD O. DOEHRING: Department of Geology, University of Wyoming, Laramie

Pages
43-66

Keywords
erosion, San Dimas, San Gabriel, California, drainage, forest fire, slope

Abstract
The San Gabriel Mountains consist of highly fractured crystalline rocks which weather to azonal, sandy loams. The climate is Mediterranean, and rainstorms of high yield and high intensity are common. The predominant vegetation except at the higher elevations is chaparral. These factors along with a mean slope angle of 68% produce high erosion rates.

Normal erosion, by definition is not influenced by previous forest fires; it consists of two major processes. The sliding of material from the slopes into slope channels and areas of low relief is most active during the summer when soil moisture and cohesion are low. During winter months fluvial erosion, the second process, scours the debris from the channels. In recent years of normal erosion, the amount of material moved into intramontane areas during the summer months far exceeds the amount of sediment removed from these areas during the winters. The relatively high drainage density and lack of substantial intramontane deposits of alluvium and colluvium indicate the operation of some process other than those of normal erosion.

A detailed study of the accelerated erosion during a seven year period following a forest fire in the San Dimas Experimental Forest has shown that this fire-flood sequence has, in a short time, removed the products of some 40 years of normal erosion. This process is thought to be the essential non-normal element in the formation of the present landforms.

Bedrock configuration and altitude, Red Rock Pass, outlet of Lake Bonneville, southeastern Idaho

J. STEWART WILLIAMS and JAMES H. MILLIGAN Utah State University, Logan, Utah

Pages
67-72

Keywords
Lake Bonneville, Red Rock Pass, channel, Marsh Creek, Snake River, alluvium, bedrock surface

Abstract
The bedrock threshold over which Lake Bonneville drained to the Snake River is covered with post-lake alluvium so that its exact position, configuration, and altitude cannot be seen. The altitude is of particular significance in the history of the lake.

The bedrock surface was explored using a 12-channel seismograph. Control for the shot points was provided by a plane-table map of the channel between marginal bedrock outcrops.

The threshold lies beneath the highway where it swings across the channel. The elevation of its crest probably exceeds 4755 feet by 1 or 2 feet over most of its length, but this cannot be determined more exactly because of the presence of the highway fill. The width of the channel on the crest is about 800 feet. The threshold trends N 85¡ W, oblique to the channel, which trends N 30¡ W. The slopes approaching the threshold are generally steeper at the east side of the channel, perhaps indicating that the threshold is slightly lower on the east end, and that in the last stages of flow, the stream was at the east side of the channel.

The post-lake fill is created in part by slope wash and small intermittent streams from the sides of the channel. The major part of the fill is contributed by the right-hand fork of Marsh Creek which furnishes mostly fine-grained alluvium by its erosion of Tertiary and Quaternary beds that flank the bedrock ridge in which the channel is cut. The depth of the fill on the threshold is about 20 feet.