Contributions to Geology 2.2

Coralline algal microatolls near Cozumel Island, Mexico

DONALD W. BOYD University of Wyoming, Laramie
LOUIS S. KORNICKER Texas A & M College, College Station, Texas
RICHARD REZAK and Shell Development Co., Houston, Texas

Pages
105-108

Keywords
microatolls, coralline algae, Cozumel Island, Mexico

Abstract
Microatolls of coralline algal encrustations were discovered during a reconnaissance study of environments around Cozumel Island, 14 miles off the northeast coast of the Yucatan Peninsula. One hundred-sixteen of them were counted in one small area along the northeastern shore in a belt approximately 100 feet wide, 150 yards offshore, and parallel to shore. The microatolls are roughly circular with diameters between 12 and 25 feet. The outer margin of each is outlined by an algal ridge several inches wide and a few inches above sea level. This rim, enclosing a shallow "lagoon", is especially prominent because it supports a luxuriant growth of upright, non-calcareous algae.

Many of the microatolls are within 15 feet of one another; the deepest passages observed between them are 14 feet deep. The microatolls form the upper rims of cylindrical structures with vertical to slightly overhanging sides. The sides are covered by coralline algae including Goniolithon solubile, Archaeolithothamnium episporum, Lithothaminium sejunctium, Epilithon membranaceum, Lithophyllum sp., and Lithoporella sp. Living coral colonies cover only a small percent of the vertical sides, but most samples show coral beneath the algal encrustation. The origin of the microatoll foundations is speculative.

Superposed deformations in Precambrian rocks near South Pass, Wyoming

RONALD G. WORL Department of Geology, University of Wyoming, Laramie

Pages
109-116

Keywords
metamorphism, deformations, Precambrian, South Pass, Wyoming

Abstract
The Anderson Ridge quadrangle (Pl. 1) was mapped on a reconnaissance scale while ten square miles in the western part of the quadrangle were mapped in detail. In the detailed area, Palmer Draw (Pl. 2), a peraluminous granite stock and related pegmatites have been emplaced in pelitic and quartzo-feldspathic metasediments. The metasediments are mainly quartz-biotite-plagioclase and quartz-biotite-plagioclase-paragonite porphyroblastic "peanut" schists with a contact zone surrounding the granite stock.

The metasediments of the Palmer Draw area have been complexly folded and faulted and their field relations and geometry indicate three episodes of deformation. The area was divided into two fields of approximate structural homogeneity for analysis. The fabric of field 1 indicates flexural folding of S, a well-defined foliation parallel to the original bedding, around a nearly horizontal fold axis Bsl. This is thought to represent the first episode of deformation. In field 2 the fabric of S and L2 lineations, minor fold axes, indicate flexural folding of S around a nearly vertical fold axis Bs2. This episode of folding affected primarily the steep limbs of the previous Bs folds. Field relations and the projection pattern of the L3 lineations, biotite traces, suggest a penetrative surface S1. S1 is planar through the Bs2 minor folds which they transect at various angles. This and other evidence indicates that S1 is later than the Bs2 folds and is superimposed on the Bs folds. The last episode of deformation then involved faulting and passive folding by movements on S1.

It is thought that the granitic emplacements preceded the Bs2 folding. This is consistent with evidence of a thermal metamorphism preceding a regional metamorphism.

The Bs1 and Bs2 folds are thought to be Precambrian in age. S1 may be Laramide in age and related to the overthrusting of the western slope of the Wind River Range.

Preliminary results of a restudy of Corosaurus alcovensis case, the only known new world nothosaur

RAINER ZANGERL Chief Curator of Geology, Chicago Natural History Museum, Chicago, Illinois

Pages
117-124

Keywords
nothosaur, Corosaurus, alcovensis, Alcova, Triassic

Abstract
Restudy of the type specimen and preliminary study of additional material of alcovensis led to the conclusion that Corosaurus is the most advanced nothosaur known to date. Its shoulder and pelvic girdles show specializations resembling those of Liassic plesiosaurs. The structure of the limbs also shows advanced aquatic specialization, but not in the direction of plesiosaurian paddles. This conclusion agrees with stratigraphic evidence to suggest that the Alcova limestone is of late Triassic age.

Mineral assemblages in the Popo Agie member of the Chugwater Formation, west-central Wyoming

M. DANE PICARD and LEE R. HIGH, JR. Department of Geology, Princeton University, Princeton, New Jersey

Pages
125-128

Keywords
mineral assemblage, Popo Agie, Chugwater, Lander, Wyoming

Abstract
Two main mineral assemblages are present in the upper part of the Popo Agie member of the Chugwater (Triassic) formation near Lander, Wyoming. These are: 1) quartz, montmorillonite and analcime; and 2) quartz, illite (shredded muscovite) and feldspar. The upper zone is 45.3 feet thick and is characterized by the quartz-montmorillonite-analcime assemblage. Montmorillonite in association with analcime suggests that the analcime may have originated from volcanic material.

Helical burrows as fossil movement patterns

HEINRICH TOOTS Department of Geology, University of Wyoming, Laramie

Pages
129-134

Keywords
helical, burrows, fossil, gravity

Abstract
The helical shape of certain burrows is interpreted in terms of two components of movement which determine the helical path of the burrowing organisms. One is a horizontal component of circular motion as a result of asymmetrical burrowing of a bilaterally symmetrical animal with paired appendages. The other component is inclination to the horizontal plane resulting from an oblique orientation to gravity. Helical burrows may be formed in a variety of environments by unrelated groups of organisms but individual types of burrows may have taxonomic or environmental significance in restricted parts of the geological column. A possible application in determining the direction of gravity at the time of the formation of these burrows would be more generally valid.

Origin of the Mowry shale

JOHN C. DAVIS Department of Geology, University of Wyoming, Laramie

Pages
135-146

Keywords
Mowry, shale, Cretaceous, Wyoming, marine

Abstract
The Mowry shale is a hard, highly siliceous black shale which is stratigraphically the highest Lower Cretaceous formation in Wyoming. W. W. Rubey has suggested that the Mowry shale originated as an altered volcanic ash, basing his ideas on a similarity between chemical analyses of the Mowry and analyses of bentonite. More recent analyses indicate that this similarity does not exist, and that the Mowry is the result of normal marine sedimentation. The presence of siliceous radiolarian tests and the absence of volcanic glass shards suggests that excess silica in the Mowry was derived from organisms and not from ash.

Mineralized xenoliths in the southern Wet Mountains, Colorado

ROBERT E. BOYER The University of Texas, Austin, Texas

Pages
147-150

Keywords
metamorphosed, xenoliths, Wet Mountains, Colorado, mineralized, Precambrian

Abstract
Three partially assimilated and strongly metamorphosed xenoliths occur within Precambrian granite near the southern end of the Wet Mountains, Colorado. The xenoliths contain calc-silicate rocks similar to types bordering the granite. Replacement deposits of sulphide mineralization rich in zinc occur within the xenoliths. The mineralization is of Precambrian age associated with the late stage of the granite intrusion. Although the deposits are not of commercial value, they afford an interesting example of mineralization by replacement.

Origin of the Wasatch Formation, Cumberland Gap area, Wyoming

JOHN C. LAWRENCE Department of Geology, University of Utah, Salt Lake City

Pages
151-158

Keywords
Wasatch, Cumberland Gap, Wyoming, Uinta Mountains, Wind River Mountains

Abstract
The main body and the New Fork and upper tongues of the Wasatch formation and the Fontenelle, middle, and upper tongues of the Green River formation are recognized in the Cumberland Gap area. The "main body of tbe Wasatch formation" corresponds to Oriel's (1962) LaBarge member of the Wasatch formation. The main body is a flood plain deposit derived from tectonically active areas to the weet of the basin. Both the New Fork and the upper tongues of the Wasatch formation are delta deposits that extend from their source areas, the Wind River Mountains and tbe Uinta Mountains respectively. The upper tongue and probably the New Fork are divisible into beds deposited in terrestrial areas and areas of lacustrine fluctuation. The terrestrial beds (nearest the source) are brilliant red beds, and the beds deposited in areas of lacustrine fluctuation (furthest from the source) are drab-green and variegated red and green beds. The New Fork and upper tongues of the Wasatch formation were deposited in response to tectonic uplift and not in response to a retreat of the lake shore. During Green River time, the first significant uplift was in the Wind River Mountains (as marked by the New Fork tongue) and this was followed by major uplift in the Uinta Mountains (as marked by the upper tongue of the Wasatch formation).