Contributions to Geology 5.1

Preliminary report on the Precambrian stromatolites in the Nash Formation, Medicine Bow Mountains, Wyoming

SAMUEL H. KNIGHT Department of Geology, University of Wyoming
DAVID K. KEEFER 5920 Taft Court, Arvada, Colorado

Pages
1-12

Keywords
stromatolites, Nash Formation, Medicine Bow Mountains, Wyoming, bioherms, metasediments, Precambrian

Abstract
The field relations and megascopic characters of stromatolites and bioherms occurring in the lower half of the Nash Formation are described and illustrated. The Nash Formation as defined is part of a succession of metasediments aggregating 25,000 feet thick occurring in the Precambrian complex of the Medicine Bow Mountains. Radiometric dating gives the age of this succession as ranging from 1.9 billion years to 1.65 billion years B.P. The Nash Formation is believed to be not less than 1.7 billion years old.

Field studies of the stromatolites and bioherms support the conclusion that they are organo-sedimentary features formed by lime-secreting algae or algal-like organisms.

Structural analysis of the Precambrian rocks southeast of Encampment, Wyoming

CLINTON S. FERRIS, JR. Mineral Division, Kerr-McGee Corporation, Casper, Wyoming

Pages
13-20

Keywords
Precambrian, Encampment, Wyoming, synform, foliation, gneiss

Abstract
Approximately 20 miles of Precambrian terrain forming the lower northeast flank of the Sierra Madre Mountains between Beaver Creek and the Encampment River were mapped. The bulk of the Precambrian rock consists of gray and pink quartzo-feldspathic gneisses. Gabbroic dikes intruded the gneiss and were largely metamorphosed to amphibolites. The gray gneiss and the orthoamphibolite were veined by pink quartzo-feldspathic aplites and pegmatites, some of which are gneissic. Pink gneiss also locally replaced gray gneiss. In the western part of the area some quartzo-feldspathic gneisses and dikes contain kyanite.

Equal-area projections of foliation attitudes, together with field observations, suggest that the structure of the gneiss in the northern part of the area is that of a northerly plunging synform. This structure probably antedates intrusion by the amphibolite. Penetrative componental shearing, post-dating intrusion by amphibolites, resulted in structural transposition which largely obliterated the synformal structure in the south, west, and east. As a result, foliation in rocks of the western and southwestern transposed areas strikes northwest and dips steeply either north or south. In the southern and southeastern transposed areas foliation dip remains steep, but the strike gradually changes to east-west and northeast, suggesting a flexture of the rock mass after transposition.

Pliocene vulcanism, southern Absaroka Mountains, Wyoming

D. L. BLACKSTONE, JR. University of Wyoming, Laramie, Wyoming

Pages
21-30

Keywords
volcanic, cone, ejecta, Pliocene, Absaroka, Wyoming, Wiggins

Abstract
A volcanic cone comprised of volcanic ejecta and basalt flows is located in south-west quarter of T. 45 N., R. 108 W., Park County, Wyoming. The base of the cone is at an elevation of 10,800 feet, and rests upon Oligocene Wiggins Formation. Whole rock analysis of the flow gave a K-Ar age of 3.6 (+ 1.0) x 106 years. Total chemical analyses are included.

Petrography of Precambrian rocks from a 3.05-kilometer-deep borehole, Wind River Mountains, Wyoming

RICHARD J. EBENS and SCOTT B. SMITHSON Department of Geology, University of Wyoming, Laramie, Wyoming

Pages
31-38

Keywords
Wyoming, Wind River Mountains, borehole, gneiss, mineralogy

Abstract
A borehole drilled through 3.05 km of Precambrian rock is located on the west flank of the Precambrian core of the Wind River Mountains, Wyoming. Rock types cored are granitic and range from quartz-dioritic gneiss to quartz-monzonitic gneiss; texture ranges from fine grained to porphyroblastic. With increasing depth hornblende, clinopyroxene, and orthoclase appear in the granitic mineral assemblages, but orthoclase alternates with maximum microcline at different depths. Mineralogy suggests an increase in grade with depth; however, retrograde features are found sporadically throughout thc borehole. Final mineral assemblages attained seem to be largely conditioned by local variations in concentration of H2O rather than just P and T. The rocks cored are granitic in generaI character but highly heterogeneous in detail.

A Fortran IV program for the determination of rotation axes from fabric data

JAMES C. KELLEY Department of Geology, University of Wyoming, Laramie, Wyoming

Pages
39-44

Keywords
fabric, rotation axes, computer program

Abstract
Graphical techniques for the analysis of fabric data are not wholly satisfactory due to their inherent imprecision. The difficulties of the graphical approach may be overcome by direct mathematical analysis. Given a simple assumption, rotation axes exhibited by fabric data may be determined by a least squares criterion.

Lamination deformed by burrowers in Flathead Sandstone (Middle Cambrian) of central Wyoming

DONALD W. BOYD Department of Geology, University of Wyoming, Laramie, Wyoming

Pages
45-54

Keywords
Flathead, sandstone, Wind River, Wyoming, burrow, Germany,

Abstract
The upper strata of the Flathead Sandstone (Middle Cambrian) exposed in a road cut in Wind River Canyon, Wyoming, contain abundant burrows with associated deformed laminae. The burrows are long, slender, sharply defined cylinders of sandstone normal to stratification. In certain laminated intervals burrows are 25 mm to 50 mm apart, and downbending of laminae near each burrow results in a distinctive pattern of convex-upward laminae between adjacent burrows. A similar phenomenon has been described in Germany.

In unoriented hand specimens, deformed laminae can be mistaken for concave-upward increments associated with U-shaped burrows, with resulting misidentification of stratigraphic top. In outcrop, downbent laminae cause difficulty in determining whether burrows originally had funnel-shaped openings.

Experiments suggest that extent and type of downbending adjacent to Flathead burrows reflect subsurface removal of sediment rather than drag caused by downward penetration of the burrower. Continuity of sharply defined burrow walls through deformed intervals implies upward extension of stabilized walls in response to continued sedimentation during the burrower's lifetime.

Preliminary gravity study of the southern Laramie Mountains: anorthosite areas and adjacent basins

DENNIS S. HODGE Department of Geology, University of Wyoming, Laramie

Pages
63-68

Keywords
anorthosite, Laramie Mountains, gravity, Precambrian, Sherman Granite

Abstract
The southern Laramie Mountains complex which is a broad asymmetrical anticline with Precambrian rocks exposed in the core, is bounded by the Denver-Julesburg and Laramie Basins. A Bouguer gravity anomaly map covers syenites, norites, hornblende gneiss, Sherman Granite, anorthosite, and flanking sedimentary basins. Minimum values of -220 mgal are obtained west and east of the range in sedimentary basins. In the north, a high value of -108 mgal is found over hornblende gneiss and a low value of -150 mgal is found in the Bull Camp Peak Granite gneiss; a gravity profile indicates that the Granite gneiss-hornblende gneiss contact dips moderately northeast. An unusual occurrence is the positive anomaly associated with a hornblende syenite body. Relatively low values ranging from -145 to -165 mgal are found over anorthosite and high values of -175 mgal are obtained over the Sherman Granite to the south. A hypothetical model indicates that the density contrast between the Sherman Granite and anorthosite exists to a depth of about 7 km. Values in Precambrian rocks decrease from -108 mgal in the north to -175 mgal to the south.

Gravity anomalies within the Precambrian core of the uplift are much greater than anomalies between the Precambrian and adjoining basins which have sedimentary sections up to 9000 feet thick in the area studied. A general decrease in Bouguer anomalies within the Precambrian from the Laramie Mountains to the Sierra Madre is attributed to westward thickening of the crust due to isostatic compensation.

The interpretation of folded transposition layering near Twin Mountains, Laramie Range, Wyoming

JOHN S. KING Department of Geological Sciences, State University of New York at Buffalo
RONALD B. PARKER Department of Geology, University of Wyoming

Pages
63-68

Keywords
fold, Laramie Range, Twin Mountains, Wyoming, sedimentary layering, metamorphic

Abstract
The detailed study of a megascopic fold in the southern Laramie Range of Wyoming demonstrated that the prominent foliation present in the metamorphic series is not preserved sedimentary layering. This is so in spite of the fact that the map units are of approximately equal thickness and are continuous over significant distances within the mapped area. Furthermore an intimate relationship between one of the fine grained foliated metamorphic rock units and a pegmatite of similar mineral paragenesis suggests derivation of the latter from the former by recrystallization controlled by the structure and by the mineral association of the rock type involved.