Contributions to Geology 3.1

Some criteria for recognition of braided stream deposits

H. THOMAS ORE Geology Department, Idaho State University, Pocatello, Idaho

Pages
1-14

Keywords
braided streams, foreset, paleocurrent, longitudinal bar, transverse bars

Abstract
Observations of modern and ancient braided stream deposits supplemented by stream table experiments show certain criteria to be useful in recognizing the deposits in the stratigraphic record. Braiding commonly results where partially incompetent streams form longitudinal bars by depositing coarse elements of bed load which trap finer elements. Transverse bars formed of better sorted sediment are tabular, wedge-shaped bodies with foreset cross-strata. They may either be the only depositional form or may be deposited on downstream ends of longitudinal bars.

External geometric criteria are of less use than internal ones. Longitudinal bars exhibit asymmetric inclined surfaces, terraces, and transverse bar wedges on their downstream ends. Transverse channels commonly cut across longitudinal and transverse bar surfaces.

Internal geometric features are easily recognizable and yield paleocurrent information. Statistically treated preferred orientations of flat-surfaced pebbles give population characteristics and allow estimates of stream direction during deposition. Larger pebbles are more reliable indicators of downstream direction. Two types of sediment, separable by sorting characteristics, characterize braided stream deposits. Upstream ends of longitudinal bars have coarser, more poorly sorted sediment than do downstream ends and transverse bars. Sorting characteristics aid in recognizing braided stream deposits. Discontinuous, horizontal stratification is common in the deposits. Foreset cross-strata exhibit grading along the inclined laminae, and the sediment is more poorly sorted at the bottoms than at the tops. Foreset, bed-roughness and trough-fill cross-stratification characterize braided stream deposits. The foreset variety is most abundant, can be a valuable current indicator and has certain high-dispersion population characteristics. Deformed stratification is present locally.

Braided stream deposits are commonly found where abundant sediment and high discharges were available during deposition.

Non-paleontological methods of correlation of rocks of Tertiary age in Wyoming: Part III - the petrographic calendar

R. S. HOUSTON Department of Geology, University of Wyoming, Laramie

Pages
15-26

Keywords
volcanic, Yellowstone, Absaroka, bulk compositions, Tertiary

Abstract
A review of bulk compositional changes in phenocryst minerals and rock fragments of volcanic origin in volcanic sedimentary rocks of Tertiary age in Wyoming indicates that the cyclic sequence of volcanic eruptions in the Yellowstone-Absaroka Volcanic Field is reflected to some degree in the composition of volcanic sedimentary rocks of the basins. Rocks of late early Eocene age, upper Eocene age, early lower Oligocene age, middle Miocene age, and Pliocene age have consistent bulk compositions. This suggests that petrographic studies of volcanic sedimentary rocks may be useful in stratigraphic correlation.

Pseudo rib-and-furrow marks in the Chugwater (Triassic) Formation of west-central Wyoming

M. DANE PICARD Department of Geology, University of Nebraska, Lincoln, Nebraska
LEE R. HIGH, JR. Department of Geology, Rice University, Houston, Texas

Pages
27-32

Keywords
paleocurrent, Chugwater, Triassic, Wyoming, ripple mark

Abstract
Ripple marked surfaces with the same surface expression as rib-and-furrow marks are found in the Chugwater formation of Wyoming, but the paleocurrent directions obtained from these two current direction indicators are mutually perpendicular. These pseudo rib-and-furrow marks are apparently the result of truncation of ripple marks in which the crests of the laminae forming the ripple mark and the crest of the ripple mark are not parallel. The mechanism (or mechanisms) which produced this angular difference is unknown. Observations of the Chugwater structures suggest that rib-and-furrow marks and pseudo rib-and-furrow marks originate in several ways, and that their usefulness as a paleocurrent direction indicator depends on their mode of formation and their relationship to other paleocurrent direction indicators.

Cooper Hill - a gravity slide in the northeastern Medicine Bow Mountains, Wyoming

JOHN S. KING Department of Geological Sciences, State University of New York at Buffalo

Pages
33-37

Keywords
Cooper Hill, Medicine Bow Mountains, slide, Arlington thrust

Abstract
Gravity emplacement is suggested to explain the anomalous physical and structural relations of three blocks of Precambrian rock resting on Tertiary sediments along the east flank of the Medicine Bow Mountains in southeastern Wyoming. The largest of these blocks is Cooper Hill - about 2 miles long and 1/2 mile wide. During Laramide time, the Medicine Bow Mountains were thrust east along the northwest trending Arlington thrust fault whose position may be demonstrated in much of the area. Near Cooper Hill, however, ground cover is heavy and loss of control on the position of the fault trace results. Several lines of evidence indicate the position of the fault trace is west of Cooper Hill close to the main slope break of the mountain front which defines the fault trace farther north. A structural plot of the main Precambrian complex compared with one of Cooper Hill indicates that the Hill is structurally independent.