Portland State University. Department of Geology
Richard E. Thoms
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geology
Geology -- Oregon -- Grant County, Geology -- Oregon -- Crook County, Ammonoidea
1 online resource (vi, 183 pages)
Thirty three species of ammonites are recorded from the composite type section of the Weberg Member of the Snowshoe Formation in the Suplee area, Grant County, Oregon. Holcophylloceras burkei, Euhoploceras westi, E. tubereulosum and Strigoceras taylori are described as new, while four new species belonging to the genera Sonninia [?], Bradfordia, Pseudotoites and Witchellia are not formally named. Three ammonite zonules characterize the ammonite sequence of the type Weberg composite section. The sequence correlates with parts of the standard lower and middle Bajocian (Jurassic) of northwestern Europe. In addition, four associations (paleo-communities) of benthic mega invertebrates, the Gervillia, Protocardia, Isocyprina and Bositra buahii associations are delineated.
The type Weberg composite section is a record of a local marine transgression westward onto an island system. The section also represents sedimentation over an irregular pre-Snowshoe topographic high, and indicates a progressive change from proximal to distal source of pre-Snowshoe sediments, from high to low energy conditions, and perhaps a slight deepening of the ocean bottom.
Ammonites are rare in the lower division of the Weberg Member, locally present in fine sandy limestones of the lower part of the upper division, abundant and most diverse in silt-rich, clay-poor limestones of the upper part of the upper division, and locally abundant in mudstones of the Warm Springs Member. Recurrent associations of certain ammonite species, strong correlation of the associations with lithofacies and biofacies, and pervasive faunal differences of ammonites between facies indicate in general that the distributional patterns of the ammonites reflect spatial life-habitats. The spotty yet widespread geographic occurrence of several ammonite species suggest they had undergone extensive planktic dispersal, thus ocean currents probably played an important role in their distributional patterns.
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Taylor, David G., "Biostratigraphy of the Type Weberg Member, Snowshoe Formation, Grant County, Oregon" (1977). Dissertations and Theses. Paper 2618.
Figure 2. Geological map of the Suplee area
Thesis_large-map.pdf (117675 kB)
Figure 4. Correlation of stratigraphic columns of the type section and reference sections of the Weberg Member
thesis_map1.pdf (23844 kB)
Figure 5. Columnar section of the type Weberg composite section