Contributions to Geology 29.1

Multituberculate mammals from the Lower Cretaceous Cedar Mountain Formation, San Rafael Swell, Utah

JEFFREY G. EATON Department of Geology, Museum of Northern Arizona, Flagstaff, Arizona 86001
MICHAEL E. NELSON Division of Sciences, Northeast Missouri State University, Kirksville, Missouri 63501

Pages
1-12

Keywords
multituberculate, Utah, Paracimexomys, Cretaceous, Cedar Mountain, ptilodontoids

Abstract
Late Albian multituberculates recovered from the Cedar Mountain Formation in central Utah are diverse and include several species of Paracimexomys. Two new species of Paracimexomys are named, and two other new unnamed species are referred to the genus, one with query. Paracimexomys and Cimexomys are tentatively assigned to the Ptilodontoidea. Two multituberculate taxa are described and not assigned to any genus. These two taxa suggest that at least one family other than the unnamed family represented by Paracimexomys is present in the sample. The diversity of ptilodontoids by the late Albian suggests an older origin for the suborder.

Cenozoic history of the Saratoga Valley area, Wyoming and Colorado

JOHN MONTAGNE Department of Earth Sciences, Montana State University, Bozeman, MT 59717

Pages
13-70

Keywords
Wyoming, Colorado, Cenozoic, Medicine Bow, Laramide, Hanna, Coalmont, thrust, White River

Abstract
The Saratoga Valley area is a northwest trending synclinal depression extending northward into Wyoming from the North Park Basin, Colorado, between the Medicine Bow Mountains and Sierra Madre-Park Range. Broad structural outlines were established during Late Cretaceous time by contractional deformation of the Laramide Orogeny. The synclinal structure deepens with plunge north and south of a centrally elevated divide within the basic trough. By Paleocene time, sediments stripped from the higher parts of the area were deposited in swampy basins to the north and south, forming the Coalmont and Hanna Formations. These deposits overlapped truncated edges of older formations and filled the Saratoga Valley to moderate levels.

Renewed contractional activity during late Paleocene-early Eocene time downfolded North Park Basin and its new sedimentary blanket along previous trends. The surrounding region was similarly affected, and Independence Mountain thrust fault was emplaced across the trend of previous structure between North Park and Saratoga Valley. Erosion and quiescence followed, as clays, silts, and tuffaceous sands of the late Eocene White River Formation occupied terrain of high relief northeast of North Park.

In late Oligocene, extensional deformation warped the landscape, renewing erosion, and probably allowing the North Platte River to establish a channel northward through the Saratoga trough. Tributaries were diverted into this new drainage, and deposition of coarse basal debris-flow and fluvial units of the early to middle Miocene Browns Park Formation spread northward from the bounding mountains, north of Saratoga. Another extensional pulse caused the area north and northeast of Saratoga to drop abruptly, defining the true boundary of the basin and interrupting the previous deposition. Ponding lead to fining of the basin sediments as marginal debris mingled with Platte River fluvial conglomerates of the central valley area. Eolian ash saturated the terrain. Browns Park deposits filled Saratoga Valley to the 9000 foot level and extended across the range margins. The Medicine Bow surface was cut by streams at the cessation of Browns Park deposition during late Miocene time.

Between late Miocene and Pleistocene time, the area was altered by extensional faults and warps probably related to regional uplift. Horsts rose at least 800 feet along the west side of the valley as Browns Park sediments were correspondingly downfaulted into half grabens, such as Big Creek Park, within the flank of the Park Range. All streams were rejuvenated; most were superimposed. Glaciers occupied the bordering highlands and fed outwash streams that covered the valley floors with coarse rocks. Landslides and surface cutting complete the modern scene.

Progressive contact metamorphism of the Middle Proterozeic Castner Marble, Franklin Mountains, West Texas

WILLIAM F. THOMANN Department of Geosciences, Texas A&I University, Kingsville, Texas 78363
ROBIN L. HOFFER Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79963

Pages
71-80

Keywords
mineral assemblages, Castner, Texas, Franklin, intrusive, metamorphism

Abstract
The Proterozoic Castner Marble occurs as roof pendants within units of the Red Bluff Granite Complex. This formation, which is the oldest recognized metasedimentary unit in the Franklin Mountains, Texas, was a siliceous, dolomitic limestone with interbedded mudrocks prior to mineralogical alteration by granite intrusions. Units of the epizonal Red Bluff Granite Complex engulfed and contact metamorphosed the Castner Marble resulting in mineralogical alteration and recrystallization of the original carbonate beds to marble, and mudrocks to hornfels. Despite metamorphism, primary sedimentary textures and structures are very well preserved throughout the formation. Diagnostic equilibrium mineral assemblages in the marble indicate three zone assemblages of increasing grade toward intrusive contacts. All mineral assemblages include calcite, phlogopite, dolomite as well as diagnostic minerals of:

A. Low Grade: tremolite + actinolite + andradite + epidote
B. Medium Grade: diopside + andradite
C. High Grade: (a) forsterite + diopside and (b) wollastonite + diopside

Highest temperatures during contact metamorphism were between 600¡C and 620¡C, with a lithostatic pressure of at least 0.5 kilobars.