Southern Rocky Mountain Colorado Plateau Distal Ejecta

Southern Rocky Mountain Colorado Plateau Distal Ejecta

Please proceed to revised paper at http://www.scribd.com/tmcelvain_1

 


 

 

 

Southern Rocky Mountain Colorado Plateau Impact Structure and Distal Ejecta

The red dots are locations where I have collected samples of the Ogallala Formation, and the yellow dots are locations where I have found evidence of shock metamorphism that I believe is related to an impact structure.

 

The immense crater formed by the comet or asteroid left a crater approximately six hundred miles in diameter including the Southern Rocky Mountains and Colorado Plateau. An impact of this size would have to have deposited a layer of distal ejecta. The orange dots on the above map on the high plains mark the locations that I collected or attempted to collect samples of the basal eolian sand in the Ogallala Formation.

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

The blue flags on the above map mark the locations where I have found planar microstructures (PM's) in the basal eolian sand of the Ogallala Formaton that fit the scale of planar deformation features (PDF's).

 

Ogallala Formation

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

The above illustration marks the present location of the Ogallala Aquifer published in Upper Arkansas River Conversation Project Reconnaissance Study, August 2005, Prepared by Wright Water Engineers, Inc. and Spronk Water Engineers, Inc.

 

The Ogallala formation and the gravels associated with it are an enigma. The slope of the Ogallala formation is 10 to 11 feet per mile, or approximately 1 foot in 528 feet, or approximately 1 inch in 50 feet. A stream flowing across this plane would barely move much less carry gravel or sand some 300 miles from the source. One easy assumption is that all the sand and gravel is distal ejecta; however, there are other problems with this assumption. There is fossil evidence in some locations of evolution over the 20 plus or minus years since the base of the Ogallala was laid down. I do not know how this dilemma could be solved except by reworking of the sediment, or that the impacts were not coeval but happened over millions of years. At present it is enough to say that the age of the earliest deposition conforms to the 21.5 plus or minus year old age of the proposed Southern Rocky Mountain and Colorado Plateau Impact Event.

The following excerpts from the article Downloaded from gsabulletin.gsapubs.org on 20 May 2009 illustrate the magnitude of the enigma concerning the Ogallala Formation.

Geological Society of America Bulletin Post-Paleozoic alluvial gravel transport as evidence of continental tilting in the

 

Post-Paleozoic alluvial gravel transport as evidence of continental
tilting in the U.S. Cordillera
Paul L. HellerKenneth Dueker Margaret E. McMillan

Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

ABSTRACT

The western United States contains three thin but remarkably widespread alluvial conglomeratic units that record episodes of large-scale tilting across the U.S. Cordilleran orogen in post-Paleozoic time. These units are: (1) the Shinarump Conglomerate of Late Triassic age exposed in northern Arizona and adjacent parts of Utah, Nevada, and New Mexico; (2) Lower Cretaceous gravel deposits that overlie the Morrison Formation throughout the Rocky Mountain region; and (3) the gravel-rich parts of the Miocene-Pliocene age Ogallala Group in western Nebraska and adjacent southeastern Wyoming. Paleoslopes of the rivers depositing these units were in the range of 104 to 103, based on paleohydraulic calculations. However, depositional thickness trends of these units are not sufficient to have generated such steep paleoslopes. Thus, long wavelength tilting of the Earth’s surface must have occurred for these gravels to be transported. Although these units were deposited adjacent to large tectonic features, including an evolving and migrating continental arc, and, for the Ogallala Group, the northward-propagating Rio Grande Rift, the tilting occurred over wavelengths too broad to be directly generated by these features. These widespread gravel units attest to the interplay between the creation of subduction-related isostatic and dynamic topography and continental sedimentation. Hence, paleotopography, as determined from calculated transport gradients of sedimentary deposits, provides a

E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Present address: University of Arkansas at Little Rock, Department of Earth Sciences, Little Rock, Arkansas 72204, USA.

means of relating constructional landforms to mantle-driven processes.

Keywords: paleohydraulics, tectonics, dynamic topography, U.S. Cordillera, conglomerate, paleotopography, syntectonic sedimentation.

INTRODUCTION

A common characteristic of synorogenic alluvial conglomerates is the relatively limited distance they prograde from their uplifted source areas out into the adjacent basins— most are found within a few tens of kilometers of their associated mountain fronts (Fig. 1, A– G). The restricted distance of gravel deposition reflects the long-term balance between the rate of delivery of coarse sediment supply and basin subsidence rate that acts to trap the gravel. Since on an elastic plate both supply and subsidence are proportional to the size of adjacent mountain belts, it is no surprise that basin sedimentation patterns have a similar length scale.

In contrast, a few conglomeratic units seen in the U.S. Cordillera are unusually widespread in distribution (Stokes, 1950; Stewart et al., 1972a; Heller and Paola, 1989), being deposited over many hundreds of kilometers downstream from their source areas (Fig. 1), yet being relatively thin over their entire extent. These gravels are related to the temporal and spatial pattern of subduction and late Cenozoic uplift of the central and southern Rocky Mountains. We argue that the occurrence of these thin, widespread gravel units does not reflect climatic controls, but must record times of large-wavelength/low-amplitude tilting of the continental interior. Thus, we believe they are the stratigraphic records of deep processes affecting the western U.S. during post-Paleozoic time.

 

Planar Microstructures possible Planar Deformation Features

The following photomicrographs are of quartz grains found in the basal eolian sand in the Ogallala Formation collected from locations marked with the blue flag in the above map. In most of the photomicrographs the quartz grains have two intersecting sets of fresh planar microstructures (PM's) that fit the scale of planar deformation features PDF's. I do not see how the shocked sand grains could have eroded from a shocked terrain. I believe the sand was deposited in a cloud of fine grained ejecta and was deposited over this enormous area. The sand probably was reworked by the wind which removed the fines. With regard to the upper beds of silt sand and conglomerate they are most likey fluvial deposits, but how they reached their final destination is still and ennigma.

Distal Evecta - Southern Rocky Mountain Impact Structure

 

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure

 

Distal Ejecta - Southern Rocky Mountain and Colorado Plateau Impact Structure